Prey-driven control of aquatic beetle predator assemblages

Trophic interactions are critical determinants of community structure and ecosystem function. In freshwater habitats, top predators are traditionally viewed as drivers of ecosystem structure, shaping populations of consumers and primary producers. The temporary nature of small water bodies makes them dependent on colonization by many organisms, particularly insects that form highly diverse predator assemblages. We conducted two mesocosm experiments to assess how prey (zooplankton) abundances influenced colonization and assemblages of natural populations of aquatic beetles. In the first experiment we removed zooplankton from mesocosms weekly via water changes, whereas in the second experiment we inoculated mesocosms with zooplankton, and both experiments had unmanipulated controls. We collected colonizing adult aquatic beetles weekly, which were identified to species, and periodically assayed zooplankton populations. Our results from both experiments showed that adult predaceous diving beetles (Dytiscidae) preferentially colonized mesocosms with higher abundances of zooplankton, whereas abundances of the omnivorous water scavenger beetles (Hydrophilidae) did not differ between treatments in both experiments. In the first experiment when we removed zooplankton via water changes, dytiscids preferentially colonized the control mesocosms throughout the duration of the water changes and for over one month after water changes ended, once zooplankton abundances in both treatments equilibrated. In the inoculation experiment, dytiscids again initially preferentially colonized inoculated pools, with colonization of the two treatments converging as zooplankton abundances converged. In both experiments, differential colonization of mesocosms by dytiscids, higher species richness in mesocosms with abundant zooplankton, and species-specific responses to zooplankton abundances generated distinct beetle assemblages between treatments. We experimentally demonstrate that zooplankton populations can be proximate regulators of predator populations and assemblages via prey density-dependent predator recruitment. Our results provide support for the importance of prey populations in structuring predator populations and the role of habitat selection in structuring communities. We indicate that traditional views of predators as drivers of ecosystem structure in many systems may not provide a comprehensive picture, particularly in the context of highly disturbed or ephemeral habitats.