The Perturbation Resistance of Early Permian Terrestrial Communities Characterized by Low Tetrapod Herbivore Diversity

In his work on trophic network evolution in terrestrial communities, Everett Olson suggested that a fundamental transition occurred in the Permian, with communities characterized by low tetrapod herbivore diversity being replaced by those with higher herbivore diversity. Based on modeling experiments with trophic networks reconstructed for eight Permian and Triassic communities from the Karoo Basin, South Africa, we previously suggested that this replacement may have occurred because disturbances are better able to propagate through carnivore-dominated communities, leading to widespread extinctions. To test this hypothesis, we reconstructed trophic networks for two Early Permian communities with low tetrapod herbivore diversity, the geographically- and time-averaged fauna from the entire Wellington Formation, Oklahoma, and the subset of that community which occurs at the Waurika 1 locality, a more spatially and temporally restricted assemblage lacking some faunal components such as insects. These networks were subjected to simulated bottom-up perturbations using the CEG model (Roopnarine, 2006), with the prediction that they should be vulnerable to collapse even at low disturbance levels. Waurika 1 was susceptible to collapse at most perturbation levels, although it displayed slightly more resistance than the highly unstable Early Triassic Lystrosaurus zone community from South Africa. The Wellington Formation fauna, however, displayed an unexpectedly high degree of perturbation resistance, which may stem from its high insect herbivore diversity. If this is the case, then the radiation of tetrapod herbivores during the Permian may have had only minor implications for community function and perturbation resistance despite significantly altering trophic structures. Alternatively, Olson may have underestimated the diversity of community types that existed during the Early Permian. Finally, these new data underscore the unusual vulnerability of the Lystrosaurus zone community, raising the question of how strongly this property is related to its existence in the immediate aftermath of the end-Permian extinction.