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.

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

Some experts considered the revised Red List as further evidence that the Earth is undergoing the first wave of mass extinction since dinosaurs died out 65 million years ago. 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% 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.
• Nearly 80% 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.
• Sea mammals are also highly vulnerable. 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.

Scientists say that there are many drivers of species extinction and all of them stem either directly or indirectly from human activity. 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.

The IUCN describes itself as the world's oldest and largest global environmental network. It is made up of more than 1,000 government and nongovernment organizations and almost 11,000 volunteer scientists in more than 160 countries. The research for the report took five years and involved more than 1,700 scientists around the world.

Check out Do Something's environment and animal welfare sections for more info.