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 predicted by the intermediate disturbance hypothesis (18). In addition, the strong light gradient in these light-dependent communities allows a potential interaction between growth rate and disturbance such as that postulated by Huston (56). This review evaluates alternative explanations of the apparently general pattern of diversity in relation to depth on coral reefs. The significant effects of disturbances of many kinds on coral reefs are well enough established (18) to conclude that models based on competitive equilibrium are unlikely to apply to coral reef community structures. Therefore, the review concentrates primarily on nonequilibrium mechanisms, although many of the potential equilibrium mechanisms are discussed. The central question of the review is whether the intermediate disturbance hypothesis alone is sufficient to explain the general patterns or whether the interaction of disturbance with growth rate must be considered. To evaluate these questions, I review published data on the major types of biotic and abiotic disturbances on reefs, as well as factors related to the intensity of competition and the rate of competition-induced population changes. Where possible, I describe experiments that address these questions. However, the reef environment is so complex that it is virtually impossible to carry out a field experiment that manipulates only one factor while holding all others constant. At present, the critical experiments have not been carried out that would evaluate all of the questions addressed in this review, and thus consistency of observations with patterns predicted by the hypothetical controlling factors is the strongest test available.

Depth range
0- 90 m

Mesophotic “mentions”
10 x (total of 9591 words)

Classification
* Focused on 'mesophotic' depth range
* Focused on 'mesophotic coral ecosystem'

Fields
Ecology
Community structure
Biodiversity
Disturbances

Focusgroups
Overall benthic (groups)
Scleractinia (Hard Corals)
Algae (Macro, Turf and Crustose Coralline)
Protists (non-Symbiodiniaceae)