Coral-algal phase shifts on coral reefs: Ecological and environmental aspects

This paper briefly reviews coral-algal phase shifts on coral reefs, with particular regard to summarizing the exogenous and endogenous factors in support of a proposed conceptual model, and to identifying critical information gaps. A phase shift occurs on a coral reef when the cover of a substrate by scleractinian corals is reduced in favor of macroalgal dominance, and resilience of the former condition is retarded because of ecological processes and/or environmental conditions. The change is often, but not always, associated with a perturbation such as coral bleaching, outbreaks of a coral-eating species, or storm damage. The new state is generally associated with some combination of reduced herbivory (from disease and/or fishing) and nutrient enrichment, although the relative importance of these factors is under debate and may vary among locations and even across single reefs. Disturbances that result in a state of generally low biotic three-dimensional structural complexity often precede a phase shift. Following such a disturbance, the system will pass to a state of higher biotic structural complexity, with either macroalgae or coral dominating. As the community progresses towards larger and more three-dimensionally complex corals or macroalgae, it exhibits greater resistance to shifting dominance from one state to the other. Studies of the phase-shift phenomena have been generally conducted at scales that are small relative to the sizes and inherent variability of whole coral reefs and systems of reefs. There is an urgent need for studies aimed at quantifying and simulating cause and effect aspects of the phase shift, including human-environment coupling, particularly in support of coral reef decision-making.