UCB Moorea Class: Biology and Geomorphology of Tropical Islands

Herbivory plays an important role in shaping coral reef community structure. Herbivores consume algae on the reef, helping to prevent coral dominated reefs from “phase shifting” to algae dominated ones. Here I examine how the presence of an herbivorous sea urchin, Diadema savignyi, influences algae growth on Porites sp. coral heads using inclusion and exclusion cages. Corals were either caged with D. savignyi or no D. savignyi, or not caged at all. Treatments were applied for 25 days and percent algae cover was estimated weekly while algae length was measured on day 25. After 25 days, variation in algae cover and algae length were not significantly associated with D. savignyi presence. Corals kept in the presence of D. savignyi had the same amount of algae as corals that D. savignyi had been excluded from. These results are identical to a similar experiment that was performed in the laboratory in which corals were placed in a tank either with or without D. savignyi. These results suggest that at the current time, D. savignyi does not have a strong impact on reducing algae growth in the coral reefs of Moorea, French Polynesia. This may be due to the health of the reef, the ability of live corals to prevent new algae settlement and growth, or the presence of superior fish herbivores.