IN MOST PLACES in the world, the human population is growing, and people are building on land that was previously wilderness or countryside. As human development expands, space for other kinds of animals shrinks. Sometimes it shrinks so much that there is no place left for them to live.

This is what happened to the dusky seaside sparrow, a bird of the Florida marshlands. In the middle years of the twentieth century, as the marshes were flooded for mosquito control and filled in for buildings, the sparrow's numbers went steadily down. Finally birdwatchers could count only six—all males. One of these was captured and put in a cage at Walt Disney World, where it died on June 16, 1987. With its death, the dusky seaside sparrow became extinct.

Extinction—natural and unnatural

Extinction is not a new phenomenon. A species has...