Slate's Explainer: Determining Species Extinction

Slate senior editor Andy Bowers explains how scientists determine when a species has become extinct. The federal government recently pulled Yellowstone National Park's grizzly bears from the endangered species list.

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ALEX CHADWICK, host:

Whatever decision is made regarding the Yellowstone grizzlies and the threatened species list, environmentalists fear that many animals and plants are becoming extinct. Actually, it's sometimes hard even for field scientists to tell if a species has already disappeared or is just very rare. When do experts declare the death of a species? Here's an Explainer from Andy Bowers of the online magazine Slate.

ANDY BOWERS (Slate): It's a little tricky. The World Conservation Union used to operate under the 50-year rule, which held that an animal could be declared extinct only if it hadn't been seen in more than 50 years. In the 1990s, though, the rules were tightened and clarified. Today, the World Conservation Union will label a species extinct only if, quote, "there is no reasonable doubt that the last individual has died," unquote.

In general, scientists must now show the repeated efforts of survey a species' known habitat failed to turn up any individual sightings or evidence of its continued survival. The new rules mean certain species are more likely to be declared extinct than others. A large mammal that lives in open grasslands will be easier to survey than a small animal that lives underground or in a swampy forest, like the ivory-billed woodpecker that apparently reappeared from near-extinction in Arkansas last year.

A significant loss of an animal's known habitat can also be used as evidence for extinction, but only when scientists can show that the animal would not be able to survive in another environment.

Does a species ever re-emerge from official extinction? It happens. The Fernandina rice rat and the Vietnamese warty pig have both gotten reprieves. And one of the most famous species rediscoveries took place in 1938 with the identification of the coelacanth, a giant prehistoric fish presumed to have been extinct for 65 million years.

CHADWICK: Andy Bowers is a senior editor at the online magazine Slate, and that Explainer was compiled by Daniel Engber.

NPR's DAY TO DAY continues. I'm Alex Chadwick.

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