Size matters: Predator outbreaks threaten foundation species in small Marine Protected Areas

<div><p>The unanticipated impacts of consumers in fragmented habitats are frequently a challenge for ecosystem management. On Indo-Pacific coral reefs, crown-of-thorns sea stars (<i>Acanthaster</i> spp.) are coral predators whose outbreaks cause precipitous coral decline. Across large spatial scales, <i>Acanthaster</i> densities are lower in large no-take Marine Protected Areas (MPAs) and reefs subject to limited human exploitation. However, using a combination of observational and manipulative experiments, we found that <i>Acanthaster</i> densities within a network of small, no-take MPAs on reef flats in Fiji were ~2–3.4 times greater inside MPAs than in adjacent fished areas and ~2–2.5 times greater than the upper threshold density indicative of an outbreak. This appeared to result from selective <i>Acanthaster</i> migration to the coral-rich MPAs from fished areas that are coral-poor and dominated by macroalgae. Small MPAs can dramatically increase the cover of foundation species like corals, but may selectively attract coral predators like <i>Acanthaster</i> due to greater food densities within MPAs or because the MPAs are too small to support <i>Acanthaster</i> enemies. As coral cover increases, their chemical and visual cues may concentrate <i>Acanthaster</i> to outbreak densities that cause coral demise, compromising the value of small MPAs. An understanding of predator dynamics as a function of habitat type, size, and fragmentation needs to be incorporated into MPA design and management.</p>