Compounded perturbations yield ecological surprises

All species have evolved in the presence of disturbance, and thus are in a sense matched to the recurrence pattern of the perturbations. Consequently, disturbances within the typical range, even at the extreme of that range a s defined by large, infrequent disturbances (LIDs), usually result in littl e long-term change to the system's fundamental character. We argue that mor e serious ecological consequences result from compounded perturbations with in the normative recovery time of the community in question. We consider bo th physically based disturbance (for example, storm, volcanic eruption, and forest fire) and biologically based disturbance of populations, such as ov erharvesting, invasion, and disease, and their interactions. Dispersal capa bility and measures of generation time or age to first reproduction of the species of interest seem to be the important metrics for scaling the size a nd frequency of disturbances among different types of ecosystems. We develo p six scenarios that describe communities that have been subjected to multi ple perturbations, either simultaneously or at a rate faster than the rate of recovery, and appear to have entered new domains or "ecological surprise s." In some cases, three or more disturbances seem to have been required to initiate the changed state. We argue that in a world of ever-more-pervasiv e anthropogenic impacts on natural communities coupled with the increasing certainty of global change, compounded perturbations and ecological surpris es will become more common. Understanding these ecological synergisms will be basic to environmental management decisions of the 21st century.