Effects of predation on intraspecific aggregation of prey and prey diversity in a subtidal marine food web

The aggregation model of coexistence suggests that intraspecific aggregation in local patches decreases richness locally, but increases richness regionally. While this model has largely been applied to competing species, it may also apply to predator–prey systems if predators influence intraspecific aggregation in their prey. Predators may influence intraspecific aggregation in prey communities by affecting prey immigration, emigration, and survival (both directly through predation and indirectly by modifying prey interactions and behavior). We performed a field experiment to determine whether predators influence the composition and aggregation of their macroinvertebrate prey in a marine subtidal food web. We quantified prey colonization of eight treatments that differed in the composition and richness of predators. We found between 50% and 70% of species exhibited positive intraspecific aggregation. We then asked whether the observed patterns in intraspecific aggregation among prey influenced local (alpha) diversity and turnover between local patches (beta diversity). Intraspecific aggregation was positively correlated with both alpha and beta diversity. The positive relationship between intraspecific aggregation and beta diversity was expected, however, the positive relationship between intraspecific aggregation and alpha diversity was not and runs counter to the negative relationship found in competitive communities. We suggest that the positive relationship we observed could occur because aggregation can increase prey survival and can generate a type III functional response.

► Intraspecific aggregation influences local and regional species diversity.
► We tested whether predators could influence prey aggregation in marine food webs.
► We found aggregation of marine invertebrate prey depended on predator composition.
► Increased aggregation increased both local and regional prey diversity.
► Behavioral responses of mobile prey to predators will influence marine food webs.