Endangered Birds

October 1, 2006

Calculating extinction: endangered species such as Asia's Red-crowned Crane are threatened by human activities


Stuart Pimm

Human activities have caused some 500 bird species worldwide to go extinct over the past 500 years, and twenty-first-century extinction rates likely will accelerate to approximately ten species per year unless societies take action to reverse the trend, according to a new report.

Without the influence of humans, the expected extinction rate for birds would be roughly one species per century, says Stuart Pimm, professor of conservation ecology at Duke's Nicholas School of the Environment and Earth Sciences and one of the report's principal authors.

Pimm's team, which pioneered the approach of estimating extinction rates on a per-year basis, calculated that since 1500—the beginning of the major period when Europeans began exploring and colonizing large areas of the globe—birds have been going extinct at a rate of about one species per year, or 100 times faster than the natural rate. The rate has speeded up in recent times. "Increasing human impacts accelerated the rate of extinction in the twentieth century over that in the nineteenth," the report said. "The predominant cause of species loss is habitat destruction."

The researchers derived their estimates using a large database of threatened and endangered species compiled by Bird Life International in Cambridge, England. They also used a compilation by report co-author Alan Peterson of the first scientific descriptions of bird species.

The new assessment considerably exceeds previous scientific estimates that 154 bird types disappeared during that past 500 years, according to the researchers. One factor contributing to such large differences in estimates is that "more than half of the known species of birds were not discovered until after 1850, an important point that previous estimates of extinction rates have failed to take into account," says Peter Raven, president of the Missouri Botanical Garden and co-principal author of the report. "One can't register a bird as extinct if it was not known to exist in the first place."

The report, which appears in the online edition of the Proceedings of the National Academy of Sciences, is not all bleak, Pimm says. "The good news in this report is that conservation efforts are reducing extinction rates to about one bird species every three or four years." But he adds that even this improved rate "is still unacceptable."

Of the 9,775 known species of birds, "an estimated additional twenty-five would have gone extinct during the past thirty years if it were not for human intervention," Raven says. But, despite conservation efforts, "some 1,200 more species are likely to disappear during the twenty-first century. An equal number are so rare that they will need special protection or likely will go extinct, too."