Omnivore density affects community structure through multiple trophic cascades.

Omnivores can dampen trophic cascades by feeding at multiple trophic levels, yet few studies have evaluated how intraspecific variation of omnivores influences community structure. The speckled dace (Rhinichthys osculus) is a common and omnivorous minnow that consumes algae and invertebrates. We studied effects of size and size structure on top-down control by dace and how effects scaled with density. Dace were manipulated in a mesocosm experiment and changes in invertebrate and algal communities and ecosystem function were monitored. Omnivores affected experimental communities via two distinct trophic pathways (benthic and pelagic). In the benthic pathway, dace reduced macroinvertebrate biomass, thereby causing density-mediated indirect effects that led to increased benthic algal biomass. Dace also reduced pelagic predatory macroinvertebrate biomass (hemipterans), thereby increasing the abundance of emerging insects. The effect of dace and hemipterans on emerging insects was mediated by a non-linear response to dace with peak emergence at intermediate dace density. In contrast with recent studies, omnivore size and size structure had no clear effect, indicating that small and large dace in our experiment shared similar functional roles. Our results support that the degree to which omnivores dampen trophic cascades depends on their relative effect on multiple trophic levels, such that the more omnivorous a predator is, the more likely cascades will be dampened. Availability of abundant macroinvertebrates, and the absence of top predators, may have shifted dace diets from primary to secondary consumption, strengthening density-dependent trophic cascades. Both omnivore density and dietary shifts are important factors influencing omnivore-mediated communities.