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The actions of humans are major determinants (Fig. 1) of the biological composition of the Earth’s ecosystems at scales ranging from the genetic diversity of local populations to the level of landscapes. Although the anthropogenic impacts of modern industrial societies are obvious, early settlers using simple technologies also altered their environments dramatically through such practices as slash-and-burn clearing and the cultivation of crops in the rain forests of Central America; the diversion of riverwater for irrigation in the Near East; the intentional use of fire in the woodlands and prairies of North America’s temperate zones, and alternations in the assemblages of harvested and unharvested species in the far north (Turner et al. 1990)