Thresholds and Multiple Stable States in Coral Reef Community Dynamics

Multiple stable states occur when more than one type of community can stably persist in a single environmental regime. Simple theoretical analyses predict multiple stable states for (1) single species dynamics via the Allee effect, (2) two-species competitive interactions characterized by unstable coexistence, (3) some predator-prey interactions, and (4) some systems combining predation and competition. Potential examples of transitions between stable states on reefs include the failure of Diadema antillarum and Acropora cervicornis to recover following catastrophic mortality, and the replacement of microalgal turf by unpalatable macroalgae after rapid increase in the amount of substratum available for colonization by algae. Subtidal marine ecosystems in general, and reefs in particular, have several attributes which favor the existence of multiple stable states. Studies of transitions between states often need to rely upon poorly controlled, unreplicated natural “experiments,” as transitions typically require pulses of disturbance over very large spatial scales. The stability of a state must often be inferred from analyses of the dynamics of participants at that state, as generation times and the potential for further extrinsic disturbance preclude the use of persistence as an indicator of stability. The potential for multiple stable states strongly influences our interpretation of variability in space and time and our ability to predict reef responses to natural and man-made environmental change.