Global rate reaches historic proportions.

• Position: Extinction - Brief Article

Half of all living bird and mammal species will be gone within 200 or 300 years, according to Donald A. Levin, a botany professor at The University of Texas at Austin. Although the extinction of various species is a natural phenomenon, the rate occurring in today's world is exceptional--as many as 100 to 1,000 times greater than normal.

On average, a distinct species of plant or animal becomes extinct every 20 minutes. Levin points out that research shows the rate of current loss is highly unusual--clearly qualifying the present as one of the six great periods of mass extinction in the history of Earth. "The numbers are grim. Some 2,000 species of Pacific Island birds (about 15% of the world total) have gone extinct since human colonization. Roughly 20 of the 297 known mussel and clam species and 40 of about 950 fishes have perished in North America in the last century. The globe has experienced similar waves of destruction just five times in the past."

Biological diversity ultimately recovered after each of the...