Herbivory plays a crucial role in structuring coral reef communities. A decrease in abundance of herbivores increases the probability of state shift from coral to algae domination. Although both fish and sea urchins are important herbivores, many tropical reef systems have suffered from over-harvesting of fishes and therefore, a more thorough understanding the effect of urchins on community structure is essential. This study examines how population dynamics of the urchin Diadema savignyi, affect the reef community of Moorea, French Polynesia. D. savignyi occur either in high density aggregations or as cryptic solitary individuals. I am exploring both potential causes and effects of this spatial variation in density. To test whether variation in intensity of predation contributes to the observed spatial pattern, urchins were tethered at locations around the island to estimate spatial variation in survivorship. There was little temporal variability in survivorship within a location, but substantial spatial differences among the locations. Spatial heterogeneity in urchin survivorship appears to be explained well by variation in abundance of their primary predators (e.g., triggerfish). My results suggest a major influence of top-down control on the structure of the coral reef community in Moorea.