"But if you took a poll of biologists, I think most would say there are somewhere around 15 million," he told Reuters by telephone from the organisation's Swiss headquarters.

According to the Collins English Dictionary, a species is "a class of plants or animals whose members have the same main characteristics and are able to breed with each other".

The conundrum will hang over a UN conference in Brazil next week where experts will discuss ways to slow the loss of species. The United Nations agreed in 2002 to reduce the rate at which animals and plants are disappearing by 2010.

"The implication of not knowing exactly how many species there are is that we can't tell if we are actually making progress on the 2010 target," said McNeely.

WHAT WE KNOW, WHAT WE DON'T

What we know is that around 1.7 million plant and animal species have been identified and named by scientists.

There are probably few large mammals on land left to be discovered although new deer and wild pig species were found in Vietnam in the 1990s in a region that had been heavily bombed by the United States during the war between the two countries.

Most birds have been named although new ones do crop up occasionally and a few "extinct" ones, such as the famed ivory-billed woodpecker of the US south, have reappeared.

Biologists say there are still many plant, insect and fish species that have yet to be named.

"In southern Africa, there are between 3,000 and 4,000 described spiders. I would estimate this is less than 50 percent of what there is in the region," said John Leroy, co-author with his wife of a field guide to spiders in southern Africa.

Underscoring the apparent ubiquity of unnamed creatures, Leroy says unknown spiders could even be lurking in gardens.

"In Johannesburg, you could easily find a new species in your backyard, if you knew what to look for," he said, adding that new information on species had been collected, but much of it was gathering dust in laboratories.

The dearth of knowledge about species numbers stems in part from the fact that there are only so many qualified scientists out there who can actually name a new plant or animal. And many of those who are qualified focus on known species.

LOST WORLDS

The uncertainty also highlights the fact that we have not explored the planet as thoroughly as a modern atlas may suggest.

Scientists said last month they had found a "Lost World" in an Indonesian mountain jungle, home to dozens of exotic new species of birds, butterflies, frogs and plants.

"Every expedition that goes into a place that hasn't been examined before finds new species," said McNeely.

So is there any scientific consensus on these issues?

Scientists agree on the sobering fact that most species that walked, crawled, swam or flew at some point are now extinct.

"There are millions of different species of animals and plants on earth -- possibly as many as 40 million. But somewhere between 5 and 50 billion species have existed at one time or another," writes paleontologist David Raup in his book "Extinction: Bad Genes or Bad Luck?".

"Thus, only about 1 in 1,000 species is still alive -- a truly lousy survival record; 99.9 percent failure!"

Scientists also agree that current extinction rates are far higher than "natural" ones because of human activities such as pollution, habitat destruction, and greenhouse gas emissions that have been linked to climate change.

But pinning down precise extinction rates is impossible when so many species have not yet been described by science.

A new housing development in Johannesburg could sound the death knell for an unknown butterfly restricted to a small area. Or a piece of Brazilian rainforest could get chopped down and a species of toad could become extinct as a result.

Some number-crunching has been done for known species.

According to the World Conservation Union, more than 800 plant and animal species have become extinct since 1500, when accurate historical and scientific records began.

Critics, who say environmentalists are being alarmist, would point out that this is less than 0.05 percent of known species.

About one-third of described amphibian species are endangered compared to about 12 percent of bird and 23 percent of mammal species.

Scientists say other measurements can also be used.