Untangling food‐web structure in an ephemeral ecosystem

Abstract

Summary Scientists know relatively little about the ecology of ephemeral aquatic habitats beyond the importance of hydroperiod and vertebrate predators, especially regarding trophic structure. We used playa wetlands, ephemeral habitats common in arid to semi‐arid regions, as our study ecosystem. We predicted that larger, more species‐rich playas would have food webs with larger dimensions (longer food chains and more energy sources used), more variation (more niches filled) and more redundancy (niche overlap). We used geometric properties of stable C and N isotope biplots to explore: (i) what regulates trophic structure in ephemeral wetlands, (ii) the role of anthropogenic forces in altering factors controlling trophic structure and (iii) how trophic structure in ephemeral systems differs from that typical of permanent systems. We sampled aquatic animal food webs of 21 playa wetlands located mostly in the short‐grass prairies of the Pawnee National Grassland in Colorado, U.S.A. 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 key to understanding trophic structure in playas is an interaction between hydroperiod and the traits, lifespans and trophic niches of the species present. 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. The trophic structure of ephemeral aquatic habitats may not respond to disturbance in the same way as permanent systems. Indeed, food chain length in a playa can actually increase as the ephemeral ecosystem approaches the end of its hydroperiod. Measures of ecosystem size need to include more than the two dimensions of surface area when being applied to general ecological theory. Food webs increase in complexity with time, and many of the same factors that control trophic structure in ephemeral habitats could be important at least at some time in the life of a relatively permanent ecosystem.