FAUNAL DYNAMICS, CLIMATE CHANGE, AND BISON: RECONSIDERING THE TERMINAL PLEISTOCENE MEGAFAUNAL EXTINCTION EVENT IN NORTH AMERICA

Hypotheses invoking climate change as the critical causal factor underlying the terminal Pleistocene megafaunal extinction event in North America are often considered inadequate. The key difficulty facing these scenarios is the lack of significant extinctions during earlier glacial-interglacial transitions of similar intensity. This challenge is rooted in the perception that large mammal faunas present in North America near the end of the Pleistocene were relatively resilient in the face of multiple climatic swings, which in turn assumes these faunas did not differ substantively from those present during earlier, equally severe periods of climate change.

This assumption is rejected here. An important difference in latest Pleistocene faunal composition was the profusion and geographic extent of the genus Bison, particularly in the American west. Bison, the index taxon for the Rancholabrean North American Land Mammal Age, first appeared in North America south of 55°N latitude after ~240 ka. During the late Pleistocene, south of the glacial ice, the species Bison antiquus spanned the continent more widely and in greater profusion than earlier species (e.g., Bison latifrons). In the Great Plains, bison were virtually nonexistent in faunas older than ~50 ka, exceeding other large mammal taxa numerically only after ~20 ka. In the southwest, fossils of Bison are relatively common in faunas younger than ~40 ka in age, but less numerous or absent in earlier assemblages.

Living bison are massive grazing ruminants that dramatically impact biological communities; their Pleistocene forebears were still larger. The increased abundance and distribution of these large herd-dwelling ruminants through time and space in late Pleistocene North America constitutes a critical difference between end-Pleistocene communities and those of earlier, similarly intense glacial-interglacial transitions – particularly with respect to the potential for driving increased competition among large mammals for water and forage. Responses of end-Pleistocene megafaunal communities to changing climate conditions and associated resource availability would therefore have been unique. Given this uniqueness, rejection of a climatic explanation for the extinction event is presently unwarranted.