Herbivorous fishes and sea urchins help control coral community structure by grazing both micro- and macroalgae and are vital to coral reef resilience and recovery. The two major groups of herbivores present in Moorea are herbivorous fishes and the sea urchin Diadema savignyi. D. savignyi occurs in two abundance states in the lagoon habitats of Moorea: a low abundance state where urchins occur as solitary individuals and a high abundance state in which individuals occur in aggregations. Using a combination of field surveys and experiments, results indicated that the observed spatial patterns of abundance are likely alternative stable states in population abundance that can be explained by the existence of alternative attractors in the population dynamics of D. savignyi. This dissertation also assesses the changes in the functional composition of the herbivorous fish assemblage following two large disturbances on Moorea. Results showed that despite having unique functional assemblages among the fringing reef, back reef, and fore reef habitats, the herbivorous fish communities at all three habitats responded similarly to the two disturbances by moving towards communities with increased representation of scrapers. The increase in scrapers appear to be sufficient thus far in preventing a regime shift from occurring as macroalgae remains a relatively minor space holder and a massive recruitment of juvenile coral has been observed on the fore reef. I also assessed the amount of functional redundancy that exists in the herbivore community, how functional redundancy changed among functional groups due to the disturbances, and the amount of response diversity among species of the same functional group. Results indicate that not only is functional redundancy higher for “preventer” functional groups (i.e., those that prevent macroalgal establishment) than “removers” (i.e., those that remove established macroalgae) but that functional redundancy also increased following the disturbances for “preventers” but decreased for “removers”. Results also showed that a considerable amount of response diversity occurred among species of the same functional group suggesting that the herbivore community of Moorea may be able to withstand a variety of disturbances as there is large variation in how functionally similar species respond to perturbations.
Thesis or Dissertation
Department of Ecology, Evolution and Marine Biology, UC Santa Barbara