Contrasting effects of fish predation on benthic versus emerging prey: a meta-analysis

Predator-prey interactions are often studied entirely within the ecosystem of the predator. However, many prey transition between ecosystems during development, expanding the effects of predators across ecosystems. Prey are often vulnerable to predation during this transition, facing a predator gauntlet as they leave their source ecosystem. As a result of predation during this transition, predators may have stronger effects on prey fluxes to the neighboring ecosystem than on prey densities in the predator's own ecosystem. I used meta-analysis of predator (fish) and prey (invertebrate) interactions in freshwater ecosystems to test the hypothesis that fish have stronger effects on prey flux to the terrestrial ecosystem, by reducing insect emergence biomass, than on prey densities in the aquatic ecosystem, by reducing benthic insect/invertebrate biomass. Fish reduced insect emergence by 39 % on average, more than twice as strong as their reductions of benthic prey (16 % reduction; averages are variance-weighted). In fact, fish effects on benthic prey were not significantly different from zero, but were significant for emergence. These results indicate that predator effects can not only cascade from one ecosystem to another but also that effects can be stronger outside than within the ecosystem of the predator. Failure to account for this may underestimate the effects of predators on prey.