In French Polynesia the dusky farmerfish Stegastes nigricans engineers coral reef communities by farming algal turf and exerting resource control through territorial defense. These behaviors can result in indirect effects on coral by increasing coral interactions with farmed algal turf while decreasing coral interactions with mobile grazers and predators within farmer fish territories. Small-scale experiments involving farmerfish and turf removals indicated dominant boulder-like Porites corals were more vulnerable to competition with turf algae than were branching Acropora, Pocillopora, or encrusting Montipora corals. In contrast, delicate branching corals were more vulnerable to predation by mobile corallivores, and grew and survived better in the presence of farmerfish defense. We conducted a paired life table response experiment to measure these indirect effects in a demographic context using a combination of recruitment tiles and size specific population monitoring. We then removed farmerfish from half of the reefs and documented the response. We tested our experimental parameter estimates by comparing projected size distributions with spatially explicit size distributions of corals in the presence and absence of this abundant farmerfish.
Results/Conclusions
In contrast to studies showing negative effects of farmerfish on branching corals, we show that under certain conditions, S. nigricans actually facilitates the establishment, growth and survival of recovering branching corals. The disturbance history of our study area has played a pivotal role in the community changes observed. While S. nigricans usually colonizes Acropora thickets, a series of disturbances on the north shore virtually eliminated these habitats and farmerfish are found colonizing the dominant disturbance tolerant, but turf sensitive, Porites. An increase in substrate availability, when coupled with lower mortality rates, has led to enhanced recovery of important branching corals within farmerfish territories. After removal of S. nigricans, delicate Acropora, as well as more robust Pocillopora experienced sharply increased mortality, due to a combination of corallivory and grazing pressure by herbivores. Smaller individuals were more vulnerable, while larger individuals incurred partial mortality and colony shrinkage. Outside farmerfish territories, these branching corals are relegated to areas of high flow, or cracks and crevices, suggesting grazing pressure constrains recovery in the absence of this key ecosystem engineer. Our results show that disturbance can play a powerful role in modifying the sign and magnitude of interspecific effects within communities. Stegastes nigricans adversely affects branching corals in relatively undisturbed habitats, but has strong positive indirect effects on branching corals when disturbances lead to shifts in community structure of grazing populations.
