UCLA

Coral reefs are among the most diverse ecosystems globally, yet many are experiencing anthropogenic disturbances leading to reef degradation. Resilience (defined as resistance to and recovery from disturbance) to disturbance influences whether reefs shift from coral-dominated to degraded algal-dominated states. Two ecological processes that support reef resilience are herbivory and nutrient limitation. Herbivory is a strong top-down control on reefs that can be reduced by overfishing. Nutrient limitation is a bottom-up process limiting growth of algae and increased anthropogenic nutrients can enhance growth and shift competitive outcomes toward algae. I address 1) the role of functional diversity of herbivorous fishes in coral reef resilience, and 2) how nutrient enrichment degrades reef resilience by changing growth and interactions among algae.

Using field experiments, I assessed the functional diversity of herbivorous fishes by examining their foraging behavior on macroalgae on a fringing reef in Moorea, French Polynesia. I found herbivorous fishes clustered into two groups based on their relative selectivity and these groups have greater functional redundancy than complementarity.Functional diversity of herbivorous fishes has never been included in models of reef resilience and alternative stable states (ASS) despite empirical evidence of their importance in supporting reef resilience. When I incorporated herbivore functional groups into a coral reef model, I found herbivore community composition influences reef recovery and ASS. Specifically, browsers are better than grazers for promoting reef recovery. Also, ASS almost disappear for a browser-dominated community, while ASS are present even when there is no fishing pressure if the community is grazer-dominated. My findings highlight that including herbivorous fish functional groups into coral reef models is critical for recognizing complex interactions between human impacts and reef community compositions that support resilience or drive ASS of coral reefs.

Finally, I examined the effects of nutrient enrichment and macroalgal presence on algal turf growth over time. I uncovered a novel facilitative interaction where macroalgal presence increased algal turf height, which could promote shifts from short, healthy turfs to longer, less desirable turfs on coral reefs.

Overall, my dissertation quantifies resilience mechanisms and thus contributes to conserving healthy coral reefs into the future.