Climate change, not human population growth, correlates with late Quaternary megafauna declines in North America

The late Quaternary saw the extinction of a great number of the world’s megafauna (those animals >44 kg), an event unprecedented in 65 million-years of mammalian evolution. Extinctions were notably severe in North America where 37 genera (~80%) of megafauna disappeared by around the late Pleistocene/Holocene boundary (~11.7 thousand-years-ago, or ka). Scholars have typically attributed these extinctions to overhunting by rapidly expanding human populations (i.e., overkill), climate change, or some combination of the two. Testing human- and climate-driven extinctions hypotheses in North America, however, has proven difficult given the apparent concurrency of human arrival in the Americas—more specifically, the emergence of Clovis culture (~13.2–12.9 ka)—and terminal Pleistocene climate changes such as the abrupt warming of the Bølling-Allerød interstadial (B-A; ~14.7–12.9 ka) or near-glacial conditions of the Younger-Dryas stadial (YD; 12.9–11.7 ka). Testing these hypotheses will, therefore, require the analysis of through-time relationships between climate change and megafauna and human population dynamics. To do so, many researchers have used summed probability density functions (SPDFs) as a proxy for through-time fluctuations in human and megafauna population sizes. SPDFs, however, conflate process variation with the chronological uncertainty inherent in radiocarbon dates. Recently, a new Bayesian regression technique was developed that overcomes this problem—Radiocarbon-dated Event-Count (REC) modelling. Using the largest available dataset of megafauna and human radiocarbon dates, we employed REC models to test whether declines in North American megafauna species could be best explained by climate change (temperature), increases in human population densities, or both. On the one hand, we reasoned that if human overhunting drove megafauna extinctions, there would be a negative correlation between human and megafauna population densities. On the other hand, if climate change drove megafauna extinctions, there would be a correlation between our temperature proxy (i.e., the North Greenland Ice Core Project [NGRIP] δ¹⁸O record) and megafauna population densities. We found no correlation between our human and megafauna population proxies and, therefore, no support for simple models of overkill. While our findings do not preclude humans from having had an impact—for example, by interrupting megafauna subpopulation connectivity or performing a coup de grâce on already impoverished megafauna—they do suggest that growing populations of “big-game” hunters were not the primary driving force behind megafauna extinctions. We did, however, consistently find a significant, positive correlation between temperature and megafauna population densities. Put simply, decreases in temperature correlated with declines in North American megafauna. The timing of megafauna population declines and extinctions suggest that the unique conditions of the YD—i.e., abrupt cooling, increased seasonality and CO₂, and major vegetation changes—played a key role in the North American megafauna extinction event.