Refuge-dependent herbivory controls a key macroalga on coral reefs

Small-scale structural complexity shapes how consumers and primary producers interact, which can influence ecosystem trajectories. Coral reefs are some of the most structurally complex ecosystems, though their complexity is threatened owing to anthropogenic influences. Some reefs shift towards macroalgal dominance following mass coral mortality, which can hinder the recovery of corals because they compete with the faster growing macroalgae for space. Using video observations, surveys and in situ experiments on a forereef in eastern Palau, we investigated the role different microhabitats play in facilitating the persistence of the macroalga Lobophora, which is one of the strongest negative interactors with corals. Collectively, our observational and experimental data show that small crevices provide a refuge to Lobophora recruits by excluding most adult herbivorous fishes. Consequently, Lobophora is disproportionately more abundant within these concealed microhabitats on the reef, which highlights the important role of microhabitats as macroalgal nurseries from which macroalgae can spread following mass coral mortality. While a large proportion of our current understanding of grazer–algae interactions is based on research using flat surfaces, our findings demonstrate that the interactions between herbivorous fishes and benthic organisms are strongly mediated by microhabitats. It is thus important to consider the influence of structural complexity in order to understand the nuances that govern benthic regimes.