Coral mortality versus structural collapse as drivers of corallivorous butterflyfish decline

As climate change increasingly threatens biodiversity, identifying specific drivers of species loss as well as the attributes of species most vulnerable to climatic disturbances is a key challenge to ecologists and conservationists. Here we assess the effects of coral loss versus declines in structural complexity on obligate and facultative coral feeding butterflyfishes on coral reefs in the central and western Indian Ocean. In the inner Seychelles, the abundance of the obligate coral feeding group declined markedly in response to live coral mortality (r (2) = 0.48), but showed no further decline with respect to erosion of the physical matrix of the reef. Conversely, the facultative feeding group showed no decline in response to live coral loss, reflecting their feeding versatility; however they did decline in response to structural erosion of the reef framework (r (2) = 0.26). There were no significant changes in either obligate or facultative corallivore abundances at a reference location (Chagos archipelago), highlighting that butterflyfish populations are stable in the absence of habitat disturbance. While specialised coral dependant fishes are highly vulnerable to coral loss caused by climate-induced coral bleaching, the structural collapse of dead coral colonies may have significant, but more variable, impacts across a wide range of fishes. If conservation and mitigation planning are to be effective, there is a clear need to better understand the mechanisms of reef structural collapse and the dynamics of system recovery following large-scale disturbance.