by Michael L. McKinney

About the author: Michael L. McKinney is a geology professor at the University of Tennessee, Knoxville.

Population growth, wasteful patterns of consumption, and diminishing natural resources are rapidly pushing many species to the brink of extinction. Even biologists have difficulty assessing the complexity and speed of human impacts on the biological world.

In the United States, many native species were initially threatened mainly by overhunting: the killing of large game species beyond their capacity to reproduce. The buffalo disappeared from the eastern United States in the early 1800s, and elk, panthers, wolves, and a few other large species disappeared by the middle part of that century. Though some species, such as wolves, are recovering, others, like the passenger pigeon, are gone forever.

The second stage of human impact in the United States, as in...