The implications of trophic competition among predators: A case study of coral reef fishes

Background/Question/Methods

Typical Lotka-Volterra models of predator-prey dynamics assume that predators act independently within the community, without intra-guild competition. For example, doubling of predators leads to a doubling of predation threat. New evidence from coral reefs, however, calls into question the generality of this assumption. Fish assemblages of remote (and unfished) coral reefs show evidence of inverted trophic pyramids, namely with a higher biomass of apex predators relative to the biomass of lower trophic guilds. In these cases, doubling the predator biomass is unlikely to double predation threat. I consider here the energetic constraints emerging from inverted trophic pyramids among fishes, specifically considering the role of competition among predators for food in modulating community dynamics. Further, I investigate the dynamical changes associated with reductions in predator biomass, mimicking the shifts observed in fish assemblages along gradients of fishing pressure. I combine results from analytical and simulation models with empirical data from across the tropical Pacific to address these questions.

Results/Conclusions

Data-driven models of tropical fish assemblages can produce inverted trophic pyramids only in the presence of strong competition for food among predators. Such trophic competition among predators results in a prey assemblage that is dynamically controlled from the bottom up. As predators are removed from the system (as by fishing, in this case study), competition among predators reduces. Interestingly, as predator density reduces, control of the prey assemblage shifts to occur from the top down. Amendments to theories of functional responses and trophic dynamics provide the framework to explore these results. These theoretical results suggest that in predator-rich environments, changes in predator density should have little impact on the size of prey populations (characteristic of bottom-up control). In contrast, prey populations in predator-depauperate environments should show more intimate coupling with predator population size (characteristic of top-down control). Data of fish assemblage structure from a large gradient of predator biomass across the tropical Pacific conform to this prediction.