Extinction: Bye Bye, Birdie ...

In April 2004, a computer and electronics professor named David Luneau paddled a canoe through a swamp forest in eastern Arkansas and captured a blurry video of a crow-sized bird perching on the trunk of a tupelo tree and then flying off into the woods. The bird had large white patches on the trailing edges of its wings and a vee of white stripes on its back -- characteristic features of the ivory-billed woodpecker, last seen in the United States 60 years before and widely believed to be extinct.

Since then, numerous search parties have been launched to comb that patch of forest for more evidence of the bird's existence, and scientists have been examining the video frame by frame and debating whether it really depicts an ivory-billed woodpecker or just a more common, similar-looking pileated woodpecker. Has this lost creature revealed itself to human eyes again after six decades -- or is the bird a figment of our wishful thinking? One thing is certain, says Duke University conservation biologist Stuart Pimm: "If it survives, it's a lonely bird."

Lonely, except that in one sense it has lots of company: species that are lost, or nearly so, are increasingly common because human activities are driving them to extinction 1,000 times faster than the normal rate, according to the just-released report, Global Biodiversity Outlook 2. The report echoes the United Nations' Millennium Ecosystem Assessment, published last year, and proclaims that a "sixth mass extinction" is under way, the worst loss of species since the dinosaurs died out 65 million years ago.

Such dire claims have attracted some skeptics, however. Mostly journalists and economists, they start with the argument that nobody even knows how many other species share the planet, so how can anyone claim to know what the extinction rate is? Taxonomists have named and described around 1.5 million species, but estimates of the actual total range from 5 million to over 15 million. A frequently cited mid-range estimate is 7 million species, but that's by no means an exact or universally accepted figure.

Because of this uncertainty, says Pimm, the aim should be simply to calculate a relative extinction rate rather than the absolute number of disappearing species. Pimm and a group of colleagues first laid out these ideas in a 1995 paper in Science that has become probably the most widely accepted approach to quantifying species loss.

Only a few of those 1.5 million described species are known well enough to assess how they're doing; what's known about many species derives from single specimens hiding in dusty museum cabinets somewhere. So in order to say something meaningful about extinction rates, it's necessary to pick a well-known group of organisms and treat them as a sample of the larger total. Fortunately for Pimm, an ornithologist, birds make a good sample group. Although there are still occasional surprises, it's generally well known how many kinds of birds there are, which ones have disappeared, and when. And, he says, the fact that there are just about 10,000 species of birds in the world greatly simplifies the arithmetic involved.

About 130 kinds of birds have vanished around the world over the past century and a half. That's a pretty firm number: "We have a body count and we have names," Pimm says. There's the great auk, for example, driven to extinction in the 19th century by hunters who sought its feathers, meat, and oil. There's the Lana'i hookbill, lost in the early 1900s when its habitat was destroyed for pineapple plantations, and the New Zealand bush wren, a ground nester that proved easy prey for introduced rats and was last sighted in 1972. The bird extinction rate is about one per 10,000 species per year, or 100 extinctions per million species per year, since the middle of the 19th century. Of course, extinction is a natural process; no species lives forever. So the real question is how the current extinction rate compares to the usual rate at which species come and go (the back-ground rate).