Predators, not nutrients, reduce fitness and facilitate algal colonization on a foundation coral species
Determining the relative roles of physical factors and predation in controlling community structure is a key focus in ecology. Current environmental-stress models postulate that physical stressors weaken top-down control, yet increasing evidence is revealing consumers can be unaffected or even enhanced by variations in physical conditions. To date, little is known about how abiotic factors interact with consumers to regulate coral reef communities, particularly direct top-down control by coral predators. To test this question, colonies of key reef-building staghorn coral, Acropora cervicornis, were subjected to nutrient enrichment and predation by the short coral snail, Coralliophila abbreviata, in a 2x2 factorial experiment, as this corallivore is both abundant in the region and prefers acroporid corals as prey. Secondly, in order to identify a relationship between predator density and coral mortality under natural settings, surveys were conducted on A. cervicornis in two sites where coral restoration was underway and across all coral species by surveying seven reef sites throughout the Florida Keys reef tract.
Our study revealed partial predation by C. abbreviata alone significantly increased coral mortality, reduced growth, and altered interactions with competitive algae. Predation caused significant colony and tissue mortality of A. cervicornis (59% of corals subjected to partial predation), however no colony or tissue mortality occurred on corals without predators during this experiment. Predation hindered A. cervicornis growth and reversed the positive effects of nutrients; while nutrient-only corals exhibited the highest growth (11% more than controls), growth of corals subjected to both nutrients and predators were reduced by 80%. Predation also facilitated algal colonization on coral colonies (18.1% coral surface area), which was otherwise extremely rare (0.009% surface area). Survey results revealed C. abbreviata density explained 27% and 40% of the variation in tissue mortality for A. cervicornis and across coral species, respectively. This study demonstrates that biological stress by chronic predation may be a strong regulator of coral populations, and may override the effects of some physical factors such as nutrient enrichment. As predator abundance can be managed locally, these findings show that reducing chronic predation may facilitate coral reef resilience to physical stressors.