Multiple "stable" states and the conservation of marine ecosystems

Marine ecosystems were among the first to provide potential examples of multiple stable states. However, remarkably few of these have been explored in detail, and none have been rigorously confirmed. This may be because differences between alternative states are too subtle to document in the context of regular disturbance, because one state is naturally far more likely to occur than any other, or because most environments naturally support only one type of stable community. It is also possible that the temporal and spatial criteria required to document alternate stable states rigorously may be too difficult to meet in most circumstances. Nevertheless, the possibility of alternative stable states has recently received renewed attention in the context of marine conservation biology. People may be widening the range of habitats in which alternate stable states are possible, or they may be shifting communities to new domains of attraction that rarely occur in the absence of massive anthropogenic perturbations. The ability of people now to alter ocean ecosystems on global scales may eliminate "edge effects" that might otherwise rescue perturbed communities. Ecosystems with alternate stable states are characterized by positive feedback mechanisms that stabilize transitions; even if return to original conditions is predicted (that is, the alternate states are not stable), the same mechanisms will retard recovery. This may explain in part why return to original conditions following anthropogenic disturbance is slower than expected. Slow recovery times and transitions to new states are both potentially costly to human societies. Thus, from a conservation perspective the indefinite persistence of an alternate state may be less important than the presence of feedback loops that slow recovery. Both possibilities reinforce the arguments for application of the precautionary principle in managing marine ecosystems. (C) 2004 Elsevier Ltd. All rights reserved.