Coping with Recovery: Community Sensitivity and Resistance in the Wake of Mass Extinction

Mass extinctions represent the disturbance, and perhaps collapse, of ecosystems in response to intense perturbation. Ecosystem recovery in the wake of such events is dominated by the contingencies of survivorship and the opportunism of diversification. The question then arises of whether post-extinction recovery communities are subsets of their predecessors, and function similarly, or whether they are no-analog communities with new properties and comprising new biotic relationships. Our ongoing survey of marine and terrestrial communities spanning the Paleozoic to the Recent suggests that, during any typical time interval, community networks tend to be structured trophically so that the propagation of low intensity perturbations to species or sets of species, to other members of the community, is localized and minimal. Paleocomunities reconstructed from the Early Triassic Lystrosaurus Assemblage Zone of the Karoo Basin, however, differed significantly from preceding and successive paleocommunities with respect to such propagation. Lystrosaurus zone communities were potentially hypersensitive to disturbance, raising the questions of why and how could those communities have persisted during what was likely an environmentally unstable time. Preliminary data suggest that the sensitivity resulted from the low diversity of amniote herbivores and omnivores in the aftermath of the end-Permian extinction, and the resulting creation of long pathways in Early Triassic food webs. We explore the possibilities that these communities could have persisted because of above-average rates of evolution, and/or metapopulation structures that were capable of sustaining local populations in the face of high rates of extirpation. The transition to the Cynognathus Assemblage Zone by the late Early Triassic was marked by the recovery of amniote diversity, and a return to more resistant communities.