Eating all, but mainly the small: How predators shift prey species to favor larger-bodied taxa

Background/Question/Methods

The direct effects of predators are well-documented, leading primarily to increased mortality rates of prey species with a host of knock-on effects of prey density and life history. In simple food webs, clear patterns of prey release or trophic cascades can emerge, for example, if the direct effects are sufficiently strong. However, in more complex food webs the evidence of direct effects of predators can be more obscure due to the effects of non-linear functional responses and competition among prey species. On coral reefs, only limited evidence exists of well-defined trophic cascades, with the examples typically including echinoderms (sea stars or sea urchins) as a prominent member of the relationship. Within coral reef fisheries, the relationships between the densities of predators and their piscine prey have been documented only at the coarsest of scales. Working with a large collection of high-resolution census data of coral reef fishes from 42 islands of the tropical Pacific, we have been tracking patterns of variability in density and size structure of the fish assemblage. Capitalizing on a prominent gradient of fishing intensity across these islands, we have identified clear signals of predator effects by considering the effects of predation and competition in concert.

Results/Conclusions

In the presence of abundant predator assemblages, prey fish taxa demonstrate clear signs of increased mortality probabilities. In particular, the size-structure of prey taxa becomes more left-shifted with the prey assemblage being comprised of smaller-bodied individuals in the presence of abundant predators; we have validated that this reflects reduced longevity due to increased predation rates. When census data are considered at the trophic level (species aggregated based on food source), there are no strong density effects as a function of predator density. Instead, when census data are considered at the species level within trophic level, a systematic pattern of density is revealed. In the presence of abundant predators, the densities of smaller-bodied prey taxa are reduced more than the densities of larger-bodied taxa. The results suggest that within a trophic level, predators affect the pattern of competition. For the individuals that can pass through the predation gauntlet successfully (i.e., species with capacity to reach larger sizes), there is increased opportunity to exploit shared resources. This work provides a novel view into the interaction between predation dynamics and intra-guild competition on coral reefs.