Multiple stressors on coral reefs: A long-term perspective

Coral reefs are subject to a high frequency of recurrent biological and phy sical disturbances. The temporal and spatial scales of these are often larg e and difficult to study, so that most of our knowledge of disturbances on coral reefs comes from investigations conducted at one or a few sites, over shea periods of time. We argue that studying single events in isolation ca n be misleading and that a longer term approach is necessary for understand ing the responses of coral reef assemblages to multiple stressors. We prese nt first a brief review of the impacts of physical disturbance (e.g., cyclo nes, hurricanes) on the community dynamics of coral reefs, with special att ention to the effects of recurrent events. We then examine two unusually de tailed, long-term data sets from Heron Island, Australia, and Jamaica which demonstrate some of the complexities of multiple stressors (broadly define d as natural or man-made disturbances). Both case studies illustrate that the effect of a particular disturbance of ten depends critically on the impact of previous perturbations. Consequentl y, even the same type of recurrent stressor can have different effects at d ifferent times, depending on history. Accordingly, when the added dimension of time is considered, the distinction between single and multiple stresso rs becomes blurred. Even a single event such as a hurricane can be viewed m echanistically as a multiple stressor, with short- and long-term impacts. W e emphasize that multiple stressors often have significant effects on recru itment and regenerative processes of assemblages. These impacts are much le ss obvious than catastrophic or chronic mortality, but they play a crucial role in community dynamics over longer time scales. Importantly, chronic an thropogenic impacts can impede the ability of coral assemblages to recover from natural disasters, even where there is little detectable effect on rat es of adult mortality. Once a reef has been degraded, it is usually impossi ble to ascertain retrospectively the precise mechanisms that were involved or the relative importance of different events. A single survey will provid e a snapshot of the status of coral reefs, but a longer term approach is re quired to understand the processes underlying changes in assemblages.