Floral Diversity in Relation to Playa Wetland Area and
Watershed Disturbance
LOREN M. SMITH* AND DAVID A. HAUKOS†
*Wildlife and Fisheries Management Institute, Department of Range, Wildlife and Fisheries Management,
Mail Stop 2125, Texas Tech University, Lubbock, TX 79409, U.S.A., email l.m.smith@ttu.edu
†U.S. Fish and Wildlife Service, Texas Tech University, Lubbock, TX 79409, U.S.A.
Abstract: There are 25,000–30,000 playa wetlands in the intensively cultivated Southern Great Plains of the
United States. Knowledge of area and watershed influences on wetland flora are needed to guide their conservation. We surveyed plant-community composition in 224 playas over 360,000 km2 and examined the relationships of species richness and diversity (Shannon’s) to playa area and watershed disturbance. The relationship of increasing species diversity to increasing area is most often hypothesized to be associated with an
increasing number of habitats and/or larger populations as area increases. Watershed disturbance ( perennial grassland vs. annual cropland) was included to determine its relationship to floral diversity and bioinvasion by exotic species. For all (terrestrial and wetland) plant species and all playas, there were only marginal relationships (r 2 0.1) between area and richness and diversity. Analyses supported the hypothesis
that the number of habitats affects playa plant diversity because number of habitats changed little as playa
area increased. Tests on only wetland plant species suggested stronger (r 2 0.2) relationships between richness and area. This relationship also was not likely related to increased population size, because large playas
remain flooded longer than small playas, and therefore wetland plant species have a greater opportunity to
become established. Finally, playas with cropland watersheds had more (p 0.05) exotic species, higher diversity, and fewer perennial species than playas with grassland watersheds. Because watershed cultivation
has altered playa hydroperiod and increased frequency of disturbance, playas are associated with a flora
dominated more by annuals and exotics. Conservation efforts aimed at preserving playa wetland plant diversity and native communities should focus not only on the area of the wetland but also on the condition of its
watershed.
Diversidad Floral con Relación al Área de Humedal Playero y la Perturbación de la Cuenca Hidrológica
Resumen: En el sur de la intensamente cultivada Gran Planicie de E. U. A. existen entre 25,000 y 30,000 humedales de playa. Se requiere de conocimiento del área y de las influencias de la cuenca sobre la flora del humedal para guiar su conservación. Analizamos la composición de la comunidad vegetal en 224 playas en
360,000 km2 y examinamos las relaciones de la riqueza y diversidad (Shannon) de especies con la superficie
de playa y la perturbación de la cuenca. A menudo se postula que la relación del incremento en la diversidad
de especies con el incremento del área está asociada con el incremento en los números de hábitat y/o con
mayores poblaciones a medida que incrementa la superficie. Se incluyó la perturbación de la cuenca ( pastizal perenne vs. cultivos anuales) para determinar su relación con la diversidad floral y la invasión biológica
de especies exóticas. Para todas las especies de plantas (terrestres y de humedal) y todas las playas, solo hubo
relaciones marginales (r 2 0.1, pendientes 0.1) entre la superficie y la riqueza y diversidad. Los análisis
apoyaron la hipótesis de que el número de hábitats afecta la diversidad de plantas de playa porque hubo
poco cambio en el número de hábitats al incrementar la superficie. Pruebas con solo las especies de plantas
de humedal sugirieron relaciones más robustas (r 2 0.2) entre riqueza y área. Esta relación tampoco estuvo relacionada con el incremento en el tamaño poblacional porque las playas extensas permanecen inundadas más tiempo que las playas pequeñas, y, por lo tanto, las especies de plantas de humedal tienen una
Paper submitted December 21, 2000; revised manuscript accepted August 22, 2001.
964
Conservation Biology, Pages 964–974
Volume 16, No. 4, August 2002
Smith & Haukos
Plant Diversity in Playas
965
mayor oportunidad para establecerse. Finalmente, las playas con cuencas cultivadas tenían más (p 0.05)
especies exóticas, mayor diversidad y menos especies perennes que las playas con pastizales. Debido a que los
cultivos en la cuenca han disminuido el hidroperíodo e incrementado la frecuencia de perturbación, las playas están asociadas con una flora dominada por anuales y exóticas. Los esfuerzos de conservación para
preservar la diversidad de plantas de humedales y comunidades nativas deben enfocar no sólo el área del
humedal, sino también las condiciones de su cuenca.
Introduction
The Southern Great Plains of North America is one of
the most intensive agriculturally affected regions in
North America (Bolen et al. 1989), with 25,000–30,000
playa wetlands (Osterkamp & Wood 1987) often serving
as the remaining sites of biodiversity in a semiarid landscape (Haukos & Smith 1994a). Because playas function
as island refuges in this agricultural landscape, their conservation is critical to maintenance of biodiversity for
the entire region. Indeed, the functions and values of
playas have been recognized regionally, nationally, and
internationally ( Bolen et al. 1989; Haukos & Smith
1994a; Playa Lakes Joint Venture 1994). Individually and
jointly, a myriad of government agencies, private conservation groups, and corporations are working toward the
conservation of playas (Playa Lakes Joint Venture 1994).
An understanding of the major factors influencing biotic
diversity in playa ecosystems is essential to developing
conservation efforts.
The basic biogeographical relationship of increasing
species diversity with increasing area (Arrhenius 1921;
Preston 1962; Williams 1964; MacArthur & Wilson 1967)
is well established for many ecosystem types and, as
such, has been used as a foundation for much land-ecosystem conservation planning (e.g., Wilson 1999). The
strength of the species-area relationship in various ecosystem or community types, and the reasons that relationship occurs, is central to conservation biology. The
species-area relationship is thought to be based primarily on increasing environmental heterogeneity (e.g., habitat diversity) and/or population size as area increases
(e.g., Williams 1964; Connor & McCoy 1979; Nilsson et
al. 1988; Rosenzweig 1995). A number of studies have
demonstrated support for either of these two hypotheses (reviewed by Rosenzweig 1995), but it is difficult to
separate the contributions (or lack thereof) of habitat
variability versus population size to the area-diversity relationship (e.g., Connor & McCoy 1979; Gilbert 1980;
Nilsson et al. 1988). Moreover, the existence of a species-area relationship and the potential relative influence
of habitat diversity versus population size on plant diversity in wetland ecosystems has seldom been considered.
The unique hydrological and physical attributes of playas (for instance, playas have little habitat heterogeneity
and similar shapes) make them especially suitable land-
scapes with which to tease out habitat versus population effects on floral diversity in wetlands and to examine the strength of the species-area relationship in this
environment. Playas are shallow depressional wetlands,
nearly circular in shape, and each is self-contained in its
own watershed. A playa is not hydrologically connected
to other wetlands and therefore can be considered an island or patch. Also, there is little elevational heterogeneity as playa area increases; the floor stays level (Luo et al.
1997). Therefore, as playa area increases, the number of
habitats in the playa stay essentially the same but plant
population size increases.
One caveat to this simple habitat scenario, however, is
that larger playas, even though they have a habitat structure similar to that of small playas, remain flooded
longer because of simple surface-area relationships.
Therefore, for those plants that require extended wet
conditions to germinate and reproduce, large playas
should provide greater opportunity for establishment
and reproduction. The large number of playas in the
Southern Great Plains, along with their similar physical
attributes, allow one to examine the biogeographical influence of water permanence and plant diversity. This
knowledge is critical to efforts aimed at preserving a limited wetland flora in a semiarid environment.
The cultivation of playa watersheds also has increased
exposed soil and sedimentation and decreased the hydroperiods of these playas (Luo et al. 1997). These types
of disturbance are commonly known to invite bioinvasions by exotic flora, disrupting natural community processes (e.g., Elton 1958; Vitousek et al. 1996). Burke and
Grime (1996) found that the success of bioinvasions by
exotic plants is strongly related to the availability of exposed soil, and bioinvasion success is greatest when this
disturbance occurs with nutrient additions. Both of
these conditions—exposed soil and nutrient additions—
commonly occur in agricultural settings, and an understanding of their influence on playa ecology is essential
to efforts to acquire and restore playa landscapes.
Knowledge of the strength of potential species-area
relationships and the influence of watershed condition
on plant communities will provide a necessary filter for
setting priorities for playa conservation. We compared
playa area to plant-species diversity and richness in playas with cultivated and perennial grassland watersheds
to examine the strength of the species-area relationship
Conservation Biology
Volume 16, No. 4, August 2002
966
Plant Diversity in Playas
for playas. In so doing, we also examined the relative importance of increased population size versus habitat
variability on overall floral diversity and the influence of
water permanence on the diversity of the specific part
of the flora dependent on wetland conditions. We further examined the relationship between watershed disturbance and playa plant-community composition in the
highly agriculturalized Southern Great Plains.
Methods
Study Site
The Playa Lakes Region of the Southern Great Plains encompasses approximately 360,000 km2 of southeastern
Colorado, southwestern Kansas, eastern New Mexico,
the High Plains of west Texas, and western Oklahoma
( Fig. 1). Playa wetlands make up approximately 2% of
the total landscape ( Haukos & Smith 1994a). Playas
range from 1 ha to 250 ha and average 6.3 ha in area
(Guthery & Bryant 1982). Most playa basins are defined
by the presence of a hydric vertisol clay soil, Randall
clay, or a close series relative (Allen et al. 1972) that is
uniformly distributed throughout a circular basin (Osterkamp & Wood 1987; Luo et al. 1997).
The Southern Great Plains was originally short- and
mixed-grass prairie but today is under cultivation with
cotton, corn, wheat, and sorghum ( Bolen et al. 1989),
and domestic livestock graze most uncultivated areas.
The climate is subhumid continental, with mean annual
precipitation ranging from 63 cm in the eastern portion
of the region to 35 cm in the west. Precipitation occurs
mainly in thunderstorms peaking in May–June and September–October ( Bolen et al. 1989). Because it is rare
for playas to be connected to ground-water sources, historically they were entirely dependent upon precipitation and associated runoff to fill. With the increase in
crop irrigation since the 1940s, however, many playas in
cultivated areas have received additional sediment, nutrients, and water from the resultant runoff ( Nelson et
al. 1983; Mollhagen et al. 1993; Irwin et al. 1996; Luo et
al. 1999).
Field Studies
We randomly selected 1% of the playas in all counties
that had 100 playas in each of the five states in the region ( Haukos & Smith 1997 ). In 1995 we determined
plant-species occurrence in 224 playas using step-point
samples (species recorded approximately every 1 m; Bonham 1989) along two transects during each of two seasonal surveys. The first transect was initiated in the southeast corner of the playa, proceeding at a 45 angle to the
west side of the basin. The second transect was initiated
on the west side of the playa at 45 to the northeast edge
Conservation Biology
Volume 16, No. 4, August 2002
Smith & Haukos
of the basin. We determined the playa edge by examining changes in soil color and slope (Luo et al. 1997). Because playas are roughly circular, both transects within a
playa were approximately equal in length.
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). We started
each sampling period in the southern portion of the region, working north, to account for seasonal differences
within the region. The dominant (50%) land use of the
surrounding watershed was classified as cropland (annual cultivation) or perennial grassland. Plants were classified as perennial or annual and as exotic or native,
based on the work of Correll and Johnston (1979),
Hatch et al. (1990), and the Great Plains Flora Association (1991). Plants were categorized as obligate or facultative wetland plants following the U.S. National Wetlands Inventory list (1996).
Data Analyses
The number of points at which a plant species was encountered was used as the number of individuals in
Shannon’s diversity calculations ( Magurran 1988), and
total species number determined richness. We used chisquare contingency analysis to compare species frequencies in playas with crop versus grassland watersheds (Zar
1996). We used t tests to compare the number of native
and exotic species, and annual and perennial species, in
playas with crop versus grassland watersheds. With t
tests we also compared the percent composition of exotics and natives and the percent composition of annuals and perennials between playas with crop and grassland watersheds.
We initially used linear regression to compare playa
area with species richness and diversity (early and late
survey data combined) using all plants for playas having
cropland and grassland watersheds as well as all playas
without regard to watershed type. We used log-transformed and untransformed measures of area, richness,
and diversity estimates in an attempt to find the strongest relationships ( Rosenzweig 1995). However, we
only present those transformations that achieved the
strongest area-diversity relationships. We calculated
playa area using diameter estimates obtained from plant
transects (Luo et al. 1997). Area estimated directly from
transects more accurately reflected actual area surveyed
than area estimated from soil maps, and therefore potentially provided stronger species-area relationships.
Finally, we conducted area diversity and richness analyses on all playas with only those species categorized as
facultative wetland or obligate wetland plants (U.S. National Wetlands Inventory 1996). Those species considered as facultative, facultative upland, or obligate upland
plants were not included in these analyses.
Smith & Haukos
Plant Diversity in Playas
967
Figure 1. Location of counties in which playa area and flora were surveyed in the southern Great Plains (U.S.A.).
The top number listed for each county is the total number of playa wetlands, and the bottom number is the number of playas surveyed in the county.
Conservation Biology
Volume 16, No. 4, August 2002
Because playa wetlands exist in a vast, semiarid agricultural region of the Southern Great Plains, maintaining
Conservation Biology
Volume 16, No. 4, August 2002
0.17
0.09
0.041
0.038
0.251
0.071
0.232
0.028
0.001
0.156
0.70
0.04
0.048
0.054
0.187
0.077
0.136
0.021
0.001
0.004
0.72
0.01
0.238
0.097
0.196
0.037
0.032
0.033
1.14
0.21
0.030
0.027
0.133
0.020
0.125
0.004
0.011
0.761
1.18
0.19
0.032
0.035
0.082
0.013
0.055
0.009
0.001
0.612
1.16
0.20
0.022
0.022
0.105
0.011
0.092
0.003
SE
Cropland playas
slope
r2
p
intercept
SE
Grassland playas
slope
r2
p
intercept
SE
* 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. Data were also analyzed for all plant species and for wetland plant species only.
Species-Area Relationships
All plant species
richness
Shannon’s
Wetland plant species
richness
Shannon’s
Discussion
All playas
Playas with cropland watersheds had higher diversity than
those with grassland watersheds, but species richness did
not vary by watershed type (Table 2). Species composition also varied by watershed type (2 182.9, p 0.001;
Appendix 1). More perennial species occurred in playas
with grassland watersheds than in those with cropland
watersheds, and more annual species occurred in playas
with crop than grassland watersheds. Percent-composition data reflected occurrence data. Playas with cropland
watersheds had greater coverage of annuals than playas
with grassland watersheds. Perennial coverage was higher
in grassland playas than in cropland playas (Table 2).
More exotic species occurred in playas with cropland
watersheds than in those with grassland watersheds,
and more native species occurred in playas with grassland watersheds than in those with cropland watersheds
(Table 2). Again, percent-basal-cover data reflected species-occurrence data. Only 6.27% of the area of playas
with grassland watersheds was occupied by exotic species, whereas 15.62% of the area with cropland watersheds was covered by exotic species. Similarly, native
species covered 68.08% of grassland playas and 58.25%
of cropland playas (Table 2).
slope
Influence of Watershed Disturbance on Flora
r2
In general, the relationships between species richness
and diversity and area for all playas were not strong:
most r2 were 0.10 and most slopes were not statistically significant. Analyses using log-log transformation
only are presented because they were generally stronger
than no transformation or the single-log area transformation. Relationships within playas with cropland or
grassland watersheds were not stronger. Only the log
area–log richness regression for the overall playas approached a situation where area explained some of the
variation in species richness (Table 1).
When upland plants were excluded from the analyses,
however, the relationship of wetland plant richness and
area improved (r 2 0.14 to 0.23; all slopes p 0.01)
( Table 1). Including only wetland plants in the Shannon’s diversity versus area analyses did not provide
stronger relationships than including all plant species in
the regressions.
Table 1. Relationship between plant-species diversity (richness, Shannon’s) and wetland area, based on log-log transformations for playas in the Southern Great Plains (U.S.A.).*
Species-Area Relationships
intercept
p
Results
0.001
0.060
Smith & Haukos
0.001
0.460
Plant Diversity in Playas
Transformation/index
968
Smith & Haukos
Table 2.
Plant Diversity in Playas
969
Plant-community characteristics of 224 playas with cropland and grassland watersheds in the Southern Great Plains (U.S.A.).
Cropland ( n 126)
Characteristic
Shannon’s diversity
Richness
No. of annual species
No. of perennial species
Composition by annuals (%)
Composition by perennials (%)
No. of exotic species
No. of native species
Composition by exotics (%)
Composition by natives (%)
Grassland (n 98)
x
SE
x
SE
t
p
1.77
19.60
8.92
9.83
29.96
46.95
4.69
14.06
15.63
58.25
0.04
0.50
0.36
0.31
1.80
2.33
0.19
0.43
1.43
2.41
1.61
19.10
6.69
10.92
11.93
62.73
2.28
15.36
6.27
68.08
0.05
0.60
0.40
0.35
1.55
2.37
0.18
0.50
1.23
2.33
2.77
0.07
4.28
2.32
6.11
4.68
9.04
1.98
4.72
2.87
0.006
0.520
0.001
0.021
0.001
0.001
0.001
0.049
0.001
0.005
their ecological integrity is key to the maintenance of
biodiversity for the entire shortgrass plains ecoregion
(Haukos & Smith 1994a). Other concerns notwithstanding, natural-resource managers often use the area of a
particular habitat as an indicator of the importance of
that habitat, because most studies show a relatively
strong positive relationship between area and the number of species occupying an area (e.g., Wilson 1999). In
this study, however, we found a relatively weak speciesarea relationship when all plant species (wetland and
upland) were included in analyses. This might indicate
to ecosystem managers that in playas size is not an important indicator for flora conservation. However, when
only wetland plant species were included in the speciesarea analysis, the relationship became stronger. Wetland
species are more rare in the High Plains than upland species (playas occupy only 2% of the landscape; Haukos &
Smith 1994a) and thus contribute, on an area basis, disproportionately more to the region’s biodiversity. Therefore, playa area should be an important factor used by
ecosystem managers to identify key conservation sites in
the Southern Great Plains.
Because habitat diversity does not increase as playa
area increases (Luo et al. 1997) and the relationship between area and plant diversity was not strong when all
plants were considered in the analyses, it appears that
habitat diversity is a more important influence on total
playa plant diversity than population size. There are only
two habitats in playas, edge and floor, because there is
little elevational change in the basin as playa area increases. This lack of habitat heterogeneity is supported
by seed-bank and extant-vegetation data showing little
plant zonation within the hydric-soil-defined playa basin
(Haukos & Smith 1994b). As indicators of habitat heterogeneity, soils can also be important in determining numbers of island species ( Johnson & Simberloff 1974).
However, playa soils are generally uniform across the basin (Allen et al. 1972; Luo et al. 1999). Unlike in playas,
habitat heterogeneity and resultant plant zonation is
common in other North American wetland systems
(Cowardin et al. 1979).
Although the number of individual plants increases as
playa area increases, it appears to have relatively little influence on the number of species occupying the playa.
Rydin and Borgegård (1988) and Nilsson et al. (1988)
found that plant-species richness varies by island area in
different Swedish lakes but that habitat diversity had little influence on plant diversity. The number of individuals was likely responsible for the diversity-area relationship in this instance ( Rosenzweig 1995). Each of these
studies involved either all woody species (Nilsson et al.
1988) or a high proportion of the flora that was made up
of woody species and long-lived perennials ( Rydin &
Borgegård 1988). Because playa flora is dominated by
annuals and short-persisting perennials (due to rapidly
changing environmental conditions), it is possible that
diversity-area relationships differ among plant communities with different life histories. Indeed, as we found for
playas, Holland and Jain (1981) found only a weak relationship between area of vernal pools dominated by herbaceous annuals and plant-species richness.
We hypothesize that the species richness–area relationship improves greatly when the analysis is limited to
wetland plant species because of habitat permanence
rather than because of population size or habitat diversity. Although large and small playas have similar habitat
structure, large playas stay flooded for longer periods of
time than small playas. Therefore, from a conservation
perspective, because large playas can stay wet longer,
they provide more opportunity for aquatic plants to colonize and successfully reproduce than small playas. Upland plant species can occur in dry wetlands and persist
in the seedbank when the wetland is flooded (e.g., Haukos & Smith 1993; Haukos & Smith 1997), but wetland
plants can seldom persist in the upland seed bank, because their requirements are never met (i.e., the upland
is seldom inundated).
Although Abbott and Black (1980) considered the potential problems of not including plant propagules (i.e.,
seed banks) in species-area studies, all such studies that
we have located have used extant flora in their analyses.
Because an annual or short-persisting perennial plant
Conservation Biology
Volume 16, No. 4, August 2002
970
Plant Diversity in Playas
species is not present during a set of surveys, however,
does not mean the species does not exist in the seed
bank. Species represented in a wetland seed bank will
appear when environmental conditions are appropriate
(Salisbury 1970; van der Valk 1981; Smith & Kadlec
1983; Haukos & Smith 1993; Bliss & Zedler 1998),
which poses unique problems for conservation biologists sampling floral diversity in wetlands.
A wetland is classified as such because it undergoes
fluctuations in its water levels from dry to relatively
deep water and can therefore accommodate terrestrial
and aquatic species. Each time the wetland undergoes
this hydrological shift, the dominant plant community
changes (van der Valk 1981; Haukos & Smith 1993). In
our study the environmental conditions of many playas
changed between early and late-season surveys. Many
playas either filled with precipitation runoff or subsequently dried between surveys. This leads to a significant change in the plant community structure within a
few weeks. Only 38% of the species, on average, that
were present early in the growing season were present
late in the season. This low similarity is also due to some
cool-season species, such as Hordeum jubatum, being
present only in the early season. These types of annual
species complete their life-history requirements and
then disappear for the remainder of that year, even if environmental conditions remain constant.
Further, this difference in species composition between surveys in playas is not actually “extinction” (Simberloff 1976). It is also not “pseudo turnover” or a sampling error ( Lynch & Johnson 1974; Simberloff 1976;
Nilsson & Nilsson 1985). The occurrence and recording
of plant species in playas is a function of a playa’s current and recent moisture regime and the timing of the
survey within the year. With only a 38% similarity in species between early and late surveys, many species can
be missed due to survey timing and environmental
changes. Therefore, the actual measurement of plantspecies presence or turnover (extinction and immigration) would be difficult in wetlands unless seed-bank
surveys were included in the analysis. Moreover, it may
be more appropriate for conservation biologists to use
seed-bank data with extant species counts than extant
species counts alone when examining species-area relationships in systems dominated by annuals and shortpersisting perennials.
Watershed Cultivation
Cultivation of playa wetland watersheds and basins has
caused increased and consistent disturbance to the
short-grass prairie landscape of the Southern Great
Plains of the United States. Because most playas ( 75%)
have cultivated watersheds ( Nelson et al. 1983), these
disturbances, on a landscape and biogeographical scale,
are related to widespread changes in playa plant com-
Conservation Biology
Volume 16, No. 4, August 2002
Smith & Haukos
munities throughout the Southern Great Plains. Not only
has this disturbance likely permitted the increase in bioinvasion by exotic species by increasing nutrient input
and the prevalence of exposed soil ( Burke & Grime
1996), it is also related to changes in the composition of
annuals and perennials. The current flora of cropland
playas is likely predisposed to domination by annual exotics in response to increasing disturbance.
The disturbance caused by cultivation of playa watersheds has caused increased sedimentation and decreased hydroperiods ( Luo et al. 1997 ). Indeed, most
playas with cultivated watersheds have lost more than
100% of their original volume. Moreover, current crop
production in the Southern Great Plains is dependent
upon irrigation, causing more-frequent water-level fluctuations in playas with cropland watersheds. Water is often pumped from playas to irrigate fields. Alternatively,
irrigation water that has been pumped from the aquifer
often flows into playas (Bolen et al. 1989). These drastic
alterations in historical wetland hydroperiods, which are
seldom considered in wetland bioinvasion events (Silvertown et al. 1999), have created an additional disturbance that allows for invasion of exotic species. In the
Northern Great Plains, Euliss and Mushet (1996) found
greater water-level fluctuations in prairie pothole wetlands surrounded by croplands than in wetlands surrounded by grasslands. They warned that these disturbances might alter the wetlands’ flora and fauna.
Once invading species become established in playas,
most produce abundant seed crops ( Haukos & Smith
1994b). Many of these species’ seeds are long-lived and
form persistent seed banks that are resilient in response
to, if not reliant on, future disturbances (Grime et al.
1981; van der Valk 1981; Smith & Kadlec 1985; Haukos
& Smith 1993). Because of these seed banks, it is unlikely that, once established, the invading species will be
lost from the current system. The widely fluctuating environmental conditions (wet-dry cycles) of playa wetlands in the agricultural regions of the Southern Great
Plains likely ensures the subsequent germination of currently established exotics.
Conclusions
The species-area relationship was much stronger for
wetland plants than for all species occurring in playa
wetlands. Larger playas had more wetland species than
smaller playas, a relationship likely related to water permanence. Therefore, any anthropogenic factor that may
alter playa hydroperiod will likely influence native playa
flora. Sedimentation, as a result of cultivation of the surrounding watershed, has drastically decreased the volume and hydroperiod of playa wetlands (Luo et al. 1997).
Watershed cultivation, and its associated influence on
playa hydroperiod, is also associated with an increase in
the prevalence of exotic species and annuals in playas
Smith & Haukos
with a cropland watershed, whereas playas with grassland watersheds have more native and perennial species. Therefore, ecosystem managers striving to maintain
the ecological integrity of playa wetlands, and, concomitantly, native floral diversity in the Southern Great Plains,
should consider playa area and condition of the watershed in their planning efforts. Protecting the native
shortgrass-prairie watershed is key to maintaining playa
wetland ecosystem health. Ecosystem managers targeting important sites of biodiversity in the Southern Great
Plains should focus on large playas with intact native
prairie watersheds. Restoration of agricultural playas
should first focus on reclamation of the upland watershed
and then on sediment removal to restore hydroperiod. Removing playa sediments prior to upland restoration will
result in only short-term benefits to hydroperiod because
cultivated watersheds rapidly erode into playas (Luo et
al. 1997).
Acknowledgments
Support for this research was provided by the Playa
Lakes Joint Venture, with funding provided by the Colorado Division of Wildlife Resources, the Kansas Wildlife
and Parks Department, the Oklahoma Division of Wildlife Resources, the New Mexico Department of Game
and Fish, the Texas Parks and Wildlife Department, the
U. S. Fish and Wildlife Service, and Phillips Petroleum
Company. L. Dierauf, J. Haskins, and J. Cornely facilitated coordination of research funding. Additional support was provided by the Caesar Kleberg Foundation for
Wildlife Conservation and the Department of Range, Wildlife, and Fisheries at Texas Tech University. R. Wipff, M.
Whitson, J. Warren, and C. Smith provided field assistance. R. Pettit, J. Wipff, and S. Jones assisted in plantspecies identification. D. Wester, R. Sharitz, and two
anonymous referees provided helpful comments on the
manuscript. This is manuscript T–9–909 of Texas Tech
University.
Literature Cited
Abbott, I., and R. Black. 1980. Changes in species composition of floras on islets near Perth, Western Australia. Journal of Biogeography
7:399-410.
Allen, B. L., B. L. Harris, K. R. Davis, and G. B. Miller. 1972. The mineralogy and chemistry of High Plains playa lake soils and sediments.
Water Resources Center report WRC-72-4. Texas Tech University,
Lubbock.
Arrhenius, O. 1921. Species and area. Journal of Ecology 9:95–99.
Bliss, S. A., and P. H. Zedler. 1998. The germination process in vernal
pools: sensitivity to environmental conditions and effects on community structure. Oecologia 113:67–73.
Bolen, E. G., L. M. Smith, and H. L. Schramm Jr. 1989. Playa lakes: prairie wetlands of the Southern High Plains. BioScience 39:615–623.
Bonham, C. D. 1989. Measurements for terrestrial vegetation. Wiley,
New York.
Plant Diversity in Playas
971
Burke, M. J. W., and J. P. Grime. 1996. An experimental study of plant
community invasibility. Ecology 77:776–789.
Connor, E. F., and E. D. McCoy. 1979. The statistics and biology of the
species-area relationship. The American Naturalist 113:791–833.
Correll, D. S., and M. C. Johnston. 1979. Manual of the vascular plants
of Texas. University of Texas at Dallas Press, Richardson.
Cowardin, L. M., V. Carter, F. C. Golet, and E. T. LaRoe. 1979. Classification of wetlands and deepwater habitats of the United States.
FWS/OBS-79/31. U. S. Fish and Wildlife Service, Washington, D.C.
Elton, C. S. 1958. The ecology of invasions by animals and plants.
Methuen, London.
Euliss, N. H., Jr., and D. M. Mushet. 1996. Water-level fluctuation in
wetlands as a function of landscape condition in the Prairie Pothole Region. Wetlands 16:587–593.
Gilbert, F. S. 1980. The equilibrium theory of island biogeography: fact
or fiction? Journal of Biogeography 7:209–235.
Great Plains Flora Association. 1991. Flora of the Great Plains. University Press of Kansas, Lawrence.
Grime, J. P., G. Mason, A. V. Curtis, J. Rodman, S. R. Band, M. A. G.
Mowforth, A. M. Neal, and S. Shaw. 1981. A comparative study of
germination characteristics in a local flora. Journal of Ecology 69:
1017–1059.
Guthery, F. S., and F. C. Bryant. 1982. Status of playas in the Southern
Great Plains. Wildlife Society Bulletin 10:309–317.
Hatch, S. L., K. N. Gandhi, and L. E. Brown. 1990. Checklist of the vascular plants of Texas. Texas A&M University Press, College Station.
Haukos, D. A., and L. M. Smith. 1993. Seed-bank structure and predictive ability of field vegetation in playa lakes. Wetlands 13:32–40.
Haukos, D. A., and L. M. Smith. 1994a. The importance of playa wetlands to biodiversity of the Southern High Plains. Landscape and
Urban Planning 28:83–98.
Haukos, D. A., and L. M Smith. 1994b. Composition of seed banks along
an elevational gradient in playa wetlands. Wetlands 14:301–307.
Haukos, D. A., and L. M. Smith. 1997. Common flora of the playa lakes.
Texas Tech University Press, Lubbock.
Holland, R. F., and S. K. Jain. 1981. Insular biogeography of vernal
pools in the Central Valley of California. The American Naturalist
117:24–37.
Irwin, R. J., P. J. Connor, D. Baker, S. Dodson, and C. Littlefield. 1996.
Playa lakes of the Texas High Plains: a contaminants survey and assessment of biological integrity. U. S. Fish and Wildlife Service, Arlington, Texas.
Johnson, M. P., and D. S. Simberloff. 1974. Environmental determinants of island species numbers in the British Isles. Journal of Biogeography 1:149–154.
Luo, H. R., L. M. Smith, B. L. Allen, and D. A. Haukos. 1997. Effects of
sedimentation on playa wetland volume. Ecological Applications 7:
247–252.
Luo, H. R., L. M. Smith, D. A. Haukos, and B. L. Allen. 1999. Sources
of recently deposited sediments in playa wetlands. Wetlands 19:
176–181.
Lynch, J. F., and N. K. Johnson. 1974. Turnover and equilibria in insular avifaunas, with special reference to the California Channel Islands. Condor 76:370–384.
MacArthur, R. H., and E. O. Wilson. 1967. The theory of island biogeography. Princeton University Press, Princeton, New Jersey.
Magurran, A. E. 1988. Ecological diversity and its measurement. Princeton University Press, Princeton, New Jersey.
Mollhagen, T. R., L. V. Urban, R. H. Ramsey, A. W. Wyatt, C. D. McReynolds, and J. T. Ray. 1993. Assessment of non-point source contamination of playa basins in the High Plains of Texas (Brazos basin watershed). Water Resource Center, Texas Tech University, Lubbock.
Nelson, R. W., W. J. Logan, and E. C. Weller. 1983. Playa wetlands and
wildlife on the Southern Great Plains: a characterization of habitat.
FWS/OBS 83/28. U. S. Fish and Wildlife Service, Washington, D.C.
Nilsson, I. N., and S. G. Nilsson. 1985. Experimental estimates of census efficiency and pseudoturnover on islands: error trend and be-
Conservation Biology
Volume 16, No. 4, August 2002
972
Plant Diversity in Playas
tween-observer variation when recording vascular plants. Journal
of Ecology 73:65–70.
Nilsson, S. G., J. Bengtsson, and S Ås. 1988. Habitat diversity or area
per se? Species richness of woody plants, carabid beetles and land
snails on islands. Journal of Animal Ecology 57:685–704.
Osterkamp, W. R., and W. W. Wood. 1987. Playa-lake basins on the
Southern High Plains of Texas and New Mexico. I. Hydrologic, geomorphic, and geologic evidence for their development. Geological
Society of America Bulletin 99:215–223.
Playa Lakes Joint Venture. 1994. Implementation plan. U.S. Fish and
Wildlife Service, Regional North American Waterfowl Management
Plan Office, Albuquerque, New Mexico.
Preston, F. W. 1962. The canonical distribution of commonness and
rarity. Ecology 43:185–215, 410–432.
Rosenzweig, M. L. 1995. Species diversity in space and time. Cambridge University Press, Cambridge, United Kingdom.
Rydin, H., and S. Borgegård. 1988. Plant species richness on islands
over a century of primary succession: Lake Hjälmaren. Ecology 69:
916–927.
Salisbury, S. E. 1970. The pioneer vegetation of exposed muds and its
biological features. Royal Society of London, Philosophical Transactions B 259:207–255.
Silvertown, J., M. E. Dodd, D. J. G. Gowing, and J. O. Mountford. 1999.
Conservation Biology
Volume 16, No. 4, August 2002
Smith & Haukos
Hydrologically defined niches reveal a basis for species richness in
plant communities. Nature 400:61–63.
Simberloff, D. 1976. Species turnover and equilibrium island biogeography. Science 194:572–578.
Smith, L. M., and J. A. Kadlec. 1983. Seed banks and their role during
drawdown of a North American marsh. Journal of Applied Ecology
20:673–684.
Smith, L. M., and J. A. Kadlec. 1985. The effects of disturbance on
marsh seed banks. Canadian Journal of Botany 63:2133–2137.
U.S. National Wetlands Inventory. 1996. National list of plant species
that occur in wetlands. U. S. Fish and Wildlife Service, St. Petersburg, Florida.
van der Valk, A. G. 1981. Succession in wetlands: a Gleasonian approach. Ecology 62:688–696.
Vitousek, P. M., D. M. D’Antonio, L. L. Loope, and R. Westbrooks.
1996. Biological invasions as global environmental change. American Scientist 84:468–478.
Williams, C. B. 1964. Patterns in the balance of nature. Academic
Press, London.
Wilson, E. O. 1999. The diversity of life. 2nd edition. W. W. Norton,
New York.
Zar, J. H. 1996. Biostatistical analysis. Prentice-Hall, Englewood Cliffs,
New Jersey.
Smith & Haukos
Plant Diversity in Playas
973
Appendix 1. Species occurrence of common ( 5%) plants in 224 playa wetlands with annual cropland (n 126) and perennial grassland
(n 98) watersheds in the Southern Great Plains, U.S.A.*
Species
Agropyron smithii
Amaranthus retroflexus
Ambrosia grayi
Ambrosia psilostachya
Aster subulatus
Bromus unioloides
Buchloe dactyloides
Chenopodium album
Chenopodium leptophyllum
Conyza canadensis
Coreopsis tinctoria
Cuscuta squamata
Cyperus esculentus
Echinochloa crusgalli
Eleocharis macrostachya
Euphorbia marginata
Grindelia squarrosa
Haplopappus ciliatus
Helenium microcephalum
Helianthus annuus
Helianthus ciliaris
Heteranthera limosa
Hoffmanseggia glauca
Hordeum pusillum
Kochia scoparia
Lactuca serriola
Lepidium densiflorum
Leptochloa fascicularis
Lippia nodiflora
Lythrum californicum
Malvella leprosa
Marsilea vestita
Melilotus officinalis
Nothoscordum bivalve
Oenothera canescens
Opuntia phaeacantha
Panicum capillare
Panicum obtusum
Phalaris caroliniana
Polygonum amphibium
Polygonum lapathifolium
Polygonum pensylvanicum
Polygonum ramosissimum
Portulaca oleracea
Proboscidea louisianica
Quincula lobata
Ratibida columnifera
Ratibida tagetes
Rorippa sinuata
Rumex altissimus
Rumex crispus
Sagittaria longiloba
Salsola iberica
Percentage
of crop
playas
Percentage of
grassland playas
Longevity
Native
status
Wetland
indicator status
25.40
39.68
85.71
3.18
39.68
15.87
33.33
71.43
55.56
30.95
18.25
11.11
15.08
72.22
76.19
8.73
23.81
6.35
10.32
34.92
65.87
9.52
3.97
27.78
72.22
26.19
8.73
12.70
30.95
19.84
75.40
14.29
2.38
8.73
69.05
0.00
6.35
20.64
15.08
23.02
24.60
74.60
19.84
6.35
7.94
5.56
3.18
3.97
62.70
12.70
61.91
17.46
20.64
42.86
17.35
67.35
9.18
14.29
9.18
88.78
29.59
53.06
19.39
14.29
7.14
6.12
27.55
76.47
15.31
36.74
8.16
6.12
18.37
74.49
4.08
20.41
36.74
50.00
13.27
13.27
7.14
60.20
21.43
71.43
19.39
10.20
8.16
68.37
26.53
10.20
36.74
1.02
9.18
5.10
29.59
19.39
19.39
7.14
5.10
11.22
25.51
32.65
5.10
17.35
9.18
20.41
P
A
P
P
A
A
P
A
A
A
A
A
P
A
P
A
A
A
A
A
P
P
P
A
A
A
A
A
P
P
P
P
A
P
P
P
A
P
A
P
A
A
A
A
A
P
P
P
P
P
P
P
A
N
N
N
N
N
E
N
E
N
N
N
N
N
E
N
N
N
N
N
N
N
N
N
N
E
E
N
N
N
N
N
N
E
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
E
N
E
FAC
FACU
FACW
FAC
OBL
FAC
FACU
FAC
FACU
FACU
FACU
FACU
FACW
FACW
OBL
FACU
FACU
FACU
FACW
FAC
FAC
OBL
FACU
FAC
FACU
FAC
FAC
FACW
FACW
OBL
FACW
OBL
FACU
FACU
FAC
UPL
FAC
FAC
FACW
OBL
OBL
FACW
FAC
FAC
FAC
NI
FAC
FAC
FACW
FAC
FACW
OBL
FACU
continued
Conservation Biology
Volume 16, No. 4, August 2002
974
Plant Diversity in Playas
Smith & Haukos
Appendix 1. (continued)
Species
Schedonnardus paniculatus
Scirpus validus
Sisymbrium altissimum
Sitanion hystrix
Solanum elaeagnifolium
Solanum rostratum
Suckleya suckleyana
Tragopogon dubius
Typha domingensis
Verbena bracteata
Vernonia marginata
Xanthium strumarium
Percentage
of crop
playas
Percentage of
grassland playas
Longevity
Native
status
Wetland
indicator status
7.94
19.84
5.56
2.38
34.13
13.49
13.49
10.32
19.05
22.22
10.32
16.67
29.59
1.02
5.10
13.27
46.94
25.51
8.16
10.20
1.02
29.59
18.37
10.20
P
P
A
P
P
A
A
A
P
A
P
A
N
N
E
N
N
N
N
E
N
N
N
N
FACU
OBL
FACU
FACU
FACU
FACU
FACW
FACU
OBL
FAC
FAC
FAC
*Longevity: P, perennial; A, annual. Native status: N, native; E, exotic. Wetland indicator status: OBL, obligate; FACW, facultative wetland; FAC,
facultative; FACU, facultative upland; UPL, obligate upland; NI, no indicator. Wetland indicator status and a complete list of all species encountered are given by Haukos and Smith (1997).
Conservation Biology
Volume 16, No. 4, August 2002