Even the natural environment was transformed. Europeans cleared vast tracts of forested land and inadvertently introduced Old World weeds. The introduction of cattle, goats, horses, sheep, and swine also transformed the ecology as grazing animals ate up many native plants and disrupted indigenous systems of agriculture. The horse, extinct in the New World for ten thousand years, encouraged many farming peoples to become hunters and herders.

The Columbian Exchange was not evenly balanced. Killer diseases killed millions of Indians. Within a century of contact, smallpox, measles, mumps, and whooping cough had reduced indigenous populations by 50 to 90 percent. The survivors were drawn into European trading networks that disrupted earlier patterns of life.