A quarter of the world's wild mammal species are at risk of extinction, according to a comprehensive global survey released Monday.
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 primate species in South and Southeast Asia – including the Hainan gibbon – are facing extinction.
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.”