When scientists try to gauge the impact of humans on bird populations since 1500–the time Europeans began to explore the world in earnest–they often cite estimates that, on average, one species has gone extinct every 4 years. Now, a group of researchers says that figure may be far too low. The good news is that conservation efforts in the 20th century have prevented many extinctions, but the authors warn that continued destruction of habitat makes even more birds likely to vanish forever in the next 100 years. Not just birders love lists. For their research, ecologist Stuart Pimm of Duke University in Durham, North Carolina, and colleagues constructed and analyzed a massive list of all species of birds and when they were first described. They came up with 9975 species alive today and 154 species that either have gone extinct or have been missing so long that they are likely gone. Most estimates of extinctions rates simply divide rough estimates of these two numbers. This approach yields a rate of 31 extinctions per million species per year (or 1.2 species every 4 years).
The problem, Pimm points out, is that the list of extinctions is incomplete. There are likely many species that went extinct before they were ever discovered; the bulk of taxonomic work by ornithologists happened in the 19th century, long after Europeans began to destroy habitat, let loose invasive species, and otherwise doom vulnerable species of birds. (Indigenous peoples such as the Polynesians also took a toll on birds before 1500, Pimm points out). By considering how long taxonomists have known about birds, Pimm and his colleagues were able to take these untold extinctions into account. The average rate since 1500 turned out to be 85 extinctions per million species per year (or 3.4 species every 4 years).
Conservation efforts have made an impact, the authors argue. Since 1975, 20 species of birds have gone extinct. Without conservation, however, another 25 species, such as the California Condor, would likely have vanished–and the extinction rate would have jumped to 150 extinctions per million species a year, the team reports online this week in the Proceedings of the National Academy of Sciences. Yet the situation could get far worse than that. As available habitat shrinks by 50%, the number of species declines by 15%. Assuming that the area of now-pristine habitats is halved–as it was in eastern North America by 1870–Pimm’s group calculates that some 1500 species will likely go extinct over the next century. Climate change could darken the scenario even further. “The scary thing is that we’re investing a lot in conservation, but it’s reasonable to expect extinction rate to increase,” says Gregory Butcher, director of bird conservation at the National Audubon Society in Washington, D.C. He adds that the situation is worse for other groups because they are not as well studied or protected as birds.
July 18, 2006
Original web page at ScienceNow