Extinction Rate Across The Globe Reaches Historical Proportions

Although the extinction of various species is a natural phenomenon, the rate of extinction occurring in today's world is exceptional -- as many as 100 to1,000 times greater than normal, Dr. Donald A. Levin said in the January-February issue of American Scientist magazine. The co-author is Levin's son, Phillip S. Levin, a National Marine Fisheries Service biologist who is an expert on the demography of fish, especially salmon.

Levin's column noted that on average, a distinct species of plant or animal becomes extinct every 20 minutes. Donald Levin, who works in the section of integrative biology in the College of Natural Sciences, said research shows the rate of current loss is highly unusual -- clearly qualifying the present period as one of the six great periods of mass extinction in the history of Earth.

"The numbers are grim," he said. "Some 2,000 species of Pacific Island birds (about 15 percent 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 five past mass extinctions, probably requiring several million years in each instance. As for today's mass extinction, Levin said some ecologists believe the low level of species diversity may become a permanent state, especially if vast tracts of wilderness area are destroyed.

Other experts, in contrast, say breaking up today's vast ranges into smaller habitats could promote the evolution of new species. That's because populations of the same type of organism that are separated from each other may diverge over time. As populations are reduced in size, genetic changes may accumulate more rapidly. Another reason diversity may rebound -- as it normally does after a major extinction episode -- is that disturbances caused by human beings do not eliminate habitats, but merely change them.