Half the world's mammals are declining in population and more than a third probably face extinction, said an update Monday of the "Red List," the most respected inventory of biodiversity.

A comprehensive survey of mammals included in the annual report by the International Union for Conservation of Nature (IUCN), which covers more than 44,000 animal and plant species, shows that a quarter of the planet's 5,487 known mammals are clearly at risk of disappearing forever.

But the actual situation may be even grimmer because researchers have been unable to classify the threat level for another 836 mammals due to lack of data.

"In reality, the number of threatened mammals could be as high as 36 percent," said IUCN scientist Jan Schipper, lead author of the mammal survey, in remarks published separately in the US-based journal Science.

The most vulnerable groups are primates, our nearest relatives on the evolutionary ladder, and marine mammals, including several species of whales, dolphins and porpoises.

"Our results paint a bleak picture of the global status of mammals worldwide," said Schipper.

The revised Red List, unveiled at the IUCN's World Conservation Congress in Barcelona, is further evidence that Earth is undergoing the first wave of since dinosaurs died out 65 million years ago, many experts say.

Over the last half-billion years, there have only been five other periods of mass extinction.

The Red List classifies plants and animals in one of half-a-dozen categories depending on their survival status.

Nearly 40 percent of 44,838 species catalogued are listed as "threatened" with extinction, with 3,000 of them classified as "critically endangered," meaning they face a very high probability of dying out.

There were a few slivers of good news showing that conservation efforts can prevent a species from slipping into the category from which there is no return: "extinct."

The black-footed Ferret, native to the United States, was moved from "Extinct in the Wild" to "Endangered" after it was successfully introduced into seven U.S. states and Mexico.

The European bison and the wild horse of Mongolia made similar comebacks from the brink starting in the early 1990s.

But these remain exceptions that highlight the need to act before other species populations dwindle beyond the threshold of viability, experts say.

"The longer we wait, the more expensive it will be to prevent future extinctions," said Jane Smart, the head of the IUCN's Species Programme. "We now know what species are threatened, what the threats are and where."

The window of opportunity for great apes and monkey appears to be closing far more quickly that scientists realised, the new study shows.

"I was blown away when I saw the results, even though I was deeply involved in the work," said Michael Hoffman, a mammal expert at Conservation International who helped compile the Red List.

"Nearly 80 percent of primates in Asia are threatened with extinction, overwhelmingly because of hunting and habitat loss."

A voracious appetite in China for traditional medicines and prestige foods is the main driver of primate loss in Southeast Asia, he said.

Sea mammals are also highly vulnerable. "The situation is particularly serious ... for marine species, victims of our increasingly intensive use of the oceans," said Schipper.

Mile-wide fishing nets, vessel strikes, toxic waste and sound pollution from military sonar kill up to 1,000 air-breathing, ocean-dwelling mammals every day, previous research has shown.

There are many drivers of species extinction and all of them stem either directly or indirectly from human activity, scientists say.

Overwhelmingly, the main threat is habitat loss, with hunting and pollution major factors as well.

But climate change is also emerging as a menace.

Species dependant on sea ice such as polar bears and harp seals, for example, are especially vulnerable to shrinking ice cover in the Arctic Circle.

Scientists are also alarmed by "catastrophic declines" in fresh-water amphibians and some mammals caused by poorly understood infections, said Schipper.

More than 60 percent of Tasmanian devils, for example, have been wiped out in the last decade by a disfiguring facial cancer that spreads through physical contact.

"Disease has always had a role to play in affecting populations, but now we are seeing diseases that are highly pathogenic," said Hoffman.

With 11,000 volunteer scientists and more than 1,000 paid staff, the IUCN runs thousands of field projects around the globe to monitor and help manage natural environments.

More than 8,000 ministers, UN officials, NGOs, scientists and business chiefs began brainstorming Sunday for 10 days in the Spanish city of Barcelona on how to brake this loss and steer the world onto a path of sustainable development.