BARCELONA, Spain -- At least one-quarter of the world's mammals in the wild are threatened with extinction, according to an international survey released yesterday that blames the loss of wildlife habitat, hunting and poaching for the steep declines.

The survey, assembled over five years by 1,700 researchers in 130 countries, is the most comprehensive yet to assess the status and future of mammals on every continent and in every ocean.

The "baiji," or Chinese river dolphin, faces extinction and already might have joined the species that have vanished from Earth.

Others are not far behind, such as the "vaquita," a small porpoise that is drowning in fishing nets in the northern Gulf of California; the North Atlantic right whale; and various monkeys and other primates hunted by poachers in Africa.

Scientists have determined that about one-quarter of the world's 5,487 species of mammals are threatened with extinction. The proportion of marine mammals in trouble appears to be higher, with an estimated one-third facing a serious threat of being wiped out. Many are killed when they are struck by ships or entangled in fishing gear.

About one-half of the world's remaining apes, monkeys and other primates face threats from hunting or destruction of forests to make way for farming, said Russell A. Mittermeier, president of Conservation International.

"Chimp and gorilla meat fetches a higher price in many markets in Central African cities than beef or chicken because it's considered a luxury item," Dr. Mittermeier said. "We are losing many of these animals that otherwise could survive because they cling to relatively good habitat."

Scientists find mammal extinction worrisome because a diversity of species stabilizes the planet. Each extinction disrupts this balance and ripples through the food chain, making it difficult for other species -- including humans -- to survive.

The bleak assessment was released in Barcelona at the World Conservation Congress, a meeting of 8,000 scientists, conservationists, business leaders and representatives from governmental environmental ministries. It was part of a larger update to the Red List of all threatened species maintained by the International Union for the Conservation of Nature, which hosts this gathering every four years.

The Red List has several categories, including: extinct; extinct in the wild; and threatened with extinction, including critically endangered, endangered and vulnerable.

Jonathan Baillie, director of conservation programs for the Zoological Society of London, said a sampling shows that 24 percent of all vertebrates -- those animals with a backbone -- appear to face extinction.

To track the health and abundance of all species is too massive a job, Dr. Baillie said, but he suggested that such sampling might be tantamount to creating a Dow Jones industrial average index for the planet's biodiversity.

Holly Dublin, who leads the conservation union's species survival commission, said more details of the Red List would be unveiled this week in Barcelona, as scientists and officials work on plans to reverse the downward slide of so many species.

Researchers have sought to make the Red List the most trusted assessment of species vulnerability by accumulating the best scientific information without regard for legal definitions or the politics of any nation. The Red List once was published as a book, but the endangered list has grown so long -- now 44,838 species -- that it is now an online catalog at www.iucn.org/redlist.

Still, the assessments are far from an exact science. The La Palma giant lizard was thought to have become extinct in the past 500 years after the Romans brought rats to the Canary Islands off the coast of Africa. But this lizard was rediscovered in 2007, clinging to steep cliffs out of reach of rats. It is now listed as critically endangered.

The assessment of marine mammals, the first completed since 1996, did not fully factor in global warming, scientists said. The results of this study will be published later this week in the journal Science.

The prospects for these animals might be worse than even the global numbers suggest, said Jan Schipper of Conservation International, who was the lead author of the Science paper. The problem is a surprising lack of information about 836 mammals.

"If you don't know where they are or how many there are, then it's hard to determine if they have viable populations or [are] threatened with extinction," Mr. Schipper said.

Given this uncertainty, as many as 36 percent of land mammals and 61 percent of whales, seals and other marine mammals could be threatened with extinction.