PREDICTING PALEOCOMMUNITY RESISTANCE TO PERTURBATION AND EXTINCTION CASCADES

Ecological theory predicts that species may become locally or globally extinct in response to changes in community structure or dynamics, whether stimulated by abiotic or biotic disturbances. Studies of extinction in the fossil record generally focus on the magnitudes of extinction, and the susceptibility of taxa to perturbations of various sorts. The extent to which paleocommunity dynamics and biotic interactions affect paleocommunity resistance to extinction remains largely unexplored. This paper addresses the role that community structure and dynamics might play in the propagation of perturbations throughout a paleocommunity. We examined 8 terrestrial faunal zones from the Karoo Basin of South Africa spanning the Middle Permian – Middle Triassic. General observations of modern food web structure were used to parameterize probabilistic reconstructions of food webs (paleotrophic networks) for the 8 zones, which were then perturbed by simulated disruptions of primary productivity. Results show significant differences in the responses of communities from the different zones. There is no trend of increasing or decreasing resistance, but there is a general correspondence between simulated community resistance and observed levels of extinction from the fossil record.

There are two general modes of community response. Most zones, e.g. the Eodicynodon zone, exhibit a strong relationship between resistance and level of perturbation, which is predictable on the basis of linear interpretations of the simulation model. Several zones however, notably Tapinocephalus, Dicynodon and Lystrosaurus exhibit either markedly lower resistance even at moderate perturbation levels, or in the latter case, significant variation at all levels of perturbation. Lystrosaurus zone communities are extremely sensitive to the level of perturbation, and to the fine-grained composition of the perturbation, suggesting that those communities would have been inherently unstable. We further explore the bases for resistance with theoretical explorations of paleotrophic network topologies and internal dynamics of trophic interactions.