Algae-herbivore interactions on the coral reefs of Moorea, French Polynesia

Herbivory is an important process to coral reef ecosystems. The relative role of herbivores in preventing the shift from coral domination to algal domination on reefs has been relatively unstudied. I determined the relative effects of herbivory versus nutrient enrichment on the algal assemblage at Moorea, French Polynesia by experimentally manipulating herbivore density, nutrient concentrations, and water flow rates. I found that herbivory is the most important factor influencing algal biomass and composition. Nutrient enrichment increased the nutrient content of algal tissue and some measures of biomass, whereas flow rate played a minor role in structuring the algal assemblage.; There are two types of herbivores, herbivorous fish and sea urchins, which differ widely in a variety of behavioral, morphological, and physiological traits. These differences likely have important influences on their relative effects on algae. I determined factors that influence the abundance of fish and urchins on two spatial scales. I found that, on a large scale, the availability of habitat was positively related to the densities of both fish and urchins. On a smaller scale, urchin densities were related to algal production, but fish densities were not related to either habitat or algal production. Further analysis of the behavioral responses of fish indicates that fish responded behaviorally to increased algal production by foraging at faster rates in areas where algal production was higher.; I also determined the relative effects of fish and urchins on algae and the influence of algal production on the relative effects of these herbivores. I found that herbivorous fish influenced five of six algal response variables while urchins influenced only two variables. The relative effects of fish on algal biomass were greater in areas of higher algal production, indicating that fish are able to sense and exploit increase algal production by consuming a larger proportion of algal biomass in areas of higher algal production.; Combined, my results indicate that fish primarily drive patterns in algal biomass, cover, and composition on these reefs and patterns in their effects are influenced by patterns in algal production.