Most recent explanations for extinctions have focused on abiotic mechanisms such as climatic change, bolide impact, and eustatic sea-level change. The structure and dynamics of the ecological systems that are responding to these disturbances have generally been overlooked. The internal structure of an ecosystem, especially interdependencies among its 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 this process, by examining how disturbances propagate among interconnected nodes on lattices. Our models suggest that the magnitude of ecosystem 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. Disturbances may act to maintain ecosystems at low levels of overall integration. Paleontological and neontological evidence for long-term ecosystem stability, co-evolutionary interactions, and mass extinction selectivity all emphasize the potential importance of interdependence in ancient and modern ecosystems.
