Introduction

As we have just seen, North America lost most of its large mammals a little over 10,000 years ago - just after humans arrived on the scence. A little over two years ago Josh Donlan and a group of colleagues outlined a ``a bold plan for preserving some of our global megafaunal heritage - one that aims to restore some of the evolutionary and ecological potential that was lost 13,000 years ago, and which offers an alternative vision for twenty-first century conservation biology'' [1, p. 913].

Our vision begins immediately, spans the coming century, and is justified on ecological, evolutionary, economic, aesthetic, and ethical grounds. The idea is to actively promote the restoration of large wild vertebrates into North America in preference to the `pests and weeds' (rats and dandelions) that will otherwise come to dominate the landscape. This `Pleistocene re-wilding' would be achieved through a series of carefully managed ecosystem manipulations using closely relatedspecies as proxies for extinct large vertebrates, and would change the underlying premise of conservation biology from manageing extinction to actively restoring natural processes. (emphasis added)

The authors propose a truly radical approach to conservation. They propose a wholesale restructuring of North American ecosystems, primarily in the Great Plains, to something they argue will approximate the conditions at the end of the last ice age. They base their proposal on four observations:

• Nowhere on earth is free of human influence, and our influence is increasing.

• Environmentalists are often portrayed as the bearers of bad news, making it difficult to build a constituency for conservation.

• Human populations are declining in many parts of the Great Plains, which offers opportunities for future conservation.

• ``Humans were probably at least partly responsible for the late Pleistocene extinctions in North America.

They also argue that the arrival of the first humans in North America, roughly 13,000ybp, provides a less arbitrary baseline for conservation than the arrival of Columbus in 1492.

Last year Donlan et al. [2] provided a much more extended development of the idea and provided a much more complete discussion of the idea and its justification. They point out that ``[f]or the past 200 million years, large carnivores have been dominant features of most ecosystems...Any thoughtful natural historian should wonder about how the loss of these large vertebrates subsequently influenced biodiversity and ecosystem function'' [2, pp. 661-662]. As the authors demonstrate, the body size distribution of those animals that went extinct at the end of the Pleistocene is dramatically different from the body size distribution of those that survived (Figure 1).

Figure 1: Body size distribution of North American animals that went extinct (top) and that survived (bottom) the end-Pleistocene extinction in North America.