The leading environmental information network says 15,589 species are now known to be in a perilous position.

Science has understood for some years that an eighth of all birds and a quarter of all mammals are in jeopardy.

But the latest Red List shows a third of amphibians and almost 50% of turtles and tortoises are on the brink, too.

The IUCN, which can call on the expertise of some 10,000 scientists across the globe, believes the threat facing global biodiversity is escalating.

It lists the 15,589 species as either Critically Endangered, Endangered or Vulnerable.

"The fact that we know more makes the situation look worse, of course, because we can list more and more species in trouble. But that isn't why the declining trend is accelerating - it is a real phenomenon," Dr Simon Stuart, who has authored a Global Species Assessment (GSA) to accompany the Red List, told BBC News.

The GSA shows trends in biodiversity over four years since the last major analysis was done in 2000. It highlights in particular the trouble now facing amphibians and cycads, an ancient group of plants.

"A lot of cycads are valuable in the horticultural trade; people want to collect them. Many have small distributions anyway, so economic [exploitation] and habitat loss is the last straw. Some are down to one individual."

Over-exploitation and habitat loss are pressures working against many species - but so is the competition from introduced (or alien) animals and plants. Human-induced climate change is thought to be an increasingly significant problem, too.

The GSA includes the first complete assessment of amphibians that was reported in the journal Science last month. These creatures (which include frogs, toads, salamanders and caecilians, or legless amphibians) are the most threatened of all vertebrate groups. They also have more species on the verge of extinction.

In total, 21% of amphibians are Critically Endangered or Endangered, whereas the proportions for mammals and birds are only 10% and 5% respectively.

The Red List is only a partial picture of the world's biodiversity.

Estimates for the total number of species on Earth vary wildly; there may be 10 million, there could be 100 million. What is certain is the limited number of species catalogued by science - barely two million.

While the status of known vertebrates is relatively well documented (roughly 40% assessed), poor information exists on the animals and plants that inhabit freshwater and marine habitats, for example.

And science has only just begun to scratch the surface in understanding how many bacterial and fungal species may be out there.

"We can continue to assess and bemoan the loss of the world's biodiversity or we can act. We must refocus and rethink the way in which society must respond to this global threat," said Dr Achim Steiner, the IUCN's director-general.

"While most threats to biodiversity are human-driven, human actions alone can prevent many species from becoming extinct.

"There are many examples of species being brought back from the brink including the southern white rhino and black-footed ferret, and thousands of dedicated people around the world are doing their utmost to reverse the extinction rate."

Dr Stuart added: "Where we have been able to concentrate conservation focus, we often get very good result. Good things are achievable."