Playas of the Great Plains (Peter T. Flawn Series in Natural Resource Management and Conservation)

PLAYAS OF THE GREAT PLAINS NUMBER THREE Peter T. Flawn Series in Natural Resource Management and Conservation PLAYA...

Author: Loren M. Smith

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PLAYAS OF THE GREAT PLAINS

NUMBER THREE

Peter T. Flawn Series in Natural Resource Management and Conservation

PLAYAS OF THE GREAT PLAINS

loren m. smith

UNIVERSITY OF TEXAS PRESS AUSTIN

The Peter T. Flawn Series in Natural Resource Management and Conservation is supported by a grant from the National Endowment for the Humanities and by gifts from the following donors:

Jenkins Garrett Edward H. Harte Houston H. Harte Jess T. Hay Mrs. Lyndon B. Johnson Bryce & Jonelle Jordan Ben F. & Margaret Love Wales H. & Abbie Madden

Sue Brandt McBee Charles Miller Beth R. Morian James L. & Nancy H. Powell Tom B. Rhodes Louise Saxon Edwin R. & Molly Sharpe Larry E. & Louann Temple

Copyright © 2003 by the University of Texas Press All rights reserved Printed in the United States of America First edition, 2003 Requests for permission to reproduce material from this work should be sent to Permissions, University of Texas Press, Box 7819, Austin, TX 78713-7819. The paper used in this book meets the minimum 䊊 requirements of ANSI/NISO Z39.48-1992 (R1997) (Permanence of Paper). LIBR ARY OF CONGRESS CATALOGING-IN-PUBLICATION DATA

Smith, Loren M. Playas of the Great Plains / Loren M. Smith. p. cm. — (Peter T. Flawn series in natural resource management and conservation ; no. 3) Includes bibliographical references and index. ISBN 0-292-70534-4 (cloth : alk. paper) — ISBN 0-292-70177-2 (pbk : alk. paper) 1. Wetland ecology—High Plains (U.S.) 2. Playas— High Plains (U.S.) I. Title. II. Series. QH104.5.G7 S65 2003 577.680978 — dc21 2003002153 Cover photo by Wyman Meinzer, courtesy of the U.S. Fish and Wildlife Service.

FOR JANIECE

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CONTENTS

List of Illustrations ix List of Tables xiii Preface xv PLAYAS AND THEIR ENVIRONMENT CHAPTER 1

What Is a Playa? 3

CHAPTER 2

Origin and Development 29 ECOSYSTEM ASPECTS

1

43

CHAPTER 3

Flora 45

CHAPTER 4

Fauna 66

CHAPTER 5

Structure, Function, and Diversity 108 CONSERVATION ASPECTS

139

CHAPTER 6

Historical, Cultural, and Current Societal Value of Playas 141

CHAPTER 7

Threats to Proper Function of Playas 160

CHAPTER 8

Conservation Past, Present, and Future 177 Appendix 202 References 219 Index 247

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LIST OF ILLUSTR ATIONS

FIGURE 1.1

FIGURE 1.2 FIGURE 1.3 FIGURE 1.4

FIGURE 1.5 FIGURE 1.6 FIGURE 1.7 FIGURE 1.8 FIGURE 1.9 FIGURE 1.10 FIGURE 2.1

FIGURE 2.2 FIGURE 2.3

An area of the central Southern High Plains showing the highest density of playas in the Great Plains 4 The Great Plains of North America with three major grassland zones 5 Aerial view of an individual playa illustrating its circular form 9 Cross-section drawing of typical Southern High Plains playa showing little elevation change in the basin 10 Location of the Southern High Plains, or Llano Estacado, in the Great Plains 11 Nebraska’s four areas of playas 16 Playas with pits excavated in them to aid with irrigation of the surrounding watershed 17 A modiﬁed playa in the Comanche National Grassland, Baca County, Colorado 18 Modiﬁed playa in Rainwater Basin of southcentral Nebraska 19 Agriculture in the western Great Plains is dependent on irrigation 27 A typical circular-shaped playa in the Southern Great Plains and typical oblong-shaped playa of the Rainwater Basin 30 The northwestern escarpment, or “caprock,” of the Southern High Plains 32 Sections of topographic maps from the vicinity of Petersburg, Texas 34

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FIGURE 2.4

A “buffalo wallow” in the eastern portion of the Texas Panhandle (1904) 37 Lunette on the southeast side of a Rainwater Basin playa 41 A typical Southern Great Plains playa showing two vegetation zones 52 Windrow of seeds in a playa 53 Counties sampled for ﬂora description in the Southern Great Plains 60 Vegetation zones in Rainwater Basin wetlands 64 Common invertebrates sampled in playas 68 Three of the most common amphibians in Great Plains playas 73 Playas throughout the Great Plains serve as important migration habitat for more than 30 species of shorebirds 84 Generalized shape of migration corridor for many waterfowl species in the Central Flyway during spring 86 Dabbling ducks often occur in tremendous densities on playas during migration 88 Dabbling duck species form pair bonds while wintering in playas 92 Grasshopper sparrows, mallards, and American coots nesting in playa basins 99 Hispid pocket mice and black-tailed prairie dogs, common mammals in the playa watershed 106 Suggested food web of a wet playa 121 Decomposition rates of pink smartweed 123 Graphical illustration of the weak relationship between playa area and total species richness 132 Southern High Plains escarpment 143 Archaeological site in a saline lake wetland 147 Cattle watering in a Texas Panhandle playa (1904) 152

FIGURE 2.5 FIGURE 3.1 FIGURE 3.2 FIGURE 3.3 FIGURE 3.4 FIGURE 4.1 FIGURE 4.2 FIGURE 4.3

FIGURE 4.4

FIGURE 4.5 FIGURE 4.6 FIGURE 4.7 FIGURE 4.8

FIGURE 5.1 FIGURE 5.2 FIGURE 5.3

FIGURE 6.1 FIGURE 6.2 FIGURE 6.3

LIST OF ILLUSTRATIONS

FIGURE 6.4 FIGURE 6.5 FIGURE 7.1 FIGURE 7.2 FIGURE 8.1 FIGURE 8.2 FIGURE 8.3

Abandoned farmstead in a wheat ﬁeld in the western High Plains 153 Urban playa in Lubbock, Texas 157 Cross section of a playa showing sediment inﬂuence 163 Playas associated with cattle feedlots receive signiﬁcant fecal runoff 170 Education programs for schoolchildren to promote playa conservation 182 A native vegetation buffer strip being established in a playa watershed 188 A playa in Texas that has been “protected” through a conservation easement 195

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LIST OF TABLES

TABLE 1.1 TABLE 1.2

TABLE 3.1 TABLE 3.2 TABLE 4.1 TABLE 4.2 TABLE 4.3 TABLE 4.4

TABLE 4.5 TABLE 5.1 TABLE 5.2 TABLE 5.3 TABLE 5.4

Number and total area of playas in 54 counties 14 Mean daily temperature and mean monthly precipitation for selected playa regions, 1961– 1990 22 Common playa algae and macroalgae in the Southern High Plains 46 Playa vegetation classiﬁed into 14 physiognomic types 57 Amphibians inhabiting playas of the Great Plains 74 Shorebirds associated with Great Plains playas during migration 81 Waterfowl species observed migrating through the playa lakes 85 Esophageal foods from hunter-shot northern pintails and from ducks collected while observed feeding 94 Mammal species likely associated with Great Plains playas 104 Range of water variables found in playas of the Southern High Plains 111 Mean aboveground standing crop of ﬁve common playa wetland plants 117 Relationship between plant species diversity and playa area in the Southern Great Plains 131 Relationship between plant species diversity and playa area when only wetland plant species are included in the analysis 133

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TABLE 5.5

Mean plant species diversity and richness for all plant species and wetland species in playas 135 Sediment depth, sediment volume, and volumeloss ratio of playas in the ﬁne- and mediumtextured soil zones 164

TABLE 7.1

PREFACE

I

n December 1983, the ﬁrst time I visited the Southern High Plains, I was interviewing for an assistant professor position at Texas Tech University. Upon landing at the airport in Lubbock, I remarked to my host, Henry Wright, “Wow, this is a ﬂat place.” I had been warned by others before my visit that “you will probably ﬁnd this one of the ﬂattest places you’ve ever been.” Certainly many people ﬁnd ﬂat landscapes less visually appealing than, for example, mountains. But it is this very “level” landscape that promotes the formation of playa wetlands and what draws me to the Plains. From a natural history perspective they are one of the least studied ecoregions in North America. I have had much help in the past 19 years studying playas and prairie ecology. In particular I want to thank the United States Fish and Wildlife Service and the Caesar Kleberg Foundation for Wildlife Conservation. Their ﬁnancial support was there at the beginning and continues through to this day. We owe a great deal of what we know about playas to these two groups. Jeff Haskins with the Fish and Wildlife Service has been very supportive and helpful especially from this ﬁnancial standpoint. Other agencies or groups that have funded some of our studies on playas and their ecology include the Texas Parks and Wildlife Department and the Playa Lakes Joint Venture, through some of its partners (in addition to those listed above): Kansas Wildlife and Parks Department, Colorado Division of Wildlife, New Mexico Department of Game and Fish, Oklahoma Department of Wildlife Conservation, Phillips Petroleum, and Ducks Unlimited. I have been fortunate to have been associated with a number of hardworking graduate students over the years whose work on playas and their associated biota has made much of this book possible. In reverse chronological order they include Doug Sheeley, David Price,

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Jim Bergan, David Haukos, Gene Rhodes, Hong-Ren Luo, Craig Davis, Jim Cathey, Jim Anderson, Warren Conway, Matt Gray, Lisa Brennan, and Dana Ghioca. Craig Davis and Jim Anderson reviewed some of the invertebrate information, and Matt Gray helped with computer graphics. Jim Anderson also reviewed decomposition information. David Haukos and Ted LaGrange of the U.S. Fish and Wildlife Service and Nebraska Game and Parks Commission, respectively, provided essential data on regional playa issues and ﬁne camaraderie on numerous ﬁeld trips. My wife, Janiece, and our children, Clayton and Jessica, helped on numerous occasions in the ﬁeld and tracked down essential information contained in this book. Janiece also proofed several tables. John Taylor helped with some historical and cultural insight especially by directing me to his “Auntee” de Baca’s book. I thank Rick Gilliland for the gift of the frontispiece. Kay Arellano graciously typed the book manuscript. Several individuals provided help with maps, ﬁgures, or photographs including Ted LaGrange, Randy Stutheit, Carlton Britton, Jerry Winslow, Jim Ray, Jim Steiert, David Haukos, Mike Gilbert, and Monte Monroe of the Southwest Collections at Texas Tech. Mike Fritz and Rick Schneider checked Nebraska fauna lists. Dianne Hall provided helpful comments on playa invertebrates and biogeography. Most of the published literature on playas exists on studies conducted in the Southern Great Plains. Playas farther north in the Plains deserve more study, especially those in Nebraska, and I hope I have not slighted them too much. The more extensive treatment of some topics may appear unbalanced compared to others (e.g., birds vs. algae), but this is primarily related to availability of information. C. C. (Tex) Reeves Jr., Gene Wilde, Mike Gilbert, and Jim Ray read portions of the manuscript, while David Haukos, Bob Fullilove, and Ted LaGrange provided comments on the entire manuscript. I certainly appreciate their thoughts. Lynne Chapman and William Bishell at the University of Texas Press provided assistance throughout the editorial process. I am thankful for the help of all listed above, but as always I accept responsibility for potential errors that are present.

PLAYAS OF THE GREAT PLAINS

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PLAYAS AND THEIR ENVIRONMENT

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CHAPTER 1

WHAT IS A PLAYA?

“P

laya” and its synonym “playa lake” are a couple of those vague terms like “swamp” or “marsh” that are generally used to describe some type of wetland. Playa is also a Spanish word with an English translation of shore or beach. The translation provides little help in describing a playa. If the titular question is asked relative to a particular geographic region, such as New Mexico, it becomes somewhat easier to answer, though the result is still not certain. When individuals use wetland terms like swamp, playa, or lake, their intended meaning generally applies to a local region, but to someone from outside these local areas the terms may carry a different sense. A farmer in Wisconsin, for example, would likely form a much different mental image of “lake” than a West Texas farmer. The Texas farmer might envision a wet, shallow low spot in a ﬁeld or pasture (such as the playa in ﬁg. 1.1), whereas the Wisconsinite would likely see a deep ﬁshing lake. Many wetland ecologists also have little understanding of what a playa is. Mitsch and Gosselink in their book Wetlands deﬁne playa as a “term used in the southwestern United States (U.S.) for marshlike ponds similar to potholes but with a different geologic origin” (2000, 41). In the National Research Council report Wetlands characteristics and boundaries, a playa lake is deﬁned as a “shallow depression similar to a prairie pothole, abundant on the Southern High Plains on a tableland south of the Canadian River in Texas and New Mexico, characterized by annual or multiyear cycles of drydown and ﬁlling” (1995, 288). Among other incorrect assumptions, both of these deﬁnitions make the naive assertion that playas are similar to prairie potholes. Although Mitsch and Gosselink were quite correct that playas and prairie potholes differ in their geologic formation and the National Research Council report was correct that the hydroperiod (the

4

PLAYAS AND THEIR ENVIRONMENT

Figure 1.1 An area of the central Southern High Plains showing the highest density of playas ( 1/sq mi) in the Great Plains. (Photo by author.)

length of time a wetland has surface water) of a playa is erratic, playas, as this description will illustrate, are uniquely different from prairie potholes and any other wetland system. The need to equate playas to prairie potholes probably arose from the fact that more has been written about the latter, and most other wetlands in the United States, than about playas and that both of these wetland types are found in the Great Plains (ﬁg. 1.2). Indeed, the paucity of literature on playas is likely because they occur primarily on private land in the more sparsely populated portions of the Great Plains, where there is little governmental ownership of wetlands. Thus, they have received less study than wetlands in more densely populated regions. Although one of the most endangered ecoregions in North America, the Great Plains itself is poorly understood relative to other U.S. ecoregions (Samson and Knopf 1996). Geologists also have not reached a general consensus on the meaning of the term playa. Motts stated, “Most American geologists would probably consider a playa to have four characteristics: (1) an area occupying a basin or topographic valley of interior drainage, (2) a smooth barren surface that is extremely ﬂat and has a low gradient, (3) an area infrequently containing water that occurs in a region of low

WHAT IS A PLAYA?

5

rainfall where evaporation exceeds precipitation, and (4) an area of fairly large size (generally more than 2,000 –3,000 feet [610 –914 m] in diameter)” (1970, 9). Motts continued, “The barren surface, devoid of vegetation and abundant gravel, is a distinctive feature of a ‘playa’. . . . Thousands of small, topographically enclosed areas ranging from a few feet to several hundred feet in diameter are scattered throughout western United States, yet one would hesitate to call them playas” (9). This deﬁnition would exclude most of the playas in the Great Plains, where playas are the most numerous. More recently Rosen deﬁned playas from a geologic perspective: “as an intracontinental basin where the water balance of the lake (all sources of precipitation, surface water ﬂow, and groundwater ﬂow minus evaporation and evapotranspiration) is negative for more than

Figure 1.2 The Great Plains of North America with three major grassland zones (after Küchler 1975). The highest density of playas occurs in the southern short-grass prairie gradually decreasing into the mixed-grass prairie. However, some playas exist in the tall-grass prairie of Nebraska.

6

PLAYAS AND THEIR ENVIRONMENT

half the year, and the annual water balance is also negative” (1994, 1). He further stated: “The playa surface must act as a local or regional discharge zone. Evidence of evaporite minerals will generally be present in parts of the basin” (1). This deﬁnition also would exclude those playas of the Great Plains. Noting that this classiﬁcation would not encompass playas in western Texas and eastern New Mexico (but not other portions of the Great Plains) he created a “special case,” which he termed “recharge playa.” As the special case name implies, these playas would not receive water (discharge) from ground sources (e.g., springs) but could supply water (recharge) to underground aquifers. This “special case” is true for the overwhelming majority of playas not just in Texas and New Mexico but for all the Great Plains. From a numbers and area perspective, the most abundant group of playas in the world, those of the Great Plains, should not be listed as a “special case”; rather, those that occur elsewhere covering less area probably should be the exception to the rule. Moreover, most geologists that have studied the playas in the Great Plains have not adopted Motts’s or Rosen’s deﬁnition (e.g., Osterkamp and Wood 1987; Gustavson et al. 1994; Reeves and Reeves 1996). Regardless of these previously mentioned misunderstandings, the terms “playa” and “playa lake” have generally referred to various types of shallow wetlands in prairie, semiarid, or arid environments throughout the world (Neal 1975; Bolen et al. 1989; Rosen 1994). As noted above, their ecology, hydrology, and geology, however, varies greatly among geographic regions. Playas have even been hypothesized to have once existed on Mars (Hartmann 1998, 24, 26). However, for the most numerous group of playas, those found in the Great Plains, I deﬁne them as shallow, depressional recharge wetlands occurring in the Great Plains region that are formed through a combination of wind, wave, and dissolution processes with each wetland existing in its own watershed. As the words depressional and recharge imply, Great Plains playas only receive water from precipitation and runoff. Naturally water is only lost through evaporation, transpiration, and recharge. Within the Great Plains, playas as thus deﬁned, occur with highest densities in the High Plains portion of the Southern Great Plains of eastern New Mexico, western Texas, the Panhandle of Oklahoma, southeastern Colorado, and southwestern Kansas (ﬁg. 1.2) but are also scattered throughout some northern portions of the Plains and

WHAT IS A PLAYA?

7

western mountain states (Motts 1970; Osterkamp and Wood 1987; MacKay et al. 1990; Brough 1996; LaGrange 1997). Because the vast majority of playas occur in the Great Plains, from Wyoming and Nebraska to Texas and New Mexico, and most scientiﬁc study of playas has occurred there, the description of playas is focused on that geographic region. DESCRIPTIONS OF GREAT PLAINS PLAYAS CLASSIFICATION

The most commonly used system to classify wetlands in the United States today is the Department of Interior’s Cowardin et al. (1979) system. It is a hierarchical method similar to taxonomic classiﬁcation with wetlands being categorized by system, class, plant community and substrate, water regime, and water chemistry. Other modiﬁers exist to note physical alterations to wetlands such as excavations and dikes. Following the Cowardin et al. classiﬁcation, playas in the Great Plains are categorized as palustrine or lacustrine systems. Palustrine wetlands generally are dominated by woody plants or persistent emergent plants. (Rooted herbaceous plants that protrude above the water’s surface are emergent vegetation.) Any nontidal wetland with more than 30% persistent emergent vegetation is palustrine. Palustrine wetlands that do not have greater than 30% persistent vegetation must be less than 8 hectares (20 ac), less than 2 meters (about 6.6 ft) in the deepest portion of the basin, and contain no active wave-formed shoreline. Lacustrine wetlands are generally larger than 8 hectares but this system cannot have persistent emergent plants exceeding 30% of the basin. This type of wetland can be placed in either littoral (less than 2 m water depth with no persistent plants) or limnetic (greater than 2 m deep) classes. These wetlands can be less than 8 hectares if an active wave-formed shoreline is part of the wetland boundary or if the deepest part of the basin exceeds 2 meters in water depth. Because as Guthery and Bryant (1982) noted, the average area of a playa in the Southern Great Plains is 6.3 hectares (15.5 ac), and most playas are shallow ( 2 m), the majority of Great Plains playas are palustrine. Fewer are lacustrine littoral playas, and even fewer are lacustrine limnetic playas; but exact percentages have not been determined. Most playas are further classiﬁed as “emergent vegetated wet-

8

PLAYAS AND THEIR ENVIRONMENT

land.” Again, however, lacustrine littoral wetlands cannot have persistent emergent vegetation exceeding 30% of the basin, so their vegetation is necessarily classiﬁed as nonpersistent (e.g., annual). Some playas also may be classiﬁed as possessing “aquatic bed” vegetation, which refers to plants living completely in or on the water. Rooted submerged and/or other ﬂoating plants, including algae, are aquatic bed vegetation. Within the palustrine system, there are even a few playas that can be placed into the “scrub/shrub” or “forested” wetland class. Scrub/shrub are woody plants less than 2 meters (6.6 ft) high, whereas forested wetlands have trees taller than 2 meters. A few may not have any vegetation but have open water with “unconsolidated bottoms.” Today, the water regime in most playas is termed “temporarily” or “seasonally” ﬂooded. Temporarily ﬂooded playas may contain water for only a few weeks during the growing season while seasonally ﬂooded playas have water present during extended periods during the growing season. Historically playas probably held water for longer periods than today; this will be discussed later. There are a few “semipermanently” ﬂooded playas. These wetlands will have water in them throughout most years. Other commonly used modiﬁers for playas, in this classiﬁcation system, are related to human-caused physical modiﬁcations such as “excavations” and “dikes.” Indeed, for many playas, portions of the same wetland can be classiﬁed differently due to physical modiﬁcations, such as excavations. APPEARANCE

Although the Cowardin et al. classiﬁcation system allows various wetland types to be clearly categorized by function, and compared among regions, it does not greatly enhance the reader’s ability to develop an accurate vision of a Great Plains playa. To recount, Great Plains playas are mainly freshwater wetlands, dependent on precipitation from storms (or irrigation runoff) for surface water, self-contained in their own closed watershed, and not recharged by elevated groundwater (e.g., ﬁgs. 1.1 and 1.3). Wetlands in the Great Plains that have springs or receive groundwater additions to their surface water are not generally considered to be playas. Because playa watersheds are not connected to one another and storms can be very localized in the Great Plains, a playa in one location may be full of water while only a short distance away other playas will be

WHAT IS A PLAYA?

9

Figure 1.3 Aerial view of an individual playa illustrating its circular form. (Photo by author.)

dry. They are shallow, usually only 1.5 meters (5 ft) deep, at most. Playas have erratic hydroperiods, drying and ﬁlling with water frequently within most years. These water ﬂuctuations usually promote diverse herbaceous plant growth. However, whether the vegetation is annual or perennial, terrestrial or aquatic, depends on how long the playa has been with or without water (Guthery et al. 1982; Haukos and Smith 1997). Playas are also small, with 87% being less than 12 hectares (30 ac) in area in the Southern Great Plains (Guthery and Bryant 1982). Although the average playa is small, the range in individual playa area is large with some less than a hectare to some more than 4 square kilometers ( 1 sq mi). Although scientists know little about the area of individual playas elsewhere in the Great Plains, the playas of southwest Nebraska are also small (LaGrange 1997). The remaining Rainwater Basin playas of south-central Nebraska may be a bit larger than those farther south in the Great Plains. Most playas in the Southern Great Plains appear almost perfectly circular (ﬁg. 1.3). Indeed, Luo (1994) devised equations to calculate playa wetland area while studying sedimentation rates; the resulting

10

PLAYAS AND THEIR ENVIRONMENT

Figure 1.4 Cross-section drawing of typical Southern High Plains playa showing little elevation change in the basin.

formulas were very similar to the simple geometric equation used to calculate the area of a circle. North of the Southern Great Plains, playas often do not appear as circular and may be more irregularly shaped. This suggests playas in northern portions of the Great Plains possibly may have been formed through a combination of different processes than those in the south. Interestingly, though, some playas in the central and northern Plains share some unique physical features with those in the south. Many of the playas in the Rainwater Basin of Nebraska and the Southern Great Plains of Texas and New Mexico have small ridges or dunes on their east and south sides termed lunettes (Kuzila and Lewis 1993; Sabin and Holliday 1995). These are addressed in Chapter 2 when playa formation is discussed. Western Great Plains playas are also structurally simple in that there is generally a gentle slope starting from the edge of the hydric soil of the wetland to a level bottom, from which the elevation does not change (Luo et al. 1997; ﬁg. 1.4). This is different from many other wetland types, such as riverine oxbows or prairie potholes, which have relatively irregular horizontal shapes and at least some elevational heterogeneity as one travels across the wetland basin. DISTRIBUTION AND NUMBERS

Until relatively recently, few playas were thought to exist in the Great Plains outside the Southern Great Plains (USFWS 1988). The Southern Great Plains of southeastern Colorado, southwestern Kansas, western Texas, the Oklahoma Panhandle, and eastern New Mexico was traditionally referred to as the “Playa Lakes Region” by numerous agencies and in many scientiﬁc papers (e.g., Nelson et al. 1983; USFWS 1994). Although not occurring in the same high density as farther south, playas do exist, essentially continuously, into northwestern Kansas, northeastern Colorado, eastern Wyoming, and western Nebraska (Holpp 1977; Osterkamp and Wood 1987; Brough 1996; LaGrange 1997). The majority of these

WHAT IS A PLAYA?

11

Great Plains playas occur in the High Plains extending from western Nebraska and eastern Wyoming south to western Texas and eastern New Mexico. True to playa form, however, they cannot be categorized that easily because some playas also exist farther east in the Low Plains. Indeed, the Rainwater Basin wetlands of south-central Nebraska were classiﬁed as playas by LaGrange (1997). The highest density of playas occurs in the Southern High Plains, in an area south of the Canadian River known as the Llano Estacado (ﬁg. 1.5). The largest plateau in North America at 82,000 square kilometers (31,700 sq mi), the Llano Estacado has been described as one

Figure 1.5 Location of the Southern High Plains, or Llano Estacado, in the Great Plains. This region contains more than 20,000 playas, the most in the Great Plains. (Modiﬁed from Sabin and Holliday 1995, courtesy of Annals of the Association of American Geographers, Blackwell Science Ltd.)

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PLAYAS AND THEIR ENVIRONMENT

of the largest featureless landscapes in the United States (Holliday 1991). The plateau is surrounded by relatively abrupt escarpments on the west, north, and east sides ranging from 50 to 200 meters (160 – 660 ft). On the south side, however, the plain gradually fades into the Permian Basin enough so that it is difﬁcult to determine the Llano edge. Elevation of the Southern High Plains declines from approximately 1,500 meters (5,000 ft) in the northwest to 725 meters (2,300 ft) in the southeast. Playas are the most ubiquitous geomorphic and hydrological feature on the Llano Estacado (Sabin and Holliday 1995) with playa densities approaching 1 per 2.6 square kilometers (1/sq mi) (Guthery et al. 1981). It is difﬁcult to imagine such a wetland density when one is at ground level, where the Llano is so ﬂat that a person may not be able to see a playa that exists just a few hundred meters distant. From the air, however, especially after a rain when most playas have water, the sight is revealing (ﬁg. 1.1). Within the Southern High Plains, playa density and area varies as a result of differences in soil texture and annual precipitation. The average area of playas increases from the southwestern portion of the region to the northeast, which follows precipitation patterns (Grubb and Parks 1968; Allen et al. 1972). However, soil texture also varies across this gradient with the southern one-third of the Llano being coarse textured, the middle being medium textured, and the northern third being ﬁne textured (Allen et al. 1972; Sabin and Holliday 1995). Although the size of playas is greatest in the northeast Llano, the density of playas is highest in the medium-textured soil zone according to Guthery et al. (1981) or the coarse soils according to Sabin and Holliday (1995). This contradiction in playa density between Guthery et al. and Sabin and Holliday might simply be related to the manner in which playa density and soil zones were delineated in the two studies. Although a few other wetland types also occur on the Llano, including several riparian areas called “draws” and approximately 40 large “saline lakes” (Reeves 1976, 1990), they do not approach the numerical importance of playas. Sometimes saline lakes have been called playas, but they are not similar in hydrology, origin, or form to Great Plains playas and therefore are not typically considered to be playas by ecologists or regional geologists (Sabin and Holliday 1995). The exact number of playas existing in the Great Plains is unknown but certainly exceeds 25,000. Most of the estimates (or guesses) of the number of playas have been made for the Southern

WHAT IS A PLAYA?

13

Great Plains or that area traditionally called the Playa Lakes Region, as deﬁned above. Osterkamp and Wood (1987) suggested there were about 30,000 whereas Curtis and Beierman (1980) counted 24,600, and Reddell (1965) listed 37,000. These suggestions did not include playas outside the Southern Great Plains, such as in northeastern Colorado, northwestern Kansas, eastern Wyoming, and Nebraska. The estimate of 25,390 playas derived by Guthery and Bryant (1982) appears defendable for a portion of the so-called Playa Lakes Region because they actually counted playas in a 54-county region of the Southern Great Plains (table 1.1), using county soil-survey maps and other studies (Schwiesow 1965; Dvoracek and Black 1973), and conducted ﬁeld checks to verify map information. These playas comprise an area of approximately 165,000 hectares (410,000 ac). Areas that had been mapped with Randall, Lofton, or Ness clays were included as were those designated as “intermittent lakes” on soils maps. Guthery and Bryant (1982) did not include some potential playas that had Randall ﬁne sandy loam soils, or count playas in all counties of the Playa Lakes Region. Therefore, these estimates, because of their limited geographic coverage and other soil restrictions, should be considered conservative for the Southern Great Plains. The conservative nature of this estimate is further supported by more recent estimates of playa abundance by Sabin and Holliday (1995, 300), who derived playa numbers from topographic maps. They suggested that 25,000 playas was a realistic estimate for the Southern High Plains, an area smaller than that surveyed by Guthery and Bryant (1982). But as Sabin and Holliday noted, playas continue to defy accurate estimation by scientists: “Not all playas appear on topographic maps and not all depressions are seasonally dry lake basins. Soil surveys are helpful because of the unique soils found in the playas, but not all regions of the Southern High Plains have reasonably current published surveys” (1995, 290). Further, the Department of Interior’s National Wetlands Inventory, which has determined numbers and area of many of the different wetland types in the United States, has yet to complete the inventory of playas existing throughout the Great Plains. If playas outside the traditionally deﬁned Playa Lakes Region are included, estimates of playa numbers and area further increase. Four areas of playas are reported to exist in Nebraska: the Southwest Playas, Rainwater Basin, Todd Valley, and Central Table (LaGrange

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PLAYAS AND THEIR ENVIRONMENT

Table 1.1 Number and total area of playas in 54 counties of the Southern Great Plains

State, County

Number of Playas

Hectares

Texas Andrews Armstrong Bailey Briscoe Carson Castro Cochran Crosby Dallam Dawson Deaf Smith Donley Floyd Gaines Garza Gray Hale Hansford Hartley Hemphill Hockley Howard Hutchinson Lamb Lipscomb Lubbock Lynn Moore Ochiltree Oldham Parmer Potter Randall Roberts Sherman Swisher Terry

298 676 598 787 535 621 395 925 220 702 451 114 1,783 65 283 752 1,383 345 123 9 1,171 185 167 1,280 18 934 842 195 590 75 455 69 564 20 219 910 532

1,877 5,746 1,932 4,966 7,132 7,998 734 7,401 1,157 2,864 5,694 682 16,439 85 1,893 5,054 9,418 2,805 1,289 37 3,396 1,513 1,081 5,422 95 6,280 3,715 1,747 6,260 1,200 4,022 1,960 6,723 40 2,048 8,145 1,225

Acres

4,636 14,193 4,772 12,266 17,615 19,756 1,815 18,278 2,858 7,074 14,069 1,684 40,605 210 4,676 12,482 23,263 6,928 3,184 91 8,388 3,738 2,669 13,405 235 15,503 9,172 4,316 15,462 2,964 9,935 4,840 16,606 99 5,058 20,117 3,022

WHAT IS A PLAYA?

15

Table 1.1 (continued) State, County

Number of Playas

Wheeler Yoakum Colorado Baca Kansas Grant Haskell Meade Morton Seward Stanton Stevens New Mexico Curry Lea Quay Roosevelt Oklahoma Beaver Cimarron Texas Total

Hectares

Acres

10 38

— 76

— 187

198

675

1,668

232 701 712 58 294 676 133

752 2,755 3,645 430 1,734 1,900 746

1,857 6,805 9,004 1,062 4,284 4,692 1,843

524 1,175 228 535

3,553 2,036 2,002 2,140

8,775 5,030 4,945 5,286

84 264 237

732 1,193 1,951

1,807 2,947 4,818

25,390

166,395

410,994

Source: Modiﬁed from Guthery et al. 1981.

1997; ﬁg. 1.6). The area of Rainwater Basin playas remaining in southcentral Nebraska is estimated at 13,807 hectares (34,103 ac; Raines et al. 1990) in approximately 400 relatively large wetlands and an unknown number of smaller basins (Schildman and Hurt 1984). In the Todd Valley and Central Table regions, there are approximately 716 hectares (1,769 ac) and 1,418 hectares (3,503 ac) remaining, respectively (LaGrange 1997, 13). The amount of playa habitat remaining in southwestern Nebraska is unknown, as is the area in northwestern Kansas, northeastern Colorado, and eastern Wyoming. Brough (1996), however, noted that there were at least 450 playas in the Powder River Basin of Wyoming. Regardless of the exact number and area, when one considers there are more than 25,000 playa wetlands, covering more than 180,000 hectares (445,000 ac), in a mostly semiarid

16

PLAYAS AND THEIR ENVIRONMENT

Figure 1.6 Nebraska has four areas of playas. (Modiﬁed from LaGrange 1997, courtesy of Nebraska Game and Parks Commission.)

to arid, highly agriculturalized environment with few other wetlands, it is obvious playas are a keystone ecosystem central to the ecological integrity of the entire Great Plains. PLAYA MODIFICATIONS

Most playas have been hydrologically and ecologically altered in some form or fashion in the Great Plains. Unlike playas in the Rainwater Basin of Nebraska (LaGrange 1997) and a large expanse of wetlands throughout the midwestern United States (Prince 1997), however, it has been difﬁcult to tile or gravity-drain playas in the western Plains. Because the western Plains are so ﬂat and distance to surface drainage features, such as creeks or draws so great, it has been difﬁcult to effectively divert water completely away from these playas. However, terraces established in the watershed, in the name of soil conservation, have prevented signiﬁcant amounts of water from entering playas. Most commonly, land managers wishing to eliminate playas have leveled some with soil or intentionally pro-

WHAT IS A PLAYA?

17

moted soil erosion into others. The extent of this problem is just now being realized but the actual magnitude of wetland loss has not been quantiﬁed. Throughout the Great Plains most playas less than 4 hectares (10 ac) are farmed when they are dry. Other playas have been hydrologically modiﬁed. Guthery and Bryant (1982) estimated that 33% of all playas in the Southern Great Plains had been modiﬁed. Playas have been trenched or ﬁlled for road construction, used for catchment of cattle feedlot runoff, and for urban wastewater and stormwater storage. They have even been used as dumps for municipal and agricultural trash. However, the most common form of hydrological modiﬁcation has been pit excavation. Many of the larger playas have had deep pits dug into them to aid in irrigation of crops in the surrounding watershed (ﬁg. 1.7). As noted below, irrigation agriculture is practiced extensively throughout the region, and initially many playas were integrated into a row-ﬂood system (Bolen and Guthery 1982). Because playas are downhill from everything in the watershed, water collects there from precipitation and irrigation runoff (often termed

Figure 1.7 Many playas have had pits excavated in them to aid with irrigation of crops in the surrounding watershed. Note irrigation pump in the background. (Photo by author.)

18

PLAYAS AND THEIR ENVIRONMENT

Figure 1.8 A modiﬁed playa with pits and trenches in the Comanche National Grassland, Baca County, Colorado. (Photo by author.)

“tailwater”). By constructing deep pits in playas, the typical surface area of the water is decreased, evaporation losses are thus minimized, and water pumping costs are lowered relative to the cost required to pump water from the regional aquifer. Pits also have been constructed to reduce the surface-water coverage of playas permitting easier cultivation of the basin or to provide more consistent water for livestock (ﬁg. 1.8). Of the playas larger than 4 hectares (10 ac) in the Southern Great Plains, 69% have had pits constructed in them (Guthery and Bryant 1982). The percentage of playas with pits in central and eastern Nebraska (also called “dugouts” in Nebraska) is also very high (LaGrange 1997) (ﬁg. 1.9). Guthery et al. (1981) noted that most of the construction of pits in the Southern Great Plains had occurred from the mid-1960s through the 1970s. With changes in irrigation strategies, from traditional furrow ﬂooding to the current use of center pivots and underground drip irrigation, the rate of construction of new pits has been greatly reduced, and the consequent need for maintenance (e.g., dredging out sediments) of pits already constructed has diminished. These pits and trenches, as described in subsequent chapters, have a great inﬂuence on not only the hydrology but also the fauna and ﬂora depending on playas. The hydrology of a wetland shapes the entire ecosystem.

WHAT IS A PLAYA?

19

PLAYA SOILS

The majority of playas can be characterized by the presence of a speciﬁc hydric soil in their basin. In the Southern Great Plains most soils are Vertisol, with the Randall series occurring most frequently. However, the Lipan, Ness, Lofton, Stegall, and Pleasant series also indicate playa presence on county soil maps (Allen et al. 1972; Guthery and Bryant 1982; Nelson et al. 1983; Sabin and Holliday 1995; USDA 1996). These soils may not be substantially different from Randall but simply classiﬁed differently by different soil-survey teams. In Nebraska, the most common soils lining the basins of Todd Valley and Rainwater Basin playas are also nearly impervious clays in the Butler, Filmore, Scott, and Massie series (Gilbert 1989). The hydric soils of the Central Table and Southwest playas have been primarily listed as Scott and Filmore, although the taxonomy of some of these soils has been changed recently and are now listed in the Lodgepole series. Sometimes a playa exists on the landscape with the characteristic hydric soil but does not occur on the county soils map or occurs on the map without a soil designation. For example, in Baca County, Colorado playas often do not have a soils designation on the soils maps but are indicated merely as “intermittent lake.”

Figure 1.9 Modiﬁed playa in Rainwater Basin region of south-central Nebraska. (Photo by T. LaGrange, courtesy of Nebraska Game and Parks Commission.)

20

PLAYAS AND THEIR ENVIRONMENT

The clay mineralogy of Southern Great Plains playa soils is similar to that existing in the surrounding watershed soils (Allen et al. 1972). The dominant clays of playa soils and those in the immediate watershed are montmorillonite and illite (Allen et al. 1972; Dudal and Eswaran 1988). The clay fraction of playa basin soils usually exceeds 50%, frequently 80% at the playa center. These clay soils often undergo gleying, indicating soil that has experienced frequent inundation (Haukos and Smith 1996). Gleying is a feature indicative of hydric playa soils. Redox concentrations, such as bodies of iron/manganese oxides, are also indicative of hydric soils (USDA 1996). These concentrations include soft masses, nodules, pore linings, and concretions formed under anaerobic conditions caused by ﬂooding (USDA 1996, 26). Moreover, the clay of Southern Great Plains playa basins has a darker color easily distinguishable from soil in the surrounding uplands (Luo et al. 1999). The high clay content of playa soils makes them only very slowly permeable relative to the soils of the surrounding watershed. Playas, therefore, have an excellent water-holding capacity (Bruns 1974). This hydrologic feature is what makes them so important from a landscape and ecological standpoint. As playa soils dry, they form large cracks, and eroded soil from the surrounding watershed slumps into the cracks. When the soil becomes wet again, the sediments become mixed with the subsoil. For example, in an attempt to reconstruct vegetation in playas from the past 120 years, Rhodes and Smith (unpublished data) surmised that different depths of sediments from soil cores could be aged using Cesium137. The different-aged sediments could then be taken into a greenhouse and, because many upland and wetland plants have long-lived seeds (e.g., van der Valk and Davis 1979), the seeds could then be germinated. This would allow examination of recent historic changes in playa plant communities. However, the Cesium results were nonsensical. Because playa sediments mix so thoroughly during wind and ﬂooding events, older sediments are not necessarily deeper than younger ones and there is no consistent pattern in the mixing. THE GREAT PLAINS PLAYA SET TING

To comprehend playas and their ecology, one must understand their setting in the Great Plains landscape. Prior to European settlement, the Great Plains or prairie regions of the midcontinent of North America existed from east of the Mississippi

WHAT IS A PLAYA?

21

River to the front range of the Rocky Mountains and from Saskatchewan and Manitoba to Central Texas (ﬁg. 1.2). Today, the grasslands of the Great Plains are some of the most endangered ecosystems in North America (Samson and Knopf 1996), because of their conversion to one of the most intensively cultivated regions in the world (Samson and Knopf 1994). Climate in the Plains where playas occur is temperate in the northeast, to semi-arid in the west, and dry steppe in the south. Given the great latitudinal variation in the range of playas in the Great Plains, it is to be expected that the climate varies accordingly (table 1.2). The major consistency throughout the entire region, however, is that precipitation comes mainly from thunderstorms, is typically highest in May and June, stays relatively high although variable through summer, and then drops off in October. Most often, therefore, playas usually ﬁll with water in spring and summer. Average annual precipitation varies from a low in Midland, Texas, of 38 centimeters (15 in.) to a high in Grand Island, Nebraska, of 63 centimeters (25 in.), with amounts being higher in the eastern portions of the High Plains than in the west. However, in the Plains such “averages” are deceiving. Extremes in precipitation are the rule, average years uncommon, and droughts frequent in the Great Plains. Winters are relatively dry, although snow depths can often be substantial ( 25 cm; 1 ft). Potential evaporation rates vary from 284 centimeters (112 in.) in Midland, to 230 centimeters (90 in.) in Dodge City, to 165 centimeters (65 in.) in Grand Island. Evaporation and precipitation greatly inﬂuence the length of the playa hydroperiod, the native vegetation present, and the agricultural crops that are grown. PRAIRIE VEGETATION AND RECENT CHANGES

Prior to cultivation by European settlers, the Great Plains was a large continuous grassland, the largest vegetative province in North America (Sampson and Knopf 1994). Its vastness was interrupted with woody plants only rarely along stream and river courses (Weaver 1968). The region also was endowed with abundant wetlands (e.g., Stewart and Kantrud 1972; Prince 1997), including playas (Bolen et al. 1989). Ecologists, geologists, and economists have divided the Great Plains into three general grassland zones. From east to west is the tallgrass, mixed-grass, and short-grass prairie. Although many individuals have identiﬁed these three grassland zones, there is little general agreement on their actual boundaries and extent.

Table 1.2 Mean daily temperature and mean monthly precipitation for selected locations throughout the Great Plains where playas occur, 1961–1990 Kansas

Nebraska

Month

Texas

North Platte

Grand Island

C°

C°

cm

C°

cm

C°

1.2

1.24

1.7 6.4

1.57 3.96

cm

Dodge City

Goodland

Amarillo

Lubbock

Midland

cm

C°

cm

C°

cm

C°

cm

1.0

1.04

1.27 1.55

8.4

2.44

3.8 6.2 10.7

0.77 1.73 2.26

1.02

0.98 3.00

1.7 4.0

5.8

0.1 3.9

8.4 13.2

1.57 1.47

5.8 2.4

0.91 1.09

5.6 2.6

1.17 1.83

2.5

3.05

3.2

4.80

Apr.

9.0

5.05

10.5

6.35

12.6

5.21

9.7

3.30

13.8

2.51

16.2

2.46

18.1

2.11

May

14.6

8.71

16.3

9.70

17.9

7.70

15.0

8.86

18.6

6.30

20.8

5.97

22.7

5.03

June

20.0

8.56

22.0

9.93

23.6

7.87

20.7

8.10

23.4

9.40

25.1

6.99

26.4

3.94

July

23.3

7.77

24.8

7.19

26.8

8.23

24.2

7.29

25.9

6.65

26.7

6.02

27.8

4.32

Aug.

22.2

4.42

23.3

7.16

25.7

6.93

22.9

4.57

24.7

8.18

25.5

6.38

27.1

4.29

Sept.

16.3

4.17

17.7

7.24

20.6

4.85

17.8

3.99

20.6

5.05

21.7

6.60

22.9

6.65

Oct.

9.4

2.49

11.3

3.43

4.2

3.25

11.2

2.29

14.7

3.48

16.3

4.72

17.8

4.42

Nov.

1.9 4.3 8.9

1.68 1.19 49.02

3.11 3.7 10.0

2.64 1.80 63.25

6.2

2.11

1.75 1.04 46.23

7.8 2.7 13.8

1.75 1.09 49.68

1.91

1.65 54.58

3.7 1.4 10.5

9.9

0.3 12.9

4.8 15.6

1.35 47.37

11.4 7.0 17.4

1.75 1.42 38.00

Jan. Feb. Mar.

Dec. Annual

Source: National Oceanic and Atmospheric Administration 1999.

WHAT IS A PLAYA?

23

Researching various grassland publications for this description I examined at least 15 different vegetation maps for the Great Plains. All varied to some degree, especially in regards to the eastern and western boundaries of the tallgrass and short-grass prairies, respectively. For example, Steinauer and Collins (1996, 40) had the tallgrass prairie extending into Indiana but the short-grass prairie just barely touching eastern New Mexico, whereas Wright and Bailey (1982, 83) did not have the tallgrass prairie extending east of the Mississippi River but did include the short-grass prairie of eastern New Mexico. For purposes of this paper I have redrawn the three general grassland zones roughly following Küchler (1975) (ﬁg. 1.2). The area containing playas from western Nebraska to the southern end of the Llano Estacado is considered short-grass prairie. The uncultivated uplands and playa watersheds in this region are dominated by grasses such as gramas (Bouteloua spp.) and buffalo grass (Buchloë dactyloides) with scattered occurrence of wheatgrass (Agropyron spp.), three-awns (Aristida spp.), yucca (Yucca spp.), prickly-pear (Opuntia spp.), and various other forbs (Küchler 1975). Where sandy soils predominate, mixed stands of sandsage (Artemisia ﬁlifolia), sand shinnery oak (Quercus havardii), bluestems (Andropogon spp.), grama grasses, and yucca occur. Percent declines in native short-grass prairie, largely due to cultivation, have ranged from 80% in Texas to 20% in Wyoming (Samson and Knopf 1994, 419). The majority of the Rainwater Basin playas of south-central Nebraska occur in the mixedgrass prairie with uplands historically dominated by bluestems, wheatgrass, and needle grass (Stipa spp.) (Küchler 1975). Native mixed-grass prairie has declined by 77% in Nebraska and 30% in Texas (no data were available for Oklahoma) (Samson and Knopf 1994, 419). The eastern edge of the Rainwater Basin playas and the Todd Valley playas are in tallgrass prairie that was historically dominated by bluestems, switchgrass (Panicum virgatum), and Indian-grass (Sorghastrum nutans) (Küchler 1975). Samson and Knopf (1994, 418) estimated that 82 –99% of the tallgrass prairie has been cultivated, a loss unsurpassed in any other major North American ecosystem. The prairie grasses of the Great Plains evolved with ﬁre and herbivory. However, with changes in ﬁre frequencies, altered historic grazing (herbivory) regimes, and intentional plantings by land managers, exotics and native woody plants have encroached into the remaining prairie. Although climate is likely the overriding factor in creating the Great Plains grasslands, ﬁre has historically played an

24

PLAYAS AND THEIR ENVIRONMENT

important role in preventing woody plant encroachment and rejuvenation of the grassland (Wright and Bailey 1982, 82). Wright and Bailey (1982, 81) suggested that a natural (nonhuman-caused) ﬁre frequency of 5 to 10 years was reasonable throughout the Plains grasslands. Many of these ﬁres burned the numerous scattered wetlands, including playas. Herbivory by native large and small mammals was also important in the maintenance of Plains grasslands (Bragg and Steuter 1996; Weaver et al. 1996). Bison (Bison bison) are commonly listed as one of the dominant grazing forces affecting historic grassland vegetation and rightly so. But herbivory by elk (Cervus elaphus), prairie dogs (Cynomys spp.), pronghorn (Antilocapra americana), and insects, among other native species, was also substantial and inﬂuential on the composition and structure of prairie vegetation. In the Southern High Plains, honey mesquite (Prosopis glandulosa) and junipers (Juniperus spp.) have moved into the grasslands. In the mixed- and tallgrass prairies, eastern red cedar (Juniperus virginiana), various oaks (Quercus spp.), and other woody plants have expanded into the prairie. Woody vegetation has also increased along riparian areas, which have had their ﬂows drastically altered as a result of reservoir construction and withdrawals for human consumption and irrigation (Friedman et al. 1998). Native species such as cottonwood (Populus deltoides) and willow (Salix spp.) have often greatly expanded their coverage. But of much worse ecological consequence has been the spread of exotics like saltcedar (Tamarix pentandra) and Russian olive (Elaeagnus angustifolia). Although Russian olive is a serious threat to Plains ecosystem integrity (Olson and Knopf 1986), until recently it was included in shelterbelt and wildlife cover plantings by the U.S. Department of Agriculture and is still by some state forestry agencies. These same native and exotic species now also exist along the margins of many Great Plains wetlands where they were historically absent. Among the many exotic grasses and forbs (too many to list here) that have been introduced into the prairie are crested wheatgrass (Agropyron cristatum), Old World bluestem (Bothriochloa ischaemum), and Kentucky bluegrass (Poa pratensis). Many of these grasses were introduced to “improve” grazing by domestic livestock or to establish cover on previously farmed or eroded sites. The introduction and expansion of exotic grasses continues today, most recently through the Conservation Reserve Program (CRP) of the U.S. Department of Agriculture.

WHAT IS A PLAYA?

25

CURRENT AGRICULTURE AND POPULATION TRENDS

The dominant agricultural crops cultivated in the southern portion of the Southern High Plains are cotton, grain sorghum, and winter wheat (TDA 2001). Corn, grain sorghum, and winter wheat are grown in the northern and central Southern High Plains, with the area of cotton decreasing (TDA 2001). In the High Plains of Colorado, Kansas, and Oklahoma wheat and grain sorghum are predominant but some corn and sunﬂowers are also grown (CDA 2001; KDA 2001; ODA 2001). In southwestern Nebraska wheat and sorghum are important crops, but in some counties (e.g., Perkins) areas of corn are increasing (NDA 2001). Corn and soybeans are dominant throughout the Todd Valley and Rainwater Basin of Nebraska (NDA 2001). Essentially all grasslands in the Great Plains not enrolled in the CRP are grazed by domestic livestock. As a result of Title XII of the 1985 Food Security Act, and its successors, a signiﬁcant portion of the cultivated Great Plains was taken out of crop production for a period of at least 10 years through the CRP. In exchange for annual rental payments, landowners throughout the United States replaced annual crops on qualiﬁed highly erodible lands with perennial cover to reduce soil erosion and reduce surplus production in some areas. Because most of the Great Plains is highly erodible, from the forces of wind and water, these lands easily qualiﬁed for the CRP. Indeed, the highest density of CRP lands in the nation occur in the Great Plains (Licht 1997, 120). For example, over 900,000 hectares ( 2 mil. ac) alone were enrolled in the Southern High Plains of Texas (Berthelsen et al. 1989). On most of the enrolled land in the Great Plains annual crops were replaced with perennial grasses. As subsequent requirements of the CRP were modiﬁed and 10-year contracts expired, some of the previously enrolled land was put back into crop production. The loss of some of this perennial grass cover came as a result of ﬂuctuations in the agricultural economy (e.g., rising crop prices). In addition, the same previously established exotic perennial grass cover no longer completely qualiﬁed for the CRP. Unfortunately, in most of the Great Plains exotic grasses had been allowed to be planted under the 1985 program (Licht 1997, 121). As the initial contracts expired after 10 years, the CRP required that a higher percentage of native grasses be planted in those original ﬁelds. By

26

PLAYAS AND THEIR ENVIRONMENT

helping restore native species, this requirement was a positive revision to the program. In states, such as Kansas, that had the foresight to plant a higher percentage of native grasses to begin with, qualifying lands for reenrollment was not as great a problem as elsewhere in the Plains. Most of the crops in the western Great Plains, where playas occur, depend on irrigation for successful production because of unpredictable and scarce precipitation (Nall 1990). Irrigation is possible because of the existence of the world’s largest aquifer. The Ogallala Aquifer underlies much of the western Great Plains region from South Dakota to the Southern High Plains of Texas and New Mexico (Reeves and Reeves 1996). The presence of the aquifer, and, at least historically, inexpensive natural gas to allow pumping, encouraged much of the expanded cultivation of the Great Plains grasslands since the early 1900s (Nall 1990). Given the current rate of water use, however, much of the western Great Plains will likely see extreme shortages by the year 2020 potentially resulting in severe social and economic consequences (Luckey et al. 1988). For example, that portion of the Ogallala Aquifer south of the Canadian River in Texas and New Mexico receives little recharge. The aquifer is essentially being mined to sustain crop production. At average pumping depths ranging from 30 to 200 meters (100 – 600 ft) (Bolen et al. 1979), the Ogallala dropped by more than 15 meters (approx. 48 ft) in the Southern High Plains between 1930 and 1980 (Weeks 1986). The drought of the 1990s, the worst on record throughout much of the Southern High Plains, has resulted in continued high use of aquifer water. Between 1993 and 1997 there was an average decline of more than 2 meters ( 6 ft) throughout most of the region (Donnell 1998). In Lea County, New Mexico, the Associated Press (1998) reported that it would take 1,900 years to replace (recharge) the water pumped from the aquifer in the last half century. Even with widely hailed water-saving technological advances such as centerpivot and drip irrigation to replace traditional furrow ﬂooding, and the enrollment of extensive CRP acreages, aquifer levels continue to decline. Indeed, center pivots have allowed irrigation to expand to sloping lands that did not allow traditional furrow irrigation, further decreasing aquifer levels (ﬁg. 1.10). In some areas of the Southern High Plains, water levels have dropped so far that it is no longer economically feasible or hydrologically possible to irrigate. Landowners in such areas have had to revert to dryland farming, an economically

WHAT IS A PLAYA?

27

Figure 1.10 Agriculture in the western Great Plains is dependent on irrigation (center-pivot in playa in this illustration) from the groundwater of the Ogallala Aquifer. Much of the southern portion of the aquifer recharges at a very slow rate and pumping can be equated with mining. (Photo by author.)

much riskier proposition than irrigated agriculture (Nall 1990). Numerous farms have gone out of business because of the effects of the declining aquifer (Associated Press 1998). In some areas north of the Southern High Plains, where the Ogallala historically has not been tapped at such a high rate, irrigated agriculture is expanding west. For example, in southwestern Nebraska the number of center pivot irrigation systems has been increasing. Irrigation permits the expansion of crops such as corn that require more water than do traditional crops like wheat. This agricultural shift is increasing soil erosion compared to dryland farming methods and causing a more highly fragmented prairie and playa environment. Along with the difﬁculties faced by agriculture such as declining aquifer levels, increased energy costs, and recent low commodity prices, there has been a decline in the human population throughout the Great Plains. “Depopulation” has occurred throughout much of the Great Plains since the early 1900s, although the demographic declines have gone largely unnoticed by the remainder of the country

28

PLAYAS AND THEIR ENVIRONMENT

(Nickels and Day 1997). In exemplifying what has occurred throughout the rural Great Plains, Nickels and Day (1997) reported human population trends in 67 counties in the Texas portion of the Great Plains since the early 1900s. From 1900 to 1930 the number of farms in the Texas Great Plains increased along with population in 64 of the 67 counties. However, since the 1930s and the Dust Bowl, the population in most of the 67 counties declined, and 54 of the 67 counties lost population in the 1980s. This trend persisted into the 1990s but slowed. Of the 67 counties studied by Nickels and Day (1997), 43 lost population in the 1990s (U.S. Bureau of the Census 2002). Depopulation has been mirrored in other western Great Plains states such as Nebraska and Oklahoma, which lost population in 50 of 52 and 22 of 23 Plains counties, respectively, during the 1980s (Popper 1992). The depopulation of the Plains of Texas has been due to many interrelated factors including younger people leaving rural environments, the agricultural economy, and, in many areas, declining water availability for agriculture (Nickels and Day 1997). As a result of these changes, many of the Plains rural areas are becoming poverty-stricken because of fewer employment opportunities, aging populations, and subsequent declining tax bases (Davidson 1990). The declining tax base has resulted in fewer local governmental services. Fewer educational opportunities, reduced police and ﬁre protection, and limited health care are notable. The declining population and human fortunes of the Great Plains has led some to propose reverting much of the private land to federal ownership and creating a large prairie-bison preserve (Matthews 1992; Popper 1992). This proposal has not been met with open arms by most of the private landowners in the western Great Plains (Licht 1997, 115), but, as suggested in the ﬁnal chapter, modiﬁcations of the idea may offer some potentials for regional economies.

CHAPTER 2

ORIGIN AND DEVELOPMENT

A

lthough most people from forested and mountainous areas outside of the Plains may ﬁnd the western prairies aesthetically “boring,” geologists consider the High Plains one of the most interesting and challenging arenas in North America. This is especially true of the Southern High Plains, where most playas occur, and where the formation of this huge plateau and its underlying aquifers continue to be discussed. Similarly, unlike other wetlands such as estuaries or prairie potholes, which can be accurately aged in geologic terms and form through known ﬂuvial and glacial events, respectively, the age of playas and the processes responsible for their origin and formation continue to be debated. As Reeves and Reeves (1996, 195) noted, some of the origin/formation confusion results from semantics. “Origin” is quite different from “development.” Thus, the origin of a playa, its starting point, may be different from how the playa subsequently developed (Wood and Osterkamp 1984). Similarly, the “playa basin” is often different than the actual “playa” as deﬁned by its hydric soils. A playa and its associated “ﬂoor” are generally inclusive of the area deﬁned by hydric soils whereas a playa basin includes the nonhydric soil immediately adjacent to hydric soil including the sloped watershed. Further, it is likely that the formative processes of playas in the northeastern areas, like the Rainwater Basin in Nebraska, differ from those in the west and south. This difference might be inferred from variations in shape alone. The almost perfectly circular playas in the western Plains vary from the irregularly shaped playas in the Rainwater Basin (ﬁg. 2.1). Data concerning formation of Rainwater Basin playas are also sparse, but playa occurrence along some paleodrainage features suggests a different combination of processes may be involved in their formation than in High Plains playas (Starks 1984).

30

PLAYAS AND THEIR ENVIRONMENT

Figure 2.1 Top photograph illustrates a typical playa in the Southern Great Plains in its near circular appearance, whereas the bottom photograph illustrates the typical oblong appearance of Rainwater Basin playas on a northwest-tosoutheast long axis. (Top photo by author; bottom photo by T. LaGrange, courtesy of the Nebraska Game and Parks Commission.)

REVIEW OF HYPOTHESES — SOUTHERN GREAT PLAINS

Nelson et al. noted, “Playa basin soils are predominately clay [primarily Randall clay] and strikingly similar regardless of location within the Southern Great Plains, reﬂecting similar formative processes” (1983, 45). Although soils may reﬂect similar playa formation, many agents have historically been implicated in the origin and formation of Southern Great Plains playas and

ORIGIN AND DEVELOPMENT

31

few have been agreed upon. These agents include large mammals wallowing on the prairie soils (Gilbert 1895; Reeves 1966) to wind deﬂation (Evans and Meade 1945). Subsidence, primarily the collapse of carbonates in solution underlying the Plains or evaporites, also has been proposed (Johnson 1901; Baker 1915; Theis 1932; Finley and Gustavson 1981; Paine 1994). The associated importance of piping and the subsidence around geologic “pipes” (vertical linear features leading to the subsurface through basin soils) has further been implicated (Rubey 1928; Frye 1950; Reeves 1966). The possibility of playa origins from meteorites has even been suggested (Evans 1961). The debate among geologists continues, so I summarize the major opinions provided in the most recent publications of Osterkamp and Wood (1987), Wood and Osterkamp (1987), Gustavson et al. (1994, 1995), Sabin and Holliday (1995), and Reeves and Reeves (1996). DISSOLUTION PROCESSES

For the Southern Great Plains, Osterkamp and Wood suggested that “any depression on the High Plains surface that periodically stores and transmits water to the subsurface may develop into a playa basin” (1987, 217). They cited examples of initiation of several playas in the past 60 years. Once the water begins accumulating in a depression, other processes take over and lead to playa enlargement. Osterkamp and Wood (1987) concluded that the principal processes involved in this next aspect of playa development were dissolution of carbonates and movement of particulates (including organic matter) with the water percolating into the subsurface. Under this scenario, organic matter is carried below the basin soil surface by water that has been ponded in the depression. Organic matter is then oxidized, releasing carbon dioxide, which reacts with inﬁltrating water to form carbonic acid. The carbonic acid then causes dissolution of carbonates. Most of the Southern High Plains plateau topsoil is underlain by a calcium carbonate substance known as “caliche” (Reeves and Reeves 1996). Caliche is actually a thick, hard, calcium carbonate deposit that makes up what is locally termed the “caprock” (ﬁg. 2.2). (“Caprock” is also a local synonym for the entire Southern High Plains plateau, or Llano Estacado. This hard limestonelike material, or caliche, is not rock at all but of sufﬁcient form and texture that it is mined throughout the region for use as a roadbed material and base for building foundations.) Because organic matter from plants collect

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Figure 2.2 The northwestern escarpment, or “caprock,” of the Southern High Plains, north of Clovis, New Mexico. (Photo by author.)

in the basin, the process continues and there is progressive development of the playa in an outward fashion. Osterkamp and Wood noted numerous examples of carbonate (caliche) dissolution below playas. Accordingly, subsidence, the lowering of the land surface into these dissolution voids, occurs mainly at playa margins allowing the playa to enlarge. At the playa margins, therefore, are different landslopes deﬁning the playa edge and ﬂoor. However, Reeves and Reeves (1996) noted, on the basis of drilling data, not all playa basins in the Southern High Plains indicate carbonate dissolution below them. It appears, therefore, that dissolution of carbonates is important in the continued formation of some playas, but not all. Wood and Osterkamp (1987) suggested that this type of formation process would lead to the near circular appearance of playas in the western Great Plains. Deﬂation, as a result of wind, they felt, would have led to playas with a more irregular oblong appearance because winds do not affect playas equally from all directions. Most winds in the Southern High Plains approach from the west and southwest. Wood and Osterkamp did acknowledge, however, “that many of the initial or ‘proto’ basins were of eolian origin . . .” (1987, 224). Origin here correctly refers to initiation of the playa depression, and formation to the subsequent development of the playa ﬂoor. Often the two processes of initiation and development are different, but not necessarily so. Wood and Osterkamp (1987) further discounted the importance of wind deﬂation in playa development by arguing that in addition to the circular shape of playas not being consistent with the theory of wind deﬂation, the existence of many playas in a linear arrangement along

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33

substrate fracture zones is not consistent with deﬂation hypotheses. Moreover, the apparent random occurrence of playas relative to topography and slope outside of these fracture zones, the lack of lunettes (dunes on the lee sides of playas) adjacent to most playas, the inadequate lunette volume relative to playa volume, the high clay content of lunettes (suggesting lunette formation after playa formation), and the movement of radioactive clay particles into the unsaturated zone at the edge of playas during aquifer recharge events all suggest that wind was not of primary importance in playa development. WIND FORCES

Reeves and Reeves (1996) and Sabin and Holliday (1995) argued, however, that the simple presence of lunettes on the east and south sides of some playas (about 5% in the Southern High Plains) indicates that wind deﬂation has been important in the development of some basins (ﬁg. 2.3). Deﬂation of playa depressions is hypothesized to occur after playas have had sufﬁcient water depths to have either prevented plant growth or enhanced decomposition of existing plants. The playas then dried, exposing unprotected basin soils to wind erosion. Anyone who has lived in this region for a reasonable length of time has seen this occur. Moreover, the above argument suggests wind is important in formation and maintenance but not necessarily origin. It is possible that wind simply maintains the existence of some playas by removing naturally collected sediments. Contrary to Osterkamp and Wood (1987), Sabin and Holliday (1995) proposed that wind erosion was the primary force enhancing playa development rather than dissolution and subsidence. They based their claim on a geographic analysis of playa frequency and size relative to soil texture. Comparing the frequency of playas and their size, as determined from topographic maps, to soil texture of the surrounding watershed, they noted that substrate texture was signiﬁcantly related to the occurrence and distribution of playas. Further, soil texture was related to playa area, depth, and shape. Soil particle size is smallest in the north-northeastern portion of the Southern High Plains and largest in the south-southwest. The soils in the north are clay— clay loam, becoming sandy farther south with loamy sand—sandy loam soils occurring in the southern portions of the region. (However, precipitation also varies along the soil-texture gradient with precipitation being greatest in the northeastern portion of the Llano and lowest in the southwestern region. The importance of

Figure 2.3 Sections of topographic maps from the vicinity of Petersburg, Texas. Lunettes are present on the southeast corners of several of the playas as indicated by contour lines. Smaller lunettes are present in the lower map section, and large lunettes exist in the upper section.

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soil texture versus precipitation would therefore appear difﬁcult to distinguish.) Sabin and Holliday (1995) found the highest numbers of playas on topographic maps in the coarse-textured soils of the southwestern Southern High Plains. The next highest density of playas was found in the northeastern portion of the region. As revealed in Chapter 1, these results do not agree with counts by Guthery et al. (1981) who used soils maps to estimate playa density and occurrence (Nelson et al. 1983). Guthery et al. found highest densities of playas in medium- and ﬁne-textured soils of the east-central Southern High Plains. Sabin and Holliday did not address this anomaly. Counting methodology, therefore, along with semantics, has also inﬂuenced interpretation of formative processes in playas. Sabin and Holliday (1995) also found that the greatest variability in individual playa basin area and the largest playas occurred in the northern areas of the Southern High Plains where ﬁne-textured soils exist. Playas with the largest range in basin depth also occurred in the ﬁne-textured soil zones, but the largest playas were not necessarily the deepest. Again, characterizing playas is not easy. Sabin and Holliday felt that wind deﬂation affected playa depth and that the depth of a playa was therefore related more to the depth and soil texture of the local substrate than to the surface area of a playa. Under this hypothesis, when winds hit resistant layers of the deeper substrate, wind force expands the playa’s surface area horizontally rather than deepening it vertically. The playas occurring on the ﬁnest-textured soils also were the most round relative to playas in other soil textures (Sabin and Holliday 1995). According to Sabin and Holliday (1995), the playas were apparently less round in coarse soils because erosion occurred more easily than in ﬁne-textured soils. Roundness is greater in ﬁne-textured soils because this less permeable soil promotes surface runoff of water, rather than inﬁltration in coarse soils, maintaining playa roundness. Therefore, according to their theory, wind erosion aids in deepening and expanding basins, but water erosion acts equally around the circumference of the basin making playas more round because the ﬁnesttextured soils are least permeable. These water-eroded soils that wash into the basin are further deﬂated by wind. Sabin and Holliday stated, “While the eluviation and dissolution hypothesis of Osterkamp and Wood (1987) suggests that playa area and depth should increase with increasing permeability, the reverse is actually the case” (1995, 300).

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PLAYAS AND THEIR ENVIRONMENT

Again, however, their measurements were made from topographic maps, not from the hydric soil–deﬁned wetland, which may yield different results. Sabin and Holliday’s explanation of playa development and formation is essentially an expansion and elucidation of a slightly earlier theory proposed by Gustavson et al. (1994); however, the latter did not present any data. MULTIGENIC IDEAS

Reeves and Reeves (1996) suggested that no single course of events could be substantiated in the origin and development of all playas in the Southern High Plains. They proposed that playas originated wherever there were depositional (elevational) lows or “irregularities” in the surface of the Plains. Many of these low areas could have then collected water and attracted herds of large mammals such as bison (modern bison or prehistoric Bison antiquus) and other extinct species (mastodon, horse, etc.) (ﬁg. 2.4). These herds of large mammals would be attracted not only to the water but also to mud for wallowing. They would then transport large amounts of basin soils out of the depression. Presumably, as noted earlier, during the times the basins did not contain water, the basin soils would be subject to wind erosion because either the water or large mammal activity (or both) did not allow vegetation to persist and prevent this erosion. Following Reeves and Reeves (1996), these “young” playas, classiﬁed by them as Type I basins, then generally developed through carbonate dissolution and gradual subsidence of underlying substrates (as suggested in Osterkamp and Wood 1987) into older and often larger Type II basins. Some wind deﬂation, piping and subsidence around pipes, and eluviation also contributed to Type II basin development (Reeves and Reeves 1996). The main differences between Reeves’s (1990) Type I and Type II basins are that the former generally do not have a lunette and have a minimum of lacustrine ﬁll (i.e., ancient lake deposits). Some small basins in the Southern Great Plains have substantial lacustrine ﬁll and are therefore actually older and considered Type II playas (Reeves and Reeves 1996). Therefore, size alone is not a sufﬁcient distinguishing feature between Type I (younger) and Type II (older) basins (Reeves and Reeves 1996). Also, unlike the Type I basin, the hydric soil—in this case, Randall clay—is inset into the lacustrine ﬁll in Type II basins. Reeves and Reeves (1996) found no evidence of steep sloped bottoms or semicircular playa margins, which

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Figure 2.4 Photo from 1904 in the eastern portion of the Texas Panhandle (Gould 1906). Gould refers to this as a “buffalo wallow.” However, note the almost circular shape. Although bison may have wallowed here, it is unlikely they gave it a circular shape.

would be indicative of solution and large-scale dome collapse of underlying material. Further, they did not ﬁnd subsurface ﬂuvial channels or deposits in Type II basins. Gustavson et al. (1994, 1995) also supported, for the most part, a multigenic theory of playa origin. They stated, “These landforms are the result of a series of intermittently active processes, including wind, ﬂuvial erosion and lacustrine deposition, pedogenesis, dissolution of soil carbonate, salt dissolution and subsidence, and animal activities, that collectively produced the typically shallow and roughly circular playa basins on the High Plains” (Gustavson et al. 1994, 12). However, the emphasis they placed on factors that were most important in playa origin and formation varied from the other studies. Gustavson et al. (1995) noted that wind deﬂation was the primary force in playa formation with the other factors being less important. Some of the differences of opinion that currently exist among geologists relate to views of whether or not playas change as they age in geologic time. In other words, some small playas remain, in the terminology of Reeves’s (1990), Type I playas and do not progress to Type II

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(Gustavson et al. 1995; Hovorka 1995). Hovorka (1995), for example, concluded that playa basins in an area northeast of Amarillo, Texas, have changed little over geologic time. Reeves and Reeves (1996) have taken exception to this conclusion by noting that many of the basins in that area are still forming. Osterkamp and Wood (1987) also concluded that playa formation was continuing and that most basins were not in a static state of existence. (Indeed, if it were not for the fact that we humans are ﬁlling the playas in so rapidly, this continuing formation would be a comforting fact. But formation simply cannot keep up with human-induced sedimentation of the basin [Luo et al. 1999].) Moreover, Reeves and Reeves (1996) noted that many outwardly appearing Type I basins are actually older in geologic perspective and should be classiﬁed as Type II. As noted, surface area alone is not a good indicator of playa basin age. Based on data from drilling, they concluded that playas in the region studied by Hovorka decreased in surface area with playa depth. Finally, Reeves and Reeves (1996, 203 – 204) noted that for playas to remain in a “non-evolving” “steady state” throughout “much of Quaternary time” would have required consistent uniformity in climate (precipitation, wind, evaporation, temperature), sedimentation factors (rate, mineralogy, particle size, seasonality, environment), playa surface characteristics (permeability, piping, ﬂora, hydroperiod, gilgai, etc.), and geologic factors (depths of Quaternary sands, caliche depth, Permian salt depth, fracture locations, etc.). The likelihood of these conditions remaining static is inﬁnitesimal and contrary to existing geologic, ecologic, and hydrologic data. However, it therefore seems odd to be placing playas in two age categories (i.e., Type I and II). The process of formation is continuing and accordingly the ages would be continuous. In addition to the lunettes that exist adjacent to about 5% of the Southern High Plains playas, some playas also appear to have formed more straightened margins on their north, east, and south edges (Gustavson et al. 1995). This has been attributed to wave action initiated by wind (Reeves 1966; Price 1972). These observations are consistent with seasonal prevailing winds in the region. Northerly and westerly winds are common in fall and winter, while southerly winds are prevalent in late spring and summer (Gustavson et al. 1995). In particular, on the north side of some playas it is common to see eroded “scarps.” Scarps are simply steep eroded elevational changes, similar to a step, generally less than a half meter high. It is believed that these scarps exist more on the north sides of playas because southerly

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39

winds occur more frequently in summer when playas are more likely to contain water. This creates a more powerful erosional force than wind alone and encourages formation of a scarp. SOUTHERN GREAT PLAINS SUMMARY

Although there is no consensus by geologists on the relative importance of individual factors (i.e., wind, dissolution) in playa origin and formation in the Southern Great Plains, it appears there is now agreement that several forces are responsible for origin and formation of playa basins. So often scientists are looking for one factor as causing an event; seldom is the real world so simple. Geologists may never nail down the actual importance of individual factors in overall Southern High Plains playa origin and formation, especially given the anthropogenic changes that have occurred in the Great Plains landscape. For example, it is probably not possible to know what contribution large mammals made to the origin of playas given they are not likely to be present again in those numbers. However, it would be possible for geologists to date a large number of playas in a systematic fashion over a wide area using drilling data. Only this type of study would clarify much of the speculation about playa age and the relative importance of formation factors. OTHER PLAYAS IN THE WESTERN GREAT PLAINS

Other playas in the Central and Northern High Plains also often appear circular and possibly were formed under similar processes to those in the Southern High Plains (Frye 1950). But, in the Powder River Basin of Wyoming, Brough noted that the playas were mainly oval or elliptical in shape: “Elongation of many playas in the direction of the prevailing winds indicate that eolian processes contribute to both the geomorphic characteristics and formation of the playas in the Powder River Basin of Wyoming” (1996, 44). These playas, however, may have an aspect to their origin that is distinct from other playas in the Great Plains. This region of Wyoming is known for its coal deposits. Brough noted, “The location of the playas, disturbances in some of the soil horizons, and the presence of clinkers support the idea that these playas were formed by the ignition and burning of coal beds and the sequential subsidence in the overburden to form the topographical depressions” (1996, 91). His use of the word “form” here probably is similar to my earlier use of “ori-

40

PLAYAS AND THEIR ENVIRONMENT

gin” in that some of the playa depressions being discussed here originated through coal ignition, and other processes such as wind led to their continued formation. Few other studies on playa origin and formation exist for other areas of the High Plains. R AINWATER BASINS OF NEBR ASKA

As noted earlier, the Rainwater Basin playas in south-central Nebraska, to the east of the High Plains, generally lack the circular appearance of other Great Plains playas, have irregular shapes, and may have formed through a combination of other processes because of different landforms and external forces (Kuzila 1994). Certainly less geologic data on their origin exists than for playas in the Southern High Plains. Kuzila (1994) noted that many of the smaller wetlands in the Rainwater Basin region were not generally discernible on topographic maps because they may only be 1 meter (3 ft) deep (similar to High Plains playas) relative to the surrounding upland. These wetlands, however, are easily documented on soil survey maps, as noted by the presence of Butler, Filmore, Scott, and Massie soil series (Gilbert 1989; Kuzila 1994). Moreover, unlike playas in the remainder of the Great Plains, some of the Rainwater Basin playas have been naturally “breached” (Starks 1984). In other words, they have some external drainage where the water may reach a certain depth and “spill” into riparian features (e.g., streams, creeks). Similar to other Great Plains playas, many Rainwater Basin wetlands also have lunettes on the south and east sides of the basin (ﬁg. 2.5). Starks (1984) found that 51 of 120 wetland basins he surveyed had lunettes. This occurrence is a much higher frequency than that found for playas in the Southern High Plains (about 5%). The high frequency of lunettes, however, could be an actual occurrence or an artifact of sampling. Remember from Chapter 1 that only about 10% (or 400) of the Rainwater Basin playas remain. The others have been drained or ﬁlled. Recent data on the remaining playas may contain lunettes at a higher frequency than if historic data (prior to drainage) were used. The few studies on Rainwater Basin playa origin and formation have come from just a few depressions. Krueger (1986) studied one basin and 16 adjacent drill holes. He concluded the basin was originally formed during the Wisconsonian period and later modiﬁed by wind and wave action. Kuzila (1994) examined two basins and their stratig-

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Figure 2.5 Lunette on the southeast side of a Rainwater Basin playa. (Photo by R. Stutheit, courtesy of the Nebraska Game and Parks Commission.)

raphy. He found that the present landscape of the Rainwater Basin region was a result of loess (windblown silt) deposition tending to smooth out the more rugged paleolandscape. Indeed, almost the entire Rainwater Basin region falls within the area known as the Central Loess Plains. Low points in that paleolandscape, however, remain and collect precipitation runoff. These low points and shapes, or morphologies, of present-day playas therefore appear inherited from that historic landscape (Kuzila 1994). Similar to Krueger, who invoked wind and water in subsequent basin formation, Kuzila felt that the Rainwater Basin playas formed from water erosion and subsequent wind deﬂation of those eroded materials. Starks (1984) noted that the southern extent of glaciation did not reach his study area in the Rainwater Basin region and therefore glaciation was not likely involved in basin formation. Some of the Rainwater Basin playas appear to have a northwest to southeast orientation (Starks 1984). This may be related to paleodrainage patterns (Stutheit et al. 2001). If windblown silts (loess) were deposited after these drainage patterns developed, this could have closed the drainage off, allowing formation of this type of closed playa

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PLAYAS AND THEIR ENVIRONMENT

basin morphology. These processes of wind and water action appear related to formation of large ( 50 ha) basins, but it is unclear whether they are involved in smaller basin development (M. C. Gilbert, personal communication). The relative importance of wind and water processes may also vary on a geographic basis within the Rainwater Basin region. For example, the playa basins in the western portions of the Rainwater Basin appear smaller and shallower than those to the east, which are larger and deeper (M. C. Gilbert, personal communication).

ECOSYSTEM ASPECTS

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CHAPTER 3

FLOR A

W

hen most ecologists consider “ﬂora” they usually think of vascular plants; sometimes, as an afterthought, algae come to mind. Fewer yet consider fungi, mosses, bacteria, and viruses even though their diversity may surpass that of the higher taxa (Wilson 1999). There are few studies investigating these latter groups in Great Plains playas and only one mention of lichens; Bartz (1997) noted the occurrence of one ground lichen (Xanthoparmelia wyomingensis) in playas of northeast Wyoming. Further, other than the known frequent occurrence of avian botulism (Clostridium botulinum) and avian cholera (Pasteurella multocida) in playas, the primary information regarding playa bacteria is associated with human health concerns in urban playas (Westerﬁeld 1996; Warren 1998). These lakes bear little resemblance to their rural counterparts in that urban lakes remain inundated for years and receive much different forms of watershed runoff. Westerﬁeld (1996) used microbiological media that would select for determination of bacteria harmful to humans—total coliforms, fecal coliforms, and enterococci—in Lubbock city playas. He found 11 species (Aeromonas hydrophila, Aeromonas trota, Aeromonas veronii, Enterobacter cloacae A, Enterococcus faecalis, Escherichia coli, Plesiomonas shigelloides, Pseudomonas aeruginosa, Pseudomonas mendocina, Pseudomonas stutzeri, and Serratia marcescens), many of which can cause serious human health problems through various methods of contact such as direct consumption, inhalation, or through a break in the skin. ALGAE

There have been few investigations into playa algae. One of the most extensive studies focused on the macroalgae Characeae in the Southern Great Plains. In surveys from mid-July to

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ECOSYSTEM ASPECTS

Table 3.1 Common playa algae and macroalgae in the Southern High Plains Algae (Price 1987) Ankistrodesmus falcatus Bracteacoccus minor Chlorella vulgaris Closterium sp. Euglena gracilis Gloeocystis ampla Golenkinia sp. Oocystis sp. Phacus pleuronectes Phormidium sp. Rhizoclonium sp. Scenedesmus basilensis Scenedesmus quadricauda Spirogyra rhizobrachalis

Macroalgae (Proctor 1990) Chara braunii Chara foliolosa Chara haitensis Chara hydropitys Nitella monodactyla Nitella bastinii Nitella acuminata Nitella axillaris Nitella clavata Nitella mucronata Tolypella prolifera

Staurastrum cristatum

late September, Proctor (1990, 78) found 11 species of charophytes from three genera in 64 playas (table 3.1). He noted that sedimentation, from eroded watershed topsoil (e.g., Luo et al. 1997), caused a loss in the diversity of Characeae and that Chara braunii was generally the only charophyte occurring in playas that had received substantial sediment loads. Sedimentation has drastically altered playa ecology, a theme that I will be repeating in this volume. Because most playas exist in intensively cultivated regions, the diversity of Characeae is being negatively affected by intensive agriculture throughout a large region of the Southern Great Plains. Proctor (1990) noted that C. braunii could be present as early as mid-April, depending on precipitation, and that plants remain fertile as late as December. He felt that the persistence of C. braunii was due to repeated germination of the species throughout the season rather than to the individual longevity of plants. Tolypella prolifera was the only other Characeae likely to be present earlier in the year than C. braunii although Tolypella was much less widespread. Because of C. braunii’s reproductive characteristics, producing zoospores earlier and more rapidly than most other Characeae, it can withstand envi-

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47

ronmental degradation and frequent natural environmental ﬂuctuation, thus making it the most widespread species in the region (Proctor 1990). Even with 11 Characeae species occurring in playas, there were generally only two species present (with C. braunii usually as one) in any given playa (Proctor 1990). Characeae species that occurred in more permanent water bodies in the Southern High Plains did not occur in playas, and those species that occurred in playas did not occur in the more permanent water sites (Proctor 1990). Certainly this speaks further to the importance of playas to biodiversity in the region. Similarly, bulbil-forming Characeae did not exist in playas, presumably because bulbils are not drought resistant and cannot withstand the frequent dry circumstances found in playas (Proctor 1990). (Bulbils are small bulbs permitting vegetative reproduction.) Although bulbils are most commonly produced by dioecious species, there are only two dioecious charophyte (non-bulbil-producing) species in playas (Nitella bastinii, Nitella monodactyla). Moreover, though six species of Characeae in playas are Nitella, Chara species made up more than 90% of the annual charophyte biomass (Proctor 1990; unfortunately actual biomass estimates were not presented). Also few extensive studies have examined the noncharophyte algae in playas. In an herbicidal runoff study, Price (1987) collected water and sediment samples from playas in two counties (Castro, Lubbock) of the Southern High Plains. He found 16 species after preparing those samples under laboratory conditions (table 3.1). Most were green algae and are common elsewhere. Cladophora spp. also occurs in playas but was not seen in Price’s study. Certainly further research on algae groups is needed throughout Great Plains playas. VASCULAR PLANTS

The ﬁrst survey of the vascular ﬂora of playas was by E. L. Reed (1930) for the Southern High Plains of Texas (common and scientiﬁc names follow Haukos and Smith [1997] unless noted otherwise). Reed also was the ﬁrst to remark on the importance of playa vegetation to the regional diversity of the prairie ﬂora: “The vegetation of the playas differs remarkably both in genera and species from that of the rest of the Staked Plains. In fact the playas have a ﬂora peculiar to themselves” (1930, 598). He noted that the common species of playas that held “water for any considerable period” were western water clover (Marsilea vestita), bur ragweed (Ambrosia grayi),

48

ECOSYSTEM ASPECTS

prostrate knotweed (Polygonum aviculare), Pennsylvania smartweed (Polygonum pensylvanicum), arrowhead (Sagittaria cuneata), mousetail (Myosurus minimus), blue mud-plantain (Heteranthera limosa), and a small sedge (presumably a spikerush [Eleocharis spp.]). All are still fairly common today with the possible exception of mousetail (Haukos and Smith 1997). Reed (1930) noted that buffalo grass was present only in the “shallow” playas. However, Parker and Whitﬁeld (1941) found that buffalo grass may occur in more playas but less frequently in the playa center. Many of the various opinions on the prevalence and dominance of different types of vegetation in playas is likely related to how playa boundaries are deﬁned, immediate past precipitation/irrigation events, and condition of the immediate surrounding watershed. The botanist William Penfound was one of the ﬁrst ecologists to provide observations of playa ﬂora outside of the Southern High Plains (Penfound 1953). He studied aquatic ﬂora throughout the Oklahoma Panhandle. As might be expected, he found fewer aquatic plants in the playas than in eastern Oklahoma water bodies and little similarity in the species between the regions. Following the initial descriptive studies of Reed (1930), Parker and Whitﬁeld (1941), and Penfound (1953), a series of more comprehensive vegetation surveys were conducted in various areas of the Southern Great Plains. In the playas of Texas, Rowell (1971, 1981) found 69 plant species, with Haukos and Smith (1993a) adding 17 more to his list. Haukos and Smith (1997) considered surveys by Hoagland (1991) for playas in southeastern Colorado, lists from Kindscher and Lauver (1993) and Kindscher (1994) for western Kansas, a multistate survey by Curtis and Beierman (1980) in Texas, New Mexico, and Oklahoma, and local surveys in four to ﬁve playas northeast of Amarillo, Texas, by Cushing et al. (1993) and Johnston (1995) in arriving at their ﬁgure of 282 different vascular plant species. Following that, an extensive survey of 224 playas in the ﬁve-state Southern Great Plains found another 64 species, making the current published total for that region 346 (a complete species list can be found in Haukos and Smith 1997). The prairie ecologist J. E. Weaver was possibly the ﬁrst to publish some botanical descriptions of Rainwater Basin playas (Weaver and Bruner 1954, 121). Referring to playas correctly as “depressed areas,” he and Bruner noted that the vegetation was adapted to frequent wet/ dry conditions and was very different from the surrounding upland. They noted at least 26 species. Subsequently, Erickson and Leslie

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49

(1987) studied vegetation in six playas of the Rainwater Basin region and identiﬁed 64 species. Their study was aimed at species-soil relationships for regulatory purposes and therefore did not examine plant community relationships or overall species prevalence. Following that investigation, Gilbert (1989) examined 47 playa wetlands comprising 13% of the Rainwater Basin playas remaining in 1989. He found 212 noncultivated vascular plants in those wetlands. Most (greater than 80%) of those species also occur in the Southern Great Plains (Haukos and Smith 1997). In 10 playas in eastern Wyoming, Holpp (1977, 112 –113) found 46 plant species. More recently, Bartz (1997, 24) found 35 plant species in the wetland zones of seven playas from northeastern Wyoming. The ﬂora of these Wyoming playas does not overlap as much with that found in the Southern Great Plains or in the Rainwater Basin. These playas appear to contain more sedges and rushes (Carex spp., Juncus spp.) and are very distinct from the surrounding uplands. PLANT SURVEY TIMING AND CLASSIFICATION

The description of playa plant community composition in various studies is related to the season of sampling and recent precipitation events. In the survey of 224 playas in the Southern Great Plains, Smith and Haukos (2002) found that, on average, only 38% of the species were similar between early in the growing season and late season. Because the same playas were sampled in both seasonal surveys, the change can be directly attributed to varying seasonal plant life histories (e.g., those species such as little barley [Hordeum pusilum] that complete their life cycle within a certain time of year—in this case, early spring) or the changing hydroperiod in the playa. As in other prairie wetlands, the primary inﬂuence on species composition in playas is hydroperiod, as reﬂected through the seed bank (van der Valk 1981). A playa that is dry in early spring, for example, can have its entire ﬂora turn over if the basin receives substantial rain and is inundated. In the spring the playa community may have been dominated more by plants typically occurring in uplands versus those that can only grow in submerged conditions and are considered aquatic. A plausible example of this is a playa dominated by western wheatgrass in early spring. The playa is then inundated, and after a few weeks the wheatgrass has been killed by the ﬂooding and replaced with longbarb arrowhead (Sagittaria longiloba) and blue

50

ECOSYSTEM ASPECTS

mud-plantain. The converse is also possible. A playa that contains several inches of water in early spring and contains longbarb arrowhead and blue mud-plantain may dry as a result of evaporation or a landowner pumping the water out of the playa for irrigation purposes. In this typical situation, species that require exposed, moist substrates to germinate, like barnyard grass (Echinochloa crusgalli) and smartweeds (Polygonum spp.) may become dominant within a very short period of time. Numerous variations of this changing hydroperiod occur constantly in playas, resulting in a dynamic ﬂora. This very dynamism usually prevents the establishment of plants that have long-term aquatic requirements and predictable hydrologic needs in their life history. Obviously, stable and predictable hydrologic requirements are not provided in most prairie wetlands, and for this reason playas have few endangered species. Most threatened or endangered wetland plant species have speciﬁc and predictable hydrologic requirements to allow them to complete their life cycle (e.g., bog pitcher plants [Sarracenia spp.]). Plants such as annuals and short-lived perennials thrive in this environment of rapidly changing conditions. They have long-lived seed banks or hardy underground structures (e.g., corms, tubers, rhizomes) that can take advantage of the temporally and spatially variable precipitation in a relatively rapid time frame (van der Valk 1981; Smith and Kadlec 1983). An aquatic species that required a more predictable set of environmental conditions, such as most plant species of concern to conservationists (i.e., those species whose populations may be experiencing severe declines, be threatened, or be endangered), would not last long in a playa. That is probably why playa plant communities are so ubiquitous throughout the Great Plains region. Perhaps that is also why playas (lacking in “species of concern”) are “understudied.” The plants that can take advantage of the rapidly changing conditions in Nebraska are the same as those in Kansas and Texas. Wetland plants are also used, along with soils and hydrology, in the legal determination of what is a jurisdictional wetland and what can be legally done to that wetland (i.e., Section 404 of the Clean Water Act; U.S. Army Corps of Engineers 1987). Therefore, plants have been classiﬁed accordingly. Plants that occur in and around wetlands have been classiﬁed as to their relative afﬁnity to exist in hydric conditions (Reed 1988). An understanding of this classiﬁcation scheme is key to understanding some of the issues surrounding playa plant communities and issues presented later relative to threats to playa

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ecosystems. This plant classiﬁcation list was developed and updated through the National Wetlands Inventory by representatives from the U.S. Fish and Wildlife Service, U.S. Army Corps of Engineers (the primary agency responsible for wetland enforcement under Section 404), the Environmental Protection Agency, and the Natural Resources Conservation Service (the U.S. Department of Agriculture agency currently responsible for delineating wetlands on agricultural lands, where most playas occur). Using this system, plants are initially classiﬁed into groups that are obligate (OBL) wetland species (which occur with a 99% probability in wetlands); facultative (FAC) species, which can occur in wetlands but are not restricted to wetlands (34 – 66% probability); and upland (UPL) species (99% probability of occurrence in nonwetlands). The FAC species can be further broken down into groups that are more closely associated with wetland environments as facultativewetlands (FACW; 67–99% probability of occurring in a wetland) or groups that are more closely associated with uplands, facultative-upland (FACU; 67–99% probability of occurring in nonwetlands). To achieve additional consensus among the individuals ranking the plants, further modiﬁers were added: a “” to indicate species with a greater afﬁnity to wetlands, and a “” for those having a greater afﬁnity to uplands. Species with insufﬁcient information to assign them to a category were designated as no indicator (NI). Plants are listed according to these categories by region, and those regional lists may differ from each other. For example, playa plants in New Mexico are assigned to Region 7, whereas playa plants in Texas are assigned to Region 6. Knotgrass (Paspalum paspalodes) is classiﬁed FACW in Texas but as OBL in New Mexico (Haukos and Smith 1997). These regional differences in classiﬁcation are thought to take into account ecotypic variation in species but also reﬂect the varying opinions of different individuals from agencies in the various regions. The relative dominance of OBL and FACW plants over the other categories can be used to deﬁne playa boundaries (NRC 1995). Along with hydric soil maps, agencies deﬁne the wetland boundary for regulatory and agricultural beneﬁt purposes. (Some current USDA initiatives, such as the Swampbuster, link wetland protection and federal agricultural payments to landowners.) The location of these boundaries, however, also inﬂuences how ecologists view the contributions of playas to regional biodiversity and, at the community level, how plants are distributed within the playa. As noted earlier, the discrepancies in the various counts of the number of playas was,

52

ECOSYSTEM ASPECTS

Figure 3.1 A typical Southern Great Plains playa showing two vegetation zones (playa edge on the left, level playa ﬂoor on the right) within the hydric soil–deﬁned wetland. Upland vegetation in the background is above the hydric soil and outside the wetland. (Photo by author.)

in part, due to the manner in which playa boundaries were deﬁned (e.g., topographic maps vs. soil maps). The same problem can inﬂuence how individuals view plant community composition in playas. If, for example, ecologists use topographic map slope position to deﬁne the playa, versus hydric soils, they will likely reach a much different conclusion about potential plant community structure (e.g., zonation) in playas. The change in topographic map slope position is typically higher in elevation than the hydric soil boundary in Great Plains playas. Therefore, relatively more species of UPL and FACU plants would appear in the ﬂora of a playa deﬁned from a topographic map versus a playa that was surveyed within its hydric soil boundary (ﬁg. 3.1). ZONATION

Zonation refers to the different life-forms or communities of wetland plants that are adapted to different levels of inundation (e.g., FACW vs. OBL or submergent vs. emergent macro-

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phytes). Because playas in the western and Southern Great Plains display little elevational heterogeneity, they are generally thought to display relatively little plant zonation (Haukos and Smith 1994b; Smith and Haukos 2002). The distribution of seeds is related to this zonation (van der Valk 1981; van der Valk and Welling 1988). Seeds ﬂoat or are windblown until they reach an obstacle. A change in elevation, or a band of vegetation is such an obstacle (Smith and Kadlec 1983) (ﬁg. 3.2). In the ﬂat bottom of a Southern Great Plains playa this elevational obstacle, or lack thereof, is similar across the hydric soil basin unless there has been some type of hydrologic modiﬁcation causing an elevational change. Excavations in the playa cause an elevation change in the hydric soil and permit additional zones or life-forms to occur. Haukos and Smith (1994b) examined the seed bank of eight playas along their hydric soil elevational gradient. Seed density and species composition did not vary along the hydric soil–deﬁned elevational

Figure 3.2 Windrow of seeds in a playa. Seeds ﬂoat in the air or water until a barrier is reached and then they settle out. (Photo by author.)

54

ECOSYSTEM ASPECTS

gradient ( 1 m). In the undisturbed hydric soil of these playas there were essentially just two zones, the playa ﬂoor and the narrow hydric soil edge that extends upslope for only a slight elevational increase (often 1 m). This relatively narrow zone not only creates an obstacle; it also has a different hydroperiod than the lower zone, resulting in a different plant community. Reed (1930, 60) also suggested the occurrence of two zones, the large playa ﬂoor and the relatively smaller playa edge. He noted that the plants occurring in those zones did not occur higher in the “plains.” Parker and Whitﬁeld (1941) commented on the zones of playas in the Southern High Plains but noted a third upland zone above the edge. This upland zone is outside the legally deﬁned (by hydric soils and hydric plants) playa basin and is dominated by obligate UPL and FACU plants. More recently Hoagland and Collins (1997) studied plant zonation in and around 40 playas in the Southern Great Plains. They described four vegetative zones in these playas. However, plants outside the playa basin as deﬁned by its hydric soil (Randall clay) were included in the analysis of zonation. They classiﬁed the four zones as (1) interior vegetation, (2) interior-edge vegetation, (3) upland-edge vegetation, and (4) upland vegetation. Therefore, as deﬁned by the hydric soils, Hoagland and Collins (1997) also found two zones within the playa itself. However, they attributed the disparities in conclusions about playa zonation between their study, which suggested four zones, and others suggesting fewer (e.g., Guthery et al. 1982; Haukos and Smith 1993a, 1994b) to anthropogenic inﬂuences on hydrology. While anthropogenic inﬂuences doubtlessly affect plant species occurrence and community composition (Smith and Haukos 2002), it is more likely the differences in the number of plant zones observed among these studies are simply an artifact of sampling procedure. The species that may have invaded or increased as a result of anthropogenic inﬂuence are affected by the same hydrologic factors in playas that have not had as much anthropogenic disturbance. If one samples plant communities outside (above on an elevational scale) the hydric soil–deﬁned wetland in a playa watershed he or she will ﬁnd additional vegetation zones even when the playa may have received anthropogenic hydrologic inﬂuences. The primary inﬂuence is the lack of elevational range within the hydric soil wetland (Smith and Haukos 2002). Unlike playas in the High Plains, the Rainwater Basin playas ap-

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pear to have more zones in the hydric soil–deﬁned wetland (Gilbert 1989). Weaver and Bruner also suggested as much by noting that playa depth ranged from just a “foot or two [ 0.5 m] below the general soil level . . . “ to “10 to 15 feet [2 –3 m]” (1954, 121). This greater elevational range in the hydric soil basin allows more plant zones adapted to different levels of inundation to occupy these southcentral Nebraska playas than is the case in other playas of the Great Plains. Gilbert (1989, 21) identiﬁed four zones within the hydric soil– deﬁned basin that were all correlated to their relative depth within the wetland. Finally, in eastern Wyoming Bartz (1997) studied plant community zonation in seven playas. He concluded that there could be up to three zones in the playa itself and a fourth if one considered the surrounding mixed-grass prairie of the upland. Not all playas had all three zones, and three of seven playas had only two wetland plant zones. COMMUNITY COMPOSITION

The particular plant community occupying a given playa at a given time in the Great Plains is related primarily to depth of inundation and length of inundation (hydroperiod) or lack thereof. The hydroperiod is primarily related to precipitation events (or irrigation pumping and runoff in some agricultural instances). The same size and depth playa in one area may possess a vastly different extant ﬂora than a playa a short distance away simply because it has received more, or less, precipitation. The word “extant” is used here to indicate the existing aboveground ﬂora at the time of a survey. Because playa ﬂora can change so rapidly, one must consider the extant ﬂora and the underlying seed bank to get a true picture of the potential ﬂora. Although plant community composition in playas is primarily and ultimately inﬂuenced by hydroperiod, grazing and ﬁre are important secondary inﬂuences. Unfortunately, few studies have examined the direct inﬂuence of these factors on plant community composition in playas, except to note structural habitat changes such as percentage of cover.

southern great plains studies Beyond the brief description of Southern High Plains playa plant community type by Reed (1930), Guthery et al. (1982, 519–520) described various “physiognomic” plant groupings in

56

ECOSYSTEM ASPECTS

101 playas for the same region. (Taxonomy and common names here follow Guthery et al. [1982, 519–520]; see table 3.2 for consistency in interpretation.) During surveys from June through August 1980, in three counties (Castro, Lamb, Bailey) of Texas, they noted the presence or absence of 33 plant taxa. Of these 33 they used 24 in an analysis to deﬁne 14 physiognomic types (table 3.2). Obviously the “crop” type had little potential for native vegetation. Their study was conducted during a below average (33 – 66% of normal) precipitation year, and they found the open water vegetation type, dominated by pondweeds, only in playas that had been hydrologically modiﬁed through pit excavation. In the counties Guthery et al. (1982) studied, pits were most often excavated to receive irrigation tailwater. Landowners could then repump the water for irrigation onto the land at less cost than pumping additional water from the aquifer. Pits generally hold precipitation and irrigation runoff for a longer time than if that water had been spread out over the playa, because less surface area is available for evaporation. This allows plants that require longer hydroperiods to persist in the pits. In that study the very existence of pits suggested that the playa received irrigation runoff. The constant recycling of irrigation water through the playa also permitted plants that required frequent moist conditions to exist. Therefore, modiﬁcation of playas through pit excavation was also associated with the increased prevalence of bulrush and cattail (narrow-leaved emergent type) and of woody species, primarily salt cedar and black willow (tree-shrub type) (table 3.2). The relative importance of pit excavation alone versus the reception of additional irrigation water alone in shaping playa plant communities is difﬁcult to separate, but it is really of little consequence. Both are correlated and important in the alteration of playa plant communities. Guthery et al. (1982, 523) further found that in that drought year pondweed, water clover (Marsilea sp.), smartweed (Polygonum lapathifolium), arrowhead, bulrush, and cattail only occurred in playas receiving irrigation runoff (nomenclature here follows that in Guthery et al. 1982). The spoil-bank type they deﬁned within a playa was simply where soil was borrowed from pit or ditch excavation. It created an elevational change and barrier thus allowing another habitat to be created that normally would not exist. This type was dominated by exotics. Beyond the implications of playas with pits receiving frequent

Table 3.2 Vegetation classiﬁed into 14 physiognomic types from 101 playas in Bailey, Lamb, and Castro Counties, Texas, June–August 1980 Percent Physiognomic type (1) Open water (2) Broad-leaved emergent (3) Narrow-leaved emergent (4) Mesic forb

(5) Wet meadow

(6) Johnsongrass (7) Disturbed forb

(8) Cultivation (9) Mudﬂat (10) Spoilbank

(11) Midgrass

(12) Short-grass (13) Road-pit (14) Tree–shrub

Dominant taxa Pondweed (Potamogeton spp.) Smartweeds (Polygonum bicorne, P. lapathifolium) Cattail (Typha domingensis) Bulrush (Scirpus spp.) Devilweed (Aster spinosus) Gray ragweed (Ambrosia Grayii) Barnyard grass, Red sprangletop (Leptochloa ﬁliformis) Johnsongrass (Sorghum halepense) Summer cypress (Kochia scoparia) Texas blueweed (Helianthus ciliaris) Crop Absence of vegetation Summer cypress Camphor-weed (Heterotheca spp.) Western wheatgrass (Agropyron smithii), Vine mesquite (Panicum obtusum) Buffalo grass (Buchloe dactyloides) Absence of vegetation Willow (Salix nigra) Salt cedar (Tamarix gallica) Siberian elm (Ulmus pumila)

Secondary taxa

occurrence

Arrowhead (Sagittaria longiloba) Barnyard grass (Echinochloa crusgalli) Spikerush (Eleocharis spp.)

28 36

13 Smartweeds, Barnyard grass, Spikerush

26

Smartweeds, Spikerush, Devilweed

41

17 Horseweed (Conyza canadensis) Wild lettuce (Lactuca spp.)

Barnyard grass, Water-hyssop (Bacopa rotundifolia) Tumbleweed (Salsola kali)

9

20 16 1

2

Gray ragweed

13 12 21

Source: Modiﬁed from Guthery et al. 1982; courtesy of the Transactions of the North American Wildlife and Natural Resources Conference. Note: Dominant taxa were most prevalent in terms of coverage; secondary taxa commonly occurred in a type, but had lower coverage than dominant taxa.

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irrigation runoff, pits can also concentrate naturally occurring precipitation, effectively reducing the hydroperiod for the remainder of the playa plant community. Some pits, especially north of the Southern Great Plains in the Rainwater Basin, were speciﬁcally constructed to concentrate water not for irrigation, but to reduce the hydroperiod. This allows greater cultivation of the wetland. Pits are also constructed for livestock watering, which reduces the hydroperiod for the entire wetland. Finally, at times the construction of pits breaches the clay layer of the playa resulting in more rapid water loss. As mentioned previously, Hoaglund and Collins (1997) studied plant communities in 40 playas in southeastern Colorado, northeastern New Mexico, the extreme northwestern Texas Panhandle, and the Oklahoma Panhandle. Most of the playas they studied occurred on USDA national grasslands (e.g., Comanche, Kiowa, Rita Blanca). Therefore, most of these playas occurred in a native short-grass prairie setting with little cropland agriculture but an extensive cattlegrazing history. Many of the playas on the national grasslands, although not modiﬁed hydrologically to receive irrigation runoff, have had pits, ditches, and islands constructed in them to retain water for livestock or, ostensibly, to improve the playa as habitat for wildlife (later modiﬁcations were thought to improve use by waterfowl, not other species). According to Hoaglund and Collins 1997, within the hydric soil– deﬁned basin of these playas, three grass species appeared to dominate: western wheatgrass, buffalo grass, and vine mesquite (Panicum obtusum). They were also the most widespread species in terms of percent of playa occurrence. Bur ragweed, snow-on-the-mountain (Euphorbia marginata), and frog-fruit (Lippia nodiﬂora) were important forb species. Hoaglund and Collins noted that the dominance of western wheatgrass decreased as elevation increased to the uplands but that buffalo grass was important throughout the wetland and into the upland. Many other species (69) occurred but none were as widespread as those listed above. Apparently water did not exist in the playas during the time playas were sampled. However, from their paper it is not possible to determine when during the growing season the vegetation was surveyed and, therefore, whether the vegetation was compared among playas for similar seasonal circumstances and whether it varied seasonally. This can inﬂuence perceptions of extant community composition.

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In the ﬁve-state playa ﬂora study that Smith and Haukos (2002) conducted in more than 200 playas, the overall community composition varied signiﬁcantly among six subregions of this portion of the Southern Great Plains (unpublished data). Correspondence analyses were used followed by cluster analyses to delineate the six subregions (ﬁg. 3.3). The dominant species in these communities were similar among the six subregions, though the occurrence of uncommon species within a subregion allowed groups to be delineated. These delineations could simply be the result of varying precipitation and growing season patterns causing some species to be common in one county and uncommon in another. Some of the more common widespread species ( 50% occurrence in playas) throughout the region were western wheatgrass, rough pigweed (Amaranthus retroﬂexus), saltmarsh aster (Aster subulatus), buffalo grass, lamb’s quarters (Chenopodium album), narrow-leaved goosefoot (Chenopodium leptophyllum), horse-weed (Conyza canadensis), barnyard grass, spikerush (Eleocharis macrostachya), curlytop gumweed (Grindelia squarrosa), annual sunﬂower (Helianthus annuus), Texas blueweed (Helianthus ciliaris), little barley, summer cypress, frog-fruit, cheeseweed (Malvella leprosa), spotted evening primrose (Oenothera canescens), vine mesquite, Pennsylvania smartweed, spreading yellow cress (Rorippa sinuata), curly dock (Rumex crispus), silver-leaf nightshade (Solanum elaeagnifolium), and prostrate vervain (Verbena bracteata). Most of these occurred throughout the six subregions, but their frequency varied by whether the watershed in which they occurred was dominated by grassland or cropland. Annuals occurred much more often in cropland playas than in grassland playas (Smith and Haukos 2002). For example, lamb’s quarters and rough pigweed occurred more than twice as often in playas with cropland watersheds. Furthermore, the number of annuals in cropland playas averaged 8.9, while those in grassland playas averaged 6.7 (Smith and Haukos 2002). The above species occurrence data were supported by horizontal coverage information of perennial versus annual species within a playa. Playas with cropland watersheds had an average coverage of annuals at 30% and perennials at 47%, while playas with grassland watersheds had average coverage of annuals at 12% and perennials at 63%. (The percentages do not total to 100 because of either open water that had no plant cover or bare soil with no plant cover.) Clearly

Figure 3.3 Counties sampled for ﬂora description in the Southern Great Plains (Smith and Haukos 2002) and counties grouped by ﬂoral-deﬁned ordination groups. (Figure by D. Haukos, courtesy of U.S. Fish and Wildlife Service.)

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watershed cultivation and its associated disturbances has inﬂuenced playa plant communities. Interrelated disturbances include sedimentation, irrigation (runoff into playas and pumping water from playas for surrounding crops), and hydrologic modiﬁcation through pit and ditch construction. The increased rate of disturbance in playas with cropland watersheds versus those with grassland watersheds is predisposing playas to being dominated more often by annual species (Smith and Haukos 2002). Cultivation of playa watersheds is further associated with the bioinvasion of native ﬂora by exotics (Smith and Haukos 2002). On average, there were twice as many exotic species occurring in playas with cropland watersheds than in those with grassland watersheds (4.7 vs. 2.3, respectively). Coverage data supported the species frequency data (15.6% crop vs. 6.3% grassland). Cultivation, and its associated disturbance (e.g., hydrologic modiﬁcation, irrigation), has caused increased and consistent disturbance to playas throughout the Great Plains such that it is difﬁcult to ﬁnd playas that have natural ﬂoral communities. Cultivation disturbances permitted the increase in bioinvasion by exotics likely by increasing the prevalence of bare soil and by altering moisture regimes, which change germination conditions. There are shorter playa hydroperiods because of sedimentation but also, at times, longer, because of increased moisture from irrigation runoff. Exotics have exploited this new niche in the altered playa environment. There were 65 common (occurred in greater than 5% of playas) species in Southern Great Plains playas (Smith and Haukos 2002). Ten were exotics, a substantial portion (15%) of the community. Furthermore, only one of the 10 was perennial, illustrating that exotics have played a role in shifting the balance from perennials to annuals. Moisture and bare soil disturbances are not the only factors promoting bioinvasions; increased nutrient loads can cause this as well (Hobbs and Atkins 1988; Burke and Grime 1996). As noted earlier, playas form the lowest elevation points for the vast majority of agricultural enterprises in the region. Therefore, playas often receive additional nutrient inputs from crop fertilizer and livestock fecal runoff (Irwin et al. 1996). Similar to playas with cropland watersheds, modiﬁcation of playas in grassland environments, even national grasslands, has also allowed plant community composition to change. Many playas in native grassland environments have had pits dug in them for concentrating

62

ECOSYSTEM ASPECTS

water, which, as noted, allows nonnative species that require deeper water to colonize and allows more upland-adapted species to move into the now dryer hydric soil. Wildlife managers have even created ditches, pits, and islands in playas in many national grasslands in the region with help from a nongovernmental wildlife organization. This produces similar results to irrigation and livestock watering pits. The effect of livestock grazing on playa plant communities, which occurs on many playas with cultivated watersheds and is likely on all playas with grassland watersheds, has not been documented. Obviously the prairie environment, including playas, evolved under the inﬂuence of large herbivores (e.g., bison, elk, pronghorn). How native herbivore grazing varies from domestic livestock is unknown. Plant selection and season/frequency of use are likely different. Guthery et al. (1982) suggested that grazing of playas by domestic livestock may promote buffalo grass and associated plants (e.g., gray ragweed) at the expense of western wheatgrass. However, these were correlative data, in that direct tests/experiments were not conducted. The buffalo grass vegetation type was simply associated with livestock grazing. Further, some species such as knotgrass, considered an obligate wetland plant throughout much of the region, have been promoted as a means to increase livestock grazing potential in playas (W. Wyatt, personal communication, High Plains Underground Water Conservation District, Lubbock, Tex.). Finally, though playas are frequently burned in the Southern Great Plains, few studies have examined the inﬂuence of burning on plant community composition. In cattail-dominated playas, a relatively rare playa vegetation type in the Southern Great Plains (Haukos and Smith 1997), ﬁre had little inﬂuence on community composition and its effects on habitat structure lasted no more than 4 months (Smith 1989).

wyoming studies In eastern Wyoming, Holpp (1977) characterized plant communities in 10 playas. The dominant grasses in those playas were western wheatgrass and foxtail barley (Hordeum jubatum). Numerous common dandelions (Taraxacum ofﬁcinale) were also present. The dominant sedges and rushes were common sedge (Carex ﬁlifolia) and Baltic rush (Juncus balticus). Holpp stated that “the intensity of the grazing pressure resulted in the decline of the perennial grass species and allowed the encroachment and establish-

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ment of the forb community. Thus, perennial forbs dominated the plant composition of all grazed sites” (1977, 104). Although these conclusions may be correct, similar to Guthery et al.’s (1982) study the data were collected through association not experimentation; the observations thus should be viewed as preliminary. As noted earlier, Bartz (1997) found up to three zonal plant communities in other playas from Wyoming. He termed them, from the lowest elevation in the playa to the highest: “spikerush,” “foxtail barley,” and “western wheatgrass.” As found in studies of playas elsewhere, Bartz found the most diverse (31 species) community, or zone, occurred on the edge of the upland and wetland. The ﬁve most abundant species in this western wheatgrass zone were its namesake, followed by prairie junegrass (Koelaria cristata), blue grama (Bouteloua gracilis), spikerush (Eleocharis acicularis), and salt grass (Distichlis spicata). The next two zones appeared to be distinguished based on the amount of another spikerush (Eleocharis palustris) that the playas contained, because the most abundant species in each zone was that spikerush. The foxtail barley zone also contained its namesake, and western wheatgrass, a spikerush (E. acicularis), and bur ragweed. The lowest, or spikerush, zone contained both spikerushes, bur ragweed, western wheatgrass, and shortawn foxtail (Alopercurus aequalis).

rainwater basin studies Gilbert (1989) has conducted the most extensive survey of Rainwater Basin ﬂora from 47 wetlands (probably 10% of the wetlands remaining in the region). He delineated ﬁve major plant zones, four of which could be considered to contain some hydric plants (ﬁg. 3.4; nomenclature follows Gilbert 1989 here). Beginning in the deepest or longest hydroperiod zone, Inner Marsh, he found all obligate wetland species, or as he classiﬁed them, “aquatic bed” and “drawdown” species. Aquatic bed species were dominant and required submerged conditions to survive, whereas drawdown species required moist exposed soils to germinate. The drawdown (shallow water) portion of this zone was dominated by two species of arrowhead (Sagittaria spp.), bur reed (Sparganium eurycarpum), water plantain (Alisma triviate), water-hyssop (Bacopa rotundifolia), and blue mud-plantain. The dominant aquatic bed species were duckweed (Lemna minor), water fern (Azola mexicana), and common bladderwort (Utricularia vulgaris). Occasionally a “reverse” zonation condition occurs here (T. LaGrange, personal communication). When the

Figure 3.4 Vegetation zones in Rainwater Basin wetlands as deﬁned by Gilbert (1989, 21). (Figure by M. Gilbert, courtesy of U.S. Army Corps of Engineers.)

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deep open water area dries occasionally, the area becomes dominated by Outer Marsh plants but is still surrounded by living persistent emergents. Moving up in elevation, the next zone was the Persistent Emergent zone, which was mostly dominated by robust herbaceous emergent perennials such as three species of cattail (Typha spp.) and three species of bulrush (Scirpus spp.). Again, all of these are considered obligate wetland plants. The Outer Marsh zone appeared more diverse, with dominants including ﬁve smartweed (Polygonum spp.) species, three spikerushes (Eleocharis spp.), Plains coreopsis (Coreopsis tinctoria), and western water clover (ﬁg. 3.4). With the exception of coreopsis all are considered facultative wetland plants or obligate. Hydrophytic grass dominants in this zone included two barnyard grasses (Echinochloa spp.), reed canary grass (Phalaris arundinacea), rice cutgrass (Leersia oryzoides), bearded sprangletop (Leptochloa fascicularis), and bluejoint (Calamagrostis canadensis). At least six species of forbs were subdominants. The next zone upslope was termed Transition, with western wheatgrass, foxtail barley, Carolina foxtail (Alopecurus carolinianus), meadow foxtail (Alopecurus pratensis), ticklegrass (Agrostis hyemalis), and Virginia wild rye (Elymus virginicus) as stand dominants (ﬁg. 3.4). Four sedges (Carex spp.), three ragweeds (Ambrosia spp.), dock (Rumex spp.), and horse-weed (Conyza canadensis) were among some of the other dominants (Gilbert 1989). The upland zone was dominated by either planted or native grassland typically grazed by livestock. An examination of the wetland species listed above by Gilbert (1989) also shows a relatively high percentage ( 10%) of exotic species. Given the highly altered state of Rainwater Basin playas, this is not surprising. They have been subjected to the same physical insults that other playas have with some additional threats such as drainage. These alterations have a higher occurrence in south-central Nebraska than in other playa locations.

CHAPTER 4

FAUNA

A

s with the ﬂora in playas, the animal life existing in these wetlands depends not only on the current playa hydroperiod but also on its historic hydroperiod. Similar to seeds, many invertebrates can remain dormant in playa sediments for decades reﬂecting past colonization events and hydric conditions (Hairston et al. 1995). Although seeds and invertebrates are generally considered in this light, many amphibians also may remain dormant in playa soils for years, their presence a result of past habitat conditions, with their future dependent on the proper moisture conditions to emerge. Beyond the inﬂuence of historic and current hydric conditions on playa fauna are the landscape factors that shape the composition of the existing animal communities. For example, the position of the playas in the Great Plains and North American landscape dictate which bird species can use playas during migration or breeding in dry or wet conditions. INVERTEBR ATES

Similar to other ecosystems, the understanding of invertebrate fauna in playas is far behind that for vertebrates. The simple groupings used in this chapter, with invertebrates included in one overall group versus the several groups of vertebrates, illustrates the bias and availability of faunal data for playas. This condition exists with full knowledge that invertebrate diversity and abundance far surpasses that of vertebrates, and invertebrates are key in all ecosystem processes (Wilson 1999). They are important food sources to most vertebrates inhabiting playas including amphibians, shorebirds, and waterfowl (Davis and Smith 1998a; Anderson et al. 1999a; Anderson et al. 2000), essential to playa nutrient cycling, and predators/parasites on other invertebrates as well as vertebrates.

FAUNA

67

The very diversity and inherent variability in occurrence and numbers of invertebrates has often discouraged scientists from tackling invertebrate studies in wetlands (Murkin et al. 1994). Identiﬁcation to even the family level of classiﬁcation can be difﬁcult, while estimating population size, generation time, and turnover is daunting. Although these difﬁculties have not encouraged widespread study of invertebrates in wetlands, it is these large gaps in our knowledge that should foster future studies. The chance to make important and immediate contributions to playa ecology is extraordinary. SCOPE AND SAMPLING LIMITATIONS

Almost all work on playa invertebrates has focused on playas that have water in them at the time of investigation. Lists of invertebrates occurring in playas (e.g., Sublette and Sublette 1967) typically ignore the terrestrial/dry phase of playas. However, the importance of this invertebrate community to the structure and function of playa ecosystems is undoubtedly substantial. Although some might expect invertebrate communities of dry playas to closely mirror the surrounding uplands, this assumption has not been tested. Because playa ﬂora varies along the gradient from upland to hydric soil basin (see Chapter 3), it is probable that many plant-speciﬁc, and indeed soil-speciﬁc, invertebrates also vary along that gradient. Certainly studies are warranted on invertebrate communities during the dry phase in the life history of a playa. Further, many so-called terrestrial species inhabit the emergent vegetation, above the water, or are incorporated into aquatic food-web processes as the playa ﬁlls with water (Anderson and Smith 2000). This forms an especially unique community of aquatic and terrestrial species occurring together, with undescribed new species (J. Anderson 1997). Moreover, the majority of invertebrate studies in playas have focused on “macro-invertebrates” (ﬁg. 4.1). “Macro” is a somewhat arbitrary designation related to a speciﬁc screen size through which sampled invertebrates will not pass (usually 500 – 600 µm) (e.g., Anderson and Smith 1996). Oftentimes the term macro is used without deﬁnition, leaving the reader to guess which groups may have been excluded from the community. The “microinvertebrate” fauna (e.g., rotifers) of playas is relatively unexplored, but its importance to playa ecology cannot be dismissed. The ﬁndings of the few studies on aquatic invertebrates in playas often have little in common, which is likely due to variable sampling

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Figure 4.1 Common invertebrates sampled in playas, including midge larvae, leech, and dragonﬂy larvae. (Photo by author.)

methodology, geographic area of sampling, and season of sampling. However, much of this variability is inherent in playas on a local basis (Sublette and Sublette 1967; Merickel and Wangberg 1981; Thompson 1985; Neck and Schramm 1992; Anderson and Smith 1998), which can be related to the proximity of other aquatic habitats (inﬂuencing colonization events) and those species that were able to remain dormant in playa sediments (J. Anderson 1997; Hall et al. 1999). Furthermore, the amount and type of vegetation, which regulates the density and species composition of invertebrates (e.g., Krecker 1939; Krull 1970; Downing 1981), also varies greatly among playas on a local basis (Smith and Haukos 2002). Finally, the length and timing of the playa hydroperiod has a tremendous inﬂuence on the community composition and abundance of aquatic invertebrates (Anderson and Smith 1998). COMMUNITY COMPOSITION

Even with limitations and problems with the level of taxonomic resolution of playa invertebrates, it is apparent the invertebrate fauna of playas is especially diverse and underestimated. Hall et al. (1999) summarized the data from Sublette and Sublette (1967), Parks (1975), Merickel and Wangberg (1981), Neck and Schramm (1992), Horne (1996), and Hall (1997) and identiﬁed 124 macroinvertebrate taxa, at various taxonomic levels. When data from

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Davis (1996) and J. Anderson (1997) are included, the number of taxa exceeds 170 (see appendix). Some of the taxa (e.g., arachnids) recorded by J. Anderson (1997) include unique invertebrates inhabiting vegetation protruding above the water’s surface. Hall et al. (1999) noted that the aquatic insect community of playas was especially diverse when compared to the noninsect community (84 versus 40 taxa, respectively, in the taxa they examined). However, as they found, this is in part due to the state of our taxonomic understanding. Knowledge of aquatic insect taxonomy in playas appears more advanced than that for noninsects. Hall et al. (1999) noted that the number of annelid and ectoproct (bryozoans) species is unknown because of limited collections and improper preservation of specimens. The molluscs also have been a neglected group in playa studies, with only 8 species identiﬁed (appendix). Of the noninsects, arthropods appear most diverse, but resolution of copepods and branchiopod cladocerans has only been made to the class or ordinal level. A bright spot, the nonclacloceran branchiopods (clam, fairy, and tadpole shrimp) and ostracods (seed shrimp) are relatively more well known (Moore and Young 1964; Ferguson 1967; Horne 1993, 1996). A word of caution is warranted relative to some of the aquatic invertebrate studies in so-called Great Plains playas. Some of the species listed may not occur in playas but rather in groundwater-connected salt lakes (e.g., Sissom 1976). It is often difﬁcult to determine actual collection location of species described in some taxonomic studies. Moreover, the majority of published studies on playa invertebrates have been conducted in the central Southern High Plains and invertebrate studies for the more northern playas are rare. For the Rainwater Basin region of Nebraska there has only been one published study on aquatic invertebrates. Gordon et al. (1990) examined eight playas during spring and summer and identiﬁed 39 taxa all at the family level or higher. Extrapolating results of invertebrate studies conducted on the Southern High Plains to playas outside that region is ill-advised for a variety of reasons, not the least of which are regional speciation and climatic differences (King et al. 1996). INFLUENCES ON COMMUNITY COMPOSITION

The immediate response of invertebrates to a playa ﬁlling with water can be astounding. A couple of weeks after basins have ﬁlled, the water’s surface can literally bubble with activity. After this typical initial ﬂush of high abundance and low diversity

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of individuals emerging from dormancy within the playa, the community composition begins to shift and diversity generally begins to increase. The shifts in community assemblage are related to additional intraplaya emergence and colonization from outside of the playa by other invertebrates (J. Anderson 1997; Moorhead et al. 1998). However, these colonization and persistence (e.g., dormancy, diapause) events are not necessarily mutually exclusive life-history strategies employed by aquatic invertebrates. J. Anderson (1997) found that 58% of the taxa persisting in playas through diapause also actively colonized playas as a primary life-history strategy of inhabitation. After some threshold, however, increased length of inundation (hydroperiod) can result in decreased invertebrate abundance. Seasonally ﬂooded wetlands often have higher invertebrate abundances than those ﬂooded for longer periods such as semipermanent marshes (Neckles et al. 1990; Batzer and Resh 1992). This length of inundation threshold, resulting in lower abundance and/or diversity, is likely due to biotic interactions. As the hydroperiod lengthens, competition and predation play an increasingly important role in determination of aquatic invertebrate assemblages and abundance. Sublette and Sublette (1967), Merickel and Wangberg (1981), and Moorhead et al. (1998) found that following playa ﬂooding in summer crustaceans were initially dominant and then insects possessing active dispersal capabilities become more prevalent. Many of these later-emerging and -colonizing species are predaceous. Subsequent competition and predation not only by invertebrates but also by amphibians and birds may decrease abundance of initially emerging species. For example, Davis and Smith (1998a) found that shorebirds reduced the abundance of invertebrates in some playas in the spring. Also, although overall invertebrate densities were higher in early spring, biomass and species diversity were greater by late summer and early fall in playas that had little emergent vegetation. Difﬁcult to separate from the biotic, abiotic factors such as water and air temperature and day length also have an important effect on playa invertebrate communities. Playas can ﬁll with water during any month, but given Great Plains precipitation patterns (see Chapter 1) Southern Great Plains playas typically ﬁll in late spring and summer when day lengths and temperatures are at their annual highs. In the Rainwater Basin, ﬁlling also happens in spring as snow melts. Warmer temperatures and longer photoperiods generally permit greater growth and biomass of aquatic invertebrates (Ward and Stan-

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ford 1982). Studies of aquatic playa invertebrates outside the spring and summer months are rare. Anderson and Smith (2000) found that, in general, invertebrate abundance and diversity were greater in the September-ﬂooded playas than those ﬁlled in November. This should be expected as a result of both abiotic (e.g., higher temperatures) and biotic (e.g., colonization opportunities) inﬂuences. Because of these biotic and abiotic relationships, altering the hydrology of playas has likely had a major inﬂuence on Great Plains invertebrate community composition. The construction of pits, trenches, or dugouts has created a more permanent water source in some playas by concentrating water in a smaller area reducing evaporative surface area. This has created a habitat that allows invertebrates, and other biota that require longer hydroperiods to reproduce, to invade and survive in playas thus changing biotic relationships. For example, Horne (1996) noted that Neck and Schramm (1992) found in playas with pits some species of molluscs not normally found in ephemeral pools. Further, there are reports of crayﬁsh (Cambaridae) existing in a few playas of the Southern High Plains and north in Kansas and Nebraska (Flowers 1996; Nebraska Game and Parks Commission unpublished observations), which might be related to playa modiﬁcation. Conversely, Rhodes and Garcia (1981) reported that playas with pits had fewer individuals and species of insects than playas without pits. Gray (1986) found that invertebrate abundance in playa pits peaked in late summer and early fall, coinciding with peak submergent plant biomass, a rare plant community outside of the pits. On the other hand, playa sedimentation, which has greatly reduced the volume and occurrence of playas throughout the Great Plains, also has likely had a profound effect on playa invertebrate communities by altering hydroperiods, plant communities, and benthic substrates. The implications of these hydrological inﬂuences on playa invertebrate evolution and extinction is unknown. Excellent reviews on the state of our knowledge of playa invertebrates have been done by Hall (1997) and J. Anderson (1997). FISHES

Due to the natural, ephemeral character of these wetlands, there were no known native ﬁshes in Great Plains playas prior to human intervention. With dugouts and pits excavated in playas from Nebraska to New Mexico, playas that have ﬁsh are now scattered throughout the region. Most of these introduced species

72

ECOSYSTEM ASPECTS

have been stocked by landowners on private lands for sportﬁshing. However, some have also been placed in playas on federal lands (e.g., Comanche National Grasslands, Colo.) and municipal properties (e.g., Lubbock, Tex.). Some have been stocked with ﬁsh for mosquito control (Nelson et al. 1983). Still other modiﬁed playas have ﬁsh such as black bullheads (Ictalurus melas) in them that apparently were not actively stocked by humans. Their means of dispersal to these playas is unknown (Bolen et al. 1989). Some of the more common ﬁshes introduced into modiﬁed and urban playas include yellow bullhead (Ictalurus natalis), channel catﬁsh (Ictalurus punctatus), sunﬁshes (Lepomis spp.), largemouth bass (Micropterus salmoides), common carp (Cyprinus carpio), golden shiner (Notemigonus crysoleucas), and fathead (Pimephales promelas) and brassy (Hybognathus hankinsoni) minnows (Curtis and Beierman 1980). Even rainbow trout (Oncorhynchus mykiss) have been stocked into urban playas for “put and take” ﬁsheries (e.g., by the Texas Parks and Wildlife Department). These urban playas, which now have permanent sources of water, contain all sorts of exotic fauna and ﬂora (Schramm et al. 1992). There are no studies addressing the inﬂuence of ﬁsh on native playa taxa. AMPHIBIANS

Aside from invertebrates, less is known about amphibians inhabiting Great Plains playas than any other major faunal group (A. Anderson 1997; Smith 1998). There have only been a few published ecological studies on amphibians in playas (Rose and Armentrout 1974, 1976; Anderson et al. 1999a,b). Although amphibians are key components of Great Plains wetlands, including serving as predator and prey, they have a relatively simple fauna compared to the other vertebrates. Six families of amphibians occur in Great Plains playas, with only one, but wide-ranging, species of salamander (ﬁg. 4.2; table 4.1). The anuran (toads, frogs) species with the widest geographic range in the playas are the true toads (Bufonidae) and spadefoots (Pelobatidae), probably because of their ability to withstand rapidly ﬂuctuating water levels. A Plains spadefoot can hatch in as little as 20 hours, in warm weather, and metamorphose from tadpole to terrestrial toad in less than 13 days (King 1960; Justus et al. 1977). Rapid reproduction and development is a prudent evolutionary strategy for aquatic species in playas. The Hylidae (tree, chorus, and cricket frogs) and

Figure 4.2 Three of the most common amphibians in Great Plains playas. Top— nektonic tiger salamander; middle— Great Plains toad; bottom— New Mexico spadefoot toad. (Photos by author.)

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ECOSYSTEM ASPECTS

Table 4.1 Amphibians inhabiting playas of the Great Plains Family Ambystomatidae Bufonidae

Hylidae

Microhylidae Pelobatidae

Ranidae

Common name

Scientiﬁc name

tiger salamander Great Plains toad green toad Texas toad Woodhouse’s toad northern cricket frog spotted chorus frog western chorus frog Great Plains narrow-mouth toad Couch’s spadefoot toad Plains spadefoot toad New Mexico spadefoot toad Plains leopard frog

Ambystoma tigrinum Bufo cognatus Bufo debilis Bufo speciosus Bufo woodhousii Acris crepitans Pseudacris clarkii Pseudacris triseriata Gastrophryne olivacea Scaphiopus couchii Spea bombifrons Spea multiplicata Rana blairi

bullfrog

Rana catesbeiana

Source: From Curtis and Beierman 1980; A. Anderson 1997; Nebraska Game and Parks Commission 2002, M. Fritz.

Ranidae (true frogs) generally require more frequent wet conditions than toads or spadefoots. However, scientists know relatively little about the environmental conditions that stimulate amphibians to emerge and breed in playas, and as little about their hibernation/estivation patterns. What playa amphibians lack in diversity they make up for in abundance. Their numbers can easily overwhelm all other vertebrates. Further, the biomass of the aquatic amphibians in playas often surpasses all fauna, especially in summer. Tens of thousands of larval tiger salamanders can occur in a playa covering just a couple of hectares in area or the same magnitude of toadlets (recently metamorphosed tadpoles) can emerge from the same size playa in a single night (Gray 2002). A tiger salamander may lay more than 5,000 eggs in a clutch (Rose and Armentrout 1976), but this is relatively small compared to some anurans. A single Great Plains toad female can lay more than 40,000 eggs in one clutch and can have two clutches a year (Krupa 1986, 1988, 1994). The sheer magnitude of amphibian biomass and abundance is key to the survival of their predators such as larval invertebrates, American avocets (Recurvirostra americana),

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and black-crowned night herons (Nycticorax nycticorax) as well as their prey, invertebrates and fellow amphibians. Many amphibians are cannibals (Degenhardt et al. 1996). This relatively high abundance and low diversity of amphibians, in limited aquatic habitat, is exactly what makes them unique and valuable to the Great Plains landscape. Similar to their effects on other biota, pits, trenches, or other modiﬁcations to the playa hydrology also inﬂuence occurrence of amphibian species that require more permanent sources of water. Anderson et al. (1999b) found that spotted chorus frogs in the central Southern High Plains were found more frequently in playas with irrigation pits than those without. For those species that can survive more xeric conditions (Great Plains toad, Great Plains narrow-mouth toad, Plains spadefoot toad, New Mexico spadefoot toad) there were no differences in their occurrence between playas with and without pits. The occurrence of bullfrogs is also likely related to hydrologic alteration of playas and through direct stocking by landowners. They are probably not native to Great Plains playas. The amount of vegetative cover in the playa basin also affects amphibian occurrence. The ﬁve species noted in the previous paragraph showed a positive relationship between their occurrence and increasing amounts of vegetation (Anderson et al. 1999b). This was especially apparent for the Great Plains narrow-mouth toad and the spotted chorus frog. Anderson et al. (1999b) further compared the occurrence of these ﬁve species between playas with grassland watersheds to those with cropland watersheds. Cultivation in the watershed is an indication of disturbance, such as in the amount of sedimentation, that has inﬂuenced the playa (Luo et al. 1997), which can affect not only burrowing conditions for estivating amphibians but also the water quality, length of the playa hydroperiod, and ﬂora. Anderson et al. (1999b) did not demonstrate a surrounding land-use effect on anuran species richness or frequency, but they found considerable annual variation in anuran richness and occurrence within the same playa. Frog and toad community composition changed in all 18 playas that were surveyed in two consecutive years. This led Anderson et al. (1999b) to suggest that anurans may forgo breeding in some years even when playas contain water. Therefore, recently initiated national surveys using annual auditory trends of breeding amphibians would yield results that are difﬁcult to interpret in playas because of variable annual breeding effort rather than ﬂuctuations in either species occurrence or individual species population size.

76

ECOSYSTEM ASPECTS

According to Degenhardt et al. (1986), tiger salamander larvae will eat almost any animal matter they catch (Collins 1982; Hammerson 1982). Indeed, they form cannibal morphs, and as the name suggests they consume large numbers of conspeciﬁc larvae (Holomuzki and Collins 1987). Cannibal morphs are not common (Rose and Armentrout 1976), but occurrence may be stimulated by high density of conspeciﬁc larvae (Collins and Cheek 1983). Cannibalism can be an adaptive life-history strategy allowing more rapid growth and earlier metamorphosis (Lannoo and Bachmann 1984). Larval anurans may also become predator/cannibals, but data in natural environments are scarce. Anderson et al. (1999a) studied the diets of newly emerged, breeding Great Plains toads, New Mexico spadefoot toads, and Plains spadefoot toads in playas of the central Southern High Plains. Great Plains toads consumed 43 invertebrate taxa, and New Mexico and Plains spadefoot toads consumed 20 and 12 invertebrate taxa, respectively, even though they found individual spadefoot diets were more diverse than Great Plains toads. Interestingly the highest dietary overlap occurred between the largest species, Great Plains toad, and the smallest, New Mexico spadefoot. The most important food for all three species was Carabidae beetles. These beetles are mostly nocturnal like the three anurans. Moreover, based on the occurrence of food in stomachs, it appears Great Plains toads feed soon upon emergence and during breeding, whereas spadefoots feed more after breeding. A. Anderson (1997) also found that several of the top invertebrate taxa consumed by these toads were considered agricultural pests. Given the potential high density of toads, this fact may indicate economic beneﬁts to the agricultural economy. The diet of larval anurans in playas remains to be studied. Most invertebrates consumed by adult anurans were “terrestrial,” not “aquatic” (Anderson et al. 1999a). Adult anurans were feeding on the edge of the playa or in the immediate watershed. This further illustrates the need to study terrestrial invertebrates in playas and the close links between the terrestrial and aquatic systems. Given the life history (aquatic larval and terrestrial adult) of playa amphibians, they would be ideal models for these types of investigation. Moreover, amphibians are believed to be declining worldwide with no single factor being consistently identiﬁed as the cause of decline across all regions (e.g., Blaustein and Wake 1990; Beebee 1996; Corn and Peterson 1996). The general lack of knowledge concerning Great Plains amphibians,

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their value as environmental indicators, and their potential decline should spur research on amphibians throughout the playas. REPTILES

There are few native reptiles occurring in the inundated portions of playas. The yellow mud turtle (Kinosternon ﬂavescens) is the main exception, with a distribution coinciding with the distribution of most playas. Overall, however, most of its time is likely spent on dry land while the majority of its foraging, drinking, and mating occur in water (Degenhardt et al. 1996). Further, apparently the yellow mud turtle can estivate for up to two years (Rose 1980), which accounts for its widespread occurrence in playas. The turtle does not mate until it is 6 –16 years old and has a clutch of 3 – 10 eggs (Degenhardt et al. 1996). The reptile community in dry playas or on the interface of the water and associated watershed can be roughly as diverse as the amphibians, though they are not nearly as abundant. Probably the most numerous group is the garter snake, which feed on the invertebrates and amphibians inhabiting playas. They include the red-sided garter (Thamnophis sirtalis), western ribbon (Thamnophis proximis), wandering garter (Thamnophis elegans), western plains garter (Thamnophis radix), and checkered garter (Thamnophis marcianus) snake (Holpp 1977; Curtis and Beierman 1980; LaGrange 1997). Prairie rattlesnakes (Crotalus viridis), western diamondbacks (Crotalus atrox), and, in some areas, massasaugas (Sistrurus catenatus) also occur in the watershed and dry playas. Plains hognose snake (Heterodon nasicus), bull snake (Pituophis melanoleucas), and western smooth green snake (Liochlorophis vernalis) are commonly observed in the Southern Great Plains (Curtis and Beierman 1980; Haukos and Smith 1994a). The glossy snake (Arizona elegans) and common kingsnake (Lampropeltis getulus) are also known to inhabit playas (M. Fritz, personal communication, Nebraska Game and Parks Commission). The Texas horned lizard (Phrynosoma cornutum), short-horned lizard (Phrynosoma douglassii), and collared lizard (Crotaphytis collaris) are some of the more common lizards occurring in dry playas or their watersheds. The six-lined race runner (Cnemidophorus sexlineatus) and northern prairie lizard (Sceloporus undulatus) also likely forage in dry basins and their watersheds (M. Fritz, personal communication, Nebraska Game and Parks Commission). While the above-mentioned species and many other snakes and lizards doubtlessly occur in dry playas

78

ECOSYSTEM ASPECTS

and associated watersheds, their occurrence has not been directly tied to the existence of the playa wetland. The ornate box turtle (Terrapene ornata) is common along playa margins. There are also occasional records of snapping (Chelydra serpentina) and painted turtles (Chrysemys picta) in playas. These occurrences probably result from release by humans into modiﬁed and urban playas. There are no published studies on the ecology of reptiles in dry or wet playas. BIRDS

More studies have been conducted on birds inhabiting playas than any other Great Plains faunal group. The majority of these studies, however, have been conducted in the Southern High Plains and the Rainwater Basin. Moreover, most of the research conducted on playa birds has focused on waterfowl and gallinaceous birds. Because of their importance to hunters and the agencies in charge of providing hunter opportunity and monitoring harvest (e.g., state ﬁsh and wildlife agencies, U.S. Fish and Wildlife Service), historically more funds have been made available to study these groups of birds than other groups such as passerines or shorebirds. Moreover, similar to invertebrates, the avifauna of playas has primarily been studied from an aquatic perspective. Knowledge of bird use of dry playas is limited, although use can be substantial (Smith and Haukos 1995). As should be expected, the greatest diversity and abundance of birds in Great Plains playas generally occur during spring and fall migration. This is because of the sheer number of species passing through the area, the seasonal nature of water and food availability in playas, and the importance of these wetlands in a semiarid landscape in which few other wetlands are available. The location and size of the geographic area containing playas dictates its importance to the vast majority of migratory birds in the midsection of North America. In the 17 counties of the Rainwater Basin region alone, 257 species have been documented, 176 of which use playa basins. The vast majority are migrant rather than resident birds (Gersib et al. 1990a; LaGrange 1997). For the Southern High Plains, Simpson and Bolen (1981) noted that 108 nonwaterfowl species could be found in playa basins, 63 of which were spring and fall migrants. Fischer et al. (1982) also counted nonwaterfowl species in 100 playas in the central Southern High Plains and found 116 species in 32 families, at least 90 of which were migrants. When waterfowl species and data from a local National Wildlife Refuge (Buffalo Lake) are considered, 185 species

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in 41 families could inhabit playas of the Southern High Plains (Haukos and Smith 1994a). Again the overwhelming majority of these are spring and fall migrants. Flowers (1996) found 168 species of birds associated with playas in Meade County, Kansas, most of which were present during migration. The numbers of bird species nesting in playas is less than during migration but still is impressive. Seyffert (2001, 3) listed 151 avian species nesting in the Panhandle of Texas. As noted for most other bird lists for the Great Plains region, the objective of his book was not to provide speciﬁc nesting habitat data, so it is understandably difﬁcult sometimes to determine from Seyffert’s observations actual species nesting within playas. Moreover, playas can be dry for months, or even years, with typical upland species nesting in the basins. But then it rains, wetland-dependent species begin nesting in large numbers, and the entire avian community changes within a few days. This phoenixlike event can be seen in most prairie wetlands but is particularly striking in playas. Finally, the number of species using playas during winter is also less than during migration; but densities, especially for waterfowl, in wet playas can be substantial. Because most of the wet playas north of Texas freeze in late November or December and do not thaw until late February or March, their value to aquatic birds in winter is limited. However, this does not suggest that frozen playas with emergent vegetation or dry playas do not provide important winter habitat for other migrant and resident birds. Again, biologists simply have not studied playas from this perspective. These data are needed to fully appreciate the importance of playas from an annual and landscape perspective. Because birds have typically been studied in taxonomic groups, they are separated here by common groups and discussed as they proceed through the annual cycle. Although the terms “spring migration” and “fall migration” are used throughout this chapter from an avian annual cycle perspective, the actual migration dates may occur in human designated winter and summer seasons. SHOREBIRDS

migration Thirty species of migrating shorebirds have been documented using playas in the Southern High Plains of Texas (Davis and Smith 1998a), while Flowers (1996) found 26 species in Kansas

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ECOSYSTEM ASPECTS

playas (table 4.2). In the Rainwater Basin playas, 32 species have been recorded (Gersib et al. 1990a). In Texas the playas most commonly used by shorebirds lacked dense stands of emergent vegetation ( 25% wetland cover), and the edges were dominated by mudﬂats (Davis and Smith 1998a). Migrating shorebirds in these playas spent most of their time feeding (Davis and Smith 1998b). As has been demonstrated for migrating shorebirds in other Great Plains wetlands, such as in the glacier-formed Prairie Potholes Region, wetlands are key sites for feeding shorebirds during migration (Wishart and Sealy 1980; DeLeon and Smith 1999). Shorebirds that may migrate 12,000 kilometers (arctic breeding, South American wintering) must feed almost constantly in playas to regain and store nutrient reserves for continued migration and/or breeding needs (Davis and Smith 1998b). The diets of American avocets, long-billed dowitchers, least sandpipers, and western sandpipers (species spanning the range in body size and foraging guilds of most playa shorebirds) are dominated by invertebrates in the Southern High Plains (Baldassarre and Fischer 1984; Davis and Smith 1998a). However, seeds of wetland plants can also be important, composing 6 –25% of these shorebirds’ diets (Davis and Smith 1998a). Female migratory birds often feed on more protein-rich foods than males do to meet the demands of egg production (Blem 1990). Interestingly, unlike many other migratory birds such as waterfowl, diets of migrating shorebirds did not vary between the sexes during spring. The constraints imposed by variable prairie habitat conditions and a short migration period in spring may not allow female shorebirds to discriminate among food types at this stage of migration (Davis and Smith 2001). Diets of Southern High Plains shorebirds varied between spring and fall (Davis and Smith 1998a). The diets of all four species listed above were dominated by chironomids in spring, whereas in the fall diets were more diverse and not dominated by any one particular item. Most of these seasonal diet differences were related to food availability (Davis 1996). Sometimes early in spring migration, chironomids were the only macroinvertebrate species found in a playa, whereas in autumn many more invertebrate species were present. Furthermore, within the physical size constraints of individual shorebird species and the types of foraging they use (American avocets are scythers, while least sandpipers are pecking/probers), it appears shorebirds consume invertebrates relative to their availability (Davis and Smith 2001).

Table 4.2 Shorebirds associated with playas at selected sites in the Great Plains during spring and fall migration

Species American avocet American woodcock Baird’s sandpiper Black-bellied plover Black-necked stilt Buff-breasted sandpiper Common snipe Dunlin Greater yellowlegs Hudsonian godwit Killdeer Least sandpiper Lesser golden-plover Lesser yellowlegs Long-billed curlew Long-billed dowitcher Marbled godwit Mountain plover Pectoral sandpiper Red knot Red-necked phalarope Ruddy turnstone Sanderling Semipalmated plover Semipalmated sandpiper Short-billed dowitcher Snowy plover Solitary sandpiper Spotted sandpiper Stilt sandpiper Western sandpiper Whimbrel White-rumped sandpiper Willet Wilson’s phalarope Upland sandpiper a

Davis and Smith 1998a. Flowers 1996. c Gersib et al. 1990a. b

Scientiﬁc name Recurvirostra americana Scolopax minor Calidris bairdii Pluvialis squatarola Himantopus mexicanus Tryngites subruﬁcollis Gallinago gallinago Calidris alpina Tringa melanoleuca Limosa haemastica Charadrius vociferus Calidris minutilla Pluvialis dominica Tringa ﬂavipes Numenius americanus Limnodromus scolopaceus Limosa fedoa Charadrius montanus Calidris melanotos Calidris canutus Phalaropus lobatus Arenaria interpres Calidris alba Charadrius semipalmatus Calidris pusilla Limnodromus griseus Charadrius alexandrinus Tringa solitaria Actitis macularia Calidris himantopus Calidris mauri Numenius phaeopus Calidris fuscicollis Catoptrophorus semipalmatus Phalaropus tricolor Bartramia longicauda

Southern High Plains Texas a

Meade County Kansas b

Rainwater Basin Nebraskac

x x x x

x x x x x

x x x x

x

x

x x x x x x x x x x x x x

x

x x x x x x x x x x x x

x x x x x x

x

x x x x x x x x x x x x

x x x x x x x x x x x x x x x x x x x x x x x x x x x x

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It is difﬁcult to estimate numbers or percentages of a certain species population migrating through and using the playas. Because of the inaccuracies associated with knowing the actual continental population size of a species to begin with, estimating percentages of a population is problematic. Moreover, estimating “residency time”— the amount of time a particular shorebird uses a wetland before continuing migration—in particular playas is also very difﬁcult (Skagen and Knopf 1994). Knowledge of residency time is necessary to determine how many birds are passing through the area. Further, playas are mostly scattered small wetlands with great regional variation in water availability. Although shorebird densities in an individual playa may be very high, extrapolating that density over a particular region is difﬁcult unless you know the number of wet playas and the range in shorebird densities over a sample of those playas. This type of problem does not exist in large, more continuous tracts of wetlands such as Cheyenne Bottoms in central Kansas (Helmers 1991) or Delaware Bay (Clark et al. 1993) where several hundred thousand birds have been estimated. In the Great Plains, shorebirds may perceive the regional landscape of playas as if it is a large individual wetland (Farmer and Parent 1997). With these restrictions in mind, and the high density of playas in much of the region (as noted in Chapter 1), shorebird numbers have been extrapolated to the availability of wet playas, resulting in estimates of several million shorebirds using the Southern High Plains during migration (Davis and Smith 1998a). In the Southern High Plains, the migration of most shorebird species was much more protracted in summer/fall (5 –10 weeks) than spring (2 – 4 weeks) (Davis and Smith 1998a). This pattern likely exists for playas throughout the Great Plains. The longer duration of migration in the fall could be related to different migration chronologies of the different age/sex classes (Morrison 1984). In many sandpipers (Scolopacidae), adults migrate before juveniles, and adult males migrate after adult females. Shorter spring migration also may be related to the urgency of breeding, especially for arctic breeding species, which only have a narrow window in which to complete nesting (Paulson 1983; Smith et al. 1991; O’Reilly and Wingﬁeld 1995). Although overall Southern High Plains shorebird abundance was higher in fall than spring, the spring migration was shorter and peak weekly abundances of shorebirds were much higher than in fall

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(Davis and Smith 1998a). This along with the relatively high numbers of species using the same habitat begs the question as to how these various shorebird species coexist (Davis and Smith 2001). Shorebirds have often been placed into different foraging guilds such as “small probers-gleaners,” which are represented by species like western and least sandpipers, and “gleaner-sweepers” like the American avocet (Helmers 1991). Among different foraging guilds, shorebirds typically used different habitats (e.g., mudﬂats vs. shallow water), separating out spatially (Davis and Smith 1998a). During spring, however, shorebirds within the same foraging guild used similar habitats but exhibited little temporal overlap in their weekly occurrence. The different species migrated at different times. Because peak weekly densities of shorebirds are high in spring and invertebrate biomass is less than in summer/fall, it may be beneﬁcial, from a ﬁtness standpoint, for shorebirds within the same foraging guild to migrate through the playas at different times (Davis and Smith 2001). Among other inﬂuences, factors such as migration distance, weather, foraging efﬁciency, and ﬂight characteristics also affect timing of playa use in spring. Because Helmers (1991) and Skagen and Knopf (1994) witnessed similar separation of migration chronologies within the same foraging guilds in Kansas, it is likely this pattern is typical for shorebirds using playas throughout the Great Plains.

nesting There are several species of shorebirds breeding in the immediate vicinity of playas (ﬁg. 4.3). In the Southern High Plains, American avocets, killdeer, black-necked stilts, and snowy plovers are most numerous although the stilts and snowy plovers become more rare to the north and east playa regions (Smith and Haukos 1995; Conway 2001). Most of these shorebirds nest along sparsely vegetated shorelines and avoid densely vegetated playas (Conway 2001). Conway (2001, 30) estimated that if hydrologic conditions were suitable, 258,000 avocet and 30,000 killdeer nests could be found in playas of the Southern High Plains. There are also a few records of spotted sandpiper and Wilson’s phalarope nesting in playas of the Great Plains (Flowers 1996; Seyffert 2001). Long-billed curlew and mountain plover nest regionally but are not necessarily directly associated with playas since they commonly nest above the immediate playa watershed in short-grass environments.

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Figure 4.3 Playas throughout the Great Plains serve as important migration habitat for more than 30 species of shorebirds. American avocets nest in and migrate through the playas. (Photo by W. Meinzer, courtesy of the U.S. Fish and Wildlife Service.)

winter Because most playas north of the Llano are frozen during winter, shorebird use is minimal there. In Southern High Plains playas there are generally only a few species seen in small numbers. Most winter much farther south. The one exception is the long-billed curlew, which consistently spends (roosting and feeding) the entire winter on playas of the Southern High Plains. Winter population estimates do not exist for the species here, but numbers can be substantial ( 2,000). WATERFOWL

migration Similar to shorebirds, the numbers and diversity of waterfowl using playas during migration is spectacular. At least 25 species of waterfowl use playas from Nebraska to Texas and New Mexico (table 4.3). During spring, in the Southern High Plains and other western playas, the dabbling ducks (Tribe Anatinae), lesser

Table 4.3 Waterfowl species observed migrating through the playa lakes Tribe, Common name Cygnini Trumpeter swan Tundra swan Anserini Canada goose Greater white-fronted goose Ross’ goose Snow goose Cairinini Wood duck Anatini Mallard Gadwall Northern pintail Green-winged teal Blue-winged teal Cinnamon teal American wigeon Northern shoveler Aythyini Redhead Ring-necked duck Canvasback Greater scaup Lesser scaup Mergini Hooded merganser Common merganser Common goldeneye Bufﬂehead Oxyurini Ruddy duck Source: Curtis and Beierman 1980; Gersib et al. 1990a; Flowers 1996.

Scientiﬁc name

Cygnus buccinator Cygnus columbianus Branta canadensis Anser albifrons Chen rossii Chen caerulescens Aix sponsa Anas platyrhynchos Anas strepera Anas acuta Anas crecca Anas discors Anas cyanoptera Anas americana Anas clypeata Aythya americana Aythya collaris Aythya valisineria Athya marila Aythya afﬁnis Lophodytes cucullatus Mergus merganser Bucephala clangula Bucephala albeola Oxyura jamaicensis

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Figure 4.4 Generalized shape of migration corridor for many waterfowl species in the Central Flyway during spring. This illustrates the importance of the Rainwater Basin region of Nebraska. (Figure courtesy of the Nebraska Game and Parks Commission.)

snow geese, and Canada geese are most common. In the Rainwater Basin white-fronted geese should also be added to the list (Krapu et al. 1995). Indeed, Rainwater Basin playas are believed to be critical spring migration habitat for waterfowl in the Central Flyway that have wintered in Texas and Mexico and are returning to the northern prairies and Arctic to nest (ﬁg. 4.4) (Gersib et al. 1989). However, concurrent with the high loss of playas in the Rainwater Basin and the high concentration of waterfowl, there have been the annual outbreaks of avian cholera during spring. Although this disease primarily affects waterfowl during winter in the Southern High Plains, it is most destructive during spring migration in the Rain-

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water Basin. Several hundred thousand ducks and geese have died in Nebraska from the disease since 1975, the ﬁrst year in which cholera was reported there (Stutheit 1988). Little is known about the factors associated with avian cholera outbreaks even though it has been a priority research and ﬁscal focus of the Department of Interior for the past 20 years (Windingstad et al. 1984). Smith and Higgins (1990) found that cholera epizootics were inversely related to semipermanent wetland densities in the Rainwater Basin. Smith et al. (1990) also noted that there was a positive relationship between snow amounts in the 60-day period before midMarch and the number of waterfowl dying from avian cholera. They hypothesized that the greater amounts of snow made grain less available to ﬁeld-feeding waterfowl, which in turn caused more stress on the birds. Further, Smith et al. (1990) suggested these weather conditions may increase mold and mycotoxins on the grain that is fed upon by waterfowl, thus weakening their immune system and predisposing them to avian cholera. Migration of waterfowl through the Southern High Plains in spring begins in mid-February and continues through May. Canada geese, snow geese, northern pintails, and mallards are the ﬁrst to migrate (many of which have also wintered in the region), while in late May blue-winged teal, gadwall, and northern shoveler are some of the last to continue north (ﬁg. 4.5). Geese begin arriving in the Rainwater Basin in late February, often leaving by mid- to late March or early April, depending on weather conditions. Most ducks come a bit later roughly following the same chronology as those in the Southern High Plains (Gersib et al. 1990a). As with shorebirds, the autumn migration of waterfowl is more extended and generally a reversal of the spring. Of course, this is all mediated not only by immediate seasonal weather but by past weather conditions. Past weather inﬂuences future water availability in the wetlands as well as plant and invertebrate foods available to waterfowl. Agricultural crops in these migration stopovers also affect the species and numbers of waterfowl using a particular area. Further, the occurrence and abundance of certain waterfowl species may affect the occurrence and abundance of others. With the rapid rise in the lesser snow goose population in North America over the last few decades (Batt 1997), the numbers migrating through the playas also have greatly increased. In 1974 only 15,000 snow geese were counted during spring migration in the Rainwater Basin, but by 1989 there were more than 450,000 (Gersib et al. 1990a);

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Figure 4.5 Dabbling ducks such as these northern pintails and green-winged teal reach tremendous densities on playas during spring and fall migration. (Photo courtesy of J. Steiert.)

and in 2001 well over a million were counted (T. LaGrange, personal communication, Nebraska Game and Parks Commission). It has been hypothesized that in the Rainwater Basin, the mere presence of large numbers of snow geese, directly and indirectly, precludes use of wetlands and agricultural ﬁelds by other waterfowl (Smith 1998). The same problems with estimating abundance of shorebirds migrating through the playas exists for waterfowl. Although biologists have a better handle on the continental population size of most waterfowl species (Strickland et al. 1994) than that of shorebirds, data concerning waterfowl “residency time” in different playa regions are lacking so that abundance of a species present over a particular time is extremely difﬁcult to determine. Regardless, biologists know that millions, if not tens of millions, of waterfowl use the playas during spring and fall (USFWS 1988; Gersib et al. 1990a). Benning (1987) estimated that 90% of the midcontinent white-fronted geese used the playas of the Rainwater Basin, while the U.S. Fish and Wildlife Service and the Nebraska Game and Parks Commission (1986) estimated that 50% of the midcontinent population of mallards and 30% of the continental population of northern pintails used this area during

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spring. No such published estimates exist for other playa regions in the Great Plains. Similar to shorebirds, waterfowl spend a large proportion of their time feeding during spring migration (Gersib et al. 1990b). However, waterfowl also are engaged in courtship activities and generally spend more time resting. Also, unlike shorebirds and ducks, the geese feed almost exclusively in the surrounding agricultural ﬁelds rather than in playas (Krapu et al. 1995). Expansion of agricultural production, especially corn, throughout the Great Plains has been implicated in the rapidly increasing snow goose population (Alisauskas and Ankney 1992). Corn varieties are being planted farther north, and snow geese can now stage and store nutrient reserves closer to their arctic breeding grounds. Migrating ducks feed on seeds and invertebrates produced in the playas as well as on agricultural grains. Female ducks generally spend more time feeding than males during spring migration (Krapu and Swanson 1977; LaGrange and Dinsmore 1989). Ducks are actively courting and/or have formed pair bonds during this period, requiring the males to spend more time defending their mates. This allows females to acquire speciﬁc nutrients for egg formation. Nutrients stored by waterfowl feeding in wetlands during migration are important to their subsequent breeding success (Krapu and Reinecke 1992).

nesting Waterfowl production in the various Great Plains playa regions can be substantial. In the traditionally deﬁned ﬁve-state playa region, it was hypothesized that up to a quarter million waterfowl could be produced in “wet” years (USFWS 1988). Production is likely closer to 50,000 –100,000 in average precipitation years. Historically, waterfowl production also was substantial in the Rainwater Basin playas (Evans and Wolfe 1967), but due to wetland loss and degradation, and conversion of grassland to cropland, production has been estimated at 10,000 ducklings in a normal year (U.S. Fish and Wildlife Service and Nebraska Game and Parks Commission 1986). In the playas, particularly in the Southern High Plains, the nesting period of waterfowl can extend from April through August. This extended breeding season makes it difﬁcult to estimate waterfowl production, as typically accomplished in more northern areas. In those annually sampled areas, waterfowl production is determined with

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two aerial surveys, one in May, the other in July (Strickland et al. 1994). Pair data from May is incorporated with July brood data to estimate production. Using this approach, ducklings produced in the Southern High Plains might be grossly underestimated. Class I ducklings (no fully quilled feathers present) can be seen from April through September. If a playa receives substantial enough rain to ﬁll in July, a duck pair will likely be seen courting there within a few days. It might be difﬁcult to even distinguish the male from the female because the male has often molted from full breeding plumage. In the western Great Plains, the most common waterfowl species nesting in or immediately adjacent to playas are mallard, blue-winged teal, and cinnamon teal; but redhead, ruddy duck, northern pintail, northern shoveler, green-winged teal, canvasback, and gadwall nests also can be found (Traweek 1978; Rhodes 1978, 1979; Smith and Haukos 1995; Flowers 1996; Seyffert 2001). In the Rainwater Basin playas, the most common nesting species are mallard, blue-winged teal, American wigeon, gadwall, and northern shoveler (Gersib et al. 1990a). The dabbling ducks generally nest in dense vegetation of dry playas or in the upland immediately surrounding the playa (Berthelsen et al. 1989; Smith and Haukos 1995), while diving ducks such as redhead and ruddy duck nest in vegetation heaped up in the water. Although not studied in playas, it is likely that ducklings and attending hens feed predominantly on aquatic invertebrates (Krapu and Swanson 1977). With the success of stocking programs of many state conservation agencies, giant Canada geese are breeding throughout the United States, occasionally nesting in the Rainwater Basin, and now also in some urban playas in the Southern High Plains. Breeding geese are not native to playas and, from a native species perspective, hopefully will not become widespread in rural playas. The frequent drying of playas might discourage geese from reproducing over large areas.

winter Commonly cited estimates of the numbers of waterfowl wintering in the Southern High Plains, the major wintering area in the Great Plains, range from 500,000 to 2.8 million ducks (all species) and 100,000 to 750,000 geese (mainly Canadas and snow geese) (e.g., Simpson and Bolen 1981; USFWS 1988). These counts were generally made from surveys termed the “December goose count” and the “midwinter waterfowl survey.” However, estimates

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generated from these surveys may not be particularly reliable because there was seldom consistency in the area covered by the surveys among years and because they lack a sound statistical sampling basis (Strickland et al. 1994). Historically some pilot biologists ﬂew to major regional reservoirs, and their vicinities, and then counted the waterfowl. This could lead to biased results because in dry years, when playas have less water, many ducks congregate on reservoirs whereas in wet years when playas have more water, fewer ducks may have been counted. In reality, then, there may have been more ducks in the wet year when fewer were counted. Regardless of past survey shortcomings (transects are now used), the playas of the Southern High Plains can be the most important wintering area in the Central Flyway for many species of waterfowl, especially mallards, the most numerous duck species in North America (Bellrose 1980; Bergan and Smith 1993). The duck species cited as being most common in winter are the dabbling ducks, especially mallard, northern pintail, American wigeon, and green-winged teal although northern shoveler and gadwall can also be numerous (e.g., Simpson and Bolen 1981). Because most playas are shallow palustrine basins, fewer diving ducks winter in the region. But where suitable habitat exists, ring-necked duck, lesser scaup, redhead, and canvasback can be locally abundant (table 4.2). Dabbling ducks use playas for feeding, roosting, and courting (Quinlan and Baldassarre 1984; Lee 1985; Sheeley 1988; Bergan 1990). Many species form their annual pair bonds during winter (ﬁg. 4.6) (Weller 1965). Most of the dabbling ducks that winter in playas are produced in the Prairie Potholes Region of North and South Dakota, Manitoba, Saskatchewan, and Alberta (Bellrose 1980), although some also come from the Arctic or are, as stated earlier, hatched locally. Rhodes et al. (1993, 1995) examined genetic characteristics of American wigeons and mallards throughout winter in the Southern High Plains. Although these species are often considered to come from one widespread breeding population, and thus represent one wintering population, genetic data indicate otherwise (Rhodes et al. 1991; Rhodes and Smith 1993; Rhodes et al. 1996). Wintering populations of wigeon and mallard in playas represent mixtures of genetically heterogeneous breeding populations. The hundreds of thousands of the small race of Canada geese (B. c. parvipes, B. c. hutchinsii; Bellrose 1980) that winter in playas are

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Figure 4.6 Many dabbling duck species like these mallards form pair bonds while wintering in playas. (Photo courtesy of J. Steiert.)

difﬁcult to separate into populations from a genetic standpoint (Cathey 1997). At least three wintering populations of Canada geese have been deﬁned as occurring in the Southern Great Plains (Arctic Goose Joint Venture 1991). The large-bodied group (Hi Line Population) breeds in Montana and southern Alberta (and is not at question) while the two small-bodied populations (Shortgrass Prairie, Tallgrass Prairie) breed in the Arctic from the Mackenzie River in the western Northwest Territories east to Bafﬁn Island in Nunavut (Bellrose 1980). The Shortgrass Prairie Population is thought to nest primarily in the western portion of this region, while the Tallgrass Prairie Population nests in the eastern Arctic. Shortgrass birds are thought to winter primarily in the playas, while Tallgrass geese are considered to winter to the east of the playas and along the Texas Gulf Coast, even though marked birds breeding in the different areas occurred in both wintering areas (Cathey 1997). Cathey et al. (1998) examined genetic subdivision of Canada geese breeding from the Mackenzie River to Bafﬁn Island. Although there were genetic differences between east-

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ern and western populations of small Canada geese nesting in the Arctic, there was no consistent basis for separating Shortgrass and Tallgrass Prairie Populations wintering in playas (Cathey 1997). Beginning in the late 1960s and early 1970s several studies were initiated to examine wintering and migrating dabbling ducks in the central Southern High Plains (e.g., Rollo and Bolen 1969; Soutierre et al. 1972). Studies examined many aspects of waterfowl ecology including, but not limited to, nutrient reserves in wintering mallards (Whyte and Bolen 1984a,b) and ﬁeld feeding by dabbling ducks (Baldassarre et al. 1983; Baldassarre and Bolen 1984). These studies showed how dabbling duck nutrient reserves (lipids, proteins) ﬂuctuated over the winter and how ﬁeld feeding and grain availability varied throughout the winter. Other studies examined waterfowl behavior (Quinlan and Baldassarre 1984; Lee 1985). Following those studies, investigations were initiated on environmental inﬂuences on nutrient reserves and body condition in dabbling ducks as well as the relationship between condition and overwinter survival. Initially, northern pintail diet and body condition were examined (Sheeley and Smith 1989; Smith and Sheeley 1993a,b). One of the ﬁrst ﬁndings was that the diet observed in pintails depended upon how birds were collected. Birds used in most wintering waterfowl diet studies were obtained from hunters (a logical choice to increase sample sizes and minimize ﬁeld work) or using methods that hunters typically employ (e.g., use of decoys). Results from these studies emphasized the importance of agricultural grain, especially corn, in the dabbling duck diet (e.g., Sell 1979; Moore 1980). When northern pintails were collected after they were observed feeding, they had consumed much more nonagricultural food, such as invertebrates and annual seeds from wetland plants, than birds collected using typical hunting techniques (table 4.4). The relative importance of grain in waterfowl diets had been overstated. The reason this is important is that waterfowl diets dominated solely by agricultural grains are thought to be nutritionally inferior to those containing a variety of wetland-produced foods (Haukos and Smith 1995). This can affect body condition, which is assumed to be related to survival. In wet years, more wetlands, and therefore more wetland food, should be available to ducks, thus improving their condition. Not only might this improved condition be related to the quality of food consumed (wetland foods vs. agricultural grains) but also to food availability. Greater food availability decreases energetic costs

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Table 4.4 Esophageal foods from hunter-killed northern pintails and those ducks collected (observed) while feeding in playas Aggregate % dry mass Hunter-killed (n 21)

Observed (n 26)

Corn Green matter Barnyard grass (Echinochloa crusgalli) seeds Smartweed (Polygonum spp.) seeds Dock (Rumex spp.) seeds

90.05* 4.11 0.25* 0.01* Tr a*

27.04 2.89 10.39 10.99

Little barley (Hordeum pussilum) seeds Pigweed (Amaranthus spp.) seeds Spikerush (Eleocharis spp.) seeds Goosefoot (Chenopodium spp.) seeds Other seeds Nonagricultural seeds Plant total Gastropoda Diptera Coleoptera Other animal Animal total

Tr* Tr* 0.01* Tr* 0.69* 0.96* 95.12* 0.09 4.78 Tr* 0.01 4.88*

Food

9.75 4.86 2.29 1.85 0.67 8.76 49.57 79.50 12.25 6.20 0.26 1.79 20.50

Source: Sheeley and Smith 1989; courtesy of Journal of Wildlife Management. a Trace ≤ 0.01%. *P 0.05; hunter-killed differs from observed.

of ﬁnding and consuming foods, thus improving body condition. In the pintail study, data indicated that in the wet year body condition was better, pintails did not ﬁeld feed until late winter, they molted earlier, and formed pair bonds earlier than in the dry year (Smith and Sheeley 1993a,b). Because the environmental conditions (e.g., increased wetland availability) associated with wet and dry years were not experimentally manipulated, one could not be certain that the population attributes observed were not due to chance alone. But with similar conclusions reached elsewhere for waterfowl in wet versus dry years (e.g., in Miller 1986 for the Central Valley of California), it did lend conﬁdence to conclusions and pointed to the need for future studies.

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Other studies were suggesting that as more winter habitat was available in wet years, more ducks were surviving the winter (Heitmeyer and Fredrickson 1981; Kaminski and Gluesing 1987; Raveling and Heitmeyer 1989). Bergan and Smith (1993) followed the pintail study with a radio telemetry investigation examining survival of female mallards during winter (November through February). Average survival over the winter period was about 78%. Precipitation, and therefore wetland availability, varied by year. As with pintails, body condition also varied by year with hen mallards in the poorest condition occurring during the driest year. And, as hypothesized, females in poorest body condition also had the highest mortality rates similar to black ducks (Anas rubripes) (Conroy et al. 1989) and canvasbacks (Haramis et al. 1986) in the eastern United States. Moreover, winter survival of mallards in playas was higher than that observed in the lower Mississippi Valley (94% vs. 82%) when playa data were restricted to the 70-day period examined in the Mississippi Valley (Reinecke et al. 1987). Factors associated with causes of mortality were quite different in Mississippi and western Texas. Hunting caused less mortality than did natural causes (e.g., predation, disease) in the Southern High Plains, whereas the reverse was true in the Mississippi Valley. There was little hunting pressure in the Southern High Plains and natural mortality was highest after the hunting season, in late January, which coincided with times of severe weather and avian cholera outbreaks. The next step was to examine why wet years produced conditions that had higher waterfowl condition and survival. Initially plant community response to wet- and normal-year precipitation was examined. By applying water in spring and midsummer, production of annual seed-producing plants was stimulated (Haukos and Smith 1993b). Those playas that were managed had production almost 10 times that of unmanaged playas. Waterfowl appeared to respond to the increased food availability, but daytime counts were confounded by differing availabilities of water among playas. Previous studies also showed that waterfowl spent less than 10% of their feeding time during the winter daylight hours (Quinlan and Baldassarre 1984; Lee 1985). Since feeding rates by waterfowl appeared relatively low in those previous studies and waterfowl count data were equivocal, other factors were examined that possibly inﬂuenced body condition and waterfowl use of playas with abundant seed foods. The next study examined numbers of waterbirds in playas that had

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variable levels of seed foods but similar water levels (Anderson and Smith 1999). It has generally been assumed that seed production was primarily driving the diet and use of wetlands by wintering dabbling ducks even though invertebrates are important components of wintering waterfowl diets (Fredrickson and Taylor 1982; Reid et al. 1989; Haukos and Smith 1993b). However, Anderson and Smith (1999) found there was no difference in daytime duck densities between playas with abundant seeds and those with little seed. Because the water during those surveys was murky and contained numerous molted feathers, it was thought that waterfowl might be using playas at other times. With the aid of a night-vision scope it became apparent that primary use of managed playas by dabbling ducks was occurring at night (Anderson and Smith 1999). Green-winged teal densities were 84 times higher at night than during the day. The majority of ducks in playas with dense annual seed-producing plants were feeding throughout the night. Ducks were using other wetlands during daylight hours for activities such as resting and/or courtship (Quinlan and Baldassarre 1984; Lee 1985). Diurnal sampling of waterfowl had been biasing perceptions of waterfowl ecology and use of playas (Tamisier 1976; Bergan et al. 1989; McNeil et al. 1992; Henson and Cooper 1994). Although waterfowl were selecting playas dominated by annual vegetation to feed in at night, it was unknown whether seed availability was inﬂuencing selection of managed playas. Subsequently, Anderson et al. (2000) found that green-winged teal were preferentially selecting invertebrates even though seeds were up to 10 times more abundant than invertebrates. Dabbling ducks, such as greenwinged teal, are undergoing prealternate molt during winter. They likely must meet the increased nutritional demands of molt not endogenously by using body reserves but exogenously through feeding preferentially on invertebrates. Indeed, in the year when aquatic invertebrates were more abundant, teal completed their molt earlier in the season than in the year when these foods were less abundant (Anderson et al. 2000). The reason ducks feed more at night than during the day in relatively more densely vegetated playas is a bit of a mystery. McNeil et al. (1992) proposed that waterfowl may feed at night more than during the day because night may provide the most proﬁtable or safest opportunity to feed, or that nocturnal foraging occurs only when di-

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urnal feeding has not been sufﬁcient to meet nutritional needs. As noted earlier, previous studies have shown that dabbling ducks in playas do not feed at high rates during the day. Therefore, the latter hypothesis does not hold in playas, and it is likely ducks feed more at night because obtaining food is more proﬁtable or it is safer to forage at night (Anderson and Smith 1999). Availability of invertebrate prey, hunting pressure, and predation all could cause waterfowl to feed more at night (Tamisier 1976; McNeil et al. 1995; Anderson and Smith 1999). Hunting pressure on ducks in the Southern High Plains is low (Carney et al. 1983). However, it is possible that this is a relict behavior left over from hunting pressure experienced farther north within the year or even from previous generations. Invertebrates also could be more available to foraging waterfowl at night by moving higher in the water column (McNeil et al. 1995). But in the playas the water was only 10 –20 centimeters (4 – 8 in.) deep, making the entire water column available to foraging ducks (Anderson and Smith 1999). However, there is a relatively high potential density of avian predators occurring around playas (see next section). Most of these are diurnal predators. Of the three owls, the great horned (Bubo virginianus) is the only species capable of preying on waterfowl at night. The high abundance of raptors may inﬂuence ducks to choose more open playas during the day (J. Anderson 1997). Northern harriers (Circus cyaneus) and other diurnal raptors are consistently observed making predation attempts on waterfowl in playas. Ducks can detect avian predators during the day in relatively open playas but not in densely vegetated playas (Anderson and Smith 1999). OTHER AVIAN TAXA

migration Although sandhill cranes (Grus canadensis) use salt lakes in the Southern High Plains and riverine habitat in Nebraska for roosting, several hundred thousand use playas during migration for feeding, roosting, and a source of fresh water (Iverson et al. 1985). Endangered whooping cranes (Grus americana) also use the Central Table and Rainwater Basin playas of Nebraska during spring after returning north from coastal Texas (LaGrange 1997). They are only seen infrequently in the playas of the Southern High Plains. It is much more difﬁcult to describe use of playas by other birds

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such as songbirds or raptors during migration. Few studies have been conducted on use of playas by these other birds, but also species lists for the various playa regions often do not separate migratory species versus residents or whether they are occurring in the vicinity of a playa versus the actual basin (Gersib et al. 1990a; Haukos and Smith 1994a). This lack of knowledge does not make their migrations through the playas any less spectacular than the more well-studied species. For example, several hundred bank swallows (Riparia riparia) may feed on insects emerging from a single Nebraska playa, while dozens of Swainson’s hawks (Buteo swainsoni) may sit in the wheat stubble surrounding a single Texas playa. To fully understand the importance of playas to all birds, and the avian role in the trophic structure of playa communities, future studies should examine use of playas during migration by these “terrestrial” species. Many species of concern also use playas during migration, including the whooping crane, the piping plover (Charadrius melodus), bald eagle (Haliaeetus leucocephalus), mountain plover, interior least tern (Sterna antillarum), and peregrine falcon (Falco peregrinus).

nesting Besides shorebirds and waterfowl, other aquatic birds nesting in wet playas include American coot (Fulica americana), pied-billed grebe (Podilymbus podiceps), eared grebe (Podiceps nigricallis), and white-faced ibis (Plegadis chihi) (ﬁg. 4.7) (Smith and Haukos 1995; Flowers 1996; Seyffert 2001). These species generally nest on ﬂoating mats of vegetation or in emergent vegetation on the water’s surface. Aside from the American coot, it is likely that four other species of rail nest in playas—the black (Laterallus jamaicensus), king (Rallus elegans), sora (Porzana carolina), and Virginia (Rallus limicola)—although nesting records are scarce (Flowers 1996; Seyffert 2001). Least (Ixobrychus exilus) and American (Botaurus lentiginosus) bitterns also likely nest in emergent vegetation of playas, especially in the more northern areas. With playa modiﬁcations, and the associated increased prevalence of trees and emergent vegetation (as noted in Chapter 3), there has been an increase in the occurrence of rookeries. Black-crowned night heron and snowy (Egretta thula) and cattle (Bubulcus ibis) egrets now nest in playas. Occasionally yellow-crowned night heron (Nycticorax

Figure 4.7 Grasshopper sparrows (top) and mallards (middle) commonly nest in dry playa basins and their watersheds while American coots (bottom) nest in wet playas. (Photos by author.)

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violaceus) are found nesting in trees associated with playas. Nightherons and egrets are commonly observed feeding on the invertebrates and amphibians occurring in playas. The occurrence of trees in these wetlands also has allowed nesting by songbird species not historically breeding in playas, or, for that matter, in the prairie environment throughout the Great Plains. Some of these bird species are exotic not only to wetlands and prairies but also to the North American continent. In the Southern High Plains, scissor-tailed ﬂycatcher (Muscivora forﬁcata), ash-throated ﬂycatcher (Myiarchus cinerascens), western kingbird (Tyrannus verticalus), and house sparrow (Passer domesticus) have nested in trees occurring in playa basins (Smith and Haukos 1995). In trees, and on the ground, mourning dove (Zenaida macroura) nest densities have been reported as high as 8 nests/hectare (3 nests/ac) (Nelson et al. 1983). Numerous other species also likely nest in the exotic trees associated with playas (Nelson et al. 1983; Gersib et al. 1990a; Flowers 1996). If trees continue to expand into the prairies, and their associated wetlands, these numbers will increase and grassland species numbers will likely decrease. Depending on the structure of the herbaceous vegetation, the avian community nesting in dry playas varies. Those playas that had been moist the previous autumn or early that spring may be dominated by dense annual or short-lived perennial vegetation. Redwinged blackbirds (Agelaius phoeniceus) prefer this type of vegetative community, and select curly dock as their primary nest substrate in the Southern High Plains (Simpson and Bolen 1981). Nest densities in curly dock can often exceed 13 nests/hectare (5 nests/ac) (Smith and Haukos 1995). Yellow-headed blackbirds (Xanthocephalus xanthocephalus) also nest in playas with dense emergent vegetation (Fischer and Bolen 1981; Flowers 1996). Although some confusion exists over their taxonomic classiﬁcation, eastern (Sturnella magna) and western (Sturnella neglecta) meadowlarks nest in dry playa basins, especially those that have stands of midheight grasses such as western wheatgrass and knotgrass (Smith and Haukos 1995; Flowers 1996). Northern bobwhite (Colinus virginianus) nest in playas with midheight grasses (Smith and Haukos 1995), and it is likely that Cassin’s sparrow (Aimophila cassinii) and grasshopper sparrow (Ammodramus savannarum) nest in the shorter to midheight grasses in the Southern High Plains (Berthelsen and Smith 1995). Other ground-

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nesting songbirds nest in playas, but no studies have investigated their ecology. In the late 1970s and early 1980s considerable research effort was expended on the importance of Southern High Plains playas to ringnecked pheasant (Phasianus colchicus). This research was spurred by the economic value of this exotic to individual landowners and regional economies (Bolen and Guthery 1982). Prior to the Conservation Reserve Program (see Chapter 1), playas provided most of the nesting habitat for all birds, including pheasants, even though playas occupy only 2% of the Southern High Plains landscape (Haukos and Smith 1994a). Nest densities of pheasants in playas was high, averaging 2.2 nests/hectare (almost 1 nest/ac) (Taylor 1980). However, because rainfall generally peaks in May, when pheasant and most other nesting birds are incubating eggs in dry playas, these nests are subject to loss from ﬂooding. Predation on nests in playas also can be high in this restricted habitat surrounded by agricultural ﬁelds. With the advent of the Conservation Reserve Program and the planting of large areas of grasses, pheasant nest densities exceeding those in playas were recorded: 5 nests/hectare (2 nests/ac) (Berthelsen et al. 1990). The taller, denser grasses produced the highest pheasant nest densities. The presence of playas is key to the existence of pheasant populations in the Rainwater Basin, because there is little dense herbaceous upland habitat in this region.

winter Considerable research was also conducted on ring-necked pheasants in winter. Playas with dense emergent vegetation provide key wintering habitat for pheasants, mostly from a cover perspective and to a lesser extent from a food perspective (Whiteside and Guthery 1983). Winter pheasant densities in playas as high as 11/hectare (4.4/ac) have been recorded in the Southern High Plains (Guthery and Whiteside 1984). Native bird densities in frozen or dry playas also can be high, but actual estimates are limited (Smith and Haukos 1995). Barn (Tyto alba) and short-eared (Asio ﬂammeus) owls often roost and feed in dry playas of the Southern High Plains. Their densities in emergent vegetated playas often exceed 1 owl/4 hectares (10 ac) (unpublished data). These owls are likely feeding on the abundant small mammals that exist in playa basins. Many songbird species also roost in the

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emergent vegetation of playas and feed on the seeds produced by annual plants. These playas may be dry or the seed that is being fed upon is protruding above the water’s surface. Hundreds of songbirds such as McCown’s longspur (Calcaius mccownii), Harris’s sparrow (Zonotrichia querula), and lark bunting (Calamospiza melanocorys) can be found feeding and/or roosting in a single playa (Smith and Haukos 1995). Many different raptor species (other than the two owl species mentioned above) prey upon the more than 30 species of birds and other fauna attracted to playas during winter. The raptors include prairie falcon (Falco mexicanus), peregrine falcon, bald eagle, golden eagle (Aquila chrysaetos), great-horned owl, Swainson’s hawk, roughlegged hawk (Buteo lagopus), red-tailed hawk (Buteo jamaicensis), ferriginous hawk (Buteo regalis), Cooper’s hawk (Accipiter cooperii), and, the most common, the northern harrier (Nelson et al. 1983; Smith and Haukos 1995). Several of these raptors exist around playas because of the presence of trees and fenceposts used as perch sites. Densities of raptors can often be relatively high. Curtis and Beierman (1980) counted 58 bald and golden eagles on a single playa. Nine-tenths of the midwinter population (about 400,000) of sandhill cranes winters in the Southern High Plains (Iverson et al. 1985). Although their focal roost points are in several salt lakes, a majority of the cranes spend at least some time roosting, feeding, and/or drinking water in playas. Indeed, some playas serve as major crane roosts with tens of thousands of birds. M AMM ALS

Throughout most of the playas there are no true aquatic mammals, although there can be muskrat (Ondatra zibethica) in modiﬁed Rainwater Basin playas. The lack of aquatic mammals in most present-day playas is related to their erratic and undependable hydroperiod. However, that fact aside, the mammals associated with the margins of wet playas or dry playas can be quite diverse and abundant (table 4.5). More species probably exist in the Rainwater Basin and Todd Valley playas, but lists have not been completed for those regions. With the extermination of the millions of bison and elk in the Great Plains, most mammals are now considered to be residents. However, although seldom considered, various migratory bat (Chiroptera) species feed on insects emerging from playas. Bats feed on emerging insects in playas from Nebraska to Texas, but

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unfortunately few studies have examined their composition and ecology relative to playas. Big brown (Eptesicus fuscus), red (Lasiurus borealis), and hoary (Lasiurus cinereus) bats are known to feed and water in Nebraska playas (M. Fritz, personal communication, Nebraska Game and Parks Commission). White-tailed deer are most common in playas in the northern and eastern portions of the playa range, but they are beginning to expand their range into playas farther west and south. Mule deer and pronghorn are found using playas in the western Great Plains where crop agriculture has not taken over vast tracts of uplands. Cropland agriculture has had a negative effect on pronghorn abundance and occurrence in playas (Leftwich and Simpson 1977, 1978). Not only do these ungulates use playas as a water source, they also forage and bed down in the basins. Recently exotic feral hogs have also been found in playas in the Southern High Plains. Their presence is not welcome because they destroy bird nests and root up native vegetation. Because playas often form the only habitat in such an intensive agricultural environment, some mammal populations reach relatively high densities in dry/moist playas. In the Southern High Plains, Scribner (1982, 11) found eastern cottontail densities in playas exceeding 20 individuals/hectare (8/ac). Moreover, due to the isolation of individual playas, population characteristics of eastern cottontails were playa speciﬁc (Scribner and Warren 1990). Desert cottontails also are present in Southern High Plains playas, but generally occur more often in playas in grassland settings with less dense annual vegetation than do eastern cottontails (Simpson and Bolen 1981). Native mouse and rat populations also can reach very high densities, but sadly speciﬁc studies on their ecology in playas are lacking. Scribner (1982) hypothesized that the cotton rat was the only species to exceed eastern cottontail populations in Southern High Plains playas. The existence of most mammalian predators in this highly agriculturalized environment is also tied to playas. Simpson and Bolen (1981) noted that, compared to the surrounding habitat, coyote, raccoon, striped skunk, and opossum preferred playa habitat. In the central Southern High Plains, Whiteside and Guthery (1980) found that 40% of the playas had coyotes. When one considers that the density of playas in that region can exceed 1 per 1.6 square kilometer (1/sq mi), densities are obviously high. The presence of coyotes in the Southern High Plains likely discourages the presence of red fox

Table 4.5 Mammal species likely associated with playas in the Great Plains Common name Virginia opossum Short-tailed shrew Least shrew Desert shrew Eastern mole Desert cottontail Eastern cottontail Black-tailed jackrabbit White-tailed jackrabbit Thirteen-lined ground squirrel Franklin’s ground squirrel Black-tailed prairie dog Plains pocket gopher Jones’ pocket gopher Yellow-faced pocket gopher Plains pocket mouse Merriam’s pocket mouse Hispid pocket mouse Ord’s kangaroo rat Western harvest mouse Plains harvest mouse White-footed mouse Deer mouse Northern pygmy mouse Northern grasshopper mouse House mouse Meadow jumping mouse Hispid cotton rat Meadow vole Southern bog lemming Southern Plains woodrat Muskrat Porcupine Coyote Red fox Swift fox Gray fox Bobcat Raccoon

Scientiﬁc name

Source a

Didelphis virginiana Blarina brevicauda Cryptotis parva Notiosorex crawfordi Scalopus aquaticus Sylvilagus audubonii Sylvilagus ﬂoridanus Lepus californicus Lepus townsendii Spermophilus tridecemlineatus Spermophilus franklinii Cynomys ludovicianus Geomys bursarius Geomys knoxjonesi Cratogeomys castanops Perognathus ﬂavescens Perognathus merriami Chaetodipus hispidus Dipodomys ordii Reithrodontomys megalotis Reithrodontomys montanus Peromyscus leucopus Peromyscus maniculatus Baiomys taylori Onychomys leucogaster Mus musculus Zapus hudsonius Sigmodon hispidus Microtus ochrogaster Synaptomys cooperi Neotoma micropus Ondatra zibethicus Erethizon dorsatum Canis latrans Vulpes fulvus Vulpes velox Urocyon cinereoargenteus Felis rufus Procyon lotor

A, C, E F A A F A, B, E A, B, E A, B, E F A, B, E F A, B, E A, B A, E A, E A, E A, E A, E A, E A A A A, C, D, E A A, D A, C, D F A, D F F A, B, E F A A, B, E F A, B, E A, B B A, B, E, F

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Table 4.5 (Continued ) Common name

Source a

Scientiﬁc name

Long-tailed weasel Mink Badger Striped skunk Pallid bat Big brown bat Red bat Hoary bat Feral hog Pronghorn Mule deer

Mustela frenata Mustela vison Taxidea taxus Mephitis mephitis Antrozous pallidus Eptesicus fuscus Lasiurus borealis Lasiurus cinereus Sus scrofa Antilocapra americana Odocoileus hemionus

A, C, D, F F A, B, E A, B, E B F F F G A, B B

White-tailed deer

Odocoileus virginianus

F

Source: Modiﬁed from Haukos and Smith 1994; courtesy of Landscape and Urban Planning, with permission from Elsevier Science. a A: Choate 1991. B: Curtis and Beierman 1980. C: Nelson et al. 1983. D: Guthery 1981. E: Simpson and Bolen 1981. F: data from Nebraska Game and Parks Commission. G: personal observation.

(Sovada et al. 1995). Because coyotes are not as detrimental to groundnesting birds as are red fox, the presence of coyotes likely positively inﬂuences the population dynamics of dabbling ducks and ringnecked pheasant. Red fox are more common, and coyotes less so, in central and eastern Nebraska playas. Raccoons are also common in playas. They especially prefer playas with dense emergent vegetation for denning (Juen 1981). Their occurrence is likely increased by the presence of playa modiﬁcations (such as pits), which increase the prevalence of water and denning structures. Many of the predator, rodent, and rabbit species inhabiting playas venture out of the basins for additional forage, but others can complete their entire life history in a single playa basin. Several endangered or threatened mammal species potentially occur in playas and their immediate watersheds. The recent (1998) petition by the National Wildlife Federation to the U.S. Fish and Wildlife Service to list black-tailed prairie dog as threatened or endangered has been particularly controversial throughout the Great

Figure 4.8 Hispid pocket mice (top) and black-tailed prairie dogs (bottom) are common mammals in the playa watershed and in dry playas. (Photos courtesy of M. Wallace.)

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Plains (ﬁg. 4.8). Black-tailed prairie dogs are common in playa watersheds, and in dry times dog towns expand into the basin. When playas ﬂood from precipitation and their burrows ﬂood, it is common to see prairie dogs in areas quite a distance from the nearest dog town. A declining species, swift fox are also found in playa watersheds and are often associated with prairie dog towns. Their populations have been declining throughout the Great Plains. Interestingly, one of the major causes of mortality documented thus far has been predation by coyotes. Numerous studies are now under way examining landscape and local biotic inﬂuences on swift fox populations. Possibly the most highly publicized endangered species potentially occurring in the Great Plains is the black-footed ferret (Mustela nigripes). These ferrets are closely associated with prairie dog towns, and Bolen et al. stated that “the possibility of its [black-footed ferret] existence in association with playa lakes remains real” (1979, 28). Choate (1991) noted that the last report of a black-footed ferret in the Southern High Plains was in 1963.

CHAPTER 5

STRUCTURE, FUNCTION, AND DIVERSITY

T

he entire structure of a playa and its many associated functions can change within a few days. If a playa has been dry for more than a year, often only a thin layer of grass and spindly forbs remain in the basin. Then in early May thunderstorms can occur over the same playa and, more important, over its surrounding watershed, resulting in the basin being covered with a half meter (2 ft) of water. Within days there is an explosion of emerging aquatic invertebrates such as clam shrimp, obligate wetland plants begin germinating, toads and frogs emerge and begin calling in earnest, shorebirds arrive to feed on the invertebrates, and duck pairs start courting and will subsequently nest on the playa margin. These developments are typical in the life history of a playa and occur in thousands of playas each year. The climate of the Great Plains dictates this condition. Playas need this hydrologic (ﬂooding/drying) disturbance to remain productive and be the key sites of biodiversity that they are. Playa fauna and ﬂora evolved under these circumstances. Indeed, wetlands are wetlands because they dry out periodically. Among other things, this enhances decomposition and allows other plant and animal communities to emerge and/or recolonize and coexist with the species that required a more aquatic or a more terrestrial condition. The general public, many ecologists, and natural resource managers assume the aquatic condition is more beneﬁcial probably because it appears to be more transient and full of life than the drier condition. However, this drying and ﬂooding allows two vastly different communities and all intermediate permutations to exist on the same site increasing the diversity of the wetland. Further, among other ecosystem processes, the cycling of nutrients provided by the hydrologic disturbance is necessary for the various biotic communities to exist. Although it is unwise to generalize about playas, they will be char-

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acterized here in just a few different moisture phases: dry, moist, and inundated. These generalizations are necessary because so few basic studies on playa ecosystem properties have been conducted. Seasonal differences in structure and function will also be highlighted because ecosystem properties may vary substantially on this basis. Finally, some of the attributes associated with playas will be related to the biotic diversity among playas. WATER VARIABLES

However, before one considers these system properties, the water environment in which playas occasionally exist must be examined because water variables can have a large effect on the attendant biota. For example, J. Anderson (1997, 252) noted that most aquatic playa invertebrate taxa increased in abundance as dissolved oxygen, water depth, and temperature increased and as pH and electrical conductivity decreased. When playas are wet, the variation in water variables among playas is large and related to length of inundation. Because each playa exists within its own unique watershed (i.e., each watershed has been inﬂuenced differently, and has potentially different soils and vegetation), the amount and quality of the water entering each playa varies. General water variables in playas are considered here, and the “quality” and “contaminant” issues are considered later along with threats to playa ecosystems (see Chapter 7). As for other issues related to playa ecosystem properties, most data on water characteristics exist for the Southern High Plains, relative to that existing for the more northern playas. The ﬁrst published limnological study on playa water characteristics in the Great Plains was for eastern New Mexico and adjacent western Texas by Sublette and Sublette (1967). Subsequent studies were conducted in West Texas by Parks (1975), Thompson (1985), Hall (1997), and A. Anderson (1997) among others. Although these studies report speciﬁc levels of chemical and physical water variables, seldom has the inﬂuence of water level ﬂuctuations on these variables been studied. Moreover, because these studies span about 30 years, the techniques used to determine the amount, or even the units, of a particular water variable have changed, making comparisons among different studies difﬁcult. Obviously one of the major inﬂuences on water variables is the surrounding watershed. Whether the watershed is cultivated or not is paramount. Cultivation will certainly affect many variables, espe-

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cially nutrient levels and water clarity. Not only will the watershed have a signiﬁcant inﬂuence on the water variables, but the timing of sampling will also affect these observations. As evaporation and inﬁltration occur, the remaining water changes in character. Hall et al. (1999) summarized some of the general water data available for eastern New Mexico and western Texas playas (Sublette and Sublette 1967; Parks 1975; Hall 1997) (table 5.1). Playa water turbidity was highly variable, and the water could at times be very turbid. Readings with a Secchi disk (simply a round black-and-white visibility disk) of 2 centimeters ( 1 in.) were not uncommon (i.e., after a disk was submerged greater than 2 cm, a person could no longer see it). Hall et al. (1999) felt that the turbid conditions in playas were the result of organic and inorganic matter being held in suspension by persistent windy conditions. Similarly, Sublette and Sublette (1967) noted that the turbidity in a playa was related to the amount of vegetation in the basin—the more vegetation the less turbidity as a result of the vegetation mediating the effects of waves and wind. The high amounts of solids found in water of some of the playas for which Hall et al. (1999) summarized data (table 5.1) were believed to be related to erosion. It was not stated whether the sediments resulted from wind or water actions. However, from Luo et al.’s (1999) soil particle size analysis it is known that the sediments deposited in playas are primarily waterborne (eroded from the watershed) with wind deposition accounting for substantially smaller amounts. Hall et al. (1999) noted that most playas were “nutrient-rich,” thus fulﬁlling criteria to be classiﬁed as eutrophic. This likely depends on the time since the last runoff event and the length of time the playa has been inundated. Certainly this is true of most wetlands in the Plains. Being the endpoint of the watershed, they not only receive the sediments but substantial allochthonous inputs of nutrients as well. Because most playas are shallow and exposed to relatively high wind velocities, the dissolved oxygen ranges are similar for the surface water and the sediment-water interface (table 5.1). Hall et al. (1999) found that the majority of playas they studied in the Southern High Plains of Texas had chemical oxygen demands and biochemical oxygen demands (i.e., oxygen needs to carry out chemical and biochemical processes in wetlands) both greater than 8 mg/L dissolved oxygen. The 8 mg/L is the oxygen saturation level at the elevation of Lubbock, Texas (1,000 m; ⬃ 3,200 ft). Because biochemical and chemical oxygen demands typically exceeded saturation levels, anaerobic situations can occur; but, as noted above, winds and shallow water depths probably

Table 5.1 Range of water variables found in playas of the Southern High Plains of Texas and New Mexico

Variable Physical properties

Nutrients

Cations, anions, and metals

Dissolved oxygen and oxygen demands

Measure Turbidity (NTU) Turbidity (ppm SiO2) Secchi depth (cm) Total solids (mg/L) Total suspended solids (mg/L) Total dissolved solids (mg/L) Total volatile suspended solids (mg/L) Speciﬁc conductance ( mhos) Hardness (mg/L) Hardness, total (mg/L CaCO3) Hardness, calcium (mg/L CaCO3) Alkalinity, methyl-orange (mg/L CaCO3) Total organic carbon (mg/L) Total inorganic carbon (mg/L) Total carbon (mg/L) Total Kjeldahl nitrogen (mg/L) Ammonia-nitrogen (mg/L) NO2/NO3-nitrogen (mg/L) Nitrate nitrogen (mg/L) Total phosphorus (mg/L) ortho-phosphate phosphorus (mg/L) Silica (mg/L) pH Calcium (mg/L) Magnesium (mg/L) Sodium (mg/L) Potassium (mg/L) Chloride (mg/L) Sulfate (mg/L) Arsenic ( g/L) Copper ( g/L) Dissolved oxygen, surface (% saturated) Dissolved oxygen, surface (mg/L) Dissolved oxygen, bottom (% saturated) Dissolved oxygen, bottom (mg/L) Biochemical oxygen demand (mg/L) Chemical oxygen demand (mg/L)

Range Minimum Maximum 20 45 2.0 195 10 120 <1 60.7 6.2

2,860 1,300 70.0 2,340 1,130 1,570 185 1,176.7 81.5

40 20 58 6.5 <1.0 12.3 0.36 ND a <0.02 ND 0.13 ND 6.8 5.0 1.8 1.5 0.8 0.1 1.3 0.3 <5 9 2.3 0.20 ND 0.02 <3

218 182 228 66.8 53.2 77.5 3.36 4.85 1.11 0.21 2.49 1.67 28.2 9.2 41.5 25.4 22.5 76.8 162 140.0 118 123 186.6 15.34 186.6 15.34 84

5

130

a Not detected. Source: From Sublette and Sublette 1967; Parks 1975; Hall 1997; modiﬁed from Hall et al. 1999, in Invertebrates in freshwater wetlands of North America: Ecology and management; used by permission of John Wiley and Sons, Inc.

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ECOSYSTEM ASPECTS

do not allow extended periods of anaerobic conditions. Frequent oxygenated circumstances would support rapid decomposition rates and low organic accumulation in the playa basin substrates. Pits in playas or the deeper playas such as seen in the Rainwater Basin region may have longer anaerobic periods. Hall et al. (1999) also noted that playas surrounded by grasslands had lower biochemical-to-chemical oxygen demand ratios than playas surrounded by cropland. They suggested that this may be the result of the cropland playas receiving highly labile fertilizer runoff rather than more tied-up sources occurring in plant material (e.g., structural carbohydrates). Water in most playas in the Southern High Plains is slightly alkaline with the dominant cations typically being calcium, potassium, magnesium, and sodium (table 5.1). The dominant anions in playas of this region are sulfate, bicarbonate, chloride, and carbonate. Many different metals have been found in playas, such as aluminum, arsenic, and copper (Irwin and Dodson 1991; A. Anderson 1997; Hall et al. 1999), but it is unknown whether these levels represent natural or anthropogenically elevated concentrations. STRUCTURE PRODUCERS

The relative frequency of different plant community types varies considerably within and among years so that it is difﬁcult to generalize about their occurrence beyond the major types within certain moisture regimes. Following the information on commonly occurring flora species presented in Chapter 3, playas that have been dry for longer than one year are typically dominated by short- and mid-size grasses such as buffalo grass, western wheatgrass, or little barley (e.g., Guthery et al. 1982; Haukos and Smith 1997; Bartz 1997; Hoaglund and Collins 1997). Scattered small forbs such as bur ragweed and frog-fruit may be mixed in with the grasses or they may become dominant if the playa has received a fair amount of agricultural disturbance (e.g., cultivation, overgrazing, sedimentation). As the hydroperiod is somewhat increased, other species become more dominant. Species that require longer hydroperiods beginning in spring can replace the others (Smith and Haukos 2002). If water is shallow (few cm, less than a few in.) for at least a month in spring, spikerushes often dominate playas (Haukos and Smith 2001). They are geographically one of the most widespread communities

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from Wyoming to Texas. In the heavily farmed Southern High Plains, moist conditions in spring followed by occasional precipitation or irrigation runoff (throughout the summer) encourages smartweed and barnyard grass to become dominant (Guthery et al. 1982; Haukos and Smith 1993b). This type, along with reed canarygrass, also is frequently encountered in Rainwater Basin playas (Gilbert 1989). A few playas are even dominated by persistent emergent plants such as various cattail and bulrush species. As noted in the ﬂora chapter, these types of plants require longer hydroperiods of several months, often associated with irrigation. Generally this condition also needs to be repeated sequentially over years for this type of vegetation to persist. However, a few large playas, especially in the Rainwater Basin, may exhibit this vegetation type naturally. When this wet longer hydroperiod is not repeated sequentially over several years, other emergent plants typically exist. Most often these are the shorter-lived arrowheads and mud-plantains. As with most other freshwater wetlands (Mitsch and Gosselink 2000, 394), seed germination and plant species diversity in playas is generally highest under moist-soil conditions versus either dry or submerged conditions (Haukos and Smith 1993a). Species that are present in dry circumstances can usually also germinate under moist conditions. However, as most wetland ecologists know, “dry,” “moist,” and “submerged” are not discrete and objectively deﬁned categories. A continuous moisture gradient exists in playas from very dry with very few species to very wet and deep with very few species. In between these extremes, similar to a bell curve, one should expect the highest germination and diversity. CONSUMERS

As with the producers, the major groups of consumers are highlighted here, including how they might ﬁt within the playa structure. Lists of known vertebrates and invertebrates appear in Chapter 4 on fauna. Similar to freshwater wetlands in general (Mitsch and Gosselink 2000, 397), little is known about the very small organisms making up the playa benthos. Because most depressional wetlands like playas are detrital based (with the energy ﬂow– nutrient cycle being primarily the result of decomposition of wetland plants), the benthic fauna is key to this system. However, Mitsch and Gosselink (2000, 397) noted that the smaller decomposers such as

114

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enchytraeids and nematodes were likely more important than larger decomposers in marshes compared to upland woody habitats. There are no studies of these groups in Great Plains playas; thus, the relative importance of different benthic taxa to nutrient cycling is undetermined. Moorhead et al. (1998) studied the macroinvertebrate fauna of playas in the Southern High Plains of Texas from May through midsummer. They noted that resident taxa were most abundant following initial precipitation events. Resident taxa were those with droughtresistant life history strategies. Obviously these species could emerge from the sediments and inhabit a wet playa more quickly than taxa that would have to colonize an area. The resident taxa were mainly omnivores, detritivores, and ﬁlter feeders dominated by phyllopod and ostracod crustaceans (Hall et al. 1999). These groups then declined proportionally as the 12-week studied hydroperiod lengthened. Herbivores then increased as algae and other living plant foods became more available. Similarly, predator taxa increased as the hydroperiod lengthened. Of the many predators that dispersed into the playas, most were insect species (e.g., Notonectidae, Hydrophilidae; Moorhead et al. 1998). Playas ﬁlling with water at other times of the year may not follow this pattern (Anderson and Smith 2000). In playas that ﬁlled with water during late summer and early fall, most invertebrates (density and biomass) existed in the benthos, with the water column and epiphytic samples contributing much less to the fauna. Snails (Gastropoda) were the dominant group. Although not as abundant as the benthic invertebrate community, there was also a unique group of invertebrates inhabiting the vegetation protruding above the water’s surface that has yet to be studied in any detail in freshwater wetlands (Anderson and Smith 2000). As noted earlier, little is known of the invertebrate species occupying dry playas, but it is possible that the invertebrate community structure of such playas mirrors that of a grassland watershed in instances where the watershed remains intact (Arenz and Joern 1996). In the intensively cultivated settings it is likely that playas serve as refuges to terrestrial invertebrates. Indeed, some playas are considered to be undesirable by farmers because they may harbor insects that are harmful to crops. For example, in the Southern High Plains of Texas and New Mexico it is believed that the boll weevil (Anthonomus grandis) may overwinter in dry playas, thus allowing the insect to infest surrounding cotton crops.

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The structure of the playa vertebrate community varies substantially on a seasonal basis. The amphibian community is probably often dominant in terms of numbers and biomass during the early summer months in most wet playas. This is an important aspect of playas that may set them apart from most other freshwater wetlands in North America, but comparative data are lacking. The infrequent mention of amphibians in wetland structure publications may simply be a reﬂection of past research effort. Wetland ecologists are only recently beginning to focus on the importance of amphibians to wetland structure and function (e.g., Semlitsch 2000). Larvae and adults are important in playa structure. The tens of thousands of amphibian larvae within individual playas likely have an important effect on the trophic structure. Birds also can be common in summer, but their prominence becomes more apparent during migration in spring and fall when fewer amphibians are present. Birds are likely the dominant vertebrate during summer in dry playas, but comparative information on small mammal abundance in playas is scarce. They are also the dominant vertebrate of wet and dry playas during winter. In winter when a playa is dry, most amphibians lie dormant in the soil and avian groups, such as grassland birds, that consume seeds dominate. The prominent avian groups in wet playas during migration are shorebirds, wading birds, and waterfowl. Unlike many freshwater marshes where mammals such as muskrat and beaver play a signiﬁcant role in wetland structure and function, few aquatic mammals occupy the playas of the Great Plains (Smith 1988). Where they occur in the Rainwater Basin, muskrats are likely important in macrophyte consumption. During the dry times, playas usually have other herbivorous mammals occupying them such as rabbits (Lagomorpha), small rodents (Rodentia), and larger ungulates such as pronghorn and deer. FUNCTION PRIMARY PRODUCTION

Few estimates of primary production exist for playas, especially for algae. The values that do exist for higher plants are typically for peak aboveground standing crop biomass (seasonal high vegetative biomass), which do not take into account past senescence of living material or herbivory (Smith and Kadlec 1985b). Moreover, belowground production is seldom considered. Belowground

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ECOSYSTEM ASPECTS

production, although it may be substantial, is very difﬁcult to estimate and typically involves coring and washing devices to extract and separate underground organs from the soil (Taylor 2000). For the robust emergent plants in circumstances where herbivory is not important, peak standing crop mass may provide a good index to aboveground primary production (Smith and Kadlec 1985b). However, peak standing crop is probably less accurate as an index of productivity in shorter-lived grasses and forbs (many of which are called moist-soil plants) because of senescence. Further, it is often difﬁcult, if not impossible, to determine if herbivory is signiﬁcant (Smith and Kadlec 1985a). Biologists must use cages (exclosures) to exclude vertebrates and examine herbivory effects on vascular plant primary production. This still does not get at the importance of herbivory by invertebrates, which may be substantial (e.g., Scott and Haskins 1987; Foote et al. 1988; Haukos 1992). Also, it does not take into account consumption of algae. The values presented here will therefore be conservative estimates. Obviously primary production in playas is inﬂuenced by a number of factors. Hydrology inﬂuences productivity primarily by which species can inhabit the playa. Soil type and fertility also inﬂuence production. The data on soil fertility in playas are limited, but in the Southern High Plains it appears comparable with surrounding agricultural ﬁelds (Haukos and Smith 1996). Further, the frequent drying and wetting of the soils within a year does not appear signiﬁcantly to inﬂuence levels of nitrogen and phosphorus (Haukos and Smith 1996). In six playas in the Southern High Plains that received irrigation runoff, peak aboveground standing crop of cattail was estimated at roughly 2,000 grams per square meter (20,000 kg/ha; 18,000 lb/ac) using wildlife exclosures (Smith 1988). This is similar to estimates of cattail biomass in other cattail-dominated wetlands (e.g., van der Valk and Davis 1978; Smith and Kadlec 1985a) and probably equivalent to that found in like playas in the Rainwater Basin. In eight playas in the Southern High Plains, Haukos and Smith (1993b) examined aboveground standing crop mass for ﬁve of the most commonly occurring plants. These plants existed under normal conditions and in playas that received a moist-soil management treatment for wildlife food enhancement. Because these plants were examined from a migratory bird food standpoint, vegetative and seed biomass were measured separately (table 5.2). Although production was enhanced for some species from management, overall these were

STRUCTURE, FUNCTION, AND DIVERSITY

117

Table 5.2 Mean aboveground standing crop (kg/ha) for seed and vegetative components of ﬁve common wetland plants found in eight playas of the Southern High Plains of Texas Irrigated Species Willow smartweed Pink smartweed Barnyard grass Curly dock Spikerush

Nonirrigated

Vegetative

Seed

Vegetative

Seed

11,487 6,492 2,635 3,274

730 532 346 1,233

1,074 1,378 376 1,582

55 105 45 703

1,228

66

986

28

Source: Modiﬁed from Haukos and Smith 1993b; courtesy of the Wildlife Society Bulletin. Note: Some playas received an irrigation treatment and others did not.

below-average precipitation years and the conditions found under management occur under natural conditions in the region. Indeed, the management schedule was selected to mimic natural conditions. Obviously there is a great deal of biomass available in vegetative and seed form for the herbivorous fauna of these playas as well as for the detrital food web. As one examines the playas that have had shorter and/or less frequent hydroperiods, the aboveground production is likely less, although estimates are few. Aboveground standing crops of the grasses, sedges, and forbs that inhabit these playas are likely similar to estimates for the same species on adjacent uplands. Typically they are (much) less than 500 grams per square meter (5,000 kg/ha; 4,500 lb/ac). Epstein et al. (1998) provided estimates of many upland prairie species for regions throughout the Great Plains. In northeastern Wyoming, Bartz (1997) provided aboveground standing crop estimates for seven playas in three playa vegetation zones from edge to center. The playa edge, which he deﬁned as “western wheatgrass,” had an average aboveground standing crop of 1,607 kilograms/hectare (160 g/sq m; 1,440 lb/ac). The next zone, lower in elevation, was termed “foxtail barley” with an average standing crop of 1,295 kilograms/hectare (130 g/sq m; 1,170 lb/ac), while the lowest zone, “spikerush,” had 1,259 kilograms/hectare (126 g/sq m; 1,130 lb/ac). These estimates included all species growing within a zone and were not necessarily monotypic stands. Bartz (1997) noted that the lowest two zones had the same species growing

118

ECOSYSTEM ASPECTS

in them and that some playas only had one of these zones. Given the very similar biomass estimates and species composition between the zones, it may be that two zones should be considered as one. Regardless, they illustrate the lower standing crop of the playas with these types of plant communities. Also note the similarity in spikerush standing crop found in Wyoming to that in Texas (table 5.2). Spikerush is a common community type in playas throughout the Great Plains; therefore, these standing crop estimates, as with cattail, can probably be roughly applied to similar playa communities elsewhere. CONSUMPTION OF PRIMARY AND SECONDARY PRODUCTION

The relative importance of herbivory by vertebrates has historically been considered to be inconsequential compared to other energy pathways in wetlands (see review in Smith 1988). However, often this can be attributed to anecdotal observation and lack of ﬁeld tests (Smith and Kadlec 1985a). Instances of signiﬁcant herbivory by waterfowl and aquatic mammals exist for freshwater marshes (e.g., Smith and Odum 1981; Derksen et al. 1982; Smith and Kadlec 1985a; Middleton 1990) and should be investigated for speciﬁc wetland types before being dismissed as unimportant in wetland functional pathways. The importance of vertebrate herbivory in playas dominated by cattail was investigated in the Southern High Plains of Texas using exclosures designed to exclude herbivorous waterfowl and rabbits (lagomorphs) (Smith 1988). In playas dominated by cattail there was essentially no effect of these vertebrates on cattail standing crop. The vertebrate herbivory pathway in these playas was negligible compared to other pathways (e.g., detrital components). However, because the number of playas with cattail as their dominant plant community in the Great Plains is relatively minor, these conclusions should probably not be extrapolated to other playas. The lack of aquatic mammals in the vast majority of playas might be used to imply this vertebrate component is unlikely to be important in the function of wet playas. However, the mammalian fauna is likely to be more important from an herbivory perspective in dry playas. This was probably especially true historically when large herds of ungulates (e.g., bison, pronghorn) used playas. When a playa is dry, smaller mammals (e.g., rodents, lagomorphs) might also be signiﬁcant herbivores, though studies on their inﬂuence are lacking.

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119

The tremendous numbers of migratory birds and invertebrates using wet and dry playas, and larval amphibians using wet playas, suggest their inﬂuence on primary production may be more substantial. However, data are largely circumstantial, especially for amphibians and invertebrates. Avian herbivory on nonseed vegetative components of playa vegetation is likely to be minor in most Great Plains playas. Many of the ducks and songbirds consume wetland seeds, not nonseed vegetative matter, whereas migratory geese primarily feed in the agricultural ﬁelds. (This brings up an important point, though not directly related to consumption. Geese and sandhill cranes feeding in uplands import large quantities of nutrients [particularly nitrogen and phosphorus] by depositing their feces in playas. This has been shown to be signiﬁcant in other freshwater wetlands [e.g., Post et al. 1998] and likely inﬂuences trophic structure.) Those wet playas that have greater amounts of seed attract larger numbers of waterfowl (Haukos and Smith 1993b; Anderson et al. 2000). The problem is that it is very difﬁcult to estimate the exact amount of seed consumed because some seed decomposes over time after it shatters from the seed head. The difﬁculty is not in estimating how much is available to birds immediately after the seed is produced, because it is still in the seed head, but in determining what is lost after shatter to herbivory versus decomposition. Regardless of the exact amount consumed, the percent of nonagricultural seed in the waterfowl diet (e.g., Sheeley and Smith 1989), given the large numbers of waterfowl using these playas (Anderson and Smith 1999) indicates that consumption of seed by migratory birds in playas is a signiﬁcant energy pathway. In playas just in the Southern High Plains, 24.3 million kilograms (53.5 mil. lb) of seeds and secondary production of invertebrates can be produced during wet periods (Anderson and Smith 1998). These seeds and invertebrates can supply migratory birds with over 9 million kilograms (19.8 mil. lb) of protein and 108.6 billion kcal of energy. Obviously, invertebrates can be signiﬁcant energy and nutrient sources. If migratory birds are selecting playa sites on the basis of invertebrate availability (Anderson et al. 2000), then it is likely they have a signiﬁcant inﬂuence on invertebrate groups. Again, however, it is extremely difﬁcult to estimate the amount of invertebrate biomass consumed by migratory birds. Davis and Smith (1998a) examined the effects of migrating shorebird foraging on invertebrate numbers and biomass in the Southern

120

ECOSYSTEM ASPECTS

High Plains. Using shorebird exclosure data from the spring when invertebrates were less abundant, shorebirds caused invertebrate biomass to decline. However, in autumn there was not a signiﬁcant decrease. The most numerous invertebrate-consuming predators in summer, however, may be amphibians, but there are no studies examining their effects on playa trophic structure. Finally, there is only one study examining invertebrate herbivory in playas. Haukos (1992) noted substantial and species-speciﬁc herbivory by a chrysomelid beetle (Gastrophysa dissimilis) on water smartweed (Polygonum coccinea) in a playa in the Southern High Plains. Both larvae and adults consumed leaf material. Further, among the adult beetles, females consumed more biomass than males. This invertebrate could alter energy pathways in playas dominated by water smartweed. DECOMPOSITION

Most production in playa wetlands likely enters the detrital food web. The major factors affecting decomposition of plant matter entering this energy pathway are availability of oxygen, temperature, nutritive quality of the plant litter, and the faunal and ﬂoral communities present that will act on the detritus. Although little is known about the makeup of microbial communities inhabiting playas, it is likely they are very important in the entire decomposition process consuming dissolved nutrients and changing their chemical form. These microbes are then being consumed by epiphytic algae and microscopic invertebrates. This is a complex process with many feedback loops (e.g., Gallagher 1978). Larger macroinvertebrate consumers then also play an important role on decomposition processes through their inﬂuence on plant fragmentation and predation on the microscopic invertebrates (ﬁg. 5.1). Clearly hydroperiod should inﬂuence decomposition rates through its inﬂuence on temperature and oxygen as well as through its effect on the composition of the biotic community involved in detrital food webs. However, results from other wetland types have been equivocal (e.g., Brinson et al. 1981; van der Valk et al. 1991; Wrubleski et al. 1997) with duration of hydroperiod showing variable results on decomposition rates. Where decomposition rates are relatively low in wetlands, there is generally a buildup of organic matter, typically referred to as a “duff” or “peat” layer. This is generally not seen in most

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121

Figure 5.1 Suggested food web of a wet playa. Solid lines indicate dominant pathway; dashed lines indicate pathways of secondary importance. The prominence of amphibians is apparent in the summer with birds playing a secondary role. During other seasons birds are most important and amphibians play a secondary role or are absent. (Information on invertebrates is modiﬁed from Hall et al. 1999, 653, in Invertebrates in freshwater wetlands of North America: Ecology and management, used by permission of John Wiley and Sons, Inc.)

playas and there is little organic matter on top of the soil surface. Indeed, in one of the few places it has been examined, the carbon content in Southern High Plains playa soils is relatively low (Luo 1994). Decomposition rates have only been studied for one species in playas of the Great Plains. Pink smartweed is a common species found in playas from the Rainwater Basin to the Southern High Plains. Because it is so widespread and produces large quantities of seed, Anderson and Smith (2002) examined decomposition of its seed and vegetative components in Southern High Plains playas. Certainly an average aboveground standing crop ranging between 1,400 and

122

ECOSYSTEM ASPECTS

6,500 kilograms/hectare (1,350 –5,800 lb/ac) is a tremendous amount of organic matter entering the playa ecosystem and inﬂuencing entire energy ﬂow pathways. Seeds were studied not only because of their food value to seed-eating vertebrates but also because of their potential inﬂuence on the seed bank. Seed banks in playas are the major factor inﬂuencing plant community composition following a change in hydroperiod (Haukos and Smith 1994b). Dry seeds, stems, and leaves of pink smartweed were placed in litter-composition bags (ﬂexible plastic screen bags) and placed in four playas with different hydrologic regimes. Bags were placed in the playas in late September similar to when this litter would be entering the detrital food web in a natural setting. The annual hydroperiod ranged from total submersion to ﬂooded for only 30% of the study. Two playas also had intermediate hydroperiods of 41% and 58% of the time. Various samples were removed from these hydroperiod regimes at different times so that hydroperiod inﬂuence on loss of biomass, hemicellulose, cellulose, and crude protein could be estimated. Samples were collected after 7, 14, 21, 28, 42, 56, 231, and 321 days. Sampling was conducted at a higher frequency early in the study because this is when most changes in the litter are thought to occur. As expected, nutritive quality of the different plant parts varied prior to their being placed in the different playas (Anderson and Smith 2002). To begin with, crude protein levels were highest in the leaves (22%), midrange in the seeds (10%), and lowest in the stems (4%). Cellulose and hemicellulose were highest in the stems (51% and 33%, respectively), midrange in seeds (47%, 30%), and lowest in the leaves (24%, 18%). Leaves contained the highest nutritive value (crude protein) and the fewest impediments to breakdown by herbivores. Stems provide the support and leaves ﬁx the energy. Smartweed leaves decomposed relatively rapidly through day 56 (ﬁg. 5.2). Approximately 59% of the dry mass was lost in the long-dry treatment and 40% from the completely submersed treatment. After day 56 decomposition slowed. Crude protein remained relatively high during these ﬁrst 56 days, and at the end of the study 14% remained in leaves in the shortest hydroperiod and only 4% in the continually submerged treatment. Decomposition of stems was also relatively rapid in the ﬁrst 56 days especially in the long-dry treatment (ﬁg. 5.2). Indeed, within the ﬁrst 14 days only 36% of the original stem biomass remained in that treatment. At the end of the study in the long-dry treatment only

Figure 5.2 Decomposition rates of pink smartweed leaves, stems, and seeds. Hydroperiod treatments are as follows: “Dry” had water 30% of the time; “permanent” had water the entire study; “short dry” had water 58% of the time; and “long dry” had water 41% of the study period, which started in late September and continued for 321 days. (From Anderson and Smith 2002, courtesy of Aquatic Botany, with permission from Elsevier Science.)

124

ECOSYSTEM ASPECTS

11% of the original mass remained, while in the continually submerged treatment 74% of the original mass remained. Crude protein rose rather substantially in the stems after being placed in the playas. Although not studied in this experiment, litter is often colonized by epiphytic algae and bacteria after being placed in the playa. They contain more nitrogen (protein) and in addition, the portion of the mass remaining after initial decomposition can contain proportionally more protein. One should expect that as a persistent component of the seed bank, smartweed seeds decomposed much more slowly than leaves and stems (ﬁg. 5.2). If a seed is to remain viable in the seed bank to replenish the community following a change in hydroperiod, then it must possess an evolutionary strategy to resist decomposition. In the ﬁrst 56 days the continually submerged and long-dry treatments had higher decomposition rates than the other hydroperiods. After 321 days the long-dry treatment only had 51% of its seed mass remaining, while the short-dry hydroperiod had 81% remaining. The greatest replenishment of the seed bank occurred in the shortest hydroperiod treatment. Crude protein declined about 3% until about four weeks had passed and then returned close to starting levels. Levels then steadily declined but did not vary by length of hydroperiod. Most often the long-dry treatment had the highest total mass loss for any plant part. This coincides with water-level ﬂuctuations whereby water is initially present in autumn as the playa dries, exposing the substrate to an oxygenated environment, and then is inundated again in spring as temperatures are again rising. Compared to a situation where plant parts are continuously submerged, less decomposition occurs probably because of less available oxygen and lower temperatures. DIVERSITY CONSIDER ATIONS

With some knowledge of the structure and function aspects of playas, it is possible to examine factors potentially inﬂuencing biodiversity in these wetlands. Any wetland in a large grassland/agricultural region will harbor many aquatic dependent species that cannot exist in the upland at large. The reverse is not true—many upland species can inhabit prairie wetlands when dry conditions occur in the wetland. This leads to the importance of playas as focal points of biodiversity, with many interesting facets of species diversity, dispersal, and colonization being found there.

STRUCTURE, FUNCTION, AND DIVERSITY

125

Overall, the more arid the climate of the landscape surrounding wetlands, the more striking some of the diversity and dispersal relationships may become. The gradient between upland and wetland is obviously more apparent in arid landscapes, creating unique refuge effects, which can lead to dispersal barriers, genetic isolation of the species that successfully colonize the wetland, and, therefore, potential speciation (King et al. 1996; Thomas et al. 1994, 1997, 1998). Drier conditions clearly place greater restrictions on dispersal of aquatic dependent species, and on subsequent wetland colonization, than in more mesic environments. Thus, because playas in the southwest portion of the Great Plains are in a more arid environment than those to the north and east, one might expect that the southwestern playas are more important to their local or regional biodiversity because of the unique aquatic species they may contain. Unfortunately, data needed to test this hypothesis for playas are sparse. Moreover, the potential relative contributions of wetlands to regional biodiversity should not be confused with the ability of a particular wetland to contain a certain number of taxa. Because playas exist essentially as islands in a vast agricultural landscape, other factors such as wetland density and area also come into play here as demonstrated in basic Island Biogeography relationships (e.g., MacArthur and Wilson 1967; Rosenzweig 1995). Island Biogeography theory holds that (1) the larger the island, the more species it should contain, and (2) the closer the island is to another island, the more species it should contain relative to opposing conditions of smaller island area and greater distance between islands. Density and area of playas vary across the Great Plains, allowing unique tests on the above biogeographic relationships. Finally, the condition and relative disturbance (e.g., cultivation) of the surrounding landscape may inﬂuence hydroperiod, dispersal, presence of exotics, and competitive relationships within playas. Anything that affects dispersal and hydroperiod will affect the biotic/competitive relationships within the playa. Cultivation is not only related to irrigation and erosion, and thus hydroperiod; it also provides an avenue for nonnative species to disperse. Moreover, the longer the hydroperiod, the more important competition and predation become in shaping wetland communities (e.g., Skelly 1997; Moorhead et al. 1998). Therefore, sedimentation or water withdrawals/additions, resulting in an altered hydroperiod, will affect biotic relationships and thus the species composition and abundance of individuals.

126

ECOSYSTEM ASPECTS

In the following section, the problem of actually determining diversity is ﬁrst examined. Then, biotic, biogeographic, and landscape factors that inﬂuence that diversity are discussed. SPECIES PRESENCE

When biologists survey ﬂora or fauna of a playa, they obviously count what they visibly observe. Most biogeographic studies that examine the number of species in a given area follow this simple approach (see review in Rosenzweig 1995). Because biogeographers and landscape ecologists are usually studying a distinct taxonomic group, such as lizards or ants, over large geographic ranges spanning from say 10 square kilometers (3.8 sq mi) to an entire island archipelago covering thousands of square kilometers, this approach seems reasonable. However, some species are present that are not easily observed existing as eggs, seeds, or estivating vertebrates. If biologists do not make an adequate inventory of the species present, they stand the risk of misinterpreting the factors that inﬂuence biodiversity. Wetlands frequently pose some particular concerns here that other habitats, such as oceanic islands, might not. Suppose a survey of plant species visibly present in a dry playa during June yields 15 species. When the same playa is surveyed in August while submerged, 10 species are found, only 5 of which existed in the June count. So now there are a total of 20 species for that playa; of these, 5 species were not missed in the ﬁrst survey and 10 species did not go extinct between surveys. Rather, the moisture conditions changed causing some species to disappear and others to ﬂourish. Both types were present in the soil seed bank and did not recently disperse into the playa. Even if the moisture conditions had not changed, many of the species present in the early survey would not be present in the latter; some plant species simply complete their life history within a given season. It could be said that not enough surveys were conducted. To a certain extent this is true, but it might be years or even decades for the playa being studied to go through enough moisture permutations to permit observation of all the species that might actually exist in the wetland. This is especially true in wetlands that have particularly long-lived seed banks and extant ﬂoral communities that can change quickly (Salisbury 1970; van der Valk 1981; Smith and Kadlec 1983; Haukos and Smith 1993a; Bliss and Zedler 1998). Abbott and Black (1980), studying ﬂora on the islands off the western coast of Australia,

STRUCTURE, FUNCTION, AND DIVERSITY

127

near Freemantle, were some of the ﬁrst ecologists to try to come to grips with the problem of not including plant propagules (seeds and oospores) in biogeographic studies. Techniques exist whereby one can germinate most wetland species seeds from the soil by exposing them to different environmental conditions in the greenhouse (Poiani and Johnson 1988). However, few, if any, studies have used plant propagules to test basic biogeographic tenets. This observational problem of not counting species that actually are present can be as severe with regard to fauna as it is with ﬂora. Some invertebrates can remain viable in dried wetland sediments for decades. They possess many different strategies to survive wetland desiccation (Danks 1987), but the entire group is typically referred to as an “egg bank” (Hairston et al. 1995). The “egg bank” includes persisting invertebrates such as dormant adults, larvae, cysts, and eggs (e.g., Wiggins et al. 1980; Thorp and Covich 1991; Horne 1993). Ecologists have collected soil from dry wetlands and placed it in water in different vessels commonly termed “microcosms” (e.g., Horne 1996); invertebrates emerging from the microcosms are then enumerated. From this perspective, the determination of dormant invertebrate species is similar to that used for seed banks in ﬂoristic studies. The problem of not sampling all species present extends to amphibians and, to a lesser extent, reptiles. Again, simply because a particular playa amphibian species is not observed in a playa does not conﬁrm the species is not lying in estivation in the sediments or in the adjacent watershed. Following inundation many amphibian species may appear but as with plants this varies by season. The only common aquatic reptile, the yellow mud turtle, may also occur at a high frequency in playas that had been dry. Therefore, the dynamic hydroperiod, seasonality, and composition of seed/egg/estivating species are major determinants of the extant biota in playas. Further, the above difﬁculties highlight problems that exist in using wetland data to test different biogeographic and landscape ecology theories. PERSISTENCE AND DISPERSAL

Knowledge of species’ abilities to inhabit wetlands is also necessary to understand the biotic composition of playas. Species present in a playa are there due to either relatively recent dispersal from adjacent habitats (emigration) or, as noted above, their ability to persist through variable wetland conditions. J. Ander-

128

ECOSYSTEM ASPECTS

son (1997, 151) classiﬁed aquatic playa invertebrates as “persisters” or “colonizers.” Colonizers were further partitioned into “active” or “passive” dispersers. This classiﬁcation system can probably be used for most playa biota. Active dispersal by wetland fauna to playas obviously involves either terrestrial or aerial locomotion. Passive dispersal among playas is practiced by plants and invertebrates, and possibly some amphibians. Many wetland plant propagules and invertebrate eggs and cysts can survive passage through avian intestinal tracts, and many adhere to the plumage and feet of birds (Proctor 1964; Proctor and Malone 1965; Proctor et al. 1967). Some amphibian eggs also may survive “adherence” transport. Further, adult and larval invertebrates may actively disperse on birds, mammals, and even other dispersing invertebrates (Maguire 1963; Moore and Faust 1972; Peck 1975; Daborn 1976; Boag 1986; Armitage 1995). Wind is also a very effective means by which plants and invertebrates are passively dispersed (e.g., Maguire 1963; MacArthur and Wilson 1967; Johnson 1995). Some invertebrate species can be considered to possess traits that would allow them to be classiﬁed as active or passive dispersers, and as transients or residents depending on their life history stage (J. Anderson 1997). J. Anderson (1997) found that at least 58% of the invertebrate taxa persisting in playas also possessed colonization (passive and active) abilities. Indeed, playa biota possess numerous life-history strategies for inhabiting wetlands during the aquatic phase. The frequency and success of active and passive dispersal rests generally on two factors: distance between wetlands and condition of the dispersal area. Once dispersed, actively or passively, the wetlanddependent biota must rely on aquatic conditions being available in the playa sufﬁciently long enough for them to fulﬁll their life-history needs or to persist in such a way that they can rely on the next aquatic phase. BIODIVERSITY RELATIONSHIPS

Patterns of species diversity in playas can now be examined, with knowledge of structure, presence, dispersal, and persistence nuances of playa biota. Some of the ﬁrst studies in ecology were aimed at explaining species diversity relationships and patterns (see review in Rosenzweig 2001). One of the earliest studies noted a relationship between increasing species diversity and increasing area (Arrhenius 1921). Many studies followed and several reasons for the well-known area-diversity relationship have been proposed (e.g., Pre-

STRUCTURE, FUNCTION, AND DIVERSITY

129

ston 1962; Williams 1964; MacArthur and Wilson 1967). Rosenzweig (1995) clearly summarized the literature in this arena and noted that the area-diversity relationship is most often thought to be related to number of habitats or population size (e.g., Williams 1964; Connor and McCoy 1979; Nilsson et al. 1988). As area increases, generally the number of available habitats increases allowing higher species diversity. Conversely, as area increases, so does the number of individuals decreasing the probability a species will go extinct. Many studies have demonstrated support for either of these two hypotheses (habitat vs. population size), but separating the contributions of habitat variability versus population size to the area-diversity relationship is difﬁcult (e.g., Connor and McCoy 1979; Gilbert 1980; Rosenzweig 1995). Nilsson et al. noted, “if this [habitat diversity] hypothesis is correct, we expect no relation between species number and area in cases where area and habitat diversity are uncorrelated” (1988, 686). Playas are especially suitable to test which of these two hypotheses may be more correct. As playa area increases, the number of habitats in the playa stays the same but plant population size increases (see Chapter 3 on ﬂora zonation). One caveat to the habitat hypothesis scenario, however, is that larger playas, even though they have similar habitat structure to small playas, can generally remain wet longer than small playas. Therefore, for those species that require aquatic conditions to complete their life history, large playas should provide greater opportunity for establishment and reproduction. Proximity to other habitat patches, as well as the amount of habitat disturbance (e.g., Rosenzweig 1995), also may inﬂuence diversity. Typically, the closer island habitats are to each other, the higher the relative diversity. Areas with intermediate levels of disturbance also are often thought to have greater diversity relative to areas with high or low disturbance (e.g., Petraitis et al. 1989). The large numbers of playas existing in varying sites and densities also allow examination of the importance of nearby habitat patches on species diversity. Finally, the frequent changes in hydroperiod (disturbance) within a year invites examination of the hypothesis that intermediate levels of disturbance promote higher levels of diversity.

vegetation species – area relationships Using data collected on 224 playa plant communities in the Southern Great Plains (ﬁg. 3.3), Smith and Haukos (2002) compared area to plant species diversity and richness in playas with cultivated and perennial grassland watersheds. Separate analyses

130

ECOSYSTEM ASPECTS

were conducted for playas with cropland and grassland watersheds because, as demonstrated in the ﬂora chapter, playas with cropland watersheds have more exotic and annual species than playas with grassland watersheds. Smith and Haukos (unpublished data) also examined the inﬂuence of nearby habitat patches and water-level disturbance on diversity and richness. Plant species occurrence was determined twice for each playa to account for cool and warm season species; late spring to early summer (15 May–30 June) and mid- to late summer (15 July–31 August). Because soil moisture and water levels affect playa plant community composition, playas were categorized immediately prior to each survey as dry, moist, or ﬂooded. Playa disturbance was equated with changes in soil moisture regime between early and late surveys. If soil moisture regime did not change between early and late surveys (i.e., dry-dry, moist-moist, ﬂooded-ﬂooded), this was classiﬁed as “low disturbance.” Disturbance was considered as “intermediate” if the moisture regime went from dry to moist, moist to dry, moist to ﬂooded, or ﬂooded to moist. Disturbance was considered as “high” if soil moisture regime went from dry to ﬂooded or ﬂooded to dry. The number of species counted was richness and Shannon’s was the calculated diversity index (Magurran 1988). Analyses were conducted on all playas with all plants (terrestrial and aquatic) and only those species categorized as facultative or obligate wetland plants (U.S. National Wetlands Inventory 1996). Three measures of habitat patch isolation were also used with playa area to evaluate their inﬂuence on diversity and richness: (1) the number of playas within a 4-kilometer (2 mi) radius of the sampled playa, (2) the total area of all playas within a 4-kilometer radius of the sampled playa, and (3) the distance in meters between the sampled playa and nearest playa. This was tested using 141 of the 224 playas for which digital data existed. In general, the relationships between total plant species richness and diversity and area for all playas were not strong (most r2 0.10 [r2s indicate the amount of variation explained by the variable, in this case area], and many slopes were not statistically signiﬁcant; table 5.3). Relationships within playas having cropland or grassland watersheds were not stronger (ﬁg. 5.3). Only the log area–log richness regression for the overall playas approached a condition where area explained more than 10% of the variation in total species richness. (Biogeographers often log-transform the variables to account for the nonlinear relationship between area and diversity.) However, when

Table 5.3 Relationship between plant species diversity (richness, Shannon’s) and playa area using various transformations for playas in the Southern Great Plains Grassland Playas

All Playas Transformation / Index

2

Slope

SE

Intercept

P<

r

0.027

0.064 0.002

0.024 0.002

18.45

0.002

0.008 0.240

0.086 0.002

4.130 0.047

0.879 0.067

15.42

0.001 0.486

r

2

Cropland Playas 2

Slope

SE

Intercept

P<

Slope

SE

Intercept

0.028 0.008

0.081 0.002

0.041

18.03

0.055

0.018

0.003

1.63

0.623

0.008

0.054 0.003

0.029 0.002

18.77 1.81

0.154

0.065 0.010

3.455 0.018

1.238 0.095

16.04 1.63

0.006 0.854

0.099 0.005

5.022 0.060

1.307 0.095

14.48 1.77

0.001 0.514

P<

r

No transformation Richness Shannon’s

1.73

0.071

LogArea/Index Richness Shannon’s LogArea/LogIndex Richness Shannon’s

1.65

0.092

0.105

0.022

1.16

0.001

0.055

0.082

0.032

1.18

0.011

0.125

0.133

0.030

1.14

0.001

0.003

0.011

0.022

0.20

0.612

0.009

0.013

0.035

0.19

0.761

0.004

0.020

0.027

0.21

0.460

Source: Modiﬁed from Smith and Haukos 2002; courtesy of Conservation Biology, Blackwell Science Ltd. Note: Relationships were examined for all playas (n 224), playas with perennial grassland watersheds (n 98), and those with annual cropland watersheds (n 126) because diversity varied between wetlands within the different watershed types.

132

ECOSYSTEM ASPECTS

Figure 5.3 Graphical illustration of the relationship between playa area and total species richness in the Southern Great Plains (n 224). Playas with cropland (r 2 0.099; P 0.001) and grassland (r 2 0.065; P 0.006) watersheds are presented separately and combined (r 2 0.086; P 0.001) due to plant community differences between the two types. (Figure by D. Haukos, courtesy of U.S. Fish and Wildlife Service.)

upland plants were excluded from the analyses, the relationship of wetland plant richness and area improved (r2 ⬃ 0.2) (table 5.4). Including only wetland plants in the Shannon’s diversity-versus-area analyses did not provide stronger relationships than when all species were included in the analyses.

Table 5.4 Relationship between plant species diversity (richness, Shannon’s) and playa area (various transformations) when only wetland plant species are included in the analysis Transformation / Index

Grassland Playas

All Playas r2

Slope

SE

Intercept

P<

r2

Slope

SE

Intercept

Cropland Playas P<

r2

Slope

SE

Intercept

P<

No transformation Richness

0.094

0.079

0.017

8.82

0.001

0.078

0.075

0.026

7.31

0.005

0.101

0.072

0.019

10.14

0.001

Shannon’s

0.002

0.001

0.002

1.34

0.476

0.017

0.004

0.003

1.11

0.201

0.003

0.001

0.002

1.53

0.568

LogArea/Index Richness

0.228

4.678

0.578

5.48

0.001

0.189

3.568

0.755

5.15

0.001

0.233

5.085

0.829

6.08

0.001

Shannon’s

0.059

0.271

0.072

1.10

0.001

0.054

0.224

0.096

0.97

0.022

0.029

0.195

0.102

1.31

0.058

0.196 0.037

0.238 0.097

0.032 0.033

0.72 0.01

0.001 0.004

0.136 0.021

0.187 0.077

0.048 0.054

0.70 0.04

0.001 0.156

0.232 0.028

0.251

0.041

0.17

0.001

0.071

0.038

0.09

0.060

LogArea/LogIndex Richness Shannon’s

Source: Modiﬁed from Smith and Haukos 2002; courtesy of Conservation Biology, Blackwell Science Ltd. Note: Relationships were examined for all playas (n 224), playas with perennial grassland watersheds (n 98), and those with cropland watersheds (n 126) because diversity varied between wetlands with the different watershed types.

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ECOSYSTEM ASPECTS

Because habitat diversity does not increase as playa area increases (Luo et al. 1997) and the area–plant diversity relationship was not strong when all plants are considered in the analyses, the habitat diversity hypothesis should not be dismissed. Hoaglund and Collins (1997) also found little support for an area–species richness relationship in 40 playas located primarily in grassland environments. And similar to playas, Holland and Jain (1981) also found only a small relationship between area of vernal pools in California and plant species richness. Vernal pools were dominated by herbaceous annuals. Although the number of individual plants increases as playa area increases, it appears to have little inﬂuence on the total number of species occupying the playa. Unlike the entire playa ﬂora, there was a relationship between wetland-only plant species richness and playa area. Considering the dispersal of plant species across a historic prairie landscape, and the more recent intensively cultivated monoculture, this makes sense. As noted earlier, upland plants can occur in wetlands when the wetland is dry and persist in the seed bank when the wetland is ﬂooded. Wetland plants cannot persist in the upland seed bank for long because their life history requirements are never met (i.e., the upland is seldom ﬂooded). Therefore, from a dispersal perspective playas are not a habitat island/patch for upland plants but they are for wetland plants. Because playa habitat is structurally simple and plant population size had little inﬂuence on total plant richness in this environment, it was hypothesized that population size or habitat heterogeneity did not account for the wetland plant species richness–playa area relationship. But, habitat permanence may account for the relationship (Smith and Haukos 2002). Because large playas stay wet longer, they provide more opportunity for aquatic plants to become established and reproduce than do small playas. Habitat patch proximity and total playa area were not related to total species or wetland species diversity. There was a weak relationship between the area of playas within 4 kilometers (2 mi) of the study playas and total plant richness, but it accounted for only 3.9% of the model variation. Possibly the high relative density of playas allowed similar dispersal of all species among playas. However, as with the area-richness relationship, the correlation was much higher when only wetland species richness was used in the analysis (18.1% of the variation).

STRUCTURE, FUNCTION, AND DIVERSITY

135

Table 5.5 Mean diversity and richness (SE) for all plant species and wetland species in playas sampled in spring and late summer by disturbance regime and watershed type in 224 playas of the Southern Great Plains Diversity Measure/ Watershed Type Richness Total Cropland a Grassland b Combined Richness Wetland Cropland Grassland Combined Diversity Total Cropland Grassland Combined Diversity Wetland Cropland Grassland Combined

Disturbance Regime Low

Medium

High

20.16(0.65)

18.89(1.14)

18.52(1.29)

18.79(0.76) 19.50(0.50)

18.67(1.25) 18.81(0.85)

21.00(1.73) 19.41(1.04)

11.03(0.48) 7.82(0.48) 9.48(0.36)

12.00(0.59) 8.47(1.06) 10.77(0.59)

11.00(0.90) 10.38(0.90) 10.78(0.65)

1.78(0.05) 1.60(0.06) 1.69(0.04)

1.78(0.07) 1.70(0.09) 1.75(0.05)

1.71(0.10) 1.55(0.13) 1.65(0.08)

1.49(0.05) 1.12(0.06) 1.31(0.04)

1.61(0.06) 1.20(0.07) 1.46(0.05)

1.45(0.10) 1.34(0.11) 1.41(0.08)

Note: Low no moisture change between early and late season surveys, dry and dry, moist and moist, ﬂooded and ﬂooded; medium moderate moisture change, dry to moist, moist to dry, moist to ﬂooded, ﬂooded to moist; high high moisture change, ﬂooded to dry, dry to ﬂooded. a n 126. b n 98.

Finally, there was no indication that disturbance, as demonstrated by within-year water-level ﬂuctuation, inﬂuenced playa plant diversity or richness (table 5.5). This type of water-level disturbance does not provide support for the hypothesis that intermediate levels of disturbance have the greatest species diversity. However, this might be a relict of sampling scheme. Sampling plants on a set schedule rather than a schedule driven by environmental moisture does not permit enumeration of species that change following an alteration in hydroperiod. When all plants are counted, the more numerous the changes in moisture, the more the plant community changes within a year allowing higher diversity on a given plot of land. Richness and diversity

136

ECOSYSTEM ASPECTS

were slightly higher in playas with cropland watersheds than in those with grassland watersheds. INVERTEBRATE DIVERSITY STUDIES

In 1994, Hall (1997) sampled aquatic macroinvertebrate communities in playas surrounded by cropland, grazed native grasslands, and ungrazed exotic grasslands that were entered into the Conservation Reserve Program (see Chapter 1). She sampled playas three times over a three-month period beginning three weeks after the playas had ﬁlled with precipitation runoff. Hall then examined the effects of playa area and surrounding watershed land use on aquatic macroinvertebrate richness and diversity (Fisher’s index) within each of the three time periods. Analyses were conducted on resident and transient groups, as well as on the combined total of these two groups. Hall (1997, 25) found that average species richness among sampling periods ranged from 23.6 to 32.6 with the highest richness and diversity occurring during the second sampling period. Resident species richness peaked during the ﬁrst and second surveys, whereas transients peaked in the second and third surveys. The inﬂuence of landscape (watershed characteristics) and playa area on diversity was more pronounced for richness than for diversity (Hall 1997, 33) and overall the inﬂuence of area was weak. This trend is similar to that observed in playa ﬂora. Landscape factors also had a greater inﬂuence on resident taxa than on transient taxa. This relationship was primarily apparent during the ﬁrst survey period because of the short life cycles of residents. However, the inﬂuence of land use varied greatly by species (Hall 1997, 68). Because residents were, by deﬁnition, conﬁned to a playa, it is probable that they would be more sensitive to land-use manipulations than transients would. Hall (1997, 34) felt that, compared to resident taxa, transient taxa were relatively more affected by biogeographic inﬂuences, such as area, than land-use inﬂuences, such as cultivation. Even so, playa area, or proximity to other playas, had little effect on richness or diversity. The amount of emergent vegetation in the playa had the most inﬂuence on diversity, which is a common phenomena for wetlands in general. It is likely that emergent vegetation provided habitat heterogeneity, which increased food and cover for a large number of aquatic invertebrate life-history requisites. Finally, Hall (1997, 93) hypothesized that diversity of aquatic

STRUCTURE, FUNCTION, AND DIVERSITY

137

macroinvertebrates in playas was structured primarily by disturbance. Disturbance was deﬁned as changes in water regime (which inﬂuences the egg bank of residents) and length of inundation (which inﬂuences reproduction and development of residents and transients). Biogeographic (e.g., wetland-area) and landscape (e.g., watershed) inﬂuences were considered to be of secondary importance to diversity of aquatic macroinvertebrates. However, in the present-day landscape these primary and secondary inﬂuences on diversity are not mutually exclusive. For example, land use inﬂuences sedimentation and sedimentation inﬂuences playa hydroperiod (deﬁned above as disturbance). Therefore, landscape and hydroperiod are closely tied in the present (Luo et al. 1997), and both likely inﬂuence the regionwide diversity of aquatic macroinvertebrates. On the other hand, within a landscape that historically, or currently, did not have these anthropogenic inﬂuences (i.e., cultivation/sedimentation), such as large tracts of native grassland, it might be possible to keep disturbance inﬂuences and land-use factors mutually exclusive. In this scenario, hydroperiod (disturbance) indeed might be the primary inﬂuence on aquatic invertebrate diversity rather than biogeographic and landscape issues. CONCLUSIONS

The maintenance of playa structure and function, and inherent diversity, is complex with climatic, biogeographic, landscape, and biotic interactions intertwined and inﬂuencing playa fauna and ﬂora. Many species within a given playa are turning over continuously (i.e., local extinction) due to climate inﬂuences on hydroperiod, while others remain dormant in the sediments. This turnover constantly affects playa structure (e.g., community composition) and function (e.g., production, nutrient cycling). The timing and length of the hydroperiod inﬂuences the ability of aquatic-dependent ﬂora and fauna to successfully disperse among, and inhabit, playas. Colonization from other playas, and other wetlands, probably helps maintain species populations in a metapopulation existence (e.g., Dytham 1995). (A metapopulation has a discontinuous distribution made up of disjunct habitat patches, in this case playas.) That is, populations within individual playas may be unstable, but for the time being, groups of playas are capable of persisting as a metapopulation. However, the longer a playa remains wet, the more important

138

ECOSYSTEM ASPECTS

biotic interactions such as predation and competition become for aquatic species (e.g., Skelly 1997; Moorhead et al. 1998). These biotic interactions may act in an opposite manner to that of increased time for colonization. That is, increased hydroperiod, to a point, should allow increased diversity by increasing opportunities for colonization. But longer hydroperiods allow competition and predation to increase, which in wetlands has often been shown to cause a decline in faunal diversity and abundance (e.g., Neckles et al. 1990; Skelly 1997). This is especially true for aquatic macroinvertebrates and amphibians. Therefore, anything that inﬂuences playa hydroperiod will affect the relative importance of biogeographic factors on diversity as well as the biotic relationships within playas. The interaction between biogeographic factors, as inﬂuenced by water permanence, and biotic factors inﬂuence the number of wetland species present. Besides climate, the main inﬂuence on playa hydroperiod is watershed land use through different means, principally irrigation and sedimentation. The presence of irrigation runoff may allow greater colonization opportunities. But decreasing hydroperiods in many playas resulting from sedimentation can, for many taxa, be shifting the importance away from biotic interactions. Because many playas do not remain wet as long as they once were, biotic interactions may be less prevalent. Playa plant communities have been altered, and competition and predation from an invertebrate and amphibian perspective is likely changing relative to that which occurred historically. This is causing changes in community composition and numbers of individual species. As the trend of altered hydroperiods continues, the number of playas that can serve as a colonizing source and/or dispersal corridor will change further, exacerbating wetland loss and upsetting natural ecosystem processes within the entire Great Plains.

CONSERVATION ASPECTS

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CHAPTER 6

HISTORICAL, CULTUR AL, AND CURRENT SOCIETAL VALUE OF PLAYAS

T

he value of wetlands to Paleo-Indian cultures, more recent Native Americans, historic European Americans, and today’s society is only now being synthesized. Perhaps because of mid-twentieth-century views of most Great Plains wetlands as impediments to agriculture and as sources of disease, their value and consistent importance to human cultures throughout time has not been fully appreciated (Prince 1997). Indeed, the importance of wetlands to the persistence of Paleo-Indian populations becomes important to current society from a historic/anthropologic standpoint while more recent Native American and European views of wetlands also become signiﬁcant to us from a cultural-heritage perspective. PALEO-INDIAN

The term Paleo-Indian is frequently used to indicate prehistoric cultures in North America, but it is seldom deﬁned (Bolen and Flores 1986). For purposes of this short discussion we will consider the Paleo-Indian period as that time between the late Pleistocene and the early Holocene (Holliday 1997, 15) in which humans coexisted with and hunted large mammals that are now extinct. This covers the period roughly from 12,000 to 8,000 years B.P. (Bolen and Flores 1986). Readers with more than a passing interest in this period on the Southern High Plains should examine Holliday’s (1997) detailed review on the subject. The ﬁrst discoveries of human occupation of North America, from the late Pleistocene times (10,000 –12,000 B.P.), came from the Southern High Plains of eastern New Mexico in the 1920s and 1930s (Wormington 1957; Meltzer 1983). These were the Folsom and Clovis sites, thought to be the earliest indications of human occupation of the Americas. Folsom and Clovis sites have since been joined by nu-

142

CONSERVATION ASPECTS

merous other famous archaeological locations throughout the High Plains of the western Great Plains, such as Lubbock, Midland, Plainview, Milnesand, Nall, and Lipscomb. The scientiﬁc importance of these areas remains today as archaeologists debate over the earliest inhabitation of the Americas and the mode of colonization (Marshall 2001). As noted by Holliday (1997), some of these Paleo-Indian investigations led to studies on playa origins (Judson 1950, 1953) and on the importance of wetlands to human occupation. Holliday (1997, 114) noted, however, that only three playas in the Southern High Plains have been found with Paleo-Indian artifacts in their ﬁll. The “ﬁll” (as noted earlier in Chapter 1) is typically Randall clay, or when on the playa margin, commonly Arch loam. Radiocarbon dating of this material indicates origins from the late Pleistocene and Holocene (Holliday et al. 1996). The discovery of Paleo-Indian archaeological material in the ﬁll of only three playas, however, does not suggest that Paleo-Indian occupation around playas was rare. Many other PaleoIndian sites have been found where human artifacts and extinct animal remains occur in or near playas, and adjacent lunettes, though not in the ﬁll (e.g., Wendorf and Hester 1962; Hester 1975; Holliday 1997, 144). And, as Holliday (1997) noted, certainly other Paleo-Indian archaeological materials likely exist in playa ﬁll, but it simply has yet to be discovered because of the lack of deep excavations and exposure. Other aquatic sites were also certainly important and available to Paleo-Indian cultures on the Llano Estacado at this time, including many perennial springs and streams. The ﬁrst of the three playas with Paleo-Indian materials was found in the 1930s near Miami, Texas, in the northeastern part of the Southern High Plains. The Miami playa is one of the few sites where mammoth (Mammuthus columbi) remains and Clovis points were found together in the region (Sellards 1938; Holliday et al. 1994). This playa was prehistorically ﬁlled with soil, and there was little indication of the presence of a wetland on soil surveys or topographic maps. Closer to the center of the Southern High Plains, near Lubbock, another prehistorically buried small playa (Ryan) yielded a large number of PaleoIndian artifacts (Johnson et al. 1987; Hartwell 1991, 1995). The ﬁnal playa site noted by Holliday (1997) to contain Paleo-Indian material in its ﬁll is located in the northwestern portion of the Llano, near San Jon, New Mexico, along the edge of the escarpment. This site contained Paleo-Indian artifacts as well as remains of prehistoric bison.

HISTORICAL, CULTURAL, AND CURRENT SOCIETAL VALUE 143

Figure 6.1 Southern High Plains escarpment area north of Clovis, New Mexico. The area has an important Paleo-Indian site nearby that is associated with a playa. It also has a more recent cultural signiﬁcance as the location where people of Hispanic heritage entered the Llano Estacado as early as the 1500s. The area is also known as “La Ceja” (the brow), due to the junipers forming on the edge of the Plains. (Photo by author.)

This area, known as “La Ceja” (the brow), is also important from a later historic/cultural standpoint as the place where Mexicans (from what is today New Mexico) would enter the Llano from the northwest (ﬁg. 6.1) (Morris 1997). In addition to the extinct bison and mammoths, other large mammals likely used playas on the Llano, including camel (Camelops hesternus), short-faced bear (Arctodus simus), and giant armadillo (Chlamytherum septentrionale) (Johnson 1983; Bolen and Flores 1986). As noted in the fauna chapter, truly aquatic mammals do not currently inhabit playas, with the exception of some Rainwater Basin wetlands in Nebraska. However, this was not always the case. Bolen and Flores (1986) noted that muskrats were present but were likely extirpated when playa habitat dried for long periods of time. They

144

CONSERVATION ASPECTS

also suggested that two periods of prolonged drought between 6,500 and 4,500 years ago (Holliday 1985) likely caused their demise. Along with large mammals, waterfowl and muskrats were part of the PaleoIndian diet (Johnson 1983). The Great Plains north of the Canadian River to eastern Wyoming also contain Paleo-Indian sites (Frison 1991). One site (Nall) in the western Oklahoma Panhandle was apparently associated with a playa and yielded numerous Paleo-Indian artifacts (Baker et al. 1957; Holliday 1997, 166). Holliday (1997, 167) noted that two other sites were associated with playas in northeastern Colorado (Stanford 1979; Bannan 1980; Graham 1981; Reider 1990). Along with mammoth, large bison, and camel, horse (Equus spp.) and ﬂat-headed peccary (Platygonus compressus) remains were also found. From the end of the Paleo-Indian period to 1850 there were several climatic shifts in the Great Plains, which greatly inﬂuenced human occupation as well as the fauna and ﬂora. Meltzer and Collins (1987) noted that during the middle Holocene the water table on the Llano Estacado had dropped and springs had stopped ﬂowing. Surface water was largely unavailable. Water scarcity apparently triggered the practice of well-digging by endemic peoples. The long period of higher temperatures and lower precipitation has been termed the “Altithermal” (Antevs 1955; Meltzer and Collins 1987). It occurred roughly between 8,000 and 4,000 years B.P. Apparently bison populations were small and restricted to limited areas of the High Plains during this time, which, with the climatic changes, also greatly restricted human occupation (Dillehay 1974; MacDonald 1981). Following the Altithermal, the climate moderated and the modern bison species expanded throughout the Great Plains, supporting a human population that became dependent on hunting these large mammals. 1500 TO 1860

The ﬁrst European penetration into the Great Plains occurred following a 1524 Spanish shipwreck in the Gulf of Mexico. The account of the survivors was related back to Spain by Alvar Núñez Cabeza de Vaca (Morris 1997). Cabeza de Vaca’s description of the region would have an immediate and long-lasting inﬂuence on the Native American inhabitants and ultimately on the ecology of the entire Great Plains. One of four survivors of the shipwreck, who then escaped slavery by Native Americans, Cabeza de Vaca traveled a southern portion of the Great Plains, continued west

HISTORICAL, CULTURAL, AND CURRENT SOCIETAL VALUE 145

to the Paciﬁc in present-day Mexico, and ultimately back to Spain. One of his major revelations to the royalty, and others, was the presence of vast herds of bison (a new type of cow, or “vaca”). Native American Jumano groups were dependent on the bison at this time for food and clothing (Morris 1997). He termed the people and the area the “Cow Nation,” for their association with bison. This initial brush with the Great Plains, the desire by Spain for additional wealth, and rumors of “cities of gold” fueled the famous expedition by Francisco Vásquez de Coronado. The Coronado expedition began in April 1541 near present-day Bernalillo, New Mexico, with approximately 1,800 men (including hundreds of Native American captives), hundreds of horses, and other livestock. Prior to this, there were no modern species of horses in North America. Native American land travel was strictly on foot. The introduction of the horse to Native American societies changed their entire culture and affected the way in which Native American nations interacted. From an ecological standpoint, use of horses inﬂuenced exploitation of resources, such as bison, in the Great Plains. Wetlands were a key variable in the scenario because they inﬂuenced where horses could be watered as well as where game populations would be abundant. In May 1541 Coronado’s group climbed the northwestern escarpment of the Llano Estacado, called La Ceja (i.e., the brow, probably named for the dark green junipers occurring there) (ﬁg. 6.1), near San Jon. This was probably the ﬁrst sustained European contact with playas and bison, the dominant herbivore of the Plains. Of course, Native Americans were present on the Llano at the time, hunting bison. Coronado’s conquistadores came upon an eastern Apache camp with tents made of bison hide soon after ascending to the Llano (Morris 1997). The Apache were the dominant Native American tribe on the Southern High Plains at the time and would remain so for nearly 200 more years. The expedition’s exact route across the Southern High Plains has been debated for decades, but recent archaeological ﬁndings in White River (Blanco) Canyon just south of Floydada, Texas, indicates Coronado likely took a southeasterly route, possibly following one of the few drainages, across the Llano. This route from northwest to southeast would have taken them through the highest density of playas in the Great Plains. After establishing a camp off the eastern edge of the Caprock, a contingent of the group then turned north to locate the “cities of gold.” The route taken by the smaller

146

CONSERVATION ASPECTS

contingent, which led toward present-day Dodge City, Kansas, would have also exposed them to playas north of the Canadian River. In this area of Kansas they are thought to have encountered Wichita bison hunters (Morris 1997, 110). Of course, there was no gold, the Spanish had been misled. Morris (1997, 49) noted that the conquistadores referred to the playas as “lagunas redondas”—translated to English as “round lakes,” an apt description. They obviously frequently used playas. Indeed, one of the ﬁrst artifacts supporting the hypothesis of a Coronado encampment near White River Canyon was a Spanish chain mail gauntlet found adjacent to a playa. But, then as now, the existence of water in any particular playa was unpredictable given the spotty nature of precipitation in the Great Plains. Not only was playa water variable, but the landscape was so ﬂat as to make it nearly impossible to determine if water existed in a playa just a few kilometers distant (Carrol 1941, 1951). After failing to ﬁnd the supposed riches of the Plains, the Coronado expedition, which had been camping in association with a native Jumano group (Hickerson 1994), returned to central New Mexico initially following a more southerly route than their arrival route. This way had a more reliable water source and then trailed northwest. The Spaniards and their entourage likely followed a series of salt lakes, or “salinas,” that exist around Lubbock, Texas, and extend northwest toward Portales, New Mexico. Although the water in the lakes proper is generally too salty to drink, these lakes at one time each contained reliable freshwater springs (Bru¨ne 1981), making travel back across the Llano more reassuring. Each of these lakes, which number about 40, has a unique biodiversity and a rich geologic, anthropologic, and cultural history (ﬁg. 6.2). For example, many are important sites of Native American history (such as the supposed birthplace of Quanah Parker, the famous Comanche chief), archaeological signiﬁcance, and cultural heritage (e.g., pastore sites—Hispanic sheepherding locations). Unfortunately many have been disturbed by fuel exploration and waste, and most no longer have live springs as a result of aquifer pumping under adjacent cropland (Bru¨ne 1981). The European exploration of the Plains region to the north that contained playas did not occur as early as in the Southern Great Plains and was not likely to have occurred until the 1600s and 1700s when French fur traders began using the major river courses, such as the Missouri and the Platte. Even then, of course, with their explo-

HISTORICAL, CULTURAL, AND CURRENT SOCIETAL VALUE 147

Figure 6.2 Archaeological site in a saline lake wetland in eastern New Mexico, near Portales. Flint pieces and bison bones at the site indicate that this was a kill /butchering site. (Photo by author.)

ration having been primarily restricted to streams and rivers, there is little mention by early Europeans of the playas of Nebraska or elsewhere in the northern Plains. Descriptions of the ﬂora and fauna encountered in the Coronado expedition have been translated, but it is difﬁcult to determine exactly where the members of the expedition encountered a particular species (e.g., Strout 1971; Morris 1997). From an examination of the plant species listed in these various accounts (the majority of which understandably have human food value), most appear to have been found off of the Llano to the east. The faunal lists also are of such general nature as to be of limited value in reconstructing much of the High Plains biota of the time. Numerous other Spanish explorers and fortune seekers entered the Llano Estacado and the Plains to the north from their camps and villages in present-day New Mexico throughout the latter 1500s and early 1600s. It is unlikely they reached the playas of Nebraska, but

148

CONSERVATION ASPECTS

their trips extended well into central Kansas (Morris 1997) and south into the Edwards Plateau of Texas. In the early to mid-1600s trade between individuals from New Mexican villages and Native Americans on the Plains became an annual event. Most often this involved trade of bison meat between Apaches, and other Native American tribes, and/or hunting parties made up of Hispanic ciboleros (bison hunters) (de Baca 1954). The trade of bison from the Plains to the west and south in New Spain became a huge enterprise and lasted well into the mid-1800s before the herds were destroyed from the east by AngloAmerican “buffalo-hunters.” Anglo-European inﬂuence on the western Plains was still minimal until the late 1800s, but changes were occurring in the Native American tribes as they acquired the use of horses and battled for different territories. Indeed, in the early to mid1700s the Comanche, and later Kiowa, had forced the Apache to the southwest mostly out of the playa country. Individuals involved in the trade of bison, other goods, and often slaves with the Comanche were known as Comancheros. For the most part, the New Mexicans forged strong alliances with the Comanches and were able to continue, relatively safe from retribution, harvesting bison in the Southern High Plains until the herds were destroyed. This safety did not extend to the Texans to the east or the Mexicans to the far south. Along with the annual excursions of ciboleros, from the west, there was also seasonal pasturing of domestic animals by the Hispanics of New Spain. These pastores typically herded sheep onto the Llano during the late spring and summer and then returned to New Mexico during fall and winter. Most of these sheep pasturing sites are thought to have been in the northwestern portions of the Southern High Plains in order to be located closest to the existing western Hispanic settlements. However, recent discoveries south of Lubbock, near Tahoka Lake (a salt lake), indicate some pastores also herded their animals in southern portions of the Llano. There are playas immediately adjacent to these sites, and it is probable livestock watered in them as precipitation permitted. Seasonal herding of domestic animals in this region probably began as early as the 1600s. Bolen and Flores (1986) noted that the ciboleros and pastores had established many trails and campsites throughout the region adjacent to playa lakes. Thus, the southern portion of the Great Plains containing playa lakes had a long and rich Hispanic cultural association, well before any Anglo-European or American inﬂuence. Farther north in the Great Plains, as noted, the initial European exploration was by the

HISTORICAL, CULTURAL, AND CURRENT SOCIETAL VALUE 149

French and French Canadians (Goetzmann and Williams 1992) who generally stuck to river courses when crossing that region of the Plains containing playas, and there appears to be little mention of playas or their use in present-day Colorado, Nebraska, Wyoming, or Kansas. Native American nations inhabiting the Playa Lakes Region of the Great Plains prior to 1838 included: the Apache and later Comanche on the Southern High Plains; the Wichita farther north and east in present-day Kansas; the Kiowa and Kansa on the Central Plains; the Pawnee farther north into Nebraska and the Arapaho and Southern Cheyenne on the western High Plains (Prucha 1990). Doubtlessly the playa wetlands of the Plains were well known to these Native Americans and served as important camp areas as well as important hunting grounds. When playas contained water, many species of game were attracted to them, with pronghorn and bison being the most commonly mentioned species. However, many other large mammals also likely used playas, including elk, especially in the central and northern Plains, and mule deer. These large mammals had their attendant predators, primarily the wolf (Canis lupus) but also the omnivorous Plains grizzly bear (Ursus arctos) (Licht 1997). As Americans of European descent began to spread across the Plains, the vast herds of various species, and their associated predators, were greatly reduced if not eliminated altogether. After the Louisiana Purchase, the subsequent Lewis and Clark expedition in 1803 –1806, through the northern Plains along the Missouri, “settlers” became more and more common in the Great Plains, especially in the northeast. The short-grass region of the western Plains, however, was still largely thought to be a desert, or at best, inhospitable prairie, until the late 1800s. Initially the penetration of the western Plains and their playas by Anglo settlers was along the prairie streams and rivers. In the southern Plains confusion over the sources of the Red and Canadian Rivers also affected settlement to some extent. For example, those looking for a route to Santa Fe from New Orleans, often confused the Canadian and Red. When the Canadian, a tributary of the Arkansas, was followed deep into New Mexico to its source, a more amenable route to Santa Fe became possible. However, if explorers followed the Red they became entangled in the rugged tributaries of the eastern escarpment of the Llano. The source of the Red was thus not known for decades. Moreover, trading posts were established on the Arkansas; and a trail from Missouri to Santa

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CONSERVATION ASPECTS

Fe was initiated to foster trade; much of it along stream and river courses. The Oregon and Mormon Trails through Nebraska and Wyoming were used between 1840 and the mid-1860s to take easterners and midwesterners to the West Coast and Utah, but few initially stopped to settle these prairies containing playas (Line 2000). However, by the mid-1800s settlement by farmers did begin along the Platte River in Nebraska and spread out into the Rainwater Basin. Because water availability in temporary prairie wetlands was unreliable for travelers and their livestock, and because there were few landmarks in the western High Plains, travel outside of riparian drainages was considered chancy compared to following river and stream courses (Bolen and Flores 1986). Most discussion on reliability of High Plains water by explorers and traders from the East in the early to mid-1800s took on a negative tone (e.g., Hodge 1930; Moorhead 1954). Much of the western Great Plains was referred to as the Great American Desert (Foreman 1939), which, along with the possibility of “Indian” attack, dissuaded many easterners from attempting permanent settlement. The negative view of the western Plains, and particularly the Llano Estacado, was not held by those of Spanish heritage to the west (Rathjen 1973) who continued to cross and seasonally inhabit the region. Interestingly though, a couple of famous American explorers in the early 1800s, Stephen Long and Josiah Gregg, were mentioned by Bolen and Flores (1986) as complaining about detouring around playas rather than following a direct route. The lack of playa water was often a problem for travelers, as was too much water at other times, illustrating the unpredictable nature of prairie wetlands. Moreover, variable precipitation also prevented large-scale agriculture necessary for permanent settlement. Indeed, widespread rowcrop production would be impossible in most years through most of the western Plains that have playas without a more reliable source of water. Settlement by farmers had begun, however, in south-central Nebraska. For those readers seeking a more rich and detailed review of the Southern Great Plains from the time of Coronado to the Civil War, see El Llano Estacado by Morris (1997). POST-1865

Following the Civil War, there was much more pressure on the Great Plains from Anglo-European people from the East. (For example, the author’s great-great grandfather was discharged from the Union Army in Brownsville, Texas, at the end of the

HISTORICAL, CULTURAL, AND CURRENT SOCIETAL VALUE 151

war. He then traveled north throughout the western Great Plains for 10 years before “settling” in Illinois as a farmer. His activities during this 10 years are unknown [e.g., scout, buffalo hunter].) Needless to say this inﬂux from the East certainly increased the conﬂict with Native Americans throughout the western Great Plains. Battles were numerous and widespread, with more than one anecdotally related skirmish occurring with “buffalo-hunters” seeking refuge in the only topographic relief close by, a dry playa. The extermination of the bison herds in the 1870s and early 1880s, and the interrelated conﬁnement of Native Americans, allowed settlers from the eastern United States to begin more widespread inhabitation of the region. The land of south-central Nebraska that contains the Rainwater Basin playas had enough precipitation to allow cultivation of crops in most years, but the land farther west and south in the High Plains was generally not suitable for widespread cultivation of crops. The western Plains were, however, suitable for grazing cattle, now that the bison were gone. Large ranches sprung up across the High Plains in the late 1800s. Indeed, the largest known, the XIT, covered more than a million hectares (3 mil. ac) of the western High Plains of Texas. When the playas contained water they allowed the ranchers to spread their cattle throughout much of this short-grass prairie (ﬁg. 6.3). This illustrates an initial difference in view of the value of playas between those individuals inhabiting the Rainwater Basin region and those living in the High Plains. In the late 1800s and early in the 1900s playas were primarily considered a hindrance in the Rainwater Basin and a beneﬁt in the High Plains. Edges of Rainwater Basin playas that were farmed, or playas that were full and had ﬂooded upland ﬁelds, were difﬁcult to plant or harvest. In this region farmers often spent all winter digging drainage tunnels to the nearest stream or river (Farrar 1996). This type of activity has resulted in the loss of 90% of the playas in the Rainwater Basin (see Chapter 1). More recently Swanson (1987) examined whether draining playas in the Rainwater Basin region of Nebraska was proﬁtable from an agricultural perspective. Draining a playa or concentrating playa water in a pit is done ostensibly by farmers to increase crop production by decreasing water surface area and thus increasing tillable acreage. Pit construction also increases the amount of water available for irrigation. Swanson (1987) noted that the expense of earthwork to complete drainage projects had increased markedly prior to 1985, while proﬁt derived from crop production in the area had received only

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CONSERVATION ASPECTS

Figure 6.3 Photo of cattle watering in a Texas Panhandle playa in about 1904 (Gould 1906).

slight increases. He also found that the clay soils (e.g., Scott, Massie, Filmore, Butler) of these playas had smaller crop yields when compared to surrounding upland soils. Swanson stated that “prolonged wetness of the soils during the growing season can restrict oxygen movement in the soil and inhibit root and plant growth” (1987, 8). Also, when the clay soils dry they become hard and form large cracks. These conditions also inhibit root and plant growth. Swanson (1987) then compared the amount of earthwork necessary to adequately drain a particular wetland to the yield potential of irrigated corn on those soils. Corn was used as the crop comparison because it had the highest income potential. Obviously the deeper and the longer a playa stayed wet, the larger the cost for earthwork to drain it. Under these calculations it was not proﬁtable for landowners to drain the deeper wetlands with Scott and Massie soils. Because there were few shallower seasonally ﬂooded wetlands remaining in the Rainwater Basin (most having already been drained), there were few opportunities remaining for landowners to proﬁtably drain wetlands. Finally, proﬁtability estimates of drainage did not include the increased tax cost

HISTORICAL, CULTURAL, AND CURRENT SOCIETAL VALUE 153

to landowners who now had tillable drained land. The above ﬁndings do not mean that drainage was not continuing in the Rainwater Basin, just that it was not proﬁtable. Sometimes it is hard for tradition to die. Farther west in the High Plains where precipitation is much less, playas were initially viewed in a more positive light. Here playas, because they remained moist for longer periods than the uplands, often provided excellent hay crops (Johnson 1901; Sheffy 1963). Playas were also used as water sources for early settlers and for recreation such as swimming (Jackson 1952; Brunson 1970). Because of the lower precipitation in the High Plains, farming of the western Plains began more slowly. It was not until the early 1900s that irrigation technology allowed farming to spread (Albrecht and Murdock 1985). With the drilling of wells into the Ogallala Aquifer, cultivation of the shortgrass prairie spread and the population increased tremendously for a few decades (Nall 1990; Nickels and Day 1997). Most of the western Plains had reached their peak rural populations by the 1930s. Since that time most of the High Plains regions of Texas, New Mexico, Kansas, Nebraska, and Colorado have lost human population (ﬁg. 6.4) (e.g., Popper 1992). The regions that could not support irrigation lost disaffected people who had thought farming could be sustained; and in areas where irrigation became more efﬁcient, farms became larger without needing the support of as many individuals (e.g., Nickels and Day 1997). Further, other than agriculture, the region has little outside industry to support displaced farmers (Nickels and Day

Figure 6.4 Abandoned farmstead in a wheat ﬁeld in the western High Plains stands as testament to the “depopulation” of the Great Plains. (Photo by author.)

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CONSERVATION ASPECTS

1997). High use of and declining levels in the Ogallala Aquifer (Corbett 1986) also has not allowed the rural population to be sustained. Farmers with playas in the western Great Plains have used these wetlands to recirculate groundwater by capturing irrigation runoff. The playa occurring at the lowpoint in the watershed will collect irrigation runoff, or tailwater, which can then be pumped, relatively inexpensively, back onto the cropland. Since the 1970s, row-water irrigation has been replaced to a great extent with center-pivot sprinklers that use less water. So, when crops were watered with row-water systems, farmers viewed playas positively, but with the advent of more efﬁcient systems, playas are often not seen as important to agricultural operations. VALUE TODAY

One of the more important realizations about the value of playas in the past few decades is that playas are true “recharge” wetlands. That is, playas recharge the Ogallala Aquifer, which underlies most of the region (e.g., Havens 1966; Wood and Osterkamp 1984; Mullican et al. 1994; Scanlon et al. 1994). Indeed, in the Southern High Plains, playas appear to be the only sites for aquifer recharge (e.g., Nativ and Riggio 1989; Stone 1990; Zartman et al. 1994). It is unknown whether playas farther north are sole sites of aquifer recharge, but it is likely they serve as recharge sites. The amount of water recharged to the aquifer has been debated, due to differences in how data were obtained, and sometimes depending on the interest group being served by the study. Some groups would like to show higher recharge rates so that withdrawals are easier to justify. Regardless of the rate of recharge that is being proposed, recharge is being far outpaced by withdrawals for irrigation. As Nativ and Smith (1987) noted, the signiﬁcance of the amount of recharge to the Ogallala has been a controversial topic. There are essentially two natural ways water can leave any playa within the Great Plains: inﬁltration (i.e., recharge) and evaporation. The hydric clay soils lining the basins of playas have long been thought to be mostly impervious to inﬁltration, and in the Southern High Plains, caliche may be a second barrier reducing recharge (Knowles et al. 1984). Caliche, a dense geologic formation, directly beneath most soils of the Southern High Plains, is known to be impervious to water penetration. However, as early as the 1940s geologists noted that there was less caliche under playas than under adjacent uplands and

HISTORICAL, CULTURAL, AND CURRENT SOCIETAL VALUE 155

that the lake sediments were in fact relatively permeable (White et al. 1946). Moreover, the clay soils of most playas form large cracks upon drying. These cracks, and solution channels, might allow some water to reach the aquifer before swelling shut following inundation (Nativ and Smith 1987). The lack of salt accumulations in playas and the presence of a freshwater ﬂora further suggests that water is being recharged. Many scientists feel that if water were being lost solely from evaporation, salts and minerals would be concentrated and a more halophytic ﬂora would be present (Harris et al. 1972; Stone 1984; Wood and Osterkamp 1984). In the mid-1980s, Nativ and Smith (1987) concluded that precipitation was responsible for most of the recharge to the Ogallala, but they could not be sure that playas were the focal points for that recharge. Subsequent studies by Nativ and Riggio (1989) were more conclusive. They noted that published recharge rates for the Ogallala of the Southern High Plains varied by a couple of orders of magnitude: 0.25 millimeters (0.01 in.) per year to 41 millimeters (1.6 in.) per year depending on how surface water and precipitation entered the recharge estimates. Most (89%) surface water in the Llano collects as runoff in playas (Dvoracek and Black 1973) so that when recharge rates were calculated below playas the picture became clearer. From data collected under playas, Nativ and Riggio (1989) estimated recharge rates ranging from 13 millimeters (0.5 in.) per year to 82 millimeters (3.2 in.) per year. Nativ and Riggio stated that “the Ogallala Aquifer is most likely recharged by focused percolation of partly evaporated playa lake water, rather than by slow regional diffusive percolation of precipitation” (1989, 61). Subsequent studies by Zartman et al. (1994, 1996) supported these observations, perhaps to an even greater extent. They noted that for focused playa recharge to occur, inﬁltration of the runoff must occur within the playa basin. Initial (within 1 minute) inﬁltration rates were rapid ranging from 2,490 millimeters (98 in.) per minute to 10 millimeters (0.4 in.) per minute, while long-term ( 48 hr) rates varied between less than 0.004 millimeters (0.001 in.) per minute and 996 millimeters (39 in.) per minute (Zartman et al. 1994, 299). Decreased inﬁltration rates over time were thought to be due to the swelling of the playa clays, after initial wetting, slowing the inﬁltration rate. The discovery of focused recharge to the Ogallala Aquifer primarily through playas has led to other experiments trying to speed recharge even further as an offset to declines from pumping. Urban

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CONSERVATION ASPECTS

and others at the Water Resources Center at Texas Tech University have placed geotextiles in a playa designed to essentially ﬁlter and drain playa water to a pipe on the playa ﬂoor that leads to the aquifer. This has met with limited success because the textile ﬁelds became clogged with sediments, which brings up another question. The relationship between sedimentation in playa basins and recharge rates has been frequently questioned. Because of the work showing high sediment loads in playas reducing the playa volume (Luo et al. 1997), many wonder if sedimentation is inﬂuencing recharge rates. Several factors come into play here. For example, when the coarser-grained soils of the uplands surrounding playas erode into the playa, they mix with the clay soils and often ﬁll in the cracks in the playa basin. Whether this will allow the collected runoff to inﬁltrate more rapidly, thus permitting faster recharge, is unknown. The other major factor that may inﬂuence recharge here is playa surface area. Because the hydric soil–deﬁned playa is now more shallow than it was prior to receiving these massive sediment inputs from surrounding agricultural ﬁelds, the water is spread out over a larger area outside of the wetland. This larger surface area is certainly subject to more rapid evaporation losses than a deeper playa with a smaller surface area. Also because the upland soils are more permeable, surface water is more rapidly absorbed there. Further, because the caliche is thicker under the uplands where water is now spread than under playas, it may not reach the aquifer. These relationships await further study. Another value of playas to agricultural interests revolves around irrigation. Recall from Chapter 1 that many playas from Nebraska to New Mexico have had pits dug in them. Some of these are used to concentrate water in a smaller area so the basin can be farmed but the vast majority are used either for livestock watering areas or to capture irrigation runoff. This obviously does not help the native fauna and ﬂora (Bolen et al. 1979), but it is a perceived value to local landowners. In the Southern Great Plains, Guthery et al. (1981) found that more than 70% of playas larger than 4 hectares (10 ac) had pits dug in them. Irrigation runoff and precipitation are concentrated in the pits and then repumped onto cropland more cheaply than if the water were pumped from the aquifer (Carthel 1994). Although not as prevalent as it once was due to the use of center-pivot irrigation, reuse pits are still often relied on in the Great Plains. In urban areas, playas are used not only for recreation but as part

HISTORICAL, CULTURAL, AND CURRENT SOCIETAL VALUE 157

Figure 6.5 Urban playa in Lubbock, Texas, intensively managed for recreational ﬁshing. (Photo by author.)

of the stormwater management system (Rainwater and Thompson 1994). In most instances, stormwater runoff has transformed these urban playas from temporary or seasonal wetlands into permanent lakes with a biotic community largely dominated by exotic species. These urban wetlands, however, generally support a park system where locals ﬁsh, boat, and bird-watch. Fishing, for example, is a very high-use recreational activity on playas in Lubbock, Texas (ﬁg. 6.5) (Schramm et al. 1992; Kraai 1994). Outdoor recreation is tremendously important to the local economies (USFWS 1993). The value of playas as wildlife habitat is well known, and this wildlife beneﬁt is often translated into hunting lease income by landowners. Obviously the presence of a playa with water has the potential to provide waterfowl hunters with recreation and the landowner with income (Ramsey 1987). Many playas in Nebraska and Texas are leased for this purpose. Not only do playas provide migratory bird hunting lease potential, but dry playas provide upland bird hunting opportunities. Guthery et al. (1984) noted that land with a playa on it had a much higher hunting lease income potential than land without a playa. The playa allows more ring-necked pheasants to persist, and more pheasants translates to higher lease prices. The value of hunters to rural economies is substantial with restaurants,

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CONSERVATION ASPECTS

lodging facilities, and other related businesses (e.g., gas stations and variety stores) often garnering half their annual receipts within a few weeks during the hunting season. Poor (1999) recently examined how Nebraskans valued the playas in the Rainwater Basin. She considered that these playas had value to all of the state’s citizens in terms of ﬂood control, water quality, recreation, direct economic value from things such as tourism, and intrinsic value based strictly on a wetlands existence. To determine this, Poor (1999) used a procedure called “contingent valuation method” whereby people are surveyed and asked whether they would be willing to pay additional taxes for potential improvements in natural resources for the public good. In this case, households were asked if they would be willing to pay a certain amount of additional taxes to increase the area of Rainwater Basin wetlands. The results of the survey indicated that Nebraskans would be willing to pay approximately $12 million annually (at the time of the survey) for the purchase and/or management of wetlands in the Rainwater Basin region. If a household existed in the Rainwater Basin region or they received income from agricultural activities, they were less likely to support these potential tax increases aimed at wetlands, while those households that had (1) engaged in outdoor recreation, (2) contributed to environmental organizations, (3) had higher incomes, and (4) had higher relative levels of education were more likely to consider tax increases. These results are similar to those found in other natural resource surveys throughout the country. Most citizens are willing to pay more to protect and manage our natural heritage. Another major asset of playas is their scientiﬁc value. Biometricians (biological statisticians) and editors of scientiﬁc journals dutifully remind ecologists that their studies should have as large an inference space as possible and that they should be repeatable. Nowhere else in the United States can a wetland ecologist ﬁnd a more ideal situation to accomplish these goals than in playas. To have a large inference space and/or to be able to apply the general principles discovered from ecological studies over a large area, a scientist must have environmental replicates. With more than 25,000 playas, over a vast area of the western Great Plains, all with similar hydrology, and a range of sizes mostly with similar shapes and soils, the playas represent a wetland ecologist’s scientiﬁc dream. There are no other wetland systems in North America more suited to replicated study. The Prairie Potholes, for example, may be more numerous but the varia-

HISTORICAL, CULTURAL, AND CURRENT SOCIETAL VALUE 159

tion in their shape and hydrology limits, somewhat, the inferences one can draw from an experimental study where treatments are applied. If, for example, an ecologist wanted to apply water-level manipulations (the treatment) to a set of wetlands to examine vegetative changes, it would be best to have similar wetlands with similar hydrology so that the ecologist would be certain the measured changes in vegetation were a result of treatment rather than a confounding factor due to inherent differences among the wetlands. Further, there are few better places to study metapopulation theories or biogeographic principles in wetlands. The range in wetland size and density, as well as in watershed conditions, allows unparalleled rigorous scientiﬁc tests to be applied. Finally, perhaps the most important value of playas is to the human condition. Humans need nature and to connect with other life on Earth. This well-known, innate need has been termed “biophilia” by E. O. Wilson (1984). Playas and other wild places must be preserved for future generations. To do any less with what we have been charged with conserving would be morally and ethically wrong. From this standpoint, therefore, the threats and lack of conservation efforts aimed at playas will be examined and, ﬁnally, what can be done to improve stewardship of these valuable wetlands.

CHAPTER 7

THREATS TO PROPER FUNCTION OF PLAYAS

T

he number and magnitude of threats to wetlands on a global scale is astounding (Mitsch and Gosselink 2000). This assault is no less on playas although the relative importance of certain threats, such as drainage, may be more important in other areas than in the High Plains. The presentation here is not limited to wetland loss, as is done in many government surveys, but continues beyond to all threats that compromise the hydrologic and ecological integrity of the playa wetland system. A partially functioning wetland is better than no wetland, but a partially functioning wetland is not better than a fully functional one. Seldom are these distinctions made when continental trends in wetland habitat are discussed. A wetland either exists or it does not. Certainly data on the extent of partial function or loss are difﬁcult to obtain, and loss estimates alone are usually sufﬁcient to make the point about wetland ecosystem degradation. But when one considers the impairment in function of the remaining wetlands, the conclusions are even more staggering. For example, approximately 90% of the Rainwater Basin playas in Nebraska have been lost to drainage, a harsh enough fact. However, when one considers that the vast majority of the remaining 10% have also lost some of their other functions, such as hydrologic integrity, it is apparent that there may not be any completely functional playas remaining in that region. The many functions of playas include, but are not limited to, nutrient cycling, water storage, maintenance of landscape habitat, maintenance of characteristic plant communities, maintenance of wetland habitat, and the removal, sequestration, and conversion of elements, compounds, and particulates (RWBJV 2000, 19). Obviously there are more functions but most of these can be measured and related back to the unaltered condition. Therefore, they are popular

THREATS TO PROPER FUNCTION OF PLAYAS

161

with regulatory agencies charged with mitigation/replacement actions. Many of the functions listed above do not necessarily relate to ecosystem health but to human needs and values. For example, the function of removal of elements that runoff into the wetland may help abate human-caused pollution, but it does not necessarily help with the health of the playa ecosystem. Threats to playa ecosystems can be evaluated from a physical standpoint—that is, the manner in which physical changes such as pit excavation or drainage alter the function of playas. Chemical and biotic threats to playas such as pesticide runoff or bioinvasion of the native ﬂora by exotics can also be examined. Many of the biotic/ chemical threats interact with physical threats. Indeed, this may allow the threats to form a synergism whereby the combination of a physical alteration such as pit excavation and a chemical change such as nutrient addition may actually cause increased environmental degradation. Finally, the reality of future climate change can probably be viewed as a physical/environmental change in temperature and precipitation, but these changes will also affect chemical/biotic interactions such as nutrient cycling. So, climate change is treated separately. PHYSICAL THREATS

The most obvious physical threats to a wetland are “draining” and “ﬁlling.” In the conterminous 48 American states, wetland loss has been greater than 50% since the beginnings of European settlement. On a potential bright side, though, Dahl (2000) reported the average annual rate of loss has declined in recent decades. He attributed the decline in loss rate to several factors including human views toward wetlands that are more positive today than they were historically. These views have resulted in wetland protection legislation and simple human choice slowing wetland drainage and ﬁlling. Seldom mentioned, however, is the fact that as the number of wetlands decline, the remaining wetlands are those that are more difﬁcult to destroy. The easy ones went ﬁrst. Dahl (2000, 9) found that between 1986 and 1997, the years for which most recent estimates are available, the annual loss rate was 23,700 hectares (58,500 ac). He found that 98% of all losses were in freshwater wetland areas, such as those found in the Great Plains. Losses were attributed to urban development (30%), rural development (21%), and agriculture (26%). There were some area gains in

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created freshwater ponds and reservoirs, but obviously these bear little ecological resemblance to naturally functioning wetlands. It is difﬁcult to say how these loss rates relate directly to playas because Dahl’s (2000) study was designed with a more general perspective in mind. It is difﬁcult to gravity-drain most playas in the western Plains because there is no drainage outlet, but with a relatively more dense stream network, drainage has been thoroughly accomplished in the Rainwater Basin. Playas there have declined from an approximate original 4,000 to about 400 today. In contrast to draining, ﬁlling is generally considered to be the active ﬁlling in of a wetland with materials that allow the wetland to be developed in urban settings for housing, or in a rural setting for farming. Filling a playa, however, takes on a different meaning. Except in some Kansas playas, and many Rainwater Basin playas, the ﬁlling of most other playas has not been a conscious, deliberate act. Rather, the ﬁlling of playas has been a side casualty of soil erosion resulting from cultivation. It is generally not the farmer’s intent to have soil ﬁll in a playa but a result of the playa being the lowest point in the watershed and the cultivated watershed soil being eroded downhill. Numerous long-term landowners have questioned why their playa “does not hold water as long as it used to.” Soil that washed in from their ﬁelds is not allowing the water to remain as long as it did in past decades. Most often the response from the landowner is that ﬁlling was unintentional and regrettable, given the positive memories a playa full of water invoked. This is not to say that some landowners have not consciously used soil erosion along with heavy equipment to erase a playa it simply suggests that it has been relatively rare in the western Plains. One of the major values and functions listed by ecologists for wetlands is that they are sediment traps (e.g., NRC 1995; Mitsch and Gosselink 2000). From the viewpoint of keeping the sediments from entering streams and rivers, and thus of improving the clarity and quality of that water, depressional prairie wetlands accomplish this well. However, sedimentation is likely the single largest immediate threat to the continued existence of properly functioning wetlands in the Great Plains today. Certainly coastal wetlands such as those in the Gulf of Mexico rely on sediments for their continued existence. Simply witness the loss of wetlands that is occurring today in coastal Louisiana. However, once sediment enters closed-basin depressional wetlands like playas, there are few ways for it to leave. Other than in-

THREATS TO PROPER FUNCTION OF PLAYAS

163

Figure 7.1 Cross section of a playa showing sediment inﬂuence on hydric soil–deﬁned volume. (Luo et al. 1997, courtesy of Ecological Applications.)

tentional excavation and removal by human means, only wind action is likely to remove sediments. Chronic and heavy sediment loads alter almost all important ecological functions of the remaining wetland by altering hydroperiod and dependent faunal and ﬂoral communities. Even though the threat of over-sedimentation is clear, there have been few studies examining its effects. To examine sediment effects on playa volume in the Southern High Plains of Texas, Luo et al. (1997) selected 40 playas, 20 with grassland watersheds and 20 with watersheds that had been cultivated. By comparing playas with grassland and cropland watersheds the relative importance of sediment on wetland volume could be ascertained because playas with grassland watersheds should have had less accumulation of sediments than those with cropland watersheds. The Southern High Plains is divided into three soil-texture zones proceeding from south to north: coarse, medium, and ﬁne. Sediment loads in playas were also compared between the medium- and ﬁne-texture regional soil zones. The volume of a playa was deﬁned using the area of its hydric soil, Randall clay (ﬁg. 7.1). Using the hydric soil boundary is essentially the only standard that can be employed to consistently measure volume because soils and slope vary considerably outside of the hydric soil–deﬁned wetland. Within the grassland and cropland playas, soil cores were taken from the soil surface to the underlying Randall clay and sediment depth was measured. Playas with cropland watersheds had 8.5 times more sediment than playas surrounded by grasslands (Luo et al. 1997). Further, playas surrounded by the coarser soils had much more sediment than those

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Table 7.1 Adjusted means and standard errors (SE) of basin volume, sediment depth, sediment volume, and volume-loss ratio of the 40 playas with cropland and grassland watersheds in the ﬁne- and medium-textured soil zones of the Texas Southern High Plains Land use/Soil zone (texture) Cropland Factor

Grassland

Fine

Medium

Fine

Medium

13,500 2,000

10,600 2,200

8,800 2,000

7,600 2,200

28.96 4.31

58.21 4.76

5.12 4.31

4.02 4.76

17,600 2,800

28,800 3,100

3,000 2,800

2,400 3,100

133.2 40.8

378.2 45.1

39.8 40.8

33.3 45.1

Basin volume (m3) adj. mean SE Sediment depth (cm) adj. mean SE Sediment volume (m3) adj. mean SE Volume loss ratio (%) adj. mean SE

Source: Modiﬁed from Luo et al. 1997; courtesy of Ecological Applications.

surrounded by ﬁner soils. Playas with cropland watersheds in ﬁne-textured soils averaged 29 centimeters (11.5 in.) of sediment, while those in medium-textured soil averaged 58 centimeters (23 in.) (table 7.1). Contrast that to grassland playas that had 5 centimeters (2 in.) and 4 centimeters (1.7 in.) of sediment in the ﬁne- and mediumtexture soil zones, respectively. Of the 20 playas with cropland watersheds, 18 had lost all of their original wetland soil deﬁned–volume ( 100%). Because widespread farming of the prairie around Southern High Plains playas has a relatively recent history (cultivation began on a broad scale in the 1930s and 1940s; Mason 1986), this results in a relatively high annual rate of sediment deposition: 9.7 millimeters (0.4 in.) per year and 4.8 millimeters (0.2 in.) per year in medium- and ﬁne-textured soils, respectively. Pimentel (2000, 224) noted that a loss of just 1 millimeter of soil over 1 hectare (2.5 ac) amounts to 15 tons. Determination of whether these soils eroded as a result of wind or water action was essential in addressing potential conservation mea-

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sures. Using soil particle size distribution data from a subset of the original playas, it was found that sediments deposited in playas were primarily a result of water action, not wind (Luo et al. 1999). Therefore, not only is precipitation on croplands causing substantial erosion, but irrigation of crops is as well. In 1995 an estimated 75.3 billion liters (19.9 bil. gal) per day was pumped from the Ogallala Aquifer. According to the Texas Water Resources Institute (2001), 96% was being used for irrigation of crops. Irrigation is far outpacing precipitation, indicating that irrigation agriculture is leading to a more rapid loss of volume in playas than dryland cultivation. It is likely the above-listed sedimentation rates have slowed in recent years given changes in irrigation practices and some soil conservation programs such as the Conservation Reserve Program (CRP) that result in less runoff. However, sediments are still accumulating faster than natural deepening rates (Luo et al. 1997). As certain irrigation wells are no longer economical to pump, or the wells go dry, playas in those areas will receive less sediment. However, as some areas lose irrigation, others are increasing. For example, since 1990, areas of southwestern Nebraska have increased cultivation and irrigation around playas, and associated sediment loads to those playas have likely increased. Of course, the central threat here to all playas is prairie cultivation, which, as noted in Chapter 1, has been severe (Samson and Knopf 1994). In the Plains, the processes inﬂuencing the upland cannot be isolated from the wetland. Moreover, as sediment loads continue to accrue in playas, there have been efforts to “reclassify” the soils in the basins. The U.S. Department of Agriculture’s Natural Resources Conservation Service in Texas has been reclassifying the soils in playas and naming some new soil series. Many of these new soil series are no longer classiﬁed as “hydric” thus resulting in a signiﬁcant “loss” in hydric soil acreage for the region (Haukos and Smith 2003). The true hydric soils (e.g., Randall clay) are now buried. These playas are now subject to less potential “jurisdictional” wetland regulation. The term jurisdictional here implies that the wetland is subject to governmental regulation. Criteria for jurisdictional classiﬁcation include the presence of hydrophytes, hydric soil, and a hydrology that allows these plants and soils to occur. Because of the turnover in plants in playas and the erratic hydroperiod in playas, these two factors may not always be present. Use of hydric soil classiﬁcation results in a more consistent wetland determination.

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Regardless, in most areas of the western Great Plains, government regulation of playas has seldom been considered, despite some USDA beneﬁts provided to landowners being tied to wetland protection (e.g., Swampbuster provisions) (Haukos and Smith 2003). Under these provisions, landowners are not supposed to receive government subsidy payments when jurisdictional wetlands are drained, ﬁlled, and so on. Two of the other physical changes to playas—pits and pumping— are often interrelated with irrigation practices. However, a landowner does not need a pit to pump water from a playa, and similarly not all pits are used for pumping. Pits help concentrate water (precipitation and irrigation tailwater) and make pumping more efﬁcient from the landowner’s perspective (Dvoracek 1981). Pits also can be used, especially in the Rainwater Basin, to lessen areal water coverage and increase tillable acreage. In the Southern Great Plains, most (95%) playas under 4 hectares (10 ac) have not been modiﬁed with pits, while 70% of those greater than 4 hectares have been (Guthery et al. 1981). With changes in irrigation from a predominance of row-water systems to a larger percentage of center-pivots and drip irrigation, the relevance of pits to a landowner’s irrigation system is changing in the region. That does not mean, however, that water is no longer pumped from a playa onto surrounding cropland; it simply suggests that pits are not used as frequently for catching irrigation tailwater because runoff from this source has declined greatly since 1980. The hydroperiod of the playa is altered and made more erratic by circulating irrigation water through the playa or pumping water out of it. This inﬂuences the entire natural ecological structure and function of the playa as noted in previous chapters. CHEMICAL THREATS

Playa contaminant studies have generally focused on pesticides (insecticides and herbicides) commonly used in production agriculture. Studies are usually of two types, those that have examined potential wildlife and associated habitat effects and those concerned with water quality or groundwater contamination from a human health perspective. More recently there have also been a few investigations into playa and groundwater contamination by bovine and human wastewater. The large concentrations of wildlife, especially migratory waterfowl, in playas prompted concerns in the 1980s that agricultural pesticides may be negatively affecting waterfowl populations. Organo-

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chlorine-based insecticides had been implicated in widespread declines in migratory bird populations throughout the United States, and thus organochlorines were the initial targets of study (e.g., Wallace 1984; White and Krynitsky 1986; Flickinger and Krynitsky 1987). Wallace (1984) examined organocholorine residues in wintering mallard, migrating blue-winged teal, and resident ring-necked pheasant in the Southern High Plains. He found residues of two organochlorine pesticides in mallard: p,p’-DDE and heptachlor epoxide. The heptachlor epoxide was found in 7.9% of the 89 mallards sampled, while DDE was found in 98.9%. The average DDE level was 0.62 parts per million, which he noted was relatively high. Wallace (1984) suggested that this high level was related to the intensive agriculture in the Southern High Plains. Adult females had the highest residues compared to juvenile females and males. He felt that adult females may have had higher levels of DDE as a result of a diet containing more invertebrates (which would likely have higher concentrations of DDE than plant material due to bioaccumulation) than the other age/sex classes. He also thought these concentrations could be due to the simple fact of adult females being older and having a longer exposure time to the contaminants than the other age/sex groups. Wallace (1984) also collected blue-winged teal in spring and autumn. Teal contained more types of organochlorine pesticide residues than wintering mallards: endrin, heptachlor epoxide, o, p’-DDT, o,p’DDD, p,p’-DDT, and p,p’-DDE. Heptachlor epoxide and o,p’-DDD were found in spring migrating teal but not in birds collected in the fall. Teal collected in the spring had higher levels of p,p’-DDE residues than birds migrating in autumn. This likely indicates that bluewinged teal were picking up much of these organochlorine pesticides in wintering areas of South and Central America. Wallace (1984) further examined organochlorine residues in resident ring-necked pheasant eggs. He found p,p’-DDE and heptachlor epoxide in the pheasant eggs as in wintering mallards; but heptachlor epoxide was found in only one egg, while DDE was in 96.7% of 61 eggs tested. DDE residues were higher in eggs collected from oat ﬁelds compared to wheat or alfalfa ﬁelds. He also found that eggs with the highest DDE residues had thinner eggshells than those with the lower residues. Eggshell thinning from these pesticides has been associated with population declines in many species of birds. Flickinger and Krynitsky (1987) examined organochlorine residues in wintering American wigeon, green-winged teal, northern pintail,

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and mallard on the Llano of eastern New Mexico and western Texas. From 12 playas, where they collected ducks for organochlorine residues, they also examined sediments. Six of the playas were associated with cattle feedlots. Residue analyses tested for endrin, toxaphene, dieldrin, heptachlor epoxide, p,p’-DDE, DDD, DDT, PCBs (polychlorinated biphenyls), and chlordane isomers. Two playas associated with feedlots contained organochlorine residues; each had low levels of chlordane isomers (0.05 – 0.22 ppm). Of the 48 individuals collected across four species, 46% did not contain detectable amounts of organochlorine residues. American wigeon did not contain any detectable organochlorine residues possibly due to their diet being dominated more by plant matter. Residue levels of DDE ranging from 0.09 to 1.2 parts per million were found in 44% of the birds. Heptachlor epoxide was detected in 31% of the ducks sampled. Because of elevated levels of heptachlor epoxide residue in the brain tissue of some northern pintail and green-winged teal, Flickinger and Krynitsky (1987) concluded that the birds may have been exposed to contamination. During the time of the study in the early 1980s, Flickinger and Krynitsky (1987) also had 23 adult waterfowl that they had found dead tested for avian diseases. The same four species were represented. They found avian cholera in 61% of the dead birds, with the highest prevalence in American wigeon (100%). American wigeon also had no detectable levels of organochlorines, so the authors concluded that the immediate mortality they observed in these waterfowl species over three winters was largely the result of disease. Migratory birds, however, have been killed by pesticides in the Southern High Plains. Organophosphate insecticides, parathion and methyl parathion, applied to agricultural ﬁelds were implicated in the deaths of more than 1,600 waterfowl (White et al. 1982). Few other studies involving vertebrates and contamination exist for Great Plains playas. In 1994, Kindscher (1994) collected water from four playas in Meade County, Kansas, to determine if pesticides were affecting plant distribution in those playas. He tested for the crop pesticides atrazine, terbuthyl, metribuzin, alachlor, and metolachlor. Kindscher (1994) found signiﬁcant amounts of three herbicides—metolachlor, atrazine, and terbuthyl—in the playa water but did not make ﬁrm conclusions about their inﬂuence on the playa plant communities. It is probable these herbicides were negatively inﬂuencing native ﬂora. Herbicides are obviously used much more frequently on croplands

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than grasslands. However, oftentimes herbicides are used on woody plants, or brush, encroaching on grasslands. Control of these woody plants is generally used to increase grass forage production for livestock. One of the more common of these herbicides used in the Southern High Plains sandy soils is the substituted urea herbicide, tebuthiuron. Price et al. (1989) examined whether runoff from this herbicide inﬂuenced common green algae found in playas. The halflife of tebuthiuron (which is activated by precipitation) is 12 to 15 months in areas of relatively high rainfall—102 centimeters (40 in.) to 152 centimeters (60 in.)—but is longer in areas with much less precipitation such as the Southern High Plains. Further, adsorption of tebuthiuron to soil particles increases as clay content increases. Recall that the hydric soils in playas are dominated by clays. Therefore, in most playas the half-life of tebuthiuron is going to be greater than 15 months. Price et al. (1989) examined two treatments on 11 algae species. The ﬁrst was designed to simulate early summer runoff when algae are ﬁrst becoming established. The second treatment simulated runoff later in the year after communities were established. The tebuthiuron runoff rate was based on actual runoff values determined by the manufacturer in ﬁeld studies. The ﬁrst treatment caused decreases in photosynthetic pigment, total algae cell counts, and packed cell volumes. This reduction in green algae productivity was accompanied by decreased water quality (indicated using dissolved oxygen, alkalinity, ammonia, nitrate, nitrite, orthophosphates, chloride). The presence of healthy algae was having a positive inﬂuence on water quality. The second treatment, which was applied after algae communities were well established, did not have any measurable impact on algae productivity. Likewise, there was little inﬂuence on water quality variables. To have minimal effect on playa green algaes, Price et al. (1989) recommended that if tebuthiuron was to be applied near playa watersheds, it should be done late in summer when algal communities were more resistant to the herbicide. Irwin et al. (1996) studied a broad array of contaminants in playas and salt lakes on the Llano Estacado of Texas from 1989 to 1992. Sediment, invertebrate, and wetland plant species were collected from playas associated with row-crop agriculture, cattle feedlots (ﬁg. 7.2), relatively undisturbed grassland, and municipal efﬂuent. Wetland plants were tested for carbonates, organophosphates, and metalloids (23 metals), while invertebrates were analyzed for those contami-

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Figure 7.2 Playas associated with cattle feedlots receive signiﬁcant fecal runoff resulting in high nutrient levels. Because of added water and elevated nutrients, these playas often stay ice-free in winter, attracting waterfowl. (Photo by W. Meinzer, courtesy of U.S. Fish and Wildlife Service.)

nants plus polycyclic aromatic hydrocarbons (PAHs). Sediments were tested for those four contaminant groups and organochlorine pesticides including PCBs, aliphatic hydrocarbons, and chlorophenoxy acid herbicides. Phosphorus and nitrogen compounds typically considered as nutrients for plant growth were also examined. Irwin et al. (1996) did not detect organophosphate or carbonate compounds in sediment, invertebrate, or wetland plant samples. DDE was the only organochlorine compound found in playa sediments, and sediments did not have detectable amounts of PCBs or chlorophenoxy acid herbicides. Low levels of PAHs were detected in playa sediments; the same was true for aliphatic hydrocarbons. However, oil and grease concentrations were high in feedlot sediment. Three metalloids—antimony, thallium, and tin—were not detectable above laboratory-designated levels, while aluminum, cadmium, and magnesium were not detected at levels that would indicate environmental concern. Arsenic was a trace metal of concern as high levels were detected in sediment, invertebrates, and wetland vegetation. Irwin et al. (1996) attributed high levels of arsenic to use of herbicides in cotton production. High levels of chromium were also found in sediment, plant, and invertebrate samples.

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As one might expect, ammonia nitrogen was highest in cattle feedlot and municipal efﬂuent playas when compared to the other playa types studied. Irwin et al. noted, “Ammonium could be a contributing factor of reduced aquatic diversity in cattle feedlot playas. Invertebrate diversity was lowest at cattle feedlot playas” (1996, 21). Chemical Oxygen Demand, an indicator of organic pollution, was especially in high playas receiving cattle feedlot and municipal efﬂuents. Anaerobic conditions are, therefore, commonly found in playas affected by human and animal wastes. Similarly these types of playas had extremely high levels of organic nitrogen and phosphates. In playas not inﬂuenced by feedlots or human efﬂuents, high winds, wetland plants, and the vertical mixing that accompanies these conditions prevent extended periods of anaerobic conditions. Thus, from metals and organic pollutant data Irwin et al. (1996, 41) concluded that feedlot contamination of playas clearly impaired surface water quality and reduced the diversity and abundance of plant and invertebrate populations. They further noted that water quality and wildlife habitat needed protection from cattle feedlot and municipal efﬂuent pollution. Playas often have human and livestock waste facilities associated with them because playas are convenient disposal areas (Sweeten 1994). Moreover, the density of U.S. feedlots is highest in the High Plains of Texas and in the Rainwater Basin region of Nebraska, making playas particularly susceptible to this pollution. In late July 1997, Thurman et al. (2000) collected water from 32 playas receiving runoff from cotton and corn ﬁelds in the central Southern High Plains. Herbicides from these two crops were found in 97% of the sampled playas. Diuron, ﬂuometuron, metolachlor, norﬂurazon, and prometryn were the major cotton herbicides detected, while the corn herbicides atrazine and propazine were also commonly found. At present the inﬂuence of these numerous herbicides on playa structure and function is unknown, but atrazine has recently been implicated as a possible agent in global amphibian declines (Hayes et al. 2002). Thurman et al. (2000) detected organophosphate insecticide in one of the sampled playas. The knowledge that playas were areas of focused recharge for at least portions of the Ogallala Aquifer also created concerns that nutrients and contaminants that collected in playas from agricultural and municipal activities might contaminate the aquifer. Generally the studies focused either on agricultural chemical contamination or nutrient contamination from feedlots. Zartman et al. noted that

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inﬁltration rates via the macropores in playas were fast and stated, “This rapid ﬂow presents environmental concerns as it provides the potential of movement from the soil surface to underlying groundwater for agricultural chemicals” (1996, 31). In the Central High Plains, that area north of the Canadian River in Texas and New Mexico including the Playa Lakes Region of Oklahoma, Colorado, and Kansas, the effects of irrigated agriculture on water quality in the Ogallala Aquifer were investigated by the U.S. Geological Survey (USGS) (McMahon 2000). As part of the National Water Quality Assessment Program, the objective of the study was to determine if pesticides and fertilizers applied on irrigated ﬁelds reached the aquifer. If these compounds remained within, or above, the root zone, there would be little need for concern. The USGS drilled ﬁve monitoring wells adjacent to irrigated ﬁelds from which they were able to collect water samples. Initially the survey examined tritium, a radioactive isotope of hydrogen that is present in water molecules. It can be used to age the water present in the aquifer because, as a result of nuclear weapons testing, tritium content in precipitation is much higher today than prior to World War II. Results from four of the ﬁve wells indicated recharge had occurred since 1950. Those same four wells that showed the relatively recent recharge also contained pesticides or their degradation products. The water samples were tested for the presence of 53 pesticide compounds including the commonly applied alachlor, atrazine, metolachlor, and simazine. One of the ﬁve wells contained atrazine at levels that exceeded drinking water standards, and although the monitoring wells were not human water supply wells, aquifer water in the region is important as a source for public and domestic water. Nitrate concentrations, generally an indicator of fertilizer use, were all above background levels. Two of the wells had nitrate levels that substantially exceeded drinking water standards (McMahon 2000). In a more recent USGS summary for the entire High Plains from Wyoming to Texas, Litke (2001) found that nitrate levels were higher than drinking water standards in 16% of the samples. Obviously water applied to croplands and/or runoff from precipitation is carrying some contaminants and reaching the aquifer with some compounds that may cause human health concerns. Whether nutrients associated with cattle feedlot playas enter the groundwater has also been a subject of concern (McReynolds 1994). Irwin et al. (1996) and Sweeten (1990) demonstrated that runoff from

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cattle feedlots is high in nutrients, salts, and pathogens. In a Texas Agricultural Experiment Station study, Sweeten (1994) noted that the wastewater and manure associated with cattle feedlots helped “seal” the playa basin because bacteria and ﬁne organic matter clogged soil “pore spaces.” He felt this would limit the recharge of the contaminants into the groundwater. From early soil core studies in one playa, Lehman et al. (1970) and Clark (1975) found little movement of nitrate or chloride through the playa soils. At three feedlot playas, Stewart et al. (1994) examined soil cores beneath the playas for nitrogen, phosphorus, and salts. They concluded that there was little inﬂuence of these compounds on the subsoil. Sweeten et al. (1990) also sampled groundwater near 26 feedlots on the Llano of West Texas. They suggested that nitrate and chloride levels in the Ogallala were not inﬂuenced by the cattle feedlots. This Experiment Station information appears to conﬂict with the USGS (Litke 2001) data showing elevated nitrates and salts in High Plains groundwater. However, the latter study was general in scope and did not identify point sources of the aquifer pollution. Further, not all aquifer contamination via playas has been agricultural in origin. Military and municipal pollutants have been identiﬁed in the vicinity of Lubbock, Texas. Certainly more studies are needed on point sources of aquifer pollution. Studies on the effects on playa function, however, are unequivocal. Nutrients and other substances reaching playas from sources such as feedlot and waste treatment operations are impairing proper ecosystem function. CLIM ATE CHANGE

As shown, for the most part, the gross inﬂuences of most physical and chemical factors, such as sediment or nutrient additions, on playa structure and function are relatively known. The changes currently being seen in the climate of the region and their inﬂuence on playa ecosystems are less tangible. Certainly, there is general consensus among scientists that there has been a humancaused increase in the global temperature largely through release of greenhouse gasses (e.g., Melillo 1999), but how that relates to local precipitation and temperature patterns in the Great Plains, particularly in the western reaches, is just now being proposed (e.g., Covich et al. 1997; NRC 2001a). Covich et al. (1997) were some of the ﬁrst scientists to examine the potential of climate change on wetlands of the Great Plains. Their

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High Plains designation most closely covers the region today that contains most of the Great Plains playas. Unfortunately most of their discussion for this region necessarily only brushes playa systems because of the lack of climate change studies in the region. Much more has been accomplished in the Northern Great Plains, and their glacial prairie potholes, using models that simulated changes in temperature and precipitation as a result of increased greenhouse gas concentrations (Poiani and Johnson 1991, 1993; Larson 1995; Poiani et al. 1995, 1996). In a nutshell, these projections generally show a decline in water levels and wet basins as a result of temperature increases even when accompanied by moderate increases in precipitation. Evaporation outpaced precipitation. Obviously these hydroperiod changes would have signiﬁcant negative effects on the native ﬂora and fauna inhabiting the prairie potholes. Some of the most recent climate change predictions indicate that the Great Plains will be particularly affected by global warming (NRC 2001a). Models examining semiarid regions such as the western Great Plains indicate an increased propensity for drought. Clearly this would result in decreased hydroperiods for playas. Some information exists as to how this decrease might inﬂuence ﬂora, but few predictions exist for faunal species that require speciﬁc hydroperiods to complete life history events (e.g., invertebrates, amphibians), although it is unlikely they would be positive. Alward et al. (1999) examined the potential inﬂuence of global warming on the vegetation of the shortgrass prairie, where most playas occur. They noted that global minimum temperatures were increasing at twice the rate that maximum temperatures were. Therefore, they examined the effects of increasing minimum temperature on one of the dominant shortgrass prairie grasses, buffalo grass (a perennial), a species that also occurs in playas when basins are dry. Alward et al. (1999) found that increased minimum temperatures were associated with declines in buffalo grass production and an increase in the density of exotic and native forbs. Alward et al. concluded, “Reductions in buffalo grass may make their system more vulnerable to invasion by exotic species and less tolerant of drought and grazing” (1999, 229). Recently, Fitter and Fitter (2002) found that British plants in the 1990s had started ﬂowering earlier in the year—a signal of climate change. Annuals ﬂowered earlier than perennials. If a similar situation occurred in the Great Plains, it would further alter plant community structure.

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BIOTIC THREATS

After outright habitat destruction, bioinvasion by exotics is typically listed as the next biggest proximate threat to biodiversity and ecosystem structure and function (e.g., Wilson 2002, 50). As noted in Chapter 3 on ﬂora, the presence of exotic plant species is widespread throughout Great Plains playas. This bioinvasion has been associated with disruption of hydroperiods and cultivation of the watersheds (Smith and Haukos 2002). It is likely that the effects of exotics extend beyond immediate diversity issues to further inﬂuence playa hydrology and nutrient cycling. For example, many playas in the Rainwater Basin are now monocultures of the invasive reed canarygrass. This species can completely dominate the ﬂora of shallow playas, effectively excluding native species of varying stature. Its presence then changes natural evapotranspiration patterns altering hydrology while the change in biomass, and the frequency at which it enters the detrital food web, alters natural nutrient cycling. The dense growth habit of reed canarygrass also excludes many wetland-dependent birds. Similar examples of ecosystem-altering plant species exist throughout the Great Plains playas, and others are on the horizon. For example, salt cedar and purple loosestrife (Lythrum salicaria), which are highly invasive, are now found in certain playas. Alternatively, some exotics even might be considered to provide short-term beneﬁts. The increased prevalence of the exotic barnyard grass (Smith and Haukos 2002) might allow a greater abundance of some wetlanddependent birds due to the large seed crops it provides. However, the ecological costs of this species to the entire natural playa ecosystem are unknown. The insidious effects of these exotics on playa structure and function are not entirely clear, but it is safe to say exotics are a continuing biological threat and inﬂuence not only other ﬂoral species but also, as noted, their dependent fauna. Less is known about the inﬂuence of exotic plants on playa fauna, but, as an example, Whitcomb et al. (1988) found that dominant prairie grasses and forbs had speciﬁc assemblages of leafhoppers (Cicadellidae). This disruptive change in the ﬂora affects the invertebrates and ripples through the entire ecosystem. Finally, little is known about faunal bioinvasions of playas, beyond some bird species. This is likely a reﬂection of research effort. Between global warming and exotic invasions associated with cropland agriculture, the future does not appear too bright

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for the native ﬂora of playas and their watersheds. Many native faunal species that coevolved with the ﬂora and speciﬁc hydroperiod requirements are therefore also in jeopardy. Chemical and physical inﬂuences on playas also may be associated with an increase in frequency of diseases affecting native ﬂora and fauna. For example, the ﬁrst reported cases of avian cholera in the United States were found in the Playa Lakes Region in the 1940s (Quortrup et al. 1946). It appears to be an exotic disease in this habitat. Other “new” diseases are also being found in wildlife associated with playas. Relatively recently, following cultivation of a new crop (peanuts) in the 1980s, sandhill cranes that were using playas began dying. The cause of death was consumption of a mycotoxin growing on peanuts (Windingstad et al. 1989). The peanuts were consumed by the cranes feeding on ﬁelds adjacent to playas. Friend et al. cautioned that “combating emerging disease has now become one of the adjustments that must be made to repair ecological integrity in a manner that sustains avian biodiversity and desired levels of avian populations” (2001, 298). These new emerging diseases are often seen in birds because much focus is placed on this faunal group. Certainly it is occurring in other playa biota, but it has only recently been documented. Any environmental change, or accidental exotic introduction, might allow new diseases to become a serious biotic threat (Rosenzweig 2001). Next, the records of potential conservation programs that aim to protect playas, and thus their structure and function, will be examined, as well as some new ideas for playa conservation.

CHAPTER 8

CONSERVATION PAST, PRESENT, AND FUTURE

W

ith all of the problems facing wetlands in the Great Plains, there is obviously plenty to do. Initially, the discussion here will focus on planning programs targeted at playas, and those with a regional versus a global scope. Planning programs are agency directives, initiatives, or legislation that can be used to conserve playa wetlands and their associated habitats. Then speciﬁc strategies and practices to carry out programs or directives will be suggested. Finally, views on how efforts can be refocused to allow meaningful conservation of playas will be proposed. PROGR AMS AND ENFORCEMENT

Probably the largest planning efforts regarding regional conservation of playa wetlands today are associated with the North American Waterfowl Management Plan. The North American Plan, as it is sometimes called, was initiated in 1986 over concerns of continentally declining wetlands and associated waterfowl populations (Sparrowe et al. 1989). The initial agreement was between Canada and the United States. Mexico ofﬁcially joined the effort some years later. Although originally focused on waterfowl, it has broadened in words, to include all wetland-dependent wildlife. In reality, though, seldom are wildlife groups, other than birds, considered in these efforts. In other words, if mammals or amphibians, for example, are helped as a result of the Plan’s programs, that is ﬁne, but effort is generally not directed at these taxa. Today the focus remains on wetland birds, although it has expanded to embrace other migratory bird conservation initiatives that include upland habitats within the regions targeted by the North American Plan. The North American Plan originally focused on speciﬁc geographic regions throughout North America in an attempt to reverse the declining waterfowl and wetland trends that were apparent in the

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early 1980s. These regions were thought to be critical to maintenance of continental waterfowl populations. Initially, in 1986, just a few of these regions were targeted and called “joint ventures.” Regional joint venture groups are made up of state and federal agencies, nongovernmental organizations, corporations, and private individuals striving to conserve wetlands and their associated wildlife. It is assumed that from all of these groups partnering together more conservation will occur than when all are operating separately. Both of the joint ventures that encompass a majority of playa wetlands, the Playa Lakes Joint Venture (PLJV) and the Rainwater Basin Joint Venture (RWBJV), started after the initial 1986 Plan was signed, and today most of the major wetland regions in North America are encompassed by some joint venture. Many of the regions targeted by the North American Plan had somewhat of an infrastructure in place prior to 1986. That is, many of the aforementioned groups already cooperated to complete wetland-based projects. For example, the state wildlife agencies in the Prairie Potholes Region worked closely with the U.S. Fish and Wildlife Service and nongovernmental agencies to protect wetlands critical for breeding waterfowl. However, the North American Plan made the arrangement more formal, included other groups, and targeted additional federal dollars to the effort. Prior to the joint ventures, there was little protection of playas in the Southern Great Plains but there was some playa wetland conservation occurring in the Rainwater Basin. For all practical purposes the only playas in the Southern Great Plains that were protected, albeit de facto, were those on national grasslands. Even these, however, were subject to modiﬁcation as most national grassland playas had pits excavated in them. Ironically most of this was done in the name of conservation. Pits purportedly helped waterfowl, but little consideration was given to other biotic components of playas or the pits’ inﬂuence on proper playa function. PLAYA LAKES JOINT VENTURE

One advantage that the Playa Lakes Region had in initially proposing a new joint venture to the North American Plan was the existence of an interagency cooperative group termed the Interagency Playa Lakes Disease Council, which was started in 1983. It was originally formed to address concerns about large waterfowl dieoffs. The dieoffs were associated with diseases, primarily avian cholera and avian botulism. Initially the council was made up of state

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wildlife agencies from Texas, New Mexico, and Oklahoma, the nongovernmental organization Texas Waterfowlers’ Association, and federal representation from the U.S. Fish and Wildlife Service at the regional level. The National Wildlife Disease Laboratory in Madison, Wisconsin, and Texas Tech University, given the latter’s long history of research on playas, were also partners in the group. In 1988 the above-listed entities, along with state agencies in Colorado and Kansas, proposed the formation of the PLJV (USFWS 1988). Other partners currently in the joint venture include The Nature Conservancy, USDA’s Natural Resources Conservation Service, Ducks Unlimited, Phillips Petroleum, Pheasants Forever, and the U.S. Forest Service. The area that was originally encompassed in the PLJV included those counties in the ﬁve states thought to contain playa wetlands (e.g., ﬁg. 3.1). It roughly followed the High Plains portion of the Southern Great Plains (see Chapter 1). However, today the boundaries of the PLJV have been expanded outside of that original region to capture other areas of playas and include areas that do not contain playas but that do have other important wetland resources. As laid out in the latest approved (as of this writing) Implementation Plan (PLJV 1994, 2 –3), the following are the goals and objectives of the Playa Lakes Joint Venture: The goal of the PLJV is successful accommodation of objective numbers of waterfowl, migratory birds, and other wildlife, wintering in, migrating through, and breeding in the PLR [Playa Lakes Region]. There are ﬁve general objectives in the PLJV. Objective A is no loss or further degradation of playa wetlands, saline lakes, reservoirs, tanks, riparian areas, or other wetlands in the PLR. Objective B is to have sufﬁcient high-quality wetland habitat to permit wide-spread dispersion of waterfowl within the PLR. Objective C is to have sufﬁcient seasonal food resources for waterfowl and other wetland-dependent wildlife populations in the PLR. Objective D is to have healthy and secure wetland and upland habitats to ensure optimum survival and diversity of waterfowl and other wildlife in the PLR. Objective E is to maintain successful reproduction of waterfowl and other wildlife breeding in the PLR.

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Speciﬁc habitat objectives are: —Protection of valuable historical migratory bird use areas. —Protection and enhancement of wetland areas that are adequately distributed throughout the PLR to attempt to prevent unhealthy concentrations of waterfowl. —Direct conservation of 10% of playas and associated uplands with joint-venture projects by combining acquisition, enhancement, and restoration efforts. —Indirect conservation of 10% of playas and associated uplands with extension efforts, demonstration areas, and technical guidance. —Protection and enhancement of important riparian habitats in the PLR. — Conservation of at least 10,000 acres of other wetlands and their associated habitats. Speciﬁc population objectives are: —Sufﬁcient habitat to maintain an overwintering population index of at least 3 million ducks and 500,000 geese as determined by cooperative winter inventories. —Sufﬁcient habitat to overwinter 350,000 – 400,000 sandhill cranes. —Sufﬁcient seasonal habitat during fall and spring to accommodate migrating waterfowl and other birds in the PLR. —Nesting and brooding habitat for a minimum of 10,000 breeding pairs of ducks.

Most of the goals and objectives of the joint venture were waterfowl oriented and aimed at wintering birds. Waterfowl production, which at times can be important, is less of a focus for this region. The overall goal was population oriented, while the major objectives were habitat oriented following the well-founded assumption that if the habitat is protected, the bird populations will generally also be protected. By examining progress toward meeting these objectives, it is apparent that Objective A has not been met. For example, loss and degradation of playas, saline lakes, and riparian habitat has continued, although data to track the changes are difﬁcult to obtain. Playas continue to be degraded through sedimentation, and riparian areas

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are further choked with exotic woody plants while their ﬂows have not improved. It is also unlikely Objectives B, C, and D have been successfully met because little protection or active playa management has occurred. Anything to improve negative effects on playa hydroperiod would help meet these objectives. Active management of playas would also help meet these objectives. Targets in Objective E, however, should be met because the speciﬁc population goal of 10,000 breeding pairs is conservative and should occur with no conservation effort. The speciﬁc habitat goals of the 1994 Implementation Plan are more laudable, but progress has still been minimal. Few to none of the historically important migratory bird use areas have been protected, and probably less than 1% of playas and associated uplands have been “directly” or “indirectly” conserved. For example, only two playas in Texas have been protected by purchase or easement through joint venture efforts (W. Johnson, personal communication, Texas Parks and Wildlife Department). Other playa projects that involved short-term (i.e., 10 years) projects such as fencing or cover establishment have largely expired as of this writing. Ironically, there has been more protection of “other” wetlands than playas in the region encompassed by the PLJV. The speciﬁc population goals for sandhill cranes and the various species of geese are probably currently attainable since these groups of birds generally have had high numbers wintering in the Southern High Plains prior to the formation of the joint venture and they feed extensively in agricultural ﬁelds that continue to occur throughout the region. Goals for ducks are more problematic. Precipitation and hydroperiod play a large role in “maintaining sufﬁcient habitat” for 3 million birds. Again restoring and preserving the historic hydroperiod in playas would go a long way toward meeting these goals. The lack of progress on speciﬁc 1994 objectives does not mean that it will not occur in the future. The PLJV is relatively young, and other efforts in the region through the joint venture may pay dividends. However, in the latest draft of the Implementation Plan (PLJV 2002, 2) there were no habitat or population objectives. Ironically, one of the goals is now to develop population and habitat goals. There is a strong education committee that has provided educational materials and presentations for schoolchildren and adults. Education may provide the spark needed to meet speciﬁc habitat objectives by getting individual citizens involved in playa conservation

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Figure 8.1 Education programs for schoolchildren should help playa conservation in the future. (Photo courtesy of J. Haukos.)

(ﬁg. 8.1). Indeed, education has been key in turning around the public’s perception of the importance of wetlands throughout the United States (NRC 1995). RAINWATER BASIN JOINT VENTURE

Unlike the PLJV, or any other joint venture, the Rainwater Basin Joint Venture (RWBJV) is contained within one state, Nebraska. The playa wetlands in this joint venture all occur within a 17-county area in the south-central portion of the state (see Chapter 1). Although other playas and wetland types occur adjacent to this region, the RWBJV, unlike the PLJV, has thus far resisted expanding its borders. The positive aspect of this approach is that it allows a strong focused effort on a relatively small area. Detractors, however, feel this view limits conservation efforts on other important wetlands in the vicinity.

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Conservation groups within the Rainwater Basin applied to become a joint venture in 1990 on the basis of the region’s importance to waterfowl in spring and the drastic loss of wetlands that had occurred there (Gersib et al. 1990a). The joint venture was approved shortly thereafter, and an implementation plan was put into place in 1992 (Gersib et al. 1992). In addition to the state’s Game and Parks Commission, the National Audubon Society, the U.S. Fish and Wildlife Service, The Nature Conservancy, Ducks Unlimited, the U.S. Natural Resources Conservation Service, U.S. Army Corps of Engineers, Pheasants Forever, and the U.S. Bureau of Reclamation are major partners. Numerous other local conservation clubs, landowners, and local governments also participate. The objectives as laid out in their 1992 Implementation Plan are very speciﬁc (Gersib et al. 1992, 3 –5): Goal: Restore and maintain sufﬁcient wetland habitat in the Rainwater Basin area of Nebraska to assist in meeting population objectives identiﬁed in the North American Waterfowl Management Plan. Objective 1: Protect, restore, and create an additional 25,000 wetland acres, plus 25,000 acres of adjacent upland habitat. Strategy 1—Protect 10,000 acres of existing wetlands, plus associated upland. Strategy 1A—Protect 5,000 acres of wetland habitat by implementing a cooperative Private Lands Program. Strategy 1B—Acquire 5,000 wetland acres from willing sellers by fee title or perpetual easement. Strategy 2—Restore and protect 12,000 acres of degraded or destroyed wetlands, plus associated upland. Strategy 2A—Restore and protect 6,000 acres of degraded or destroyed wetlands through a cooperative Private Lands Program. Strategy 2B—Restore and protect 6,000 acres of degraded or destroyed wetlands by fee title acquisition or perpetual easement on a willing seller basis. Strategy 3— Create and protect 3,000 acres of new wetlands, plus associated upland. Strategy 3A— Create and protect 1,500 acres of new wetlands on private land.

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Strategy 3B— Create and protect 1,500 acres of new wetlands on public land. Objective 2: Provide reliable water sources for a minimum of 1⁄ 3 of all protected wetland acres to assure sufﬁcient water quantity, quality, and distribution to meet migratory waterfowl and water bird needs. Strategy 1—Establish a Water Management Work Group to coordinate with Natural Resource Districts (NRD), Nebraska Department of Water Resources, Nebraska Natural Resources Commission, local irrigation districts and others to identify acceptable, quality supplemental water sources for RWB wetlands. Strategy 2—Assess and prioritize protected wetlands to determine which warrant supplemental water sources. Sites should: a) be cost effective and publicly acceptable, b) aid in distributing waterfowl throughout the RWB area, c) diversify the wetland types available for water bird use, and d) involve private landowner participation when available. Strategy 3—Develop an annual RWB water management program that addresses the estimated quantity of water needed annually, the timing of water delivery and distribution needs. Objective 3: Develop and implement wetland enhancement strategies to optimize those values wetlands provide to waterfowl, endangered species and other water birds. Strategy 1—Identify acceptable wetland management options and programs that assist landowners in managing wetlands on private land. Strategy 2—Use the Public Lands Work Group to identify RWB wetland management techniques and best management practices to manage wetlands on public land. Comprehensive Strategies. The following strategies apply to all objectives:

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Comprehensive Strategy 1—Develop a broad base of support and cooperation among local, regional and national interests. Comprehensive Strategy 2—Support legislative programs that assist in meeting the RWBJV and NAWMP goals. Comprehensive Strategy 3—Develop funding sources to: a) meet the estimated $3 million average annual cost of RWB habitat protection, restoration and creation, b) provide supplemental water sources necessary to ensure that 1⁄ 3 of all protected wetlands have water during migration periods, c) operate and maintain publicly owned or managed RWB wetlands. Comprehensive Strategy 4— Conduct research to ﬁll existing wetland/water bird data gaps, increase understanding of RWB wetland values and optimize protection and enhancement activities. Comprehensive Strategy 5—Adapt the North American Waterfowl Management Plan Evaluation Strategy to assess accomplishments of all phases of wetland protection, restoration, creation and enhancement in the RWB area.

The focus of the Rainwater Basin Joint Venture has been primarily on migrating waterfowl during spring. The large numbers of waterfowl, especially white-fronted geese and northern pintails that use this area during spring migration are the primary reason for the RWBJV’s focus on this season. Because of its more northerly location, the Rainwater Basin has few wintering birds. Further, the ephemeral nature of water in playas also restricts their abilities to produce large numbers of waterfowl. The RWBJV has made steady progress toward meeting acreage goals outlined in Objective 1. Indeed, on a percentage-of-acreage goal basis they have protected and restored more wetlands than many other joint ventures including the PLJV. As of 2001, the RWBJV had protected 5,741 hectares (14,187 ac) of playa and associated watershed habitat mainly through acquisition and perpetual easements. As seen in Objective 2, the RWBJV also relies on some artiﬁcial water management to provide habitat for migratory waterfowl needs during spring. The joint venture provides support and funds to pump

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groundwater into the basins during those spring migrations in which there is little available water in the region. Fluctuations in precipitation are thus ameliorated in this system, and natural processes that are inﬂuenced by hydroperiod are compromised. The RWBJV justiﬁes this management strategy with the knowledge that there are so few wetlands remaining in the Rainwater Basin and such large numbers of waterfowl dependent on them, that water pumping is necessary for the continued health of these populations. Progress on Objective 3 is more difﬁcult to judge. Identifying options and practices is obviously accomplished, but whether they are “best management practices” is less clear. Few evaluation/research projects have been conducted on practices in the Rainwater Basin playas and knowledge concerning their effectiveness is lacking (Smith 1998). Much progress has been made toward satisfying the Comprehensive Strategies, including the initiation in 2001 of research to ﬁll “data gaps.” OTHER PLAYA CONSERVATION PROGRAMS

Some of the governmental and nongovernmental programs discussed here are also conducted within cooperative projects of the joint ventures. The USDA has several programs that inﬂuence playas and their associated watersheds (upland). The previously mentioned Conservation Reserve Program (CRP) was initiated with the landmark Food Security Act in 1985 and continues today. It pays landowners annually to take highly erodible farmland out of crop production and place it into permanent cover for a 10-year period. In areas with playas, establishment of permanent cover has largely been accomplished through the planting of perennial grasses. Perennial grass cover can result in signiﬁcant protection of playa watersheds (and thus hydroperiod), but unfortunately most of the original cover established under CRP was exotic grasses. Some of these species are now invading pests. All too often exotics introduced in the name of conservation have become pests. Moreover, Licht (1997) suggested that simple land purchase would have been less expensive to the American taxpayer than making annual payments to landowners because CRP payments exceeded the lands’ appraised value. Certainly purchase is preferable to a 10-year contract when the goal is to preserve habitat. Since 1985, CRP has been modiﬁed and places greater emphasis on native grasses and forbs. Many feel CRP now has the potential to restore watersheds

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and, in general, native prairie throughout the Great Plains. However, given the problems with exotics and economics, the relative success of CRP in ecosystem restoration is thus far unknown. Direct conservation of playas can be undertaken through the Wetland Reserve Program (WRP). This is also part of the Food Security Act although the acreage available under WRP is much less than under CRP. Administered by the Natural Resources Conservation Service (NCRS), a wetland can be protected through an easement (land-deed covenant prohibiting wetland destruction or habitat degredation), which can be permanent, or for a ﬁxed-year term (e.g., 30 years). Cost-share wetland restoration projects are also available. The WRP generally provides longer-term protection than acreage enrolled in CRP. Unfortunately, few playas have been enrolled in WRP and only three landowners have ﬁled an intent for playa consideration in WRP (Haukos and Smith 2003) in the Southern High Plains, where most playas occur. Although WRP has been promoted for playa wetlands in the Rainwater Basin, efforts in the western High Plains have been, in general, weak. Some Natural Resources Conservation Service personnel have not promoted the possibility of WRP to landowners (Haukos and Smith 2003). Given the easement aspect of this program, however, the potential of WRP is substantial in the region and should be encouraged. Two other programs initiated through the Department of Agriculture’s NRCS, entitled the Wildlife Habitat Incentives Program (WHIP) and the Environmental Quality Incentive Program (EQIP), have been used with some success throughout the playa-plains region. In a cost-share (up to 75%) arrangement with the landowners, habitat practices that beneﬁt wildlife are carried out. Some of the projects that have beneﬁted playas and their associated wildlife include restoring the immediate watershed and fencing playas to prevent overgrazing by livestock (ﬁg. 8.2). Projects last from 3 to 10 years. Generally, this program has been promoted in local situations depending on the biologist present in the area. Again, this program should be promoted throughout the Great Plains. As of the summer of 2002 (this writing) it appears the potential of these programs will be greatly expanded in the new farm legislation. The United States Department of Interior, primarily through the Fish and Wildlife Service, also has several programs aimed at protecting playas and/or their associated wildlife. Some have resulted in land acquisition, as in the Rainwater Basin, or focused on short-term

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Figure 8.2 A native vegetation buffer strip being established in a playa watershed in an attempt to prevent further sedimentation and hydroperiod deterioration. (Photo by D. Haukos, courtesy of U.S. Fish and Wildlife Service.)

landowner projects, the primary method applied in the Southern Great Plains. The Waterfowl Production Area (WPA) program has been used in the Rainwater Basin region to protect playas. This program uses funds from federal duck stamps to purchase or provide long-term easements of wetlands. (These stamps are required by USFWS to hunt waterfowl and are sold by USPS to hunters and nonhunters; funds are used in habitat protection.) As its name suggests, the program is primarily for breeding waterfowl. Although production is minimal in playas, the program protects signiﬁcant wetland habitat. Unfortunately, in the Great Plains this program is not available south of Nebraska. Recently Haukos and Smith (2003) recommended establishment of Wetland Management Districts by the U.S. Fish and Wildlife Service in the Southern Great Plains. Funds from purchase of migratory bird stamps could be used to purchase playas there. This would greatly increase the playa area currently protected. The Partners for Fish and Wildlife Program and the Partners in Flight Program funded through the Departments of Interior and Agriculture have the potential to provide cost-share or direct cost coverages to private landowners for habitat management or conservation

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strategies such as watershed restoration or protection. Relatively new initiatives championed by the U.S. Fish and Wildlife Service mainly through Partners for Fish and Wildlife are entitled the Great Plains and High Plains Partnerships (Clark 1996). They form partnerships similar to joint ventures. These programs have an ecosystem perspective that focuses on the entire grassland region, its attendant avian species, and all species of concern (i.e., those species whose populations may be endangered, threatened, or experiencing severe declines). As primary sites of biodiversity in the prairies, playas can receive attention under these plans. So far these programs have resulted in a few local cost-share projects on private lands, and the potential is there for many more. In addition to these federal-private lands programs, most if not all states that have playas have programs that provide ﬁnancial and/or consulting assistance to landowners wishing to manage wildlife on their lands. In Texas, for example, there is the Landowner Incentive Program (LIP), which ﬁnancially compensates landowners for carrying out habitat management and/or protection practices. These state programs have the potential to protect large areas of playa wetlands and associated uplands. Finally, there are two main nongovernmental organizations, The Nature Conservancy and Ducks Unlimited, that have shown a major interest in playa lakes. Most of their efforts have been conducted within the joint ventures. Relatively more of their resources have gone toward protection of playas in the Rainwater Basin region than in the western High Plains. The Nature Conservancy typically focuses on purchase of properties for outright protection or on longterm conservation easements where the land is protected in various fashions with land-deed covenants. In addition to purchase, Ducks Unlimited also partners with state and federal agencies to conduct habitat projects beneﬁting wetlands and their associated wildlife. For example, Ducks Unlimited is involved in projects aimed at restoring proper hydrologic function in playas on state and federal lands. ENFORCEMENT OF WETLAND REGULATIONS

Enforcement of existing natural resource laws is certainly one way of conserving and preserving playa ecosystems. However, though a number of state and federal regulations have been aimed at playa wetland protection (e.g., Cloud 1994; Coffman 1994; Hatcher 1994; LaGrange 1997), enforcement of these laws has been

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variable. Unfortunately, it is typically not until a wetland type becomes more rare and endangered that regulations are more vigorously enforced. Possibly that is why there has been more regulatory activity on playas in the Rainwater Basin region than in the High Plains to the west. One of the most notable federal wetland regulations is Section 404 of the Clean Water Act. Essentially, the U.S. Army Corps of Engineers is responsible for regulating the deposition of dredge or ﬁll material into “waters of the United States” (Hatcher 1994, 301). Until recently, wetlands including playas generally have been included as such waters and therefore subject to Section 404 regulations. Wetlands embedded in a prairie landscape are usually further termed “isolated waters” (Hatcher 1994, 301), with consequent legal ramiﬁcations that will be discussed later. On agricultural lands, where essentially all Great Plains playas occur, the NRCS delineates (i.e., identiﬁes and makes boundary determinations) wetlands. If dredge or ﬁll material is proposed to be placed in a wetland, a permit is required from the Corps of Engineers. If discharge is being directed to a jurisdictional wetland, the Environmental Protection Agency is involved with this act. These agencies usually consult with the U.S. Fish and Wildlife Service to determine if substantial negative impacts on wildlife will occur as a result of a proposed activity, but the U.S. Fish and Wildlife Service has no regulatory authority in the matter. If a permit for wetland alteration is authorized, mitigation may be required. The concern about legal replacement (i.e., mitigation) of wetlands lost to drainage or ﬁlling has resulted in a relatively new proposed wetland classiﬁcation system that considers a wetland’s function. It has been termed the Hydrogeomorphic approach, or HGM for short (Brinson 1993; Smith et al. 1995). Consider the following: If developers cause the loss of a wetland, they often have to legally mitigate the loss by providing other wetland habitat. Historically, seldom has wetland mitigation examined whether like function was being replaced (NRC 2001b). The HGM method holds promise in that regard. The HGM approach involves ﬁnding “reference” wetlands with as many functions remaining intact as possible to the wetland under consideration (Smith et al. 1995; RWBJV 2000). Then, if mitigation is required for a certain wetland, function of the reference wetland and lost wetland can be compared to determine mitigation. However, in both the Rainwater Basin and Prairie Potholes Regions, for example, selection of reference wetlands (i.e., those least altered)

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has been difﬁcult. The vast majority of all prairie wetlands have received some impairment to proper function. A recent National Research Council (2001b) report suggested that wetland mitigation has frequently failed because sadly the mitigated wetland bears little resemblance in terms of ecological function to the original. In addition, it is important to note that Section 404 generally has little inﬂuence on wetland drainage. Rather, in most playas, Section 404 would come into play when “ﬁll” is placed in a wetland. Although active in the Rainwater Basins (Raines et al. 1990), a more prominent presence of the Corps of Engineers and EPA in the High Plains might have a positive inﬂuence on materials placed in playas. A recent (January 2001) court ruling is causing confusion over the status of “isolated wetlands” and thus the Clean Water Act’s relevance to prairie wetlands (Haukos and Smith 2003). Previously, federal jurisdiction over isolated wetlands could be based on the “migratory bird” rule. That is, isolated wetlands could be considered jurisdictional if they were used by migratory birds. The Supreme Court ruled that this basis exceeded the intent of Congress when it passed the Clean Water Act. Now there is uncertainty as to how this ruling might affect regulation of isolated natural prairie wetlands. The Migratory Bird Treaty Act (MBTA) and the Endangered Species Act (ESA), promulgated by the U.S. Department of Interior, also can be used, at times, to protect playas through protection of wildlife (Cloud 1994). The MBTA prohibits the “take” of migratory birds, unless legislated through hunting seasons or other permitted actions. Enforcement of this act has been interpreted to include habitat modiﬁcations or pollution that results in the death of migratory birds. For example, if pollutants are dumped into a playa resulting in the death of migratory birds, it is a violation of the act. The ESA prohibits the “take” of federally designated endangered or threatened species. In addition, some states have their own version of this regulation with their local species of concern. “Take,” as it relates to playas, can be interpreted as the modiﬁcation of a playa habitat that could kill or harm a threatened or endangered species that might use that wetland. As part of the original 1985 Food Security Act, the USDA, through the NRCS, has some regulatory authority over the loss of wetlands or agricultural lands (Coffman 1994). Speciﬁcally the regulatory provision is termed “Swampbuster.” If an individual converts a wetland to a situation where an agricultural crop is or can be planted on the pre-

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vious site, that person can then lose eligibility for USDA beneﬁts. It is unlikely that any individual with a playa has lost beneﬁts from violating the Swampbuster provision (Haukos and Smith 2003). Most states with playas also have their own regulations and regulatory agencies responsible for water quality, both for above- and belowground supplies (e.g., Ambrose et al. 1994; LaGrange 1997). These state agencies typically are involved with groups that place waste disposal into wetlands. For example, playas used as human sewage treatment sites and livestock feedlot waste collection sites are often under the responsibility of a state agency, sometimes along with federal groups responsible for the Clean Water Act. Haukos and Smith (2003) provided details concerning regulations of states in the Southern Great Plains. Certainly, more regulatory effort should be focused on playas by these agencies. STR ATEGIES FOR REGIONAL PLAYA CONSERVATION

Discussion of conservation strategies will initially focus at the playa level and then expand out to the landscape level. The Playa Lakes Joint Venture has concentrated more on private lands programs than on direct protection of playas (e.g., purchase or easements). The logic behind this approach is that there are more than 25,000 playas in the region and 99% of them are on private lands. It was assumed that not enough playas could be purchased or placed in easement to make as much difference in the wetlandwildlife resource as working on short-term private lands projects. However, since 1990 the overall number of properly functioning playas has continued to decline. Most private landowner agreements help wildlife and their playa habitat, but only in the short term. Many land protection opportunities through easement, “willing seller,” and/or “government foreclosure” (i.e., nonpayment of federal farm loans) have been passed up. In the Southern High Plains fewer than 10 playas have been placed in perpetual easement after foreclosures, and their actual protection from erosion, grazing, and cultivation has been minimal. Many more playas could have been protected. Moreover, land purchases/easements are also not popular with some state and federal agencies because the agencies feel their limited personnel would suffer as a result of additional management of properties. This attitude needs to change. The number of properly functioning playas is declining and with them the opportunity for conservation.

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INITIAL CONSIDERATIONS AND ECOLOGICAL FUNCTION

With this view in mind, a hierarchical approach for conservation of playas and their associated biota is proposed. Because there are so few playas that have not been hydrologically altered, ﬁrst consideration for protection (purchase, easement) should be on the few remaining playas that have intact watersheds dominated by native prairie. The native intact watershed ensures a minimum of sediment deposition and thus a relatively stable hydroperiod. A watershed dominated by native plants is also typically associated with a playa that has fewer exotic plant species. Initially, pits also should be avoided because they promote exotics and alter the playa hydroperiod. There are a few of these “natural” playa groups remaining, but their future is by no means secure. Purchase of, or easements on, these areas should be given highest priority. The next consideration in protection of playas with native plant watersheds, if there is such an option, should be playa size. All other factors being equal, larger playas should be selected over smaller playas. As seen in the species-area comparisons, the larger the playa, the more native wetland species it should be able to protect. The third factor in protection consideration should be playa density. Plant diversity was a bit larger in playas that were closer together versus farther apart. Again, given the choice of playas with similar watersheds and sizes, conservation efforts should probably be focused in areas of relatively higher playa density. Playas clustered closer together would also make them easier to manage from a logistic standpoint than those existing in a more scattered fashion. Notice much of the logic applied above to make playa conservation suggestions is based on ﬂora studies. This assumes that what follows for the ﬂora follows for the fauna. It is likely a relatively safe assumption given results of previous biogeographic studies (Rosenzweig 1995; Wilson 1999) and the fact that much of the fauna is dependent on speciﬁc ﬂora. In sum, the most powerful inﬂuence on the biota of the playa, beyond climate, is the condition of its watershed. This inﬂuence is likely through the hydroperiod. Erosion of soils into, or circulating irrigation water through, the basin alters hydroperiod, in turn altering which species can survive there. It also permits a disturbance that allows nonnative species to colonize. Pits or trenches also alter hydroperiod. Next, after hydroperiod, in descending order of importance, ecosystem managers should then consider playa size (larger being better) and playa density.

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ALTERED PLAYAS AND RESTORATION

Because the availability of playas with native grassland watersheds over a range of sizes and densities is limited, the majority of conservation options that ecosystem managers have will generally be more restricted to some set of altered conditions. The availability of properties for preservation is dictated by many factors such as the number of willing sellers or people willing to allow easements. For example, most of the playas available for potential conservation projects will have limitations such as cultivated watersheds and pits. However, that does not mean biologists should reject potential projects on playas that might be compromised by cultivation, pit excavation, small size, and/or low density. Some conservation is better than no conservation. But the above hierarchical order of factors inﬂuencing playa function should still drive decision making on these other projects within areas considered for reserves. This more common situation of altered playas is the realm of Restoration Ecology. Given the amount of all types of habitat that has been destroyed, there are many opportunities for restoration. Indeed, these opportunities prompted E. O. Wilson to state, “The next century will, I believe, be the era of restoration in ecology” (1999, 340). The following is a hypothetical playa restoration situation. Suppose a state or federal agency or nongovernmental organization has thankfully purchased a section of land, as part of a larger planned reserve, that contains a playa. The manager has been given the task of restoring the function of the playa from an ecosystem perspective. Following the aforementioned hierarchical considerations, restoring the hydrology, and thus the hydroperiod, should be the ﬁrst priority in situations where managers are trying to return playas to their original ecological functions. Therefore, initially the condition of the watershed should be examined. If the watershed is in native prairie, then the next step can be considered; however, given the statistics, it is more likely the watershed is in cultivation. Purchasing, or otherwise preserving, a playa wetland without the ability to manage at least a portion of the entire watershed circumference is folly. Everything that can be accomplished through management in the basin itself can be negated by upland inﬂuences (ﬁg. 8.3). Indeed, the entire wetland may be lost to sedimentation. As much of the upland as possible should be restored. The larger the restored area around the playa, the more resistant the playa is likely to be in the face of bioinvasions by exotics and other outside

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Figure 8.3 A playa in Texas that has been “protected” through a conservation easement. However, note the large sediment deposit that occurred since the easement was signed. One portion of the watershed was not included in the easement, and sediment continued to enter the playa from adjacent property continuing to degrade the wetland. (Photos by J. Ray, courtesy of Texas Parks and Wildlife Department.)

inﬂuences. If this is not possible, then the manager must determine what size of watershed buffer would efﬁciently protect the playa from excessive sedimentation. Indeed, this situation occurs fairly frequently when biologists are working with private landowners interested in protecting wetlands but only a portion of the adjacent watershed. Two main consider-

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ations come into play with buffer width protection: slope and soil texture. The steeper the slope and the coarser the soil, the more soil that will erode into the playa and the more negatively the playa will be inﬂuenced by sedimentation. Steeper slopes and coarser soils likely require relatively wider buffer strips than low slopes and ﬁner soil textures. However, there have been no playa studies investigating proper buffer width size or native species composition mixes that would adequately control sedimentation, even without the considerations of soil type and slope. Therefore, playa biologists have had to use their best judgment. They have used strip widths ranging from 30 meters (100 ft) to 90 meters (300 ft) and a variety of grass mixtures. Biologists should consider the soil series in the area under management, as well as the grass species that originally occurred on those soils, when restoring watersheds. Oftentimes, however, managers have planted a native, or even exotic, that is taller or more robust than the original native community in an effort to provide dense nesting cover for waterfowl and other game birds, and/or to provide greater erosion control in a smaller area. If the objective is to restore natural ecological function, this strategy should be avoided. Therefore, over the geographic range of playas, the native grasses of playa watersheds may range from a short-grass community such as buffalo grass–blue grama to a mixed-grass community that includes sideoats grama, or even to tallgrass with bluestems (see Chapter 3). Once the watershed has been protected to the practical extent possible, management consideration can then be shifted to the hydric soil–deﬁned basin. Managers can get a quick sense of how sediments have inﬂuenced the hydrology of the basin by taking a few (5 –10) soil cores at equal intervals across the basin. If sediment depths are somewhat moderate ( 12 –20 cm; 5 – 8 in.), the value of sediment removal to restoring hydroperiod, to the average playa, would be limited. However, there is no magic sediment-depth cutoff where the decision to remove or not remove sediment should be made. In small playas with relatively shallow slopes, 12 centimeters (5 in.) may signiﬁcantly reduce playa volume and thus hydroperiod. It may be suitable to remove sediments in this situation whereas in a large playa with steep slopes this would not likely improve the hydroperiod. When in doubt, the manager can use equations in Luo et al. (1997), sediment measurements, and playa edge determinations to estimate volume loss and how it inﬂuences hydroperiod. If removing sediments would signiﬁcantly improve the hydrope-

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riod, then care must be taken to preserve the hydric soils and seed bank. Damaging the hydric soils may negatively affect hydroperiod and vegetative recovery. Removing sediments involves heavy equipment such as maintainers or bulldozers, and slight mistakes with this equipment can sometimes remove hydric soil with the sediment. Moreover, if it is determined that sediment removal is appropriate, the watershed should be protected ﬁrst before sediment is removed. Otherwise the wetland could return to its previous condition within a few years. Although sediment removal has not been practiced in a large number of playas, often due to expense, its potential is now gaining favor among ecosystem managers. The next consideration for playa restoration should be physical modiﬁcations. Large pits and trenches can often simply be ﬁlled in with the original “spoil soil” that was excavated from the basin to improve hydrology. However, the hydric soil nature of the pit or trench soil should be assessed ﬁrst. Some of the pit spoil banks have been made from sediments that have eroded into the wetland. Obviously, under these circumstances, the trench or pit banks should be removed from the wetland. The pit or trench could then be allowed to ﬁll in naturally with hydric soil, as precipitation allows, or the excavated area could be carefully ﬁlled mechanically with hydric soil gathered throughout the basin. Once these factors have been restored, the playa, and its attendant biota, may require minimal management. However, natural disturbances such as ﬁre and large herbivore grazing may need to be introduced to maintain native plant communities and dependent fauna. This is true for the watershed as well as the wetland. Although grazing and ﬁre prescriptions exist for watersheds, few are available for wetlands. Also, current prescriptions used in uplands for grazing may not mimic intense “spot” herbivory that occurred historically in prairies (Stolzenburg 2002). For example, some local intense grazing may be required for species such as the mountain plover. Grazing and ﬁre is also often used to improve waterfowl use of playas in the Rainwater Basin. WATERFOWL CONSERVATION

The conservation goal of many managers for certain playas may not be to restore the wetland to its original ecological function but to focus on waterfowl. Given the funding source for much of wetland conservation, such as waterfowl stamp money, and

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the fact that fewer wetlands are available for use by waterfowl, many managers are required to focus their playa management efforts on waterfowl. This often results in the pumping of groundwater into the playa or in some cases removing water from the playa. Playa management for waterfowl should also begin with watershed protection; otherwise, after a while there will be nothing left to pump into. Several management scenarios to improve playa habitat for waterfowl have been examined (e.g., Haukos and Smith 1993b; Anderson and Smith 2000). Focus has primarily been on ways to improve the carrying capacity of the playa by improving the food resources. The technique is called “moist-soil” management (Fredrickson and Taylor 1982), whereby soils in the spring and summer are either dried out with a drawdown or irrigated (in playas with groundwater) to promote moist conditions for germination and growth of wetland plants that produce abundant seeds. Generally these prescriptions follow historical precipitation trends and obviously are most valuable to waterfowl populations in below-average precipitation years. The early (spring) irrigation promotes germination, while later (summer) irrigation is primarily used to improve seed production with some additional germination. The wetlands are then ﬂooded (artiﬁcially or with precipitation) in fall, winter, and spring to make those seed foods available to waterfowl. Invertebrates are also an important food during this time. Invertebrates are more diverse and abundant the earlier in the autumn, and the longer, the playa is inundated (Anderson and Smith 2000). This type of management generally increases carrying capacity, as determined by food production, by more than 10 times that found in unmanaged playas that are simply ﬂooded during fall, winter, and spring (Haukos and Smith 1994a; Anderson and Smith 1999). For those managers required to manage waterfowl in playas, it is often an inefﬁcient use of funds to apply water during winter or migration seasons without moist-soil management (e.g., Haukos and Smith 1993b). The major cost in the above scenario is the application of water during winter and migration. Most wetlands are ﬂooded from 20 to 45 centimeters (8 –15 in.) to promote waterfowl use. The cost of irrigations for moist soil plant production is generally only 10% of the winter, spring, or fall ﬂooding expense. Thus, for a 10% increase in cost, the wetland can support 10 times as many waterfowl (or other seed- and invertebrate-eating species). Sometimes when drought is extensive and a large percentage of wetlands have been

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lost, such as in the Rainwater Basin, it is justiﬁable to pump water in wetlands that have not been moist-soil managed for roosting birds. Of course, if the wetland is not ﬂooded, it is used by many species of migrating songbirds and upland game birds (Smith and Haukos 1995). (Detailed prescriptions for moist-soil management of waterfowl habitat exist in Haukos and Smith 1991, 1993b, and Anderson and Smith 2000.) PRAIRIE RESERVES AND PLAYAS

These efforts to protect playas, whether for ecosystem integrity or waterfowl management, should focus on reserves or complexes. As reviewed by Licht (1997, 132 –134), there have been many plans proposed within the past decade and a half concerning the creation of prairie reserves in the Great Plains. Several have been controversial, such as the Popper and Popper (1994) proposal for a huge grassland “buffalo [bison] commons.” The controversy surrounding some of these proposals is generally related to retiring/purchasing large amounts of agricultural lands and placing them in a reserve system. As might be expected, local communities initially did not view those types of proposals as being helpful to an ailing agricultural economy. Generally these proposals were viewed as a landgrab scheme and an attempt to destroy a way of life. Recently, however, renewed ways of preserving grasslands have been proposed that aim to support local economies, and the dust has settled somewhat (Licht 1997). Plans that include a substantial tourism/recreation component and that use land forfeitures or willing sellers in land acquisition will likely be met with some local support, especially if the local tax base is maintained. Any incentive that could lead to a more sustainable economy and human population for the region might have a chance of garnering the necessary local support. Licht (1997) has provided supportive arguments for creating a prairie reserve system and the economic justiﬁcation for doing so. Mostly he justiﬁed the potential establishment of such reserves on economic grounds by suggesting the diversion of government funds currently expended on failing local economies, and farm programs, to land acquisition. This justiﬁcation is also supported at the global scale for a number of other industries (Wilson 2002, 183 –184). Licht (1997) contended that changing some annual agriculture payments to land acquisition payments could be justiﬁed on economic grounds. Habitat restoration contracts, tourism, and recreation would then provide

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support for local economies. This is where much of the concern is now focused. Change, especially in the way people make a living, is often viewed with skepticism, because of the uncertainty associated with it. But, given the economic and population trends in parts of the Great Plains region, such changes should be considered in certain areas (Wallach 1985). Projected land-use need trends are much higher for recreation/tourism than for agriculture (Licht 1997, 149). Many of the reserve plans focus on one or two “keystone” species that are supposed to serve as indicators of a truly functioning grassland ecosystem. Generally the species are the so-called “charismatic megafauna.” In the Plains, bison are frequently mentioned. Certainly this measure is laudable, and probably helpful from a tourism promotion standpoint because of their viewing potential. But while the many parcels of land are being put together to support such wideranging species, focus should be maintained on those landscape features containing the key sites of existing biodiversity in the region. Preserving these sites of biodiversity in the parcel acquisition process will make it easier to support, and sustain, the large keystone species with their large space needs. In the short- and mixed-grass prairies, those sites are often the playas and riparian sites. Focusing parcel purchase on playas and other wetlands in a native grassland setting will allow preservation of biodiversity and the necessary habitat requisites for the far-ranging species. Although space/area need not be the foremost consideration in every prairie reserve design, the quality (i.e., inclusion of wetlands) of that space should be considered. The establishment of a prairie reserve network is likely a worthwhile endeavor and would lead to a more diverse, stable economy and substantial protection of Great Plains playas. END STATEMENT

To preserve properly functioning playa/prairie ecosystems, immediate action is needed on a number of fronts. Programs that focus on long-term protection and restoration through purchase or perpetual easement will make the largest contributions. This protection should be made at the landscape level where playa complexes are preserved in a continuous natural (both virgin and restored) prairie setting. The blueprint for selection of playas is provided earlier, and where those playas can be connected with native prairie to preserve large areas, a reserve or complex can be created. This might be accomplished through Wetland Management Districts,

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nongovernment preserves, or even formation of new national parks. A series of these complexes or focal sites located throughout the Great Plains would provide the necessary conservation; 2,000 to 3,000 playas (about 10%) in many reserves throughout the Great Plains would be a worthwhile goal. State and federal agencies, as well as nongovernmental organizations, must ﬁnd the renewed vigor to accomplish the task. Certainly the support of many individuals will be needed to encourage the necessary funding of these agencies and organizations to do the job. Public education and grassroots organization will be key. The many reserves could support a reasonable ecotourism and recreation industry, in its broadest sense. That said, however, everything discussed in the previous sections on improving playa ecosystems may be compromised by the burgeoning global human population (more than 6 billion), which is taking a tremendous toll on the climate, air, water, and soil needed to support not only humans but inextricably linked ecosystems (Pimm 2001). Despite the many plans on how to slow this growth, and notwithstanding some optimistic predictions that the human population will begin to level out in the year 2050 (e.g., Wilson 1999), the damage being done to natural resources must be abated to maintain the health of remaining ecosystems. As Wilson stated, “It should be obvious to anyone not in euphoric delirium that whatever humanity does or does not do, Earth’s capacity to support our species is approaching the limit” (2002, 33). Individuals should support national/global referendums and policies that beneﬁt protection of these resources (e.g., water quality acts, climate change treaties). For example, an agency or conservation group may be successful in establishing or protecting a playa reserve, but without citizen support and implementation of control of greenhouse gases, a continuing increase in Great Plains temperature could eliminate the functional hydroperiod. The acquisition would have been for naught. Numerous treatises exist on ways and reasons to support global environmental policies (e.g., Ehrlich 1997; Wilson 2002). Combining our efforts to bring about regional conservation of playa reserves with commitments to global environmental quality will insure the protection of this unique Great Plains environment. The future of playas, and indeed our own, depends on it.

APPENDIX

Macroinvertebrate Taxa Collected from Playas on the Southern High Plains

Taxa Ectoprocta (bryozoans) Oligochaeta Naididae Lumbricidae Tubiﬁcidae Limnodrilus hoffmeisteri (Claparède) Limnodrilus sp. Lumbriculidae Hirudinea Hirundinidae Erpobdellidae Erpobdella punctata (Leidy) Glossiphonidae Helobdella triserialis (Blanchard) Gastropoda Lymnaeidae Fossaria cockerelli Pilsbry & Ferriss Fossaria bulimoides (Lea) Physidae Physella bottimeri (Clench) Physella virgata Lea Planorbidae Gyraulus parvus Say Planorbella tenuis (Dunker) Planorbella trivolvis (Say) Stylommatophora

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Taxa Pelecypoda Sphaeriidae Sphaerium striatinum (Lamarck) Anostraca Branchinectidae Branchinecta lindahli Packard Branchinecta packardi Pearse Streptocephalidae Streptocephalus dorothae Mackin Streptocephalus texanus Packard Thamnocephalidae Thamnocephalus platyurus Packard Notostraca Triopsidae Triops longicaudatus (LeConte) Conchostraca Caenestheriidae Caenestheriella setosa (Pearse) Eocyzicus concavus (Mackin) Leptestheriidae Leptestheria compleximanus (Packard) Lynceidae Lynceus brevifrons (Packard) Ostracoda Candoniidae Candona patzucaro Tressler Cyprididae Cyprinotus antillensis (Broodbakker) Megalocypris gnathostomata (Ferguson) Megalocypris pseudoingens Delorme Physocypria globula (Furtos) Cypridopsidae Cypridopsis vidua (Müller) Potamocypris unicaudata Schaefer Ilyocyprididae Pelocypris tuberculatum (Ferguson) Limnocytheridae Limnocythere sanctipatricii (Brady and Robertson)

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Taxa Copepoda Diaptomidae Cladocera Amphipoda Isopoda Odonata Anisoptera Gomphidae Aeshnidae Anax junius (Drury) Libellulidae Orthemis ferruginea (Fabricius) Pantala ﬂavescens (Fabricius) Pantala sp. Plathemis lydia Drury Sympetrum corruptum (Hagen) Sympetrum sp. Tramea sp. Zygoptera Coenagrionidae Enallagma civile (Hagen) Lestidae Lestes alcer Hagen Lestes disjunctus Selys Orthoptera Acrididae Gryllidae Tettigoniidae Ephemeroptera Baetidae Callibaetis sp. Cloeon sp. Caenidae Heteroptera Aphididae Belostomatidae Belostoma ﬂumineum Say Cicadellidae Corixidae Corisella edulis (Champion)

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Taxa Corisella tarsalis (Fieber) Rhamphocorixa acuminata (Uhler) Sigara alternata (Say) Sigara sp. Trichocorixa reticulate (Guérin-Méneville) Trichocorixa verticalis (Fieber) Gerridae Gerris marginatus Say Gerris sp. Miridae Mesoveliidae Mesovelia mulsanti White Notonectidae Buenoa margaritacea Torre-Bueno Notonecta undulata Say Notonecta unifasciata Guérin-Méneville Notonecta sp. Pentatomatidae Reduvildae Rhopalidae Saldidae Saldula interstitialis (Say) Saldula pallipes (Fabricius) Veliidae Microvelia beameri McKinstry Microvelia sp. Coleoptera Cantharidae Carabidae Cerambycidae Coccinellidae Endomychidae Curculionidae Bagous sp. Lissorhoptrus simplex (Say) Listronotus ﬁliformis (LeConte) Listronotus grypidioides (Dietz) Listronotus scapularis (Casey) Notiodes aeratus (LeConte)

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Taxa Dytiscidae Brachyvatus sp. Copelatus chevrolati Aubé Copelatus sp. Cybister ﬁmbriolatus (Say) Eretes sticticus (Linnaeus) Hygrotus nubilus (LeConte) Laccophilus fasciatus terminalis Aubé Laccophilus q. quadrilineatis Horn Laccophilus sp. Liodessus afﬁnis (Say) Neobidessus sp. Thermonectus nigrofasciatus Aubé Thermonectus nigrofasciatus ornaticollis (Aubé) Uvarus texanus (Sharp) Uvarus lacustris Say Endomychidae Eylaidae Gyrinidae Dineutus assimilis (Kirby) Haliplidae Haliplus triopsis Say Haliplus tumidus LeConte Helophoridae Helophorus linearis LeConte Helophorus sp. Heteroceridae Hydrophilidae Berosus exiguus (Say) Berosus infuscatus LeConte Berosus miles LeConte Berosus rugulosus Horn Berosus stramineus Knisch Berosus styliferus Horn Enochrus hamiltoni (Horn) Enochrus sp. Hydrophilus triangularis Say

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Taxa Octhebius sp. Paracymus confusus Wooldridge Tropisternus lateralis (Fabricius) Melyridae Megaloptera Sialidae Mordellidae Salpingidae Staphylinidae Scarabaeidae Tenebrionidae Trichoptera Leptoceridae Oecetis sp. Hymenoptera Formicidae Ichneumonidae Diptera Athericidae Ceratopogonidae Culicoides variipennis (Coquillett) Forcipomyia sp. Chironomidae Ablabesmyia sp. Apedilum sp. Chironomus stigmaterus Say Chironomus sp. Clinotanypus sp. Cricotopus sp. Cryptochironomus sp. Dicrotendipes californicus Johannsen Dicrotendipes sp. Endochironomus nigricans (Johannsen) Labrundinia sp. Parachironomus sp. Polypedilum sp. Procladius bellus (Loew) Procladius sp. Tanypus sp. Tanytarsus sp.

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Taxa Tanytarsini Tanypodinae Culicidae Aedes nigromaculis (Ludlow) Culex tarsalis Coquillett Dolichopodidae Ephydridae Notophila sp. Muscidae Psychodidae Sarcophagidae Stratiomyidae Odontomyia sp. Simuliidae Syrphidae Eristalis sp. Tabanidae Tabanus sp. Tipulidae Tipula sp. Lepidoptera Arctiidae Geometridae Noctuidae Pyralidae Acarina Arrenuridae Arrenurus dentipetiolatus Marshall Arrenurus new sp. Eylaidae Eylais sp. Hydrachnidae Hydrachna sp. Ixodidae Pionidae Piona ﬂoridana Cook Araneae Clubronidae Corinnidae

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Graphosidae Linyphiidae Lycosidae Salticidae Tengellidae Tetragnathidae Thomisidae Source: Modiﬁed from Hall et al. 1999, in Invertebrates in freshwater wetlands of North America: Ecology and management; used by permission of John Wiley and Sons, Inc. a Species identiﬁed by Merickel and Wangberg (1981) as Corisella inscripta (Uhler) and later determined to be C. edulis (Champion) by R. Sites. b Individual identiﬁed by Merickel and Wangberg (1981) as Buenoa scimitra Bare and later determined to be B. margaritacea Torre-Bueno by R. Sites.

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INDEX

Page numbers in italics refer to photographs or illustrations. For speciﬁc animals and plants, look under the common names. Also look under the headings “Animals” and “Plants.” Abbott, I., 126 –127 Adherence transport, 128 Agriculture: current trends in, 25 – 28, 27, 151–154; and drainage of playas, 151–153, 160 –162; dryland farming methods, 26 –27; and fertilizers, 61, 172; and insects in playas, 114; and irrigation, 17–18, 17, 26 –27, 27, 55, 56, 58, 61, 125, 138, 151, 153, 154, 156, 165, 166; and migratory birds, 87, 89, 93 –94; and modiﬁed playas, 17–18, 17, 56, 58, 166; and pesticides, 166 –172; and pronghorn, 103; and sedimentation, 163 –165; and soil erosion, 162 –165; and Swampbuster regulation, 191–192; as threat to playas, 61, 125, 161, 162, 165 –166 Agriculture Department, U.S., 24, 51, 165, 166, 179, 186 –189, 191–192 Algae, 45 – 47, 121, 169 Altithermal period, 144 Alward, R. D., 174 American explorers, 149–150 Amphibians, 72 –77, 73, 115, 119, 120, 121, 127, 128, 177 Anderson, A. M., 75 –76, 109

Anderson, J. T., 69, 70, 71, 96, 109, 121–124, 127–128, 203 –217 Anglo-European settlers, 148 –151 Animals: amphibians, 72 –77, 73, 115, 119, 120, 121, 127, 128, 177; birds, 70, 78 –102, 84, 86, 88, 92, 99, 115, 118 –120; charismatic megafauna as keystone species, 200; consumption of primary and secondary production by, 113 – 115, 118 –120; Coronado expedition’s description of, 147; endangered species, 105 –107, 189; and exotics, 175 –76; ﬁshes, 71–72; grazing by, 23, 24, 62, 115, 116, 118, 197; invertebrates, 66 –71, 68, 76, 80, 113 –114, 119, 120, 121, 127–128, 136 –138, 198, 202 –217; livestock fecal runoff, 61; livestock feedlots, 168 –173, 170, 192; livestock watering pits, 58, 62, 156; mammals, 102 –107, 106, 115, 118, 142 – 144, 149, 177; reptiles, 77–78; wildlife habitat of playas, 157– 158. See also Diversity; and speciﬁc animals Anions, 111, 112 Aquatic bed vegetation, 8 Archaeological sites, 141–142

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Area–diversity relationships, 128 –129 Army Corps of Engineers, U.S., 51, 183, 190, 191 Australia, 126 –127 Avian botulism, 45, 178 –179 Avian cholera, 45, 86 – 87, 168, 176, 178 –179 Bacteria, 45 Bailey, A. W., 23, 24 Bartz, J. E. M., 45, 49, 55, 63, 117–118 Basins. See Playa basins Bats, 102 –103, 105 Beierman, H., 48, 102 Bergan, J. F., 95 Biodiversity. See Diversity Biometricians, 158 Biophilia, 159 Biotic threats to playas, 175 –176 Birds: conservation programs for, 177–186, 188 –189, 197–199; consumption of primary and secondary production by, 119–120, 121; diets of, 80, 89, 93 –94, 95 –97, 98, 198; diseases of, 45, 86 – 87, 168, 176, 178 –179; diversity and number of species of, 78 –79; hunting of, 93, 94, 95, 97, 157; and invertebrates, 70, 80, 83, 96, 97, 119–120, 198; migration of, 78 – 89, 84, 86, 93, 97–98, 177, 179–180, 184 –185, 188, 191; nesting of, 83, 89–90, 98 –101, 99; North American Waterfowl Management Plan, 177–186; pesticides as threat to, 166 –168; predators on, 97, 101, 102; residency time of, in wetlands, 82, 88 – 89; shorebirds, 70, 79– 84, 84, 87– 89, 98, 115, 119–120; and structure of playas, 115; waterfowl, 84 –97, 88, 92, 98, 115, 119, 166 –168, 177–186, 188, 197–199; winter populations of, 79, 84, 90 –97, 92, 101–102

Bison, 24, 28, 36, 62, 118, 142 –143, 144, 145, 147, 148, 149, 151, 200 Bison [buffalo] commons, 199 Black, R., 126 –127 Bolen, E. G., 78, 103, 143 –144, 150 Botulism in birds, 45, 178 –179 Brough, J. S., 15, 39– 40 Bruner, W. E., 55 Bryant, F. C., 7, 13, 17 Buffalo. See Bison Buffalo [bison] commons, 199 Buffalo grass, 48, 57, 58, 59, 62, 174, 196 “Buffalo wallows,” 37 Buffer width protection, 195 –196 Bulbils, 47 Bureau of Reclamation, U.S., 183 Burning of playas. See Fire Cabeza de Vaca, Alvar Núñez, 144 –145 Caliche, 31–32, 154 –155, 156 California, 134 Canada, 177 Cannibalism, 76 Caprock, 31, 32 Cathey, J. C., 92 –93 Cations, 111, 112 Cattle feedlots, 168 –173, 170, 192 Cattle ranches, 151, 152 Charismatic megafauna, 200 Chemical threats to playas, 166 –173 Choate, L. L., 107 Cholera in birds, 45, 86 – 87, 168, 176, 178 –179 Ciboleros, 148 Clark, R. N., 173 Clay soils. See Soils Clean Water Act, 190, 191, 192 Climate, 21, 22, 173 –174 Collins, M. B., 144 Collins, S. L., 23, 54, 58, 134 Colonizers versus persisters, 128 Colorado: agriculture in, 25; archaeological sites in, 144; distribution, numbers and size of playas in, 6, 10 –11, 15; ﬁshes in, 72;

INDEX

grassland zones in, 5; modiﬁed playa in, 18; and Playa Lakes Joint Venture (PLJV), 179; playa soils in, 19; vegetation in playas of, 48, 58 – 61, 60; water quality in, 172 Conservation: action blueprint for, 200 –201; altered playas and restoration, 194 –197, 195; and buffer width protection, 195 –196; Conservation Reserve Program (CRP), 24 –26, 101, 136, 165, 186 – 187; and education programs, 181–182, 182; and enforcement of wetland regulations, 189– 192; Hydrogeomorphic (HGM) method of, 190 –191; initial considerations for and ecological function, 193; moist-soil management, 198; Natural Resources Conservation Service (NRCS), 51, 165, 179, 183, 187, 190; and nongovernmental organizations, 179, 183, 189; Playa Lakes Joint Venture (PLJV), 178 –182, 185, 192; and prairie reserves, 199–200; Rainwater Basin Joint Venture (RWBJV), 178, 182 –186; regional playa conservation, 178, 189, 192 –200; state programs and regulations for, 189–190, 192; U.S. Department of Agriculture programs, 186 –189, 191–192; U.S. Interior Department programs for, 187–189, 191; waterfowl conservation, 177–186, 188, 197– 199; Wetland Management Districts proposed for, 188, 200 Conservation Reserve Program (CRP), 24 –26, 101, 136, 165, 186 –187 Consumption of primary and secondary production in playas, 113 –115, 118 –120 Contingent valuation method, 158 Conway, W. C., 83

249

Coronado, Francisco Vásquez de, 145 –146 Covich, A. P., 173 –174 Cowardin, L. M., 7– 8 Coyotes, 103 –105 Cranes, 97, 98, 102, 119, 176 Crayﬁsh, 71 CRP (Conservation Reserve Program), 24 –26, 101, 136, 165, 186 –187 Curtis, D., 48, 102 Cushing, C. E., 48 Dahl, T. E., 161–162 Davis, C. A., 69, 70, 119–120, 203 –217 Day, F. A., 28 Decomposition in playas, 120 –124, 121 Deer, 103, 105, 115, 149 Deﬂation hypotheses, 32 –33, 35 – 36, 37, 38, 42 Degenhardt, W. G., 76 Depopulation of Great Plains, 27– 28, 153 –154, 153, 200 Dikes, 8 Diseases of birds, 45, 86 – 87, 168, 176, 178 –179 Dispersal and persistence, 127–128 Dissolution process in playa development, 31–33 Disturbance and diversity, 130, 135, 137 Diversity: area-diversity relationships, 128 –129; biodiversity relationships, 128 –129; and disturbance, 130, 135, 137; importance of playas for, generally, 124 –125; invertebrate diversity studies, 136 –137; and Island Biogeography theory, 125; and keystone species, 200; and metapopulation, 137; persistence and dispersal, 127–128; and surveys of species presence, 126 –127; vegetation species-area relationships, 129–136

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Drainage of playas, 151–153, 160, 161, 162 Draws, 12 Drought, 174, 198 –199 Ducks, 84 – 87, 88, 89–91, 92, 93 – 94, 95 –97, 99, 105, 119, 167–168, 181. See also Birds Ducks Unlimited, 179, 183, 189 Dugouts, 18 Easements, 192, 193, 195 Education programs, 181–182, 182 Elk, 24, 62, 149 Emergent vegetated wetland, 7– 8 Endangered species, 50, 105 –107, 189 Environmental Protection Agency (EPA), 51, 190, 191 Environmental Quality Incentive Program (EQIP), 187 EPA (Environmental Protection Agency), 51, 190, 191 EQIP (Environmental Quality Incentive Program), 187 Erickson, N. E., 48 – 49 Erosion, 33 –36, 37, 38, 39, 40, 42, 110, 125, 162 –165, 193, 196 European explorers, 144 –149 Excavations, 8 Exotics, 24, 25, 56, 61, 62, 65, 157, 175 –76, 181, 186 –187, 193, 194, 196 Extant ﬂora, 55 FAC (facultative) species, 51 FACU (facultative-upland) species, 51–52, 54 FACW (facultative-wetland) species, 51–52 Fauna. See Animals; and speciﬁc animals Feedlots, 168 –173, 170, 192 Ferrets, 107 Fertilizers, 61, 172 Fill of playas, 37, 142, 162, 191 Fire, 23 –24, 62, 197 Fischer, D. H., 78

Fish and Wildlife Service (USFWS), 51, 78, 88, 105, 178, 179, 183, 187–190 Fishes, 71–72 Fitter, A. H., 174 Fitter, R. S. R., 174 Flickinger, E. L., 167–168 Flooding, 8, 198 –199 Flora. See Plants Flores, D. L., 143 –144, 150 Flowers, T. L., 79– 80 Food Security Act, 25, 186, 187, 191 Forest Service, U.S., 179 Fox, 103 –104, 105 Friend, M., 176 Frogs, 72, 74, 75 Garcia, J. D., 71 Geese, 85 –93, 119, 185 Geological Survey, U.S., 172, 173 Gilbert, M. C., 49, 55, 63 – 65 Gleying, 20 Global warming, 174 Gordon, C. C., 69 Gosselink, J. G., 3, 113 –114 Gould, C. N., 37 Grasses and grassland zones, 5, 21, 23 –24, 25, 26, 48, 49, 50, 51, 55, 57–59, 61, 62 – 65, 112 –113, 199, 196, 200. See also Plants Gray, P. N., 71 Grazing. See Herbivory Great Plains: Anglo-European settlers on, 148 –151; climate of, 21, 22, 173 –174; European explorers in, 144 –149; ﬁre in, 23 –24, 62, 197; grassland zones of, 5, 21, 23 –24; grazing on, 23, 24, 62, 115, 116, 118; Native Americans of, 145 –151; Paleo-Indian period in, 141–144; population of, 27– 28, 153 –154, 153, 200. See also Agriculture; Animals; Plants; Playas Great Plains and High Plains Partnerships, 189

INDEX

Greenhouse gas, 174 Gregg, Josiah, 150 Gustavson, T. C., 31, 36, 37–38 Guthery, F. S., 7, 12, 13, 17, 18, 34, 35, 55 –56, 57, 62, 63, 103, 156, 157 Hall, D. L., 68, 69, 71, 109–112, 136 –137, 203 –217 Haukos, D. A., 47, 48, 49, 53 –54, 59– 61, 129–130, 188, 192 Helmers, D. L., 83 Herbicides. See Pesticides Herbivory, 23, 24, 62, 115, 116, 118 – 120, 197 HGM (hydrogeomorphic) method, 190 –191 Higgins, K. F., 87 Hispanics, 146, 148 Historical value of playas. See Value of playas Hoagland, B. W., 48, 54, 58, 134 Hogs, 103, 105 Holland, R. F., 134 Holliday, V. T., 12, 13, 31, 33 –36, 141, 142, 144 Holpp, F. A., 62 – 63 Horne, F. R., 68, 71, 203 –217 Horses, 144, 145, 148 Hovorka, S. D., 38 Hunting, 93, 94, 95, 97, 157 Hydric soils. See Soils Hydrogeomorphic (HGM) method, 190 –191 Hydroperiods: and aquatic mammals, 102; and conservation programs, 193, 194, 196 –197; and cultivation and irrigation, 125, 138, 166; and decomposition, 120, 122, 124; and diversity, 125, 137–138; and exotics, 175; and global warming, 174; and invertebrates, 71, 114; and primary production, 116, 117; and sedimentation, 138, 196 –197; and vegetation, 49–50, 55, 58, 61, 112, 113, 117; and watershed land use, 138

251

Insecticides. See Pesticides Insects, 24, 69, 70, 78, 71, 80, 102, 114 Interagency Playa Lakes Disease Council, 178 –179 Interior Department, U.S., 7, 13, 87, 187–189, 191. See also Fish and Wildlife Service (USFWS) Invertebrates: amphibians’ consumption of, 76; birds’ consumption of, 70, 80, 83, 96, 97, 119– 120, 198; community composition of, 68 – 69; as consumers in playas, 113 –114, 120; diversity studies of, 136 –137; and hydroperiods, 71, 114; importance of generally, 66; inﬂuences on community composition, 69–71; macroinvertebrates, 67, 68 – 69, 114, 136 –137, 202 –217; microinvertebrates, 67; persistence and dispersal of, 127–128; scope of and sampling limitations, 67– 68, 68; taxa of macroinvertebrates, 202 –217 Irrigation, 17–18, 17, 26 –27, 27, 55, 56, 58, 61, 125, 138, 151, 153, 154, 156, 165, 166, 198. See also Agriculture; Irrigation runoff Irrigation runoff, 17–18, 55, 56, 58, 61, 113, 116, 154, 156, 165, 166 Irwin, R. J., 169–173 Island Biogeography theory, 125 Isolated wetlands, 191 Jain, S. K., 134 Johnston, M. C., 48 Kansas: agriculture in, 25; birds in, 79– 80, 81, 82, 83, 84; climate of, 21, 22; distribution and numbers of playas in, 6, 10 –11; ﬁlling of playas in, 162; grassland zones in, 5; invertebrates in, 71; native grasses planted in, 26; pesticides used in, 168; and Playa Lakes Joint Venture (PLJV), 179; vegeta-

252

PLAYAS OF THE GREAT PLAINS

tion in playas of, 48, 50, 59– 61, 60; water quality in, 172 Keystone species, 200 Kindscher, K., 48, 168 Knopf, F. L., 23, 83 Krueger, J. P., 40 – 41 Krynitsky, A. J., 167–168 Küchler, A. W., 23 Kuzila, M. S., 40 – 41 La Ceja (the brow), 142 –143, 143, 145 Lacustrine wetlands, 7– 8, 37 LaGrange, T. G., 11 Landowner Incentive Program (LIP), 189 Lauver, C., 48 Lehman, O. R., 173 Leslie, D. M., Jr., 48 – 49 Lewis and Clark expedition, 149 Lichens, 45 Licht, D. S., 186, 199 LIP (Landowner Incentive Program), 189 Litke, D. W., 172 Livestock. See Animals Lizards, 77–78 Llano Estacado, 11–12, 11, 31, 32, 142 –150, 143, 169 Loess deposition, 41, 42 Long, Stephen, 150 Louisiana, 162 Lunettes, 10, 33, 34–35, 39, 40 – 41, 41 Luo, H. R., 9–10, 110, 163, 196 Macroinvertebrates, 67, 68 – 69, 114, 136 –137, 202 –217 Mammals, 102 –107, 106, 115, 118, 142 –144, 149, 177. See also Animals; Bison; and other mammals Mammoth, 142, 143, 144 MBTA (Migratory Birds Treaty Act), 191 McNeil, R., 96 –97 Meltzer, D. J., 144 Merickel, F. W., 68, 70, 203 –217

Metals, 111, 112 Metapopulation, 137, 159 Mexico, 86, 145, 177 Microinvertebrates, 67 Migration of birds, 78 – 89, 84, 86, 93, 97–98, 177, 179–180, 184 – 185, 188, 191 Migratory Birds Treaty Act (MBTA), 191 Military pollutants, 173 Mitigation of wetlands, 190 –191 Mitsch, W. J., 3, 113 –114 Mixed-grass prairie, 5, 21, 23, 24 Modiﬁed playas, 16 –18, 17, 18, 19, 56, 58, 61– 62, 166 Moist-soil management, 198 Moorhead, D. L., 70, 114 Morris, J. M., 146, 150 Motts, W. S., 4 –5, 6 Multigenic theory of playa origin, 36 –39 Muskrats, 102, 104, 115, 143 –144 National Audubon Society, 183 National Research Council, 3 – 4, 191 National Water Quality Assessment Program, 172 National Wetlands Inventory, 13, 51 National Wildlife Disease Laboratory, 179 National Wildlife Federation, 105 Nativ, R., 154, 155 Native Americans, 145 –151, 147 Natural Resources Conservation Service (NRCS), 51, 165, 179, 183, 187, 190 Nature Conservancy, 179, 183, 189 Nebraska: agriculture and irrigation in, 25, 151, 165, 166; birds in, 78, 80, 81, 84, 86 –90, 97, 98, 101; climate of, 21, 22; conservation programs in, 178, 182 –188, 190 – 191; distribution, numbers and size of playas in, 9, 13, 15, 16, 41; drainage of playas in, 151, 160, 162; exotics in, 175; feedlots in,

INDEX

171; ﬁlling of playas in, 162; hunting in, 157; invertebrates in, 69, 71; lunettes on playa in, 41; mammals in, 102 –103, 115, 143; mixed-grass prairie in, 23; modiﬁed playas in, 18, 19, 58; origin and development of playas in, 40 – 42; playa soils in, 19; population trends in, 28; sedimentation in, 165; value of playas in, 158; vegetation in playas of, 23, 48 – 49, 50, 54 –55, 63 – 65, 64, 116; water variables in playas of, 112. See also Rainwater Basin playas Neck, R. W., 68, 71, 203 –217 Nesting of birds, 79, 83, 89–90, 98 – 101, 99 New Mexico: archaeological sites in, 141, 142 –143, 143, 147; birds in, 84; caprock in, 32; distribution, numbers and size of playas in, 6, 10 –11, 15; grassland zones in, 5; insects in, 114; lunettes on playas in, 10; Native Americans in, 147, 148; Ogallala Aquifer in, 26; pesticides in, 168; and Playa Lakes Joint Venture (PLJV), 179; recharge playas in, 6; vegetation in playas of, 48, 51, 58 – 61, 60; water variables in playas of, 109, 110, 111 Nickels, C. R., 28 Nilsson, S. G., 129 North American Waterfowl Management Plan, 177–186 NRCS. See Natural Resources Conservation Service Nutrients, 110, 111, 119 OBL (obligate) species, 51, 52 Ogallala Aquifer, 26 –27, 153 –155, 165, 171–173 Oklahoma: agriculture in, 25; archaeological sites in, 144; distribution, numbers and size of playas in, 6, 10 –11, 15; grassland

253

zones in, 5; and Playa Lakes Joint Venture (PLJV), 179; population trends in, 28; vegetation in playas of, 48, 58 – 61, 60; water quality in, 172 Organochlorines, 167–168, 170, 171 Osterkamp, W. R., 31–33, 35, 38 Owls, 97, 101, 102 Oxygen, 110 –112 Paleo-Indian period, 141–144 Palustrine wetlands, 7– 8 Parker, J. M., 48, 54 Parker, Quanah, 146 Parks, L. H., 68, 109, 202 –216 Partners for Fish and Wildlife Program, 188 –189 Partners in Flight Program, 188 –189 Pastores, 146, 148 Penfound, William, 48 Persistence and dispersal, 127–128 Persisters versus colonizers, 128 Pesticides, 166 –172 Pheasants, 101, 105, 167 Pheasants Forever, 179, 183 Phillips Petroleum, 179 Physical threats to playas, 161–166 Pimentel, D., 164 Piping, 31, 37 Pits and trenches, 18, 18, 56, 58, 61– 62, 151, 156, 166, 178, 194, 197 Plants: algae, 45 – 47, 121, 169; bacteria, 45; classiﬁcation of, 51–52, 52; community composition of, 55; and conservation programs, 193, 196; Coronado expedition’s description of, 147; exotics, 24, 25, 56, 61, 65, 157, 175 –176, 181, 186 –187, 193, 194, 196; grassland zones of Great Plains, 5, 21, 23 – 24; and hydroperiods, 49–50, 55, 58, 61, 112, 113, 117; lichens, 45; primary production of, 112 –113, 115 –118; of Rainwater Basin, 23, 48 – 49, 54 –55, 63 – 65, 64; Southern Great Plains studies of, 55 – 62; and structure of playas, 112 –

254

PLAYAS OF THE GREAT PLAINS

113; surveys of, 47–51, 55 – 65; vascular plants, 47– 65; vegetation species-area relationships, 129–136; Wyoming studies of, 62 – 63; zonation of, 52 –55, 63 – 65, 64. See also Diversity; Seeds Playa basins, 29, 38, 42, 196 Playa Lakes Joint Venture (PLJV), 178 –182, 185, 192 Playa Lakes Region, 10 –13, 149, 172, 176 Playas: appearance of, 4, 8 –10, 9, 10; classiﬁcation of, 7– 8; consumption of primary and secondary production in, 113 –115, 118 – 120; cross-section drawing of, 10, 163; decomposition in, 120 –124, 121; deﬁnition of, 3 –7; dissolution process in development of, 31–33; distribution and numbers of, 10 –16, 11, 16, 193; diversity considerations on, 124 –137; ﬁll of, 142; functions of, 115 –124, 160 –161; and grassland zones of Great Plains, 5; loss rate per year and decline in, 151, 161– 162, 192; modiﬁcations of, 16 – 18, 17, 18, 19, 56, 61– 62, 166; origin and development of, 29– 42, 142; primary production in, 112 –113, 115 –118; shallowness of, 7, 9, 40; shape of, 9–10, 9, 29, 30, 32, 35 –36, 37, 40; size of, 7, 9–10, 14 –15, 42, 193; structure of, 112 –115; threats to, 160 –176; value of, 141–159; water variables of, 108 –112; and wind forces, 32 –36, 34, 35, 37– 40, 42, 164 –165. See also Animals; Conservation; Plants; Soils PLJV. See Playa Lakes Joint Venture Pollution. See Threats to playas Poor, P. J., 158 Popper, D. E., 199 Popper, F. J., 199 Population: global human population, 201; of Great Plains, 27–28, 153 –154, 153, 200

Prairie dogs, 104, 105 –107, 106 Prairie Potholes Region, 190 –191 Prairie reserves, 199–200 Precipitation, 21, 22, 55, 155, 174 Predators, 97, 101, 102, 114 Price, D. J., 47, 169 Primary production in playas, 112 – 113, 115 –118 Proctor, V. S., 46 – 47 Pronghorn, 24, 62, 103, 105, 115, 118, 149 Pumping water from playas, 166 Rabbits, 103 –105, 115 Raccoons, 104, 105 Rainfall. See Precipitation Rainwater Basin Joint Venture (RWBJV), 178, 182 –186 Rainwater Basin playas: and agriculture, 25, 151, 166; area of, 15; avian cholera in, 86 – 87, 176; birds in, 78, 80, 81, 86 –90, 101; conservation programs for, 178, 182 –188, 190 –191, 197; drainage of, 151, 160, 162; and enforcement of wetland regulations, 190, 191; exotics in, 175; and feedlots, 171; ﬁlling of, 162; identiﬁcation of, by LaGrange, 11, 13; invertebrates in, 69; mammals in, 102, 115; map of, 16; modiﬁed playa in, 19; origin and development of, 29, 40 – 42; photographs of, 30, 41; with pits, 58; soils of, 19; value of, 158; vegetation of, 23, 48 – 49, 54 –55, 63 – 65, 64, 116; water variables in playas of, 112 Raptors, 97, 101, 102 Recharge, 6, 154 –156 Recreation and tourism, 153, 156, 157, 157, 158, 199–200 Reed, E. L., 47– 48, 54, 55 –56 Reeves, C. C., Jr., 29, 31–33, 36 –39 Reeves, J. A., 29, 31–33, 36 –39 Regional playa conservation, 178, 189, 192 –200 Reptiles, 77–78

INDEX

Residency time of migratory birds, 82, 88 – 89 Restoration Ecology, 194 Rhodes, M. J., 71 Rhodes, O. E., Jr., 20, 91 Riggio, R., 155 Rodents, 103, 104, 105 –107, 106, 115, 118 Rosen, M. R., 5 – 6 Rosenzweig, M. L., 129 Rowell, C. M., Jr., 48 RWBJV (Rainwater Basin Joint Venture), 178, 182 –186 Sabin, T. J., 12, 13, 31, 33 –36 Salamanders, 72, 73, 74, 76 Salt lakes (salinas), 12, 146, 147 Samson, F. B., 23 Scarps, 38 –39 Schramm, H. L., Jr., 68, 71, 203 –217 Scientiﬁc value of playas, 158 –159 Scribner, K. T., 103 Sediment removal, 196 –197 Sedimentation, 9–10, 61, 110, 138, 156, 162 –165, 163, 180, 193 –197, 195 Seeds, 53, 53, 55, 80, 102, 119, 122 – 124, 127, 197, 198 Seyffert, K. D., 79 Sheep herding, 146, 148 Shorebirds, 70, 79– 84, 84, 87, 88, 89, 98, 115, 119–120 Short-grass prairie, 5, 21, 23 Simpson, C. D., 78, 103 Skagen, S. K., 83 Smartweed, 48, 56, 57, 59, 65, 121–124 Smith, B. J., 87 Smith, D. A., 154, 155 Smith, L. M., 20, 47, 48, 49, 53 –54, 59– 61, 70, 95, 96, 119–124, 129– 130, 188, 192 Snakes, 77–78 Societal value of playas. See Value of playas Soils: and agriculture, 152; carbon content of, 121; clay soils, 20, 30, 37, 152, 154 –156, 163, 163, 165;

255

and conservation of playas, 196; erosion of, 162 –165, 193, 196; fertility of, 116; hydric soils, 10, 10, 19–20, 29, 36, 53 –55, 156, 165, 197; moist-soil management, 198; of playa basins, 29, 30, 37, 196; of Rainwater Basin, 40; and recharge rates, 154 –156; and sedimentation, 163 –164, 163; texture of, 12, 163 –164, 163, 196; types and description of, 19– 20; and wind forces, 33 –36; and zonation, 53 –55 Songbirds, 99, 100 –102, 119, 199 Starks, P. J., 40, 42 Steinauer, E. M., 23 Stewart, B. A., 173 Stormwater management system, 17, 157 Sublette, J. E., 68, 70, 109, 110 –111, 202 –216 Sublette, M. S., 68, 70, 109, 110 – 111, 202 –216 Swampbuster, 191–192 Swanson, L. D., 151, 152 Sweeten, J. M., 172 –173 Tailwater. See Irrigation runoff Tall-grass prairie, 5, 21, 23, 24 Tebuthiuron, 169 Temperature. See Climate Texas: agriculture in, 165; archaeological sites in, 142; bacteria in, 45; birds in, 79– 80, 81, 84, 86, 92, 97, 98; “buffalo wallows” in, 37; cattle ranches in, 151, 152; climate of, 21, 22; conservation programs in, 179, 181, 189, 195; distribution, numbers and size of playas in, 6, 10 –12, 14 –15; feedlots in, 171, 173; ﬁshes in, 72; herbivory in, 118; hunting in, 157; insects in, 114; invertebrates in, 114; lunettes on playas in, 10, 34–35; mammals in, 102; military and municipal pollutants in, 173; Ogallala Aquifer in, 26; pesticides in, 168 –171; playa basins

256

PLAYAS OF THE GREAT PLAINS

in, 38; population trends in, 28; recharge playas in, 6; recreational use of playas in, 157, 158; sedimentation in, 163, 164; vegetation in playas of, 5, 23, 47– 48, 50, 51, 56 – 61, 60, 117; water variables in playas of, 109, 110, 111 Texas Tech University, 156, 179 Texas Water Resources Institute, 165 Texas Waterfowlers’ Association, 179 Thompson, G. K., 109 Threats to playas: agriculture as, 61, 125, 161, 162, 165 –166; biotic threats, 175 –176; chemical threats, 166 –173; climate change as, 173 –174; drainage of playas as, 151–153, 160, 161, 162; exotics as, 175 –76, 181; feedlots as, 168 –173, 170, 192; ﬁlling of playas as, 142, 162; introduction to, 160 –161; and jurisdictional wetland regulation, 165 –166, 189–192; pesticides as, 166 –172; physical threats, 161–166; sedimentation as, 162 –165, 180 Thurman, E. M., 171 Toads, 72 –76, 73 Tourism. See Recreation and tourism Trenches. See Pits and trenches Turbidity of water, 110, 111 Turtles, 77, 78 Types I and II playas, 36 –37, 38 Unconsolidated bottoms, 8 UPL (upland) species, 51–52, 52, 54 USDA. See Agriculture Department, U.S. USFWS. See Fish and Wildlife Service (USFWS) USGS. See Geological Survey, U.S. Value of playas: and Anglo-European settlers, 149–151; and bio-

philia, 159; contingent valuation method, 158; current value, 154 –159; and European explorers, 144 –149; from 1500 to 1860, 144 –150; and hunting, 157– 158; and irrigation, 156; in latenineteenth and twentieth centuries, 150 –154; and Native Americans, 146 –151, 147; in Paleo-Indian period, 141–144; in Rainwater Basin, 158; as recharge sites, 154 –156; for recreation, 153, 156, 157, 157, 158; scientiﬁc value, 158 –159; for stormwater management system, 17, 157; in urban areas, 156 –157; as wildlife habitat, 157–158 Vascular plants. See Plants Vegetation. See Plants Vertebrates. See Animals Wallace, B. M., 167 Wangberg, J. K., 68, 70, 203 –217 Water erosion, 36, 42, 164 –165 Water quality, 166, 172, 192, 201 Water Resource Center, Texas Tech University, 156 Water variables of playas, 109–112 Waterfowl, 84 –97, 88, 92, 98, 115, 119, 166 –168, 177–186, 188, 197–199 Waterfowl Production Area (WPA) program, 188 Watershed, 109–110, 138, 162, 163, 175, 186, 193, 197, 198 Weaver, J. E., 48, 55 Westerﬁeld, M. M., 45 Wetland Management Districts, 188, 200 Wetland regulations, 165 –166, 189–192 Wetland Reserve Program (WRP), 187 Wetlands, 50 –51, 115, 160, 161, 162, 165 –166, 187, 189–192. See also Playas

INDEX

WHIP (Wildlife Habitat Incentives Program), 187 Whitcomb, R. F., 175 Whiteside, R. W., 103 Whitﬁeld, C. J., 48, 54 Wildlife habitat, 157–158, 187. See also Animals Wildlife Habitat Incentives Program (WHIP), 187 Wilson, E. O., 159, 194, 201 Wind deﬂation, 32 –33, 35, 36, 37, 38, 42 Wind erosion, 33 –36, 39, 164 –165 Wind forces in playa development, 32 –36, 34–35, 37, 38, 39, 40, 42, 164 –165

257

Winter populations of birds, 79, 84, 90 –97, 92, 101–102 Wood, W. W., 31–33, 35, 38 WPA (Waterfowl Production Area) program, 188 Wright, H. A., 23, 24 WRP (Wetland Reserve Program), 187 Wyoming playas, 15, 23, 39– 40, 49, 55, 62 – 63, 117–118 Zartman, R. E., 155, 171–172 Zonation of vegetation, 52 –55, 63 – 65, 64

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