Body size changes of Bison and mortality age structure data document the effects of climate-driven environmental change and human hunting pressure on large mammals in North America. Morphometric and mortality data are drawn from 58 archaeological and 9 paleontological localities dating between 37,000 and 250 calBP. Proxy information on body size is based on measurements recorded on 901 adult calcanei (os calcis) and 1026 humeri. In addition, published mortality profiles from 24 archaeological faunal assemblages spanning the last 14,000 years were used to approximate the age structure of bison populations. These results suggest that dramatic diminution in bison body size occurred in several short bursts, rather than a continuous gradual decline. These periods of rapid size reduction correlate with times of ecological reorganization, when aridity-driven changes in grasslands decreased forage quality and availability. Mortality age data indicate that the decrease in body size occurred in a context where there was no evidence for a progressively severe juvenile bias in bison populations. Overall, it appears that the changes in body size were a reaction to environmental conditions rather than the result of human predation pressure.

It is generally assumed that evolution is an issue of looking at how a species fits into its environment. This over-constrains our thinking; we should look at how the species and the ecosystem evolve together. The current theories of the Pleistocene extinction (Climate change and Overkill by H. sapiens) are inadequate. Neither explains why: (1) browsers, mixed feeders and non-ruminant grazer species suffered most, while ruminant grazers like bison generally survived, (2) surviving mammal species, including both subspecies of bison, were sharply diminished in size; and (3) vegetative environments shifted from plaid to striped (Guthrie, 1980). In addition, climate change theories do not explain why mammoths and other megaherbivores survived changes of similar magnitude. Although flawed, the simple overkill hypothesis does link the extinctions and the arrival of H. sapiens. However, it omits the reciprocal impact of prey decline on H. sapiens; standard predator-prey models, which include this effect, demonstrate that predators cannot hunt their prey to extinction without themselves succumbing to starvation. An alternate scenario and computer simulation (download at http://quaternary.net) characterized by a boom/bust population pattern is presented. It suggests H. sapiens reduced predator populations, causing herbivore populations to boom, leading to overgrazing of trees and grass, resulting in environmental exhaustion and extinction of herbivores. If true, bison survival and differentiation into two sub- species, B. bison bison [plains bison] and B. bison athabascae [woodland bison,] through the Pleistocene may be accounted for thusly: environmental exhaustion selectively favors animals that could extract maximum energy from low quality forage to survive and reproduce the split into sub species is a reflection of the new vegetative environment.

Dozens of large mammals such as mammoth and mastodon disappeared in North America at the end of the Pleistocene with climate change and "overkill" by human hunters the most widely-argued causes. However, the population dynamics of humans and megafauna preceding extinctions have received little attention even though such information may be telling as we expect increasing human populations to be correlated with megafaunal declines if hunting caused extinctions. No such trends are expected if climate change was the primary cause. We present tests of these hypotheses here by using summed calibrated radiocarbon date distributions to reconstruct population levels of megafauna and humans. The results suggest that the causes for extinctions varied across taxa and by region. In three cases, extinctions appear linked to hunting, while in five others they are consistent with the ecological effects of climate change and in a final case, both hunting and climate change appear responsible.