Measuring extinction, species by species

By Alister Doyle, Environment Correspondent

OSLO (Reuters) - The Yangtze River dolphin, the Christmas Island shrew and the Venezuelan skunk frog are all victims in an alarming flood of extinctions, but how do scientists decide when such "possibly extinct" creatures no longer exist?

The United Nations says the world faces the worst spate of extinctions since the dinosaurs vanished 65 million years ago, with man-made threats such as rising populations, felling of forests, hunting, pollution and climate change.

Yet proving that any individual species has gone the way of the dodo necessarily demands long, fruitless searching.

"If there's one thing in my career I'd like to be proved wrong about, it's the baiji," said Sam Turvey of the Zoological Society of London, using another name for the Yangtze River dolphin.

Turvey spent almost 3 months this year interviewing Chinese fishermen in vain for sightings of the long-snouted dolphin, which has not been seen since 2002. Some colleagues in China are still looking.

The baiji was almost declared extinct in 2006 after an acoustic and visual survey of the river turned up nothing. Then, a blurry video gave experts pause, and it was rated "possibly extinct."

About 300 plant and animal species, including the Christmas Island shrew and the Venezuelan skunk frog are also "possibly extinct," the worst category short of extinction, according to the International Union for Conservation of Nature's (IUCN) Red List.

If Turvey's study turns up no firm evidence, it will likely push the Yangtze River dolphin into the "extinct" column, said Mike Hoffmann, who manages a global project to assess species for the IUCN and Conservation International.