Trout have weak effects on zooplankton diversity but strong effects on community biomass

Abstract The introduction and extirpation of predators can have large impacts on food web structure and ecosystem function. The magnitude of these predator effects can be influenced by species diversity and turnover along environmental gradients in space or time, which have the potential to buffer or magnify the impact of trophic interactions on ecosystem functioning. In this study, we explored the individual species and community effects of fish predators on lake zooplankton in the Sierra Nevada, CA, USA. Local and beta diversity in zooplankton communities were resilient to fish, only marginally differing among fish and fishless sites, and were more structured along elevational gradients. Observed diversity changes were attributed to species turnover in zooplankton communities, which suggests elevational gradients and fish are acting as ecological filters excluding subsets of species rather than reducing species density. We found that fish significantly reduced densities of larger‐bodied zooplankton species. Further, as larger‐bodied species densities were lower in the presence of fish, we found that the community weighted mean of body mass decreased in the presence of fish in lakes. Although species turnover largely maintained diversity over elevational gradients and in the presence of fish, such changes in larger‐bodied taxa and community biomass may have important implications in the transfer of energy throughout the food web and to connected riparian and aquatic ecosystems that are dependent on the flow of biomass and nutrients.