Dissertation Abstract

The Biogeography of Biotic Upheaval: No-analog Plant Associations and the End-Pleistocene Megafaunal Extinctions

Gill, Jacquelyn L  2012  jacquelyngill.wordpress.com

Geography, University of Wisconsin (United States), 120 pp.

The late-glacial/early Holocene transition was a period of widespread environmental upheaval, including the arrival of the first humans, the extinction of 34 genera of megafauna, melting ice sheets, and the individualistic response of plant ranges and abundances. North American pollen records from this interval are characterized by a high minimum dissimilarity from present, representing plant associations with no modern analog. I tested the hypothesis that megafaunal herbivory played a role in the formation of late-glacial no-analog plant associations, using spores from the dung fungus Sporormiella as a lake sediment proxy for the presence of megaherbivores. I first review research on modern plant-megaherbivore interactions, outlining testable hypotheses about the impacts of the end-Pleistocene extinctions on vegetation. Next, I present a new, multi-proxy record (including fossil pollen, charcoal, and Sporormiella) from Silver Lake, OH, a classic no-analog pollen site. The Silver Lake paleorecord supports the hypothesis that a combination of top-down forcings, including novel climates and megaherbivory release, contributed to the formation of the no-analog plant associations and enhanced fire regimes following the local decline of megaherbivores at 13,900 BP. I conducted a modern process analysis of Sporormiella at Konza Prairie which links Sporormiella abundances to bison presence and grazing intensity (kg/m2/year), and indicates a very local spore source area (< 100 m). Lastly, I conducted a detailed review of late-glacial vegetation dynamics and no-analog plant associations in the Upper Midwest. I described the trajectory of communities in ecological space through time using nonmetric multidimensional scaling, showing how no-analog associations emerged from individualistic taxon-level dynamics. My results suggest that a combination of local site factors and periglacial conditions influenced the response of vegetation to extrinsic abrupt forcing, including megafaunal extinction and climate change, producing novel plant associations that were unique to the Upper Midwest.