Predator identity and ecological impacts: Functional redundancy or functional diversity?

Different species occupy similar trophic positions in natural communities. However, ecologists have often overlooked the consequences of this variation for local communities by assuming that species occupying similar trophic positions are functionally similar. There have been few experimental tests of this important assumption. We tested the assumption of functional similarity by comparing the effect of six different predators (three fish and three salamander species) on an ensemble of larval anuran prey. Our experiment identified substantial variation in the impact of different predators on a variety of responses. Differences among predators in their selection of prey caused the structure of the larval anuran ensemble to vary continuously as opposed to producing discrete alternative states. Predators also differed in their ability to suppress either the total number or biomass of anurans. Thus, performance of larval anurans was dependent upon the identity of the predator. Moreover, the identity of predators judged similar and their degree of similarity depends on the specific response variable. In our study we found no predominance of weak or strong interactors, as predators appear to be evenly distributed along a gradient of interaction strengths. Knowledge of the identity of the species occupying a particular trophic position is crucial to predicting the impact of that trophic position on a community. Ecologists should not simply assume that species are functionally similar. The differing effects of different predators suggest that variation in their distribution across complex landscapes influences prey number, biomass, population dynamics, distribution, and ultimately local and regional species diversity.