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?

A researcher trains a finless porpoise (Neophocaena phocaenoides) at a research institute in Wuhan, capital of central China's Hubei province, November 6, 2006. REUTERS/Stringer

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.

It would be the first “megafauna” mammal -- one weighing more than 100 kg (220 lb) -- 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 night-time 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, known from a cloud forest habitat of 10 sq km (3.9 sq mile), has not been spotted despite repeated searches.

Despite the difficulties of proof, scientists say species are disappearing at an ever faster rate.

Some 76 mammals have gone extinct since 1500, a much faster rate than in previous centuries, and 29 are “possibly extinct” on the 2008 Red List.

BACK FROM THE DEAD

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 bluebuck antelope, which could have boosted tourism.

While most news is bleak, a few “Lazarus” species give 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.

Australian scientists were even delighted to find two dead night parrots in 2006 and 1990, taken as signs the reclusive species survives.

A few years ago the fabulous green sphinx moth, known from one Hawaiian island, was written off as extinct but then experts on another island were flabbergasted to catch one in a net.

Nevertheless, Hoffmann said Red List’s demands for evidence meant that it probably underestimated the pace of extinctions. Searches have to be rigorous, at the right seasons, and in nearby habitats, with the correct equipment.

“Scientists want to be cautious” because of the finality of extinction, Hoffman said.

“Possibly extinct” is a category so bleak that it does not even include the critically endangered ivory-billed woodpecker -- subject of speculation about a U.S. comeback after reported sightings in Arkansas in 2004.

“It has never been listed as ‘possibly extinct’ because there were sightings 20 to 30 years ago in Cuba,” Hilton-Taylor said. “There is still good habitat there.”

One result of declaring a species extinct is that it inevitably ends cash for conservation -- lending agencies such as the Global Environment Facility use Red List data.

And, when one species goes extinct, new ones become endangered, as is happening on the Yangtze River, where the finless porpoise and the Chinese paddlefish, reported to grow up to 7 meters (23 feet), are also in danger.

“The problem with the Yangtze is that the threats are still there and they are escalating,” Turvey said.

And there are wider threats. The U.N. Climate Panel said in 2007 that up to 30 percent of species will face increasing risks of extinction if temperatures rise by another 1 degree Celsius (1.8 Fahrenheit).

The panel, which says temperatures rose 0.7 C in the 20th century, also forecasts more droughts, heatwaves and rising seas linked to human emissions of greenhouse gases spurred mainly by burning fossil fuels.

In a 2006 report, Birdlife expert Stuart Butchart wrote that 150 bird species had gone extinct since 1500, or 0.3 a year. That was 30-300 times the background rate of extinctions -- a natural process deduced from fossil records.

And no one knows the number of species on earth -- one U.N.-backed study estimated 5-30 million against about 2 million documented so far. The U.N. Convention on Biological Diversity estimates they may be vanishing faster than they are found, at a rate of three per hour, the fastest in millions of years.

-- For Reuters latest environment blogs click on: blogs.reuters.com/environment/