Thresholds and Multiple Stable States in Coral Reef Community Dynamics

Multiple stable states occur when more than one type of communitycan stably persist in a single environmental regime. Simpletheoretical analyses predict multiple stable states for (1)single species dynamics via the Allee effect, (2) two-speciescompetitive interactions characterized by unstable coexistence,(3) some predator-prey interactions, and (4) some systems combiningpredation and competition. Potential examples of transitionsbetween stable states on reefs include the failure of Diademaantillarum and Acropora cervicornis to recover following catastrophicmortality, and the replacement of microalgal turf by unpalatablemacroalgae after rapid increase in the amount of substratumavailable for colonization by algae. Subtidal marine ecosystemsin general, and reefs in particular, have several attributeswhich favor the existence of multiple stable states. Studiesof transitions between states often need to rely upon poorlycontrolled, unreplicated natural "experiments," as transitionstypically require pulses of disturbance over very large spatialscales. The stability of a state must often be inferred fromanalyses of the dynamics of participants at that state, as generationtimes and the potential for further extrinsic disturbance precludethe use of persistence as an indicator of stability. The potentialfor multiple stable states strongly influences our interpretationof variability in space and time and our ability to predictreef responses to natural and man-made environmental change.