BARCELONA, Spain — A quarter of the world's wild mammal species are at risk of extinction, according to a comprehensive global survey released here today.

The new assessment — which took 1,700 experts in 130 countries five years to complete — paints "a bleak picture," leaders of the project wrote in a paper being published in the journal Science. The overview, made public at the quadrennial World Conservation Congress of the International Union for the Conservation of Nature (IUCN), covers all 5,487 wild species identified since 1500. It is the most thorough tally of land and marine mammals since 1996.

"Mammals are definitely declining, and the driving factors are habitat destruction and over-harvesting," said Jan Schipper, the paper's lead author and the IUCN's global mammals assessment coordinator.

The researchers concluded that 25 percent of the mammal species for which they had sufficient data are threatened with extinction, but Schipper added the figure could be as high as 36 percent because information on some species is so scarce.

Land and marine mammals face different threats, the scientists said, and large mammals are more vulnerable than small ones. For land species, habitat loss and hunting represent the greatest danger; marine mammals are more threatened by accidental killing through fishing, ship strikes and pollution.

Although large species such as primates (including the Sumatran orangutan and red colobus monkeys) and ungulates (hoofed animals) might seem more physically imposing, the researchers wrote that these animals are more imperiled than small creatures such as rodents or bats because they "tend to have lower population densities, slower life histories, and larger home ranges, and are more likely to be hunted."

Primates face some of the most intense pressures: According to the survey, 79 percent of primates in South and Southeast Asia — including the Hainan gibbon — are facing extinction.

Conservation International President Russ Mittermeier, one of the paper's co-authors and a primate specialist, said the animals are experiencing "a triple whammy" in the region.

"It's not that surprising, given the high population pressures, the level of habitat destruction, and the fairly extreme hunting of primates for food and medicinal purposes," Mittermeier said in an interview. He added that some areas in Vietnam and Cambodia are facing "an empty forest syndrome," where even such populous species as the crab-eating macaque or temple monkey are "getting vacuumed out of some areas where it was common."

In some cases the scientists have a precise sense of how imperiled a species has become: There are 19 Hainan gibbons left in the wild on the large island off China's southeast coast, Mittermeier said. In other instances, such as with the beaked whale and jaguar, researchers have a much vaguer idea of their numbers. Among the hoofed animals who are endangered, scientists list the Dama gazelle and the Malaysian tapir.

Technological advances — such as satellite and radio tagging, camera tracking and satellite-based GPS (global positioning system) mapping — have helped scientists gauge the status of mammals and their habitat more thoroughly. The authors of the assessment wrote that most land mammals occupy "areas smaller than the United Kingdom," while "the range of most marine mammals is smaller than one-fifth of the Indian Ocean."

The findings come as other researchers are documenting new ways that human-generated emissions of greenhouse gases affect marine mammals. In a paper published Thursday in the journal Geophysical Research Letters, a team at the Monterey Bay Aquarium Research Institute found that ocean acidification spurred by carbon emissions will cause sounds to travel farther underwater, because increasingly acidic seawater absorbs less low- and mid-frequency sound.

By 2050, the researchers predicted, sounds could travel as much as 70 percent farther in parts of the Atlantic Ocean and other areas, which might improve marine mammals' ability to communicate but also increase the amount of background noise, which could prove disorienting.

"We understand the chemistry of the ocean is changing. The biological implications of that we really don't know," said the lead author, ocean chemist Keith Hester. "The magnitude to which sound absorption will change, based mainly on human contribution, is really astounding."

The authors of the IUCN's mammals assessment said the species declines they have observed are not inevitable. "At least 5 percent of currently threatened species have stable or increasing populations," they wrote, "which indicates that they are recovering from past threats."

"It comes down to protecting habitats effectively, through protected areas, and preventing hunting and other forms of exploitation," Mittermeier said. As one example of how conservation can work, he noted that in areas where scientific researchers work, animals stand a much better chance of surviving. "Where you have a research presence, it's as good or better than a guard force," he said.

Schipper offered the model of the U.S. effort to bring back the black-footed ferret, which was essentially extinct on the North American prairie as of 1996. "Now it's endangered, which, in this case, is a huge improvement," he said. "When governments and scientists commit resources to a project, many species can be recovered."