The Evolution of an Ecosystem: Pleistocene Extinctions

Abstract

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 a herbivore population boom, leading to overgrazing of trees and grass, resulting in environmental exhaustion and extinction of herbivores. If true, bison survival and differentiation into two subspecies, (B. bison bison [plains bison] and B. bison athabascae [woodland bison,] through the Pleistocene may be accounted for thus: 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.