Mammals in global decline.

Nearly one-fourth of the world's known mammal species are threatened with extinction, according to a recent study compiled by the IUCN-World Conservation Union, which suggests that earlier estimates of the number of endangered species may have been too low. The latest update in a series known as the Red List, the report is the most comprehensive evaluation of globally threatened animals ever compiled, and the first to assess all known mammal species.

Until now, birds were the only group fully assessed. With 11 percent of all bird species facing the threat of extinction and 70 percent experiencing population declines, scientists had relied largely on the status of birds as an indicator of the level of threat to all terrestrial life-forms (see "Flying Into Trouble: The Global Decline of Birds, and What it Means," January/February 1994).

The number of mammals on the Red List - a shocking 1,096 of the 4,630 known species - has spurred calls for an intensified international focus on biodiversity loss. The report also found that nearly one-third of all 275 primate species are at risk, almost three times the number of previous estimates (see note below).

The new survey was compiled using a revised set of criteria, which the authors described as more objective than those used in previous estimates, to determine the threat of extinction. Based on 35 years of data from more than 500 scientists worldwide, the report found that 5,205 vertebrates of all kinds are endangered, including 25 percent of amphibians, 20 percent of reptiles, and 34 percent of fish. More than 100 species of marine fishes were added this year, suggesting that fish, too, could be at a higher risk than previously thought.

The Red List recognizes three distinct categories of risk: of the 1,096 species of mammals considered threatened, 169 are listed as "critically endangered," 315 as "endangered," and 612 as "vulnerable." Each classification is largely determined by the rate of a species' population decline over the past 10 years, as well as the number of surviving adults and the stability of habitat. For example, those animals with population declines of 80 percent or more over 10 years are considered critically endangered; those losing 50 percent or more are classified as endangered; and those dropping at least 20 percent are considered vulnerable.

George Rabb, chair of the IUCN species survival commission that compiled the list, said that the report should serve as a "red flag," focusing attention on the most significant factor threatening the survival of species: the destruction of habitat brought about by human population growth and economic development. Other contributing factors, he said, include pollution, overharvesting, and the introduction of foreign species. At current rates of decline, biologists fear that many mammals with niches or habitat needs that conflict with human development may soon come to depend on the tinkering hand of wildlife management and captive breeding - unable to exist without human intervention.

The report's authors stress that the listing remains largely incomplete: of the 1.7 million documented species, only 10 percent have been evaluated. And current records of threatened species may only be a small indicator of a much larger phenomenon, as some scientists estimate that the number of undocumented species, most of them invertebrates and microbes, remains in the tens of millions.