The resilience of coral reefs and its implications for reef management

Our view of ecosystems has evolved from one emphasizing determinism to an understanding that systems can exhibit dramatic, and often surprising, shifts in state. Perhaps the most well-known shift is the replacement of corals by macroalgae, but others occur when systems experience overwhelming bioerosion or heavy sedimentation. Preventing undesirable shifts in ecosystem state is a key goal of management, particularly given the need to stem the loss of ecosystem services. However, ecosystem shifts have proved difficult to predict because they can occur with little warning. Worse, the symptoms, such as loss of coral, and may be difficult to reverse because ecological feedback processes can constrain recovery. Thus, it is important to understand the factors that drive shifts in ecosystem state and the stability of such states. This is the study of resilience. A resilient reef is usually considered to be one that absorbs disturbances and recovers to a coral-rich state (though other states are also possible). We describe methods to quantify explicitly the resilience of a reef by combining models of a reef’s equilibrial dynamics with its stochastic disturbance regime. In this case, resilience can be calculated as the probability that a reef will avoid shifting to an alternate stable state in a prescribed period of time, given its current state and anticipated disturbance regime. We then discuss the opportunities to “manage for resilience.” Because many acute disturbances, such as coral bleaching, cannot be mitigated directly, the emphasis for management is to enhance processes of coral recovery through the management of watersheds, nutrient-runoff, and grazers. In addition, scientists are beginning to understand spatial patterns of the response of corals to disturbance. Although such research is at an embryonic stage, it promises to play an important role in helping to stratify the interventions of managers across the seascape