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 Yangtze River dolphin was almost declared extinct in 2006 after an acoustic and visual survey of the river turned up nothing. [File photo]

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 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 three 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 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.

It would be the first "megafauna" mammal - one weighing more than 100 kilograms - to die out since the Caribbean monk seal in the 1950s.

"To say something is extinct requires quite a lot of proof, of negative evidence, and may take many years to collect," said Craig Hilton-Taylor, who manages Red List.

Scientists working on the "possibly extinct" list rummage in the undergrowth for rare plants, frogs or rats, set up nighttime traps for bats or moths, or scour the seabed for corals.

Some experts liken the difficulties to "proving" that the mythical Loch Ness Monster does not exist.

The Christmas Island shrew has not been seen on its Australian island since 1985. The Venezuelan skunk frog has not been spotted despite repeated searches.

Seventy-six mammals have become extinct since the year 1500, a much faster rate than in previous centuries, and 29 are "possibly extinct" on the 2008 Red List.

Extinct species have often unknown economic value, such as the Australian gastric brooding frog, which incubated its young in its stomach and might have pointed to ways to treat ulcers. Or South Africa's blue buck antelope, which could have boosted tourism.

While most of the news is bleak, a few "Lazarus" species give conservationists cause for celebration - last year, a lizard presumed extinct turned up on La Palma in Spain's Canary Islands after no sightings in 500 years.