STABILIZING THE UNSTABLE: THE IMPORTANCE OF ECOLOGICAL INTERACTIONS IN THE PROCESS OF RECOVERY FROM MASS EXTINCTIONS
ROOPNARINE, Peter D., Invertebrate Zoology & Geology, California Academy of Sciences, 55 Concourse Dr, Golden Gate Park, San Francisco, CA 94118 and ANGIELCZYK, Kenneth D., Department of Geology, The Field Museum, 1400 South Lake Shore Drive, Chicago, IL 60605, proopnarine@calacademy.org
The E. Triassic was a time of delayed biotic recovery after the end Permian mass extinction, with low biotic diversity and unstable biogeochemical cycles. Here we show that the ecosystem of the Karoo Basin, South Africa was altered significantly by the extinction, and did not recover until at least the late Olenekian. We applied a numerical model of ecosystem dynamics to paleocommunities spanning the late M. Permian to the Olenekian-Anisian Cynognathus Assemblage Zone (CAZ). The model examines community stability and robustness, where stable systems return to equilibrium after minor perturbation, and robustness is the resistance to collapse under major perturbation. All Permian communities and the CAZ were stable, maintaining equilibrium under reductions of primary productivity up to ~60%. Resulting secondary species extinctions in local communities may have been ameliorated by immigration given broad geographic species distributions. Those communities were also robust, resisting collapse and elevated extinction until 60-70% reductions of primary productivity. The earliest Triassic LystrosaurusAssemblage Zone (LAZ) ecosystem was an exception. Results indicate that LAZ communities were unstable, with a tendency to unravel in disequilibrium, thus also losing robustness.
Instability and low robustness in the model LAZ is caused simultaneously by depressed herbivore diversity in the extinction's aftermath, and great diversification of carnivorous temnospondyl amphibians and small bodied amniotes. Resulting strong interactions between those carnivores and lower trophic level prey, mutual carnivory, and increased indirect competition for diminished resources all contributed to LAZ instability. This would have led to elevated rates of evolution and extinction of the destabilizing elements. Alternatively, reducing dietary breadth of the carnivores or lowering interaction strengths in the model increased model stability and robustness, suggesting that LAZ could have been stable if it was an ecologically unusual or metabolically depressed system. Both paths point to a recovery, realized in the CAZ, of increased herbivore richness and functional diversification of amniote carnivores.
