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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).
To determine the background extinction rate, scientists look to the fossil record and to genetic material, or DNA, which accumulates small changes in its sequence as it is copied and passed down from generation to generation. Because these minor copying mistakes occur at a known rate, they can act as "molecular clocks" to help establish how long ago closely related species diverged and to track other aspects of species history. This evidence suggests that under normal circumstances species survive for one million to 10 million years. If species typically lived for only one million years, then we ought to see one extinction per million species-years, or one per million species per year." And so what that tells us is that the rate of bird extinctions is a hundred times greater than it should be," says Pimm.
In their 1995 paper, Pimm and his colleagues also performed similar analyses of mammals, reptiles, frogs and toads, and freshwater clams (dividing the number of extinctions witnessed over the past century by the total number of known species in each of these groups) and came up with similar results: current extinction rates are two orders of magnitude above normal. But the real body count is likely to be even higher, because species usually don't go extinct immediately when their habitats are destroyed, exotic predators arrive, or they otherwise come to ecological harm. Instead, they often hang on for decades or even longer before disappearing forever.
Habitat destruction is a major cause of species loss and has accelerated rapidly in recent years, especially in the world's most species-rich environments -- about half the original extent of tropical moist forest has been lost, for example, most of it in the last 50 years -- so it's likely that many extinctions have not yet had time to occur. That means the number of threatened and endangered species (those that are likely to go extinct in the next few decades without human intervention to save them) might be a better assessment ofthe probable toll than simply the number of recent extinctions. The World Conservation Union (IUCN) currently lists 1,213 birds as threatened, about 12 percent of all avian species. "Which means that by the end of the century we could expect that probably a thousand species of birds might disappear," Pimm says. That would be 10 extinctions per 10,000 species per year, or 1,000 times the background extinction rate. The numbers for other well-known groups are similar, if not worse: 20 percent of the world's mammal species appear on the IUCN Red List of threatened species.
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http://www.alternet.org/envirohealth/39790/
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