Stable isotope analysis of food web complexity in playa wetlands of the high plains: Effects of habitat complexity, physicochemical state, and biodiversity

Background/Question/Methods

Understanding factors controlling food webs is challenging, especially in natural settings because diversity and food web interactions can fluctuate widely through space and time. The problem is aggravated when the focus is on relatively permanent ecosystems with diverse habitats and high species diversity. In contrast, playa lakes, which are ephemeral wetlands in the Great Plains and many other arid to semi-arid regions, offer opportunities to reveal factors controlling food webs because they have low species diversity and relatively homogenous habitat, which eliminates the need to deal with large numbers of species to track, heterogeneous habitat, and confounding factors within large watersheds.

Our study focuses on factors controlling biodiversity and food web dynamics in playas of several states in the U.S. high plains. We are exploring the ecological importance of ecosystem size and habitat diversity (albeit low in both cases compared to most permanent aquatic systems) as well as hydrologic cycles and anthropogenic disturbances. During summer 2011 and 2012, we collected macroinvertebrates and amphibians from 86 playas and food web data (stable isotopes) from 25 playas in Colorado, Wyoming, Nebraska, South Dakota, and North Dakota. Sampled playas varied in many environmental features including size, depth, turbidity, dominant solutes, plant structure, and anthropogenic impacts (roads, crops, grazing, etc.). Food web complexity is being evaluated using stable isotope analyses, and experiments in artificial playas are in progress.

Results/Conclusions

Biodiversity was affected by many of the measured factors.  Species richness showed a unimodal relationship with maximum depth of the playa. Grassland depressional playas had the highest species richness, whereas rock pools and salt lakes were the least species rich.  The presence of grazing by cattle was highly correlated with vegetation density.  Grazed/vegetation-free playas were dominated by large branchiopods (fairy, tadpole, and clam shrimps), whereas ungrazed/vegetated playas had much higher proportions of insects (water beetles, water boatmen, backswimmers, and many dipterans), amphibians (tadpoles and salamanders), and molluscs were more common.  Food web complexity, as measured by Layman’s metrics, increased with species richness and habitat complexity. 

This study highlights how playa wetlands allow us to examine and challenge some of the long-held “beliefs” and paradigms dealing with food webs, species richness, and community assembly.   Additional biodiversity and food web studies are critically needed in playas because these ecosystems are vital aquatic systems in the Great Plains, they have rarely been studied, and they are mostly unprotected by environmental laws.