Alternative states on coral reefs: beyond coral-macroalgal phase shifts

Degradation of coral reefs is often associated with changes in community structure where macroalgae become the dominant benthic life form. These phase shifts can be difficult to reverse. The debate on coral reef phase shifts has not focused on reports of coral reefs becoming dominated by other life forms following disturbance. A review of the primary and grey literature indicates that reefs dominated by corallimorpharia, soft corals, sponges and sea urchins can enter an alternative state as a result of a phase shift. Shifts can be triggered by pulse disturbances that cause large-scale coral mortality, and may become stable as a result of positive feedback mechanisms. However, they may differ from the archetypical coral-macroalgae shift, depending on the factors driving the shift; whereas coral-macroalgae and coral-urchin shifts seem to be driven by loss of top-down control through overfishing, shifts to corallimorpharian, soft coral and sponge dominance seem more associated with changes in bottom-tip dynamics. Understanding the differences and similarities in mechanisms that cause and maintain this variety of alternative states will aid management aimed at preventing and reversing phase shifts of coral reefs.