Patterns of Species Diversity on Coral Reefs

Abstract

The zonation of dominant species and growth forms on coral reefs is one of the most striking patterns found in any natural community (42, 46, 115). Numerous factors may be important in controlling coral distributions and species diversity. These include light, usually correlated with depth; sedimentation; temperature; wave energy; plankton availability; frequency of mortality caused by storms or tidal exposure; and grazing by fish and urchins. Three major studies that correlate variation in physical factors with coral diversity and species composition over depth and horizontal position conclude that the reef environment is extremely heterogeneous and that species composition and diversity are determined not by physical gradients but by microhabitat conditions and complex biotic interactions (19, 88, 89). In spite of the heterogeneity of coral reefs strong evidence suggests that predictable patterns of species diversity exist along a depth gradient. Three separate studies on well-developed reefs have found similar patterns of species diversity in relation to depth. On each reef, diversity (measured by species richness and/or H') was low near the surface and increased to a maximum between 15 and 30 m in depth. (Red Sea 0-30 m (74); Jamaica 0-30 m (58); Jamaica 15-56 m (72); Indian Ocean 0-60 m (108). Below 30 m, diversity decreased gradually at the two sites where deeper surveys were made. Coral reefs have been described recently as being nonequilibrium systems, where competitive exclusion is prevented by frequent disturbances, as pre-