DOES EXTINCTION RESISTANCE EXPLAIN CHANGES IN THE STRUCTURE OF TERRESTRIAL COMMUNITIES?

An examination of Carboniferous through Triassic terrestrial communities reveals that early communities were dominated by carnivores; most herbivores were aquatic and almost all primary productivity came from aquatic sources. This resulted in a complex trophic network with strong ties to the aquatic realm. Starting in the Middle Permian, food web structures changed. Terrestrial herbivores diversified, making terrestrial producers the main source of productivity and simplifying terrestrial food webs. This community structure persists to the Recent, whereas Carboniferous-style community structures become very rare after the Permian. Here we examine whether this turnover was caused by characteristics of species in the communities or emergent properties of the communities. We used a probabilistic trophic network model to examine how much secondary extinction can be caused within a community by perturbations of various sizes, and applied the model to eight Permian and Triassic terrestrial communities from South Africa. Seven of the communities have structures similar to those of modern communities, whereas the earliest Triassic Lystrosaurus Assemblage Zone community resembles Carboniferous communities. The seven communities of modern aspect are resistant to perturbation, requiring large disturbances to cause high levels of secondary extinction. In contrast the zone community is very vulnerable: small perturbations can cause significant secondary extinction. Therefore, terrestrial communities with Carboniferous-style organization may not have persisted because they were unstable and liable to collapse after minor disturbances. Our results suggest that the properties that make communities resistant to extinction are a complex mix of species and community-level characteristics. Finally, these findings imply that the end-Permian extinction reset terrestrial communities to a more primitive state, much as it did for marine communities.