Excluding consumers can promote coral recruitment in a remote and protected archipelago

Background/Question/Methods

Consumers, both herbivores and predators, play an integral role in structuring ecological communities and overall ecosystems. As a result of human exploitation, abundances of consumers, particularly large-bodied consumers, are declining worldwide. Ecosystems are simultaneously experiencing altered disturbance regimes, resulting in succession and recovery. On coral reefs, benthic feeding fishes (e.g. herbivores, corallivores, omnivores) are important consumers that mediate successional dynamics following disturbances like bleaching events. However, little is known about how these fishes influence coral reef succession across large spatial scales and previous studies have primarily been conducted in regions that already experience high fishing pressure. In our study, we ask the question: how does the exclusion of consumers influence coral reef benthic community succession and coral recruitment? We conducted a large-scale field experiment in the Chagos Archipelago, a remote marine protected area in the Indian Ocean with high fish and shark biomass, across eight sites up to 160km apart. We deployed 221 experimental settlement tiles using three treatments: caged (excluding large fishes), uncaged (no fishes excluded), and partially caged (cage control) tiles, and monitored coral recruitment and benthic taxa composition on the tiles after 2.5 years.

Results/Conclusions

Overall, our preliminary results demonstrate that consumers are important drivers of coral reef benthic community succession and coral recruitment. Specifically, the exclusion of fishes affected the location of where corals grew on the tile (i.e. on the exposed top surface or sheltered bottom surface of the tile). The caged tiles had 9X the number of corals than the uncaged tiles, when only considering corals growing on the top of the tiles. However, this trend reversed when evaluating the bottom of the tiles, where the uncaged tiles had 2X more corals than the caged tiles. In addition, relative recruit abundance varied by site, suggesting that site specific differences like fish composition, currents, and recruitment availability may influence consumer-coral interactions. The negative effect of consumer exposure on coral recruitment suggests that, in remote regions with high fish biomass, there is intense predation pressure on young corals that may outweigh the benefits of fish grazing down algae competitors. These results are contrary to findings in other coral reef ecosystems, where excluding/losing fishes typically decreases coral recruitment due to increased competition with algae. Thus, the consequences of fish loss for benthic reef communities may vary depending on fishing pressure and consumer community composition.