Resilience of Pacific Staghorn Coral is governed by a fish

Thicket-forming staghorn corals provide essential habitat for fishes and invertebrates on tropical reefs worldwide, but they are highly vulnerable to disturbances. Despite the potential for rapid colony growth, the abundance of staghorn coral either returns very slowly or fails to recover at all following landscape-scale disturbances. The causes of low resilience of this important coral morpho-type are not understood. The species of staghorn coral that forms thickets at the MRC site, Acropora pulchra, is no exception; the abundance of this species has not increased appreciably in the lagoons of Moorea since widespread destruction by a cyclone nearly 3 decades ago. We tested three general hypotheses that could account for the exceedingly slow recovery of thickets: (H1) staghorn propagules do not recruit to areas once occupied, (H2) staghorn recruits no longer can tolerate current abiotic conditions where they once thrived, and (H3) staghorn recruits cannot tolerate natural enemies. Observations and experiments ruled out Hypotheses 1 and 2 and supported Hypothesis 3. Recently recruited propagules of A. pulchra were observed where thickets historically thrived, indicating that the low resilience was not due to lack of larval connectivity with adult corals elsewhere. Notably, every new staghorn recruit was located in the territory of a gardening damselfish, the farmerfish Stegastes nigricans. We found that farmerfish are key to both the establishment of new staghorn colonies and the persistence of existing thickets. Farmerfish, which occur mostly with staghorn and less commonly with other corals, protect their gardens from herbivores and their coral host from corallivores. Transplanted staghorn thrived in cages or when protected by farmerfish; unprotected corals were rapidly consumed by corallivorous fishes. Because the number of farmerfish in a group increases with the size of a staghorn thicket, there is a positive feedback between farmerfish and staghorn coral that enables the thicket to persist even where corallivorous fish are abundant. The establishment of new staghorn colonies requires larvae to settle into the protected gardens of farmerfish, which can be locally rare after landscape scale disturbances. Thus the abundance, persistence and rate of recovery of staghorn thickets are governed primarily by the behavior and dynamics of a territorial fish.