Global Bird Populations Face Dramatic Decline In Coming Decades, Study Predicts

"Our projections indicate that, by 2100, up to 14 percent of all bird species may be extinct and that as many as one out of four may be functionally extinct-that is, critically endangered or extinct in the wild," said researcher Cagan H. Sekercioglu of the Stanford Center for Conservation Biology (CCB) and lead author of the PNAS study. "Important ecosystem processes, particularly decomposition, pollination and seed dispersal, will likely decline as a result."

These findings come on the heels of the November 2004 Global Species Assessment by the World Conservation Union (IUCN), which found that 12 percent of all bird species are already threatened with extinction, along with nearly one-fourth of the world's mammals, one-third of the amphibians and 42 percent of all turtles and tortoises.

"Even though only 1.3 percent of bird species have gone extinct since 1500, the global number of individual birds is estimated to have experienced a 20 to 25 percent reduction during the same period," wrote Sekercioglu and CCB co-authors Gretchen C. Daily and Paul R. Ehrlich. "Given the momentum of climate change, widespread habitat loss and increasing numbers of invasive species, avian declines and extinctions are predicted to continue unabated in the near future."

Future scenarios

The study was based on a painstaking analysis of all 9,787 living and 129 extinct bird species. Eight researchers spent a year collecting data on the conservation, distribution, ecological function and life history of every species-more than 600,000 computer entries in total. "The result is one of the most comprehensive databases of a class of organisms ever compiled," Sekercioglu said.

To forecast probable rates of extinction, he and his colleagues entered the data into a computer program designed to simulate best-case, intermediate-case and worst-case scenarios for the future.

The best case was based on the assumption that conservation measures in the next 100 years would be sufficient to prevent additional bird species from becoming threatened with extinction.

For the worst case, the researchers assumed that the number of threatened species will increase by about 1 percent per decade-that is, 1 percent in 2010, 2 percent in 2020, 3 percent in 2030, etc. "These assumptions are conservative, since it is estimated that, every year, natural habitats and dependent vertebrate populations decrease by an average of 1.1 percent," the authors wrote.

For the intermediate scenario, the scientists used statistics from 1994 through 2003 as a basis for calculating the likelihood that a non-threatened species would become threatened after a decade.

The results of the three future scenarios were dramatic. The computer forecast that between 6 and 14 percent of all bird species will be extinct by 2100, and that 700 to 2,500 species will be critically endangered or extinct in the wild. Even the middle-of-the-road intermediate scenario revealed that one in 10 species will disappear a century from now, and that approximately 1,200 species will be functionally extinct.

The study cited several reasons for the expected decline in bird populations, including habitat loss, disease, climate change, competition from introduced species and exploitation for food or the pet trade.

"Island birds are particularly at risk," the authors said, noting that one-third to one-half of all oceanic island species will be extinct or on the brink of extinction by 2100. Birds with highly specialized diets are predicted to experience more extinctions than average, they wrote, adding that some plant species also face extinction if their primary pollinators and seed-dispersers vanish.

"It's hard to imagine the disappearance of a bird species making much difference to human well-being," said Daily, an associate professor (research) in Stanford's Department of Biological Sciences and director of the CCB Tropical Research Program. "Yet consider the case of the passenger pigeon. Besides mail becoming a lot less fun to receive, its loss is thought to have made Lyme disease the huge problem it is today. When passenger pigeons were abundant-and they used to occur in unimaginably large flocks of hundreds of millions of birds-the acorns on which they specialized would have been too scarce to support large populations of deer mice, the main reservoir of Lyme disease, that thrive on them today."

Scavengers and insectivores

More than a third of all scavengers and fish-eaters are extinction-prone, according to the study, yet little is known about the potential consequences of their widespread disappearance. "Since most scavenging birds are highly specialized to rapidly dispose of the bodies of large animals, these birds are important in the recycling of nutrients, leading other scavengers to dead animals and limiting the spread of diseases to human communities as a result of slowly decomposing carcasses," the authors wrote.

As an example, the researchers pointed to India, where the collapse of the vulture population in the 1990s was followed by an explosion of rabid feral dogs and rats. In 1997 alone, more than 30,000 people died of rabies in India, more than half of the world's total rabies deaths that year.

Insect control is another important ecosystem service performed by birds, yet the study found that more insect-eating bird species are prone to extinction than any other group. "Exclusions of insectivorous birds from apple trees, coffee shrubs, oak trees and other plants have resulted in significant increases in insect pests and consequent plant damage," the authors wrote, adding that the extreme specializations of many insectivorous birds, especially in the tropics, make it unlikely that other organisms will be able to replace the birds' crucial role in controlling pests.

"The societal importance of ecosystem services is often appreciated only upon their loss," the authors wrote. "Disconcertingly, avian declines may in fact portray a best-case scenario, since fish, amphibians, reptiles and mammals are 1.7 to 2.5 times more threatened [than birds]." Invertebrates, which may be even more ecologically significant than animals, also are disappearing, they noted. Therefore, "investments in understanding and preventing declines in populations of birds and other organisms will pay off only while there is still time to act," the authors concluded.

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The PNAS study was supported by Dr. Walter Loewenstern and the Koret and Winslow foundations.