Mesopredator outbreaks in fragmented habitats lacking top predators can dramatically alter ecological processes in ways that impede conservation and management of remaining habitat. On Indo-Pacific coral reefs, outbreaks of the corallivorous crown-of-thorns sea star (Acanthaster planci) are a primary cause of coral decline, and determining the ecological context that promotes or hinders outbreaks is crucial to successful coral reef management. Although Acanthaster densities can decrease in large no-take marine protected areas (MPAs) and reef systems subject to limited exploitation, insights from mesopredator studies in other systems suggest that small reserves may be vulnerable to outbreaks when embedded within increasingly fragmented and degraded coral reef ecosystems. Using a network of locally-managed MPAs in Fiji, we provide evidence that small reserves are at special risk for Acanthaster outbreaks. This included quantifying differences in Acanthaster density between MPAs and fished areas, as well as monitoring selective migration of adult Acanthaster into multiple MPAs.
Results/Conclusions
Acanthaster densities were 100-240% greater within small MPAs than in adjacent fished areas, and selective migration of adult sea stars from nearby fished areas into MPAs where their coral food was more abundant likely contributed to this trend. Tagged Acanthaster released along the borders of each MPA exhibited persistent movements biased towards the MPA at all borders sites. Daily displacement rates of tagged Acanthaster were negatively correlated with mean coral cover along area borders. Coral cover increased 80-440% immediately (within 20 m) inside MPAs compared to immediately outside, thus attracting sea stars. Our findings suggest that key reserve features including small size and/or marked contrasts in habitat quality between protected and fished areas may concentrate Acanthaster feeding within reserves. While small MPAs can enhance coral recovery, they may be too small to support biological control of Acanthaster and inadvertently foster Acanthaster densities that cause coral demise –– potentially negating the value of small MPAs. These findings highlight the context-dependent nature of mesopredator dynamics and the need to incorporate them into MPA design and management.