Reconstructing large herbivore abundance and environmental interactions in postglacial North America

Abstract

Large herbivores drive critical ecological processes, yet their long-term dynamics and effects are poorly understood due to the limitations of existing paleoherbivore proxies. To address these shortcomings, long-term records of paleoherbivores were constructed by (i) applying new analytical techniques to existing bison fossil datasets; and (ii) examining fecal steroid data that characterize temporal changes in ungulate abundance and community composition. These paleoherbivore reconstructions were analyzed in relation to their environmental contexts to better understand herbivore-ecosystem interactions through time in three separate studies: First, spatiotemporal changes in postglacial bison distribution and abundance in North America were examined by summarizing fossil bison observations. Bison observations were compared with simulated climate variables in a distribution modeling framework to project probable bison distributions in 1000-year intervals from the Last Glacial Maximum to present in light of changing climatic drivers over time. Since the Bolling-Allerod Interstadial (14.7-12.9 ka) the geographic distribution of bison is primarily explained by seasonal temperature patterns. Second, Holocene records of bison abundance were compared to paleofire reconstructions spanning the midcontinental moisture gradient to determine the relative dominance of herbivores and fire as biomass consumers. Bison dominated biomass consumption in dry settings whereas fire dominated consumption in wetter environments. Historical distributions of herbivory and burning resemble those of Sub-Saharan Africa, suggesting a degree of generality in the feedbacks and interactions that regulate long-term consumer dynamics. Third, the utility of fecal steroids in lake sediments for reconstructing past herbivore abundance and identity was tested by (i) characterizing the fecal steroid signatures of key North American ungulates, (ii) comparing these signatures with multiproxy data preserved in lake sediments from the Yellowstone Northern Range, and (iii) comparing influxes of fecal steroids over time to historical records of ungulate biomass and use. Bison and/or elk were abundant at Buffalo Ford Lake over the past c. 2300 years. Ungulate densities in the watershed were highest in the early 20 th century and likely contributed to decreases in forage taxa and possibly increased lake production. These results demonstrate long-term ecological impacts of herbivores and highlight opportunities for continued development of paleoherbivore proxies.