Not worth the risk: apex predators suppress herbivory on coral reefs

Abstract

Apex predators are known to exert strong ecological eff ects, either through direct or indirect predator – prey interactions. Indirect interactions have the potential to infl uence ecological communities more than direct interactions as the eff ects are propagated throughout the population as opposed to only one individual. Indirect eff ects of apex predators are well documented in terrestrial environments, however there is a paucity of information for marine environments. Furthermore, manipulative studies, as opposed to correlative observations, isolating apex predator eff ects are lacking. Coral reefs are one of the most diverse ecosystems, providing a useful model system for investigating the ecological role of apex predators and their infl uence on lower trophic levels. Using predator models and transplanted macroalgae we examined the indirect eff ects of predators on herbivore foraging behaviour. We show that the presence of a model reef shark or large coral-grouper led to a substantial reduction in bite rate and species richness of herbivorous fi shes and an almost absolute localized ces-sation of macroagal removal, due to the perceived risk of predation. A smaller-sized coral-grouper also reduced herbivore diversity and activity but to a lesser degree than the larger model predators. Th ese indirect eff ects of apex predators on the foraging behaviour of herbivores may have fl ow-on eff ects on the biomass and distribution of macroalgae, and the func-tioning of coral reef ecosystems. Th is highlights that the ecological interactions and processes that contribute to ecosystem resilience may be more complex than previously assumed. Predation is arguably one of the most infl uential selectio