Community dynamics of ephemeral systems: food web drivers, community assembly, and anthropogenic impacts of playa wetlands

Abstract

Scientists know relatively little about the ecology of ephemeral wetlands and few attempts have been made to integrate the published ecological studies. We use playa wetlands, a common aquatic feature of many arid to semi-arid landscapes, to study what drives community dynamics and food web trophic structure (size, redundancy, and variability) in an extremely abundant aquatic habitat of the U.S. Great Plains. We conducted a multi-scale study to track playa wetland invertebrate communities and their food webs across space and time. We explored invertebrate diversity and richness and used geometric properties of stable isotope biplots (C13 and N15) to measure trophic structure complexity. On a large scale, playa communities are unaffected by changes in habitat size, depth, and native vegetation buffer. Artificially created waterbodies often hold communities indistinguishable from natural playas. Tilling for row crops negatively impacts playa diversity, but cattle grazing has a positive effect, perhaps reinstating functions of formerly abundant, native grazers, such as bison. Playa food webs were more complex in insect dominated communities rather than crustacean branchiopods. Insect diversity was controlled by hydroperiod stage; therefore, the key to understanding playa trophic structure is an interaction between hydroperiod and the traits, lifespan, and trophic niche of the species present. Whether a playa was artificially created or grazed by cattle had little effect on trophic structure, yet tilling for row crops expanded the food sources available for the food web. On a temporal scale, playa food webs initially increased in diversity, with declines at the end of the hydroperiod. Playas were organized in time rather than space. That is, food web complexity was more closely associated with identity of the taxa rather than total diversity. Mesocosm experiments showed that habitat structure may play a role in determining food-web complexity, but the effect was overridden by the influence of hydroperiod. Mesocosm experiments revealed insect diversity to control trophic structure complexity in playa wetlands, rather than the trophically redundant crustacean branchiopods. While playas are highly resistant and resilient habitats, they cannot survive all perturbations. Playas should be protected and have much to inform us about food web assembly and maintenance.