TERRESTRIAL COMMUNITY REORGANIZATION DURING THE EOCENE – OLIGOCENE TRANSITION WITHIN NORTH AMERICA

The Eocene-Oligocene Transition (EOT; 40 – 30Ma) represents one of the most dramatic climate change events in the earth’s history. Changes in ocean current circulation triggered the expansion of glacial ice sheets, a fall in global sea levels, global cooling, and terrestrial aridification that produced large-scale ecological turnover across nearly all major faunas. The best-documented terrestrial record of the EOT is preserved in the White River Group (WRG) deposits of western North America. These communities are useful for studying the long-term effects of climatic perturbation at the ecosystem scale because of the longevity of the genera preserved within; many of the major families originate in the late Eocene and persist across the EOT boundary until the middle Oligocene. To understand how these communities responded to global climatic changes, the WRG faunas were taxonomically grouped into biostratigraphic zones following recent treatments. Each species was assigned to an ecological guild based upon its dietary lifestyle and size data. Using predator-prey relationships, we then reconstructed trophic guild networks of the White River faunal community interactions. Using these guild networks, we applied a numerical model of ecosystem dynamics to the White River fauna to assess the community stability and robustness following varying degrees of environmental perturbation (e.g., reduction in primary productivity) resulting from the climate changes associated with the EOT. Stability is reflected in an ecosystem’s ability to return to equilibrium after minor perturbation, and robustness is the resistance to collapse under major perturbation. Consistent with earlier studies of WRG faunal turnover utilizing different methods, our preliminary results suggest these communities experience relatively little change in stability and robustness across the EOT as compared to other EOT terrestrial faunas. Our results suggest that the communities before and after the EOT within the WRG are robust, resisting collapse until primary productivity was reduced by 60–70%.