The effects of intraguild interactions (or lack of) on prey diversity in experimental ponds food webs

Abstract

The interactions between top- and intermediate-level predators can have significant effects on community structure of food webs. The direct and indirect effects of top predators not only affect prey communities both negatively (e.g. predation) and positively (e.g. thinning effects), but can also impact intermediate-level predators (via intraguild predation [IGP]). These IGP interactions can have especially far-reaching implications when predators of increased ecological importance, such as keystone predators, are involved. Yet, the strength of these interactions are often context-dependent with relationships that are highly variable, and little is known about the factors that shape the outcomes. We conducted a mesocosm experiment to test for the potential IGP interactions among top (invertebrate) and intermediate (amphibian) predators and their effects on a diverse prey community. We used larval Boyeria vinosa (Fawn Darner), a rarely studied member of the family Aeshnidae, and Ambystoma opacum (Marbled Salamander), a known keystone predator in parts of its range, in a mesocosm study with an additive experimental design. Each mesocosm received an equal assemblage of five anuran prey species. Total prey survival was lowest when both predators were present, but A. opacum overwhelmingly reduced the survival of three prey species, lowering species diversity in all food webs when present. Species diversity was highest in the B. vinosa food web, with one prey species (Anaxyrus americanus) emerging smaller and sooner than from other food webs. We did not detect any interactions between predator species, indicating that B. vinosa may not be as voracious as other members of Aeshnidae. Our findings suggest that density mediated IGP interactions are non-existent among our focal species. Ultimately, we also demonstrate the importance of examining variability in keystone predators and how abundant, but less-known predator species affect community structure.