Predation risk, competition, and territorial damselfishes as drivers of herbivore foraging on Caribbean coral reefs

Food availability, competition, habitat complexity, and territorial damselfish shape foraging decisions of herbivorous coral reef fishes. However, relatively little is known about how predators affect herbivore diet selection. We examined diets of 2 common reef herbivores, Sparisoma aurofrenatum and Acanthurus coeruleus, in the Florida Keys, across sites of varying predator biomass. We used stable isotope analysis to understand the importance of predation risk relative to other known drivers of herbivore foraging decisions. For S. aurofrenatum, we found that greater predator biomass was associated with an increase in the diversity of resources consumed within populations. In contrast, increasing densities of damselfishes, which aggressively defend resource-rich algal gardens, were associated with lower diet diversity. However, within A. coeruleus populations, diet diversity increased with damselfish abundance, but was unrelated to predator biomass. Stomach content analyses and direct observation of diet selection in the field corroborated the stable isotope analysis. Importantly, both predator and damselfish abundance impacted diet diversity in different ways for these 2 fishes, which may be linked to differences in sociality and group foraging. A. coeruleus is more likely to forage in schools, potentially reducing predation risk and allowing them to overwhelm damselfishes and access their territories. Interestingly, damselfish abundance was positively correlated with predator biomass, suggesting that predators may influence herbivore diets indirectly via altered densities or behavior of damselfishes. Our work argues for more emphasis on the role of predation risk in affecting herbivore foraging in order to understand the implications of human-mediated predator removal and recovery in coral reef ecosystems.