ECOSYSTEM ORGANIZATION AND EXTINCTION DYNAMICS

Most recent explanations for extinctions have focused on abiotic mecha nisms such as climatic change, bolide impact, and eustatic sea-level c hange. The structure and dynamics of the ecological systems that are r esponding to these disturbances have generally been overlooked. The in ternal structure of an ecosystem, especially interdependencies among i ts components, should strongly influence its response to environmental change, by controlling the degree to which loss of one component can produce additional extinctions. We used percolation theory to model th is process, by examining how disturbances propagate among interconnect ed nodes on lattices. Our models suggest that the magnitude of ecosyst em response to disturbance is strongly non-linear. Small disturbances can produce large extinctions and large disturbances small extinctions , depending on the degree of interdependency within the system. Distur bances may act to maintain ecosystems at low levels of overall integra tion. Paleontological and neontological evidence for long-term ecosyst em stability, co-evolutionary interactions, and mass extinction select ivity all emphasize the potential importance of interdependence in anc ient and modern ecosystems.