Protection of coral reef fish delivers ecosystem-critical biocontrol of coral-eating starfish across the Great Barrier Reef.

While biological control (or biocontrol) is an established method for managing pest species in terrestrial systems, few successful applications have been reported for marine environments. Crown-of-thorns starfish (CoTS, Acanthaster ssp.) are regarded as a pest species across the Indo-Pacific, where they are voracious predators of corals and represent one of the largest causes of coral mortality on the Great Barrier Reef (GBR). The role of reef fish in moderating outbreaks of CoTS through biocontrol has recently become more widely recognized. Here we have incorporated reef fish into a meta-community model of the GBR to demonstrate the critical role that marine reserves and other fisheries regulations have had in limiting the prevalence of CoTS outbreaks and maintaining the resilience of the GBR ecosystem. Our results suggest that without these interventions, the GBR would have already passed a major tipping point to a new state characterized by few predatory fish, continuous CoTS outbreaks and substantially lower coral cover. Model projections to 2050 demonstrate the importance of maintaining protection into the future and suggest that additional gains can be made over the next decade by continuing to manually control CoTS numbers. However, beyond 2040, the escalating impacts of climate change and the underlying resilience of CoTS populations will limit the effectiveness of interventions based on biocontrol.