Evidence for indirect effects of predation on coral reef fishes

Background/Question/Methods

It is widely recognized that removal of apex predators can have broad ecological consequences for terrestrial and aquatic communities. However, the indirect effects of predator removal are oftentimes less well understood. In marine ecosystems, fisheries exploitation directly alters fish communities by targeting large-bodied predatory species. The direct effect of extraction leads to reductions in species diversity, abundance, and size-structure of the fish assemblage. Reductions of predatory species can lead to compensatory release of non-targeted species. Despite there being well-studied examples of trophic cascades in terrestrial and aquatic systems, prey release is not commonly observed in complex systems such as coral reefs. The removal of predatory species may not lead to increased prey abundance or well-defined trophic cascades on coral reefs but may still have important direct and indirect effects that strongly influence the ecology of these systems. We examine the indirect effects of predation on reef fish assemblages by comparing life-history characteristics and body condition factors of fishes along a gradient of predator biomass. This was accomplished by performing targeted collections of a subset of the most abundant reef fishes from six central Pacific islands that span a gradient of fishing pressure and associated predatory fish biomass. 

Results/Conclusions

We found significant variation across islands in the patterns of growth and somatic investment for eight common reef fishes. Responses, however, differed among species, likely reflecting the varying indirect effects of predators across trophic levels. At islands where predators are abundant, prey species from lower trophic levels (herbivores and planktivores) showed decreased growth rates, reduced longevity, and smaller size relative to islands with fewer predators. Further, indices of condition (length-specific body mass and liver mass) of these prey species varied inversely with predator density. Contrasting results were observed for species of mesopredators. These small-bodied carnivores showed lower growth rates and reduced condition on islands with fewer apex predators. This result likely is a consequence of mesopredator release in areas where the density of apex predators has been reduced, thereby leading to increased intra-specific competition among these smaller-bodied carnivores. These findings suggest that predators can indirectly affect the prey assemblage even when signs of prey release or trophic cascade are not evident. Focused efforts to identify the indirect effects of predators on the life history and condition of prey species are imperative for understanding the dynamics of complex systems and for the development of effective management and recovery plans.