Functional diversity within a morphologically conservative genus of predators: implications for functional equivalence and redundancy in ecological communities

1. The idea that sets of species may have similar effects on population, community or ecosystem processes is a prevalent theme in many areas of ecology, especially in the context of biodiversity and ecosystem function. If indeed species are functionally equivalent, limiting similarity suggests that it should be closely related, morphologically similar species using similar resources in a similar manner. 2. We assayed the functional equivalence of three congeneric, morphologically similar predatory fish species (genus Enneacanthus). Functional equivalence was evaluated using aspects of both effects of fish on a variety of prey responses and the growth responses of the fish themselves as a measure of energy consumption. Fish were matched by initial size to control for effects of body size. A strict definition of functional equivalence based on niche theory was used to delineate it from the alternative of functional diversity. 3. Based on observed effects on larval anurans, only a single species pair could roughly be judged functionally equivalent, but these two species showed the greatest differences in growth rate and, hence, metabolic demand. Using the criterion of relative yield total, again, only a single pair could roughly be judged equivalent, however, members of this alternative species pair were dramatically different in their effects on larval anurans. Thus, as previously shown for a more diverse set of species, grouping of species by similarity in effects depends upon the specific response variable. 4. Overall range of effects produced on a variety of response variables was surprising, given the similarity in morphology and autecology, strong phylogenetic affinity, and the fact that neither predator size nor growth explained significant variation. Each species appears to be interacting with the environment in a different manner, either as a consequence of differences in metabolic demand or differences in preferences or efficiency with regard to prey types. 5. Observed responses are consistent with the predictions of niche theory and support an alternative explanation for observed relationships between diversity and ecosystem function. Our work suggests that functional equivalence may be uncommon, difficult to predict a priori, and that functional diversity, not functional equivalence, may underlie observed diversity-ecosystem function relationships.