A GENERALIZED MODEL OF POPULATION DYNAMICS, FUNCTIONAL STRUCTURE AND GLOBAL STABILITY OF PERMIAN-TRIASSIC COMMUNITIES OF THE KAROO BASIN

The Karoo Basin of South Africa preserves a sequence of terrestrial ecosystems spanning the middle Permian to Middle Triassic, and includes the Permo-Triassic mass extinction (PTME). Recent revisions of vertebrate assemblage zones resolved the PTME into three phases during the end Permian-Early Triassic. Prior work modeled the transient stability of communities during the PTME, concluding that those communities would have remained relatively stable because of their functional structures. Here we present a generalized model examining the roles of community structure, strength of ecological interactions, and species life-history traits, in the global stability of a community (number of species in a community which persist over time). Recent work has shown that global stability scales negatively with community size, and the number (connectance) and strengths of interspecific interactions. Our model demonstrates that global stability also scales negatively with species population growth rates (r), and positively with the structuring of species into groups of functionally similar taxa (guilds). The application of the model to the Karoo paleocommunities during background extinction, including pre-mass extinction Permian communities and the Middle Triassic post-extinction community, shows that functional structure leads to greater global stability relative to unstructured model communities of equal taxon richness, connectance, interaction strength and r. During the PTME, advantages conferred by guilds are restricted to high r models, but high r is also associated with complex species dynamics, such as chaos, and hence lower global stability. Therefore, the loss of functional structure would not have reduced global stability during the PTME, if r remained low. In the Early Triassic LAZ community, during which diversity rebounds for the first time, functional structure lowered global stability at low r. Thus, LAZ’s functional structure would have reduced species persistence, relative to functionally unstructured communities. Therefore, the LAZ community should have been ephemeral, and would have been susceptible to the evolution of any functional structure that leads to an increase of stability, as observed in the Middle Triassic.