Perspectives from Research Funded by the
U.S. Environmental Protection Agency:
Region 7 in Kansas City (Contract CD977399-01)
and Region 8 in Denver (Contract CD968115-01)
Drs. James H. Thorp1, 2, D. Christopher Rogers1, and Brian J. O’Neill1, 2, 3
1 Kansas Biological Survey, University of Kansas, Lawrence, KS
2 Department of Ecology and Evolutionary Biology, University of Kansas
3 Present Address: Biology Department, University of Wisconsin, Whitewater, WI
The Nature of Playas
The North American Great Plains and arid West contain a very abundant and ecologically fascinating type of ephemeral wetlands called “playas” (Fig. 1 a,b). For example, nearly 22,000 playas are thought to be present in Kansas alone (Huggins et al. 2011). These seasonally astatic wetland habitats are important ecologically, municipally, and agriculturally (e.g., Stebbins, 1976; Jain, 1976; Ebert and Balko, 1984; Jain and Moyle, 1984; Zedler, 1987; Ikeda and Schlising, 1990; Witham et al., 1998; Brown and Jung, 2005; Comer et al., 2005; Schlising et al., 2007).
Ecologically these systems represent a type of “island biogeography” in that they are literally physical and temporal islands of water in a sea of land and time (Stebbins, 1976; Holland and Jain, 1981; Ebert and Balko, 1984; Rogers, 2014, 2015). These wetlands are vital to many regions for groundwater recharge (Harvey et al., 2007), like the Ogallala Aquifer, which supplies the municipal and agricultural water needs of several US states (Gurdak and Roe, 2009).
Regulatory agencies and conservation organizations have developed various plans for protecting, enhancing or constructing seasonally astatic wetlands, typically as part of restoration or mitigation (e.g., Ferren and Pritchett, 1988; California Native Plant Society, 1994; U.S. Fish and Wildlife Service, 1998; Meisler, 2001; Biebighauser, 2003). As a result, habitat monitoring has been required in these wetlands, particularly as relates to restoration and constructed habitats. Unfortunately, there has been no effort to develop a unified or consistent method for conducting biological monitoring in these habitats (De Weese, 1998).
Obligatory playa invertebrates are entirely dependent upon the aquatic environment provided by playa wetland ecosystems. These organisms depend upon the presence of water in the late summer and early spring and the complete drying and absence of water at all other times (Eng et al., 1990; Rogers, 1998, 2009; Eriksen and Belk, 1999). These wetlands depend upon intact subwatersheds, and the surrounding uplands that support those watersheds (Holland, 1978, 1988). Playa habitat is a component of the larger grassland ecosystem (Holland, 1988, 1998; Holland and Jain, 1981). Furthermore, because playa organisms are essentially opportunistic, they can use a variety of ephemeral water sources, including artificial habitats that support a similar hydroperiod to natural playa habitats (Eng et al., 1990; Rogers, 1998, 2009; Eriksen and Belk, 1999). For example, roadside ditches and some stock ponds holding water for a month or more, may also contain a plentiful community of playa-type species (O’Neill et al., in press) (Fig. 2 a, b).
Despite their seemingly harsh environments, playas support a somewhat unique fauna with dominant species that are largely absent from permanent wetlands and almost all aquatic systems with fish. These “signature species” include tadpole shrimp (Crustacea, Branchiopoda, Notostraca), fairy shrimp (Anostraca), and clam shrimp (Spinicaudata and Laevicaudata), as well as amphibians such as tadpoles of spadefoot toads (Spea) (Fig. 5a-d). Playas support many birds including migrating water fowl (e.g., Krapu, 1974; Swanson et al., 1974; Skagen and Knopf, 1994; Baker et al., 1992; Silveira, 1996; Sánchez et al., 2007), as well as amphibians (e.g., Morey, 1998; Calhoun et al., 2003; Vasconcelos and Calhoun, 2004; Trauth et al., 2006), some mammals (e.g., Cox, 1984; Zedler and Black, 1992; Vanschoenwinkel et al., 2008), numerous endemic plants (e.g., Lathrop and Thorne, 1983; Ikeda and Schlising, 1990; Stallings and Warren, 1996) and invertebrates (e.g., Thorp, 1976; Bratton, 1990; Gibbs, 1993; Collinson et al., 1995;Williams, 1997; Rogers, 1998, 2001, 2009; Fairchild et al., 2003; Kiflawi et al., 2003; Hall et al., 2004; Batzer et al., 2005), especially aquatic insects and smaller branchiopod crustaceans like cladocerans and copepods.
Permanent residents of these habitats require a suite of specialized attributes, which can include resting eggs, ability to diapause, short generation time, rapid growth rate, and high vagility of propagules or adults for dispersal (Brendonck et al., 1998, 2008; Rogers, 2009; O’Neill and Thorp, 2014).
The large branchiopods (clam, fairy, and tadpole shrimps) are a group well adapted to surviving in playa wetlands due to their highly resistant resting eggs, and they are often the dominant group of organisms early in the hydroperiod (Brendonck et al., 1998, 2008; Rogers, 2009). Numerous insect species rely heavily on these playas, while others disperse from adjacent permanent aquatic habitats. Amphibians are common in these habitats when inundated, and waterfowl often use wet playas as resting, breeding, and feeding sites. Towards the end of the hydroperiod, playas become dominated by predatory invertebrates. To our knowledge, our studies are the first of their kind to employ stable isotopes to analyze food web structure in playa wetlands.
Recommendations for Sampling and Monitoring Playas
Sampling playas is both easier and harder than working in other wetlands. It is easier because they are shallow, the area encompassed is small, and vegetation is often minimal, thereby making it easier to pull a net through the water. In contrast, it is harder for the simple reason that playas hold water for only short periods (with occasionally many months or some years between flooding), making consistent sample periods problematic in wet playas. Consequently, dry playa sampling may be required.
Wet Playa Sampling
For the most part, playa invertebrates will be on vegetation (where present) and in the water column (Rogers, 1998). Some species may accumulate around substrate that reflects light (e.g., a fence post or rock), but most will be distributed somewhat evenly throughout the playa. Although aquatic insects and tadpoles will often be found among emergent and submerged vegetation, aquatic plants are not present in all playas and are never as prevalent as in relatively permanent wetlands. Our studies throughout the Great Plains as part of our EPA Region 7 and 8, however, have focused more heavily (but not exclusively) on the large branchiopod crustacean community, because these are the most representative taxa of ephemeral wetlands.
Sampling strategies for ephemeral wetlands have not been specified for states by EPA, and the methods applicable to permanent wetlands, lakes, and rivers are generally not appropriate. We employed a modified version of Rogers (1998) ephemeral wetland macroinvertebrate bioassessment method. We recommend sweeping through the middle of the wetland using a 1-m net sweep (25 cm x 18 cm; 400 µm mesh); this samples 45 L of water. If water is <18 cm deep, extend the sweep to fulfill the 45 L requirement (e.g. water 9 cm deep, sweep for 2m). For depths >18 cm, start the sweep at the bottom and sweep up evenly through 1 m to the water surface. Preserve the captured invertebrates immediately in 75% EtOH.
We have compared quantitative and qualitative sampling from the same basin. The qualitative sample consisted of sampling until the following conditions were met: (1) all wetland areas were repeatedly sampled; (2) repeated net sweeps failed to collect new taxa; and (3) each taxon was repeatedly sampled. Quantitative samples from the middle of the wetland consistently collected a higher proportion of total taxa, and we recommend this statistically valid approach.
Once the samples are returned to the lab, the organisms can be identified with a stereo-dissecting microscope using various taxonomic keys, such as Thorp and Rogers, 2016.
Dry Playa Sampling
Understanding the community structure and diversity of a playa is challenging because sampling is normally restricted to wet periods, which can be separated by many months to a few years. Therefore, we compared the community structure of wet playas with the diversity of organisms we obtained by hatching eggs from sediments collected from the same dry playas (Fig. 4 a-b). After collecting sediment from selected playas in Kansas and Nebraska, the material was sieved in the laboratory to isolate organic particles in the 150-300 µm size fraction. This process concentrates invertebrate resting eggs so they are amenable to visual inspection under magnification (Fig. 5 a-c). The eggs were photographed using a Moticam 2500 digital camera and identified using scanning electron micrographs from the primary literature (e.g., Mura, 1992; Hill and Shepard, 1997). We were then later able to link a particular type of egg with the invertebrate that produced it.
We also hatched the eggs contained in dry sediments. Because some species will hatch only during winter/spring conditions and others only during summer conditions, we exposed the dry sediments to appropriate temperatures in laboratory incubators and then placed the sediment in culture containers. Each sediment sample was then partially inundated with distilled water chilled to 0˚C, then inundated again after 24 hr. Over the subsequent two weeks various macroinvertebrates hatched or emerged and were collected and identified. All cultures were allowed to dry completely, and then were incubated at 35˚C for one month. After this period they were partially inundated with room temperature distilled water, then inundated again after 24 hr. Over the subsequent month, more macroinvertebrates hatched or emerged and were collected and identified.
Of the 16 samples processed, we were able to culture to identifiable stage, 1 to 5 separate taxa per sample for an average sediment sample. These included all the following types of crustaceans: 6 species of fairy shrimp, 3 species of clam shrimp, and 2 species each of cladocerans, tadpole shrimp, copepods, and ostracods. We also cultured species of molluscs (snails mostly but also bivalves), culicid mosquitoes, and heterocerid beetles.
Wet vs Dry Playa Sampling
When it is possible to sample a wet playa, do so because it is always easier and generally more informative than sampling a dry playa. However, for most of the year, only dry playas will be available. The question arises then, how representative is information from dry playas in understanding the structure of wet playas? Interestingly enough, we found more crustacean and snail species in dry sampling than in wet sampling, though presumably a longer survey of the wet playas would have turned up the additional species. While the wet sampling approach resulted in a longer list of insect species, we actually found a couple of species in the dry sample cultures that were not found in the wet sampling. In general, it appears that the wet sampling generally reveals more insect species in general, while the dry sample culturing reveals more crustacean species. In general, however, our results show that Dry Sampling and Culturing can both yield important information about the health and functionality of a given habitat. Dry sampling and Culture allows us to sample at any time of the year and still yields usable data on playa functions and values. This is critical when there is a limited time to make conservation or land management decisions regarding playa wetlands.
Factors Affecting the Health of Playa Communities
Because playas and most other ephemeral wetlands lack federal environmental protection, they are often subjected to grazing pressure, row crop agriculture, and construction projects. To determine whether modified playas differed significantly from natural, relatively undisturbed playas, we sampled crustacean communities from 73 ephemeral wetlands in the high plains of Colorado, Wyoming, Montana, South Dakota, and Nebraska) (O’Neill et al., in press). Neither habitat size, habitat depth, nor whether a wetland was natural or artificially created had any recognizable effect on the crustacean community. Moreover, natural communities have great flexibility, which seems to impart resilience under some anthropogenic forces. Macroinvertebrate communities from artificial waterbodies (roadside ditches and stock ponds) were indistinguishable from those in naturally formed playa wetlands. Cattle grazing, which in some ways resembles effects of native vertebrate grazers like bison and pronghorn, was generally associated with increased invertebrate densities and richness. In contrast, tilling for row-crop agriculture, removal of grazing, and over-grazing decreased invertebrate density and richness. Overall, current conservation strategies in ephemeral wetlands may need to be revised to include artificial habitats as viable, important habitats, and cattle grazing as an essential ecosystem component in areas now lacking large native grazers, such as bison.
Playa Food Webs
Scientists know relatively little about the trophic ecology of ephemeral aquatic habitats beyond the importance of hydroperiod and vertebrate predators. Therefore, we evaluated the food webs in 21 playas located mostly in the short-grass prairies of the Pawnee National Grassland in Colorado, U.S.A. using stable isotope analyses (O’Neill and Thorp, 2014).
Our results were both predicted and unexpected. Playas with higher insect diversity had more complex trophic structures than those dominated by large branchiopods (tadpole, clam and fairy shrimps). Insect diversity seemed dependent on length of the hydroperiod and time since filling, both of which are determined by playa depth. The trophic structure of naturally created playas was marginally more complex than artificial playas, and playas buffered by native vegetation were more likely to have food webs that included unique trophic strategies. Food webs of playas influenced by row crop agriculture had a broader selection of food sources. Playas grazed by cattle had food-web structures comparable to those in ungrazed playas, suggesting that playa biota may be adapted to large mammal disturbances. Food chain length increased in some playas as the ephemeral ecosystem approached the end of its hydroperiod.
We conclude that the key to understanding trophic structure in playas is an interaction between hydroperiod and the traits, lifespans, and trophic niches of the species present.
Community Assembly Rules in Playas
Playas are excellent for studying community succession and food-web assembly, in part because they allow repeated observations of complete assembly dynamics (see O’Neill and Thorp [in review] from which this section was largely derived). This contrasts with the information based on more long-lasting ecosystems which has formed the foundation of community assembly studies in ecology. While ephemeral ecosystems may not always “follow the rules” when it comes to community assembly, they do organize themselves in discernible patterns. Community assembly in playa wetlands can proceed differently among playas depending upon the active species pool, which is in turn dependent on the nature of the playa. While density and diversity tend to increase early, the eventual decline could be due to many factors including predation, competition, and life history constraints. How well these patterns resemble those in permanent systems depends on several factors, including which taxa colonize, how they interact with current residents, and the length of the hydroperiod. The ultimate trophic structure seems more closely tied to community membership than hydroperiod stage, which could be due to specific traits and behaviors of individual organisms that are immigrating and emigrating. The unstable nature of ephemeral ecosystems means that many traditional community assembly rules need to incorporate the disconnection of temporal scales common in these systems.
The study of ecosystem processes in ephemeral ecosystems, such as community and food-web assembly has much to add to our current knowledge of how communities and food webs are assembled. Inclusion of the processes common in ephemeral ecosystems, such as the interruption of temporal mechanisms, will only enrich our understanding of community assembly and food-web processes.
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