Bye bye, birdie ... and myriad other creatures humans are wiping out.

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 a thousand times faster than the normal rate, according to the just-released 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 1.5 million. A frequently cited mid-range estimate is 7 million species, but that's by no means an exact or universally accepted figure.

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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 1 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 background rate).

DEEP BACKGROUND

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 1 million to 10 million years. If species typically lived for only 1 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.

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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 of the 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.

Not all scientists agree that hundred- to thousand-fold increased extinction rates among birds and a few other well-known groups mean that all kinds of species are disappearing at the same rates. "There are just so many differences among, even within, taxa, how species respond to the kinds of forces that are causing extinction," says Daniel Simberloff, an ecologist at the University of Tennessee in Knoxville. Do the patterns we see in birds, which make up just 0.6 percent of known species, necessarily apply to insects, which account for 54 percent?

Pimm and Simberloff were once colleagues at the University of Tennessee and know each other's work well. (Pimm even likes to joke that one of Simberloff's primary research interests is "pointing out Stuart Pimm's mistakes.") "There are certainly lots of credible scientists who don't like the extrapolation methods and would argue with aspects of them," Simberloff says.

Yet within the scientific community, the debate over extinction rates is about details, like just how much extrapolation is appropriate, rather than the big picture. "I don't know of any credible environmental scientist that doesn't think that extinctions are happening at greatly increased rates," Simberloff says. To him, high extinction rates among birds and other well-known groups are evidence enough of a biodiversity crisis--regardless of what the exact patterns might prove to be among other kinds of species.

Moreover, as researchers have begun to look more closely at those other groups of species, all evidence suggests that things are just as bad, if not worse, than studies of birds and mammals indicate. According to a Nature Conservancy study, dragonflies and beetles are more highly threatened than birds in North America. In the sea, where many scientists had long believed that species would be relatively shielded from extinction risk, more than 40 percent of a subfamily of groupers meet IUCN criteria for imperilment, says Callum Roberts, a marine conservation biologist at the University of York in the United Kingdom. Most marine species have not yet been assessed.

In a study comparing population trends among butterflies, birds, and plants in Britain, a group of ecologists led by Jeremy Thomas of the National Environment Research Council in Dorchester found that butterflies fared the worst in recent decades. Seventy-one percent of butterfly species declined over the course of the study, compared to 54 percent of birds and 28 percent of plants. The group's analyses of other types of insects, while less detailed, suggested similar patterns.

Their study involved over 20,000 volunteers who submitted more than 15 million records of species sightings--an enormous amount of effort to analyze just a few groups of organisms on a relatively small, species-poor island with a well-characterized biota, and a good illustration of why sampling is necessary, and probably always will be. It's just about inconceivable that the precise status of every species on Earth can be known, and there has to be some point at which reasonable people decide that what is known is enough.

Admittedly, what we know are still only scattered details woven into a much grander, and still largely mysterious, tapestry. Many of the largest groups of organisms, and the most species-diverse environments, remain relatively unexplored. Tropical moist forests, for example, are thought to contain half the Earth's species, and if that's true only about 1 in 20 of the species living there have been catalogued.

However, two important pieces of information about these environments are available. First, it's often possible to determine how much of a habitat has been destroyed, by means of forest surveys or satellite photos. Second, it's known that larger areas of habitat can support more species, and by contrast smaller areas contain not only fewer numbers of creatures but fewer species--a principle called the species-area relation. Specifically, an area of habitat half the size of another area doesn't host half the number of species as the larger area, but about 85 percent. Thus, say Pimm and many other ecologists, the 50 percent of tropical moist forest that's been lost so far is expected to lead to the extinction of 15 percent of tropical moist forest species.

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Scientists also use the species-area relation to predict future extinctions as habitat destruction continues. Peter Raven, president of the Missouri Botanical Garden in St. Louis, estimates that tropical moist forests will be reduced to about 5 percent of their original extent by mid-century. According to the species-area relation, that would commit more than half of the species they hold to extinction. "If you put that together with habitat destruction in temperate regions," Raven says, "you come up with something like half to two-thirds of all the species in the world becoming extinct during the course of this century"--or at least set on an inexorable path to that fate. Callum Roberts, who has been working on similar calculations for coral reefs, reports that "the species-area relationships suggest that marine species will be lost as a consequence of habitat destruction almost as fast as terrestrial species will."

Not all species are equally vulnerable to ecological threats. "It's easier to destroy a species with a small range than a big one," Pimm says, simply because it's easier to wipe out the entire area where it lives. In fact, a large proportion of species have small ranges, and they're not evenly distributed over the planet. For reasons scientists are still debating, they are clumped together in particular spots, most of which are in the tropics.

Habitat destruction in those areas could be particularly devastating, as British ecologist Norman Myers has pointed out. Myers pioneered the concept of "biodiversity hotspots," and in 2000, with input from scientists from Conservation International, he defined 25 hotspots covering just 1.4 percent of the planet's land area. The hotspots include 15 tropical forests, but also places like the Mediterranean basin and the Cape Floristic Region at the southern tip of Africa. Destroying these habitats could wipe out 44 percent of all plant species, as well as 35 percent of birds, mammals, reptiles, and amphibians.

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In a similar analysis of coral reef environments, which are among the most diverse parts of the sea, Roberts and a group of colleagues identified 10 marine biodiversity hotspots, representing just 0.012 percent of the ocean but containing a large proportion of small-range species. Moreover, since that analysis was published in 2002, researchers have been gathering evidence of unexpectedly rich concentrations of biodiversity in other parts of the ocean, such as deep-ocean seamounts and cold-water coral reefs, which are being destroyed at a rapid clip by factory trawlers. "The habitats on them are literally being clear-cut as effectively as any forest cutting in the Amazon," Roberts says.

Some scientists have objected to this use of the species-area relation, arguing that it's a tool for predicting the total number of species you'll find in an area if you sample a smaller portion of it--not for predicting the number of species you'll lose by destroying a portion of habitat. In other words, you can use the equation to make predictions about going from a smaller area to a bigger one, but not from a bigger area to a smaller one.

Yet in several different environments around the world, researchers have found that predictions of species loss based on the species-area relation align pretty well with reality. In the eastern U.S. forests, which were reduced by about 50 percent at their smallest extent (around 1870), the species-area relation predicts a loss of 15 percent of species. In fact, of 28 bird species restricted to the forest, 4 (or 14.3 percent) had gone extinct and a fifth was critically endangered as of 1995, according to Stuart Pimm and Robert Askins. (One of the extinct species was the ivory-billed woodpecker, so the best-case scenario now stands at 3 extinct and 2 critically endangered.) Likewise, in tropical forests such as the Atlantic Forest of Brazil and the island chains of Indonesia and the Philippines, where deforestation is more recent, the species-area relation accurately predicts or underestimates the number of threatened bird species--an expected result, says Pimm, because in many areas other threats such as invasive species and over-hunting also contribute to species endangerment.

Still, Simberloff says that these numbers should be taken with a grain of salt, because the species-area relation is "a very blunt tool." A great deal of habitat loss will surely lead to substantial species loss, but there are many other factors besides area that influence how many species live in a certain place, and the species-area relation doesn't say how fast species will go extinct. "All [the analyses] can say is at some point in the future there are going to be fewer species," he emphasizes.

"It's a glass-half-empty/glass-half-full situation," Pimm responds. Even if these analyses don't yield a precise number of species destined for extinction, they do give us a good sense of the magnitude of the problem. A loss of half to two-thirds of all species, as Peter Raven predicts is possible, puts the present era on par with the five previous mass extinctions in the history of life on Earth. The most recent one, 65 million years ago, wiped out the dinosaurs along with about two-thirds of all forms of life on land.

CLIMATE OF DEATH

While habitat destruction was the focus of most work on global extinction rates throughout the 1990s, recently scientists have begun to consider the biodiversity impacts of climate change. A group of researchers presented perhaps the most comprehensive effort to date to quantify these possible effects in a 2004 paper in Nature.

Led by biologist Chris Thomas (then at the University of Leeds in the United Kingdom), the group assessed the present distributions of 1,103 animal and plant species and projected how the habitat available to them would change under conditions predicted by the most commonly used computer model of climate change. As the Earth warms, boreal forest is expected to shrink toward the poles, for example, and alpine habitat will retreat up the sides of mountains.

Reasoning that habitat loss is habitat loss whether it's caused by chainsaws or the greenhouse effect, Thomas's team calculated the proportion of habitat that species are likely to lose as the climate warms, then used the species-area relation to predict the number of extinctions likely to result. They found that, depending on the assumptions of the model, 15-37 percent of the species would be on their way to extinction by 2050.

The paper generated an uproar almost immediately. Daniel Botkin, of the University of California at Santa Barbara, says the analysis makes inappropriate use of the species-area relation and is based on weak underlying data. "I've shown that we don't even know the area that is boreal forest very well," he says, pointing out that calculating the future loss of a certain habitat is pretty meaningless when we don't know its present extent.

Yet the gloomy predictions don't depend on the species-area relation, Thomas and his coauthors explained in an online follow-up to their article. The computer model predicted that 8 percent of species would have no suitable habitat left at all by 2050. Moreover, warming isn't likely to stop in 2050--in fact, the maximum temperature increase predicted for 2050 is pretty close to the minimum increase predicted for 2100. So for species that lose most of their habitat by 2050, "it doesn't take much extrapolation in the mind to realize it's not going to be more than a few decades before they've lost the rest," Thomas says. Although he views the 2004 analysis as only "a first step" to understanding the effects of global warming on biodiversity, Thomas still sees it as a pretty good indicator of the magnitude of extinctions that are likely to result from climate change: "It looks like it's going to be in the tens of percents of species."

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How do older predictions of species loss from habitat destruction line up with newer ones about extinction from climate change? No one has done a formal analysis, and Thomas says no one knows yet how much the two groups of species at risk will overlap. But Pimm reluctantly ventures the conclusion that the losses may prove to be additive, because habitats likely to shrink most as the planet warms, like those on mountaintops and in the polar regions, also tend to be remote and thus relatively unaffected by habitat destruction. "Global warming is going to start knocking off the species that we thought might survive," Pimm says.

STRANGERS IN A STRANGE LAND?

Of course, nature is full of surprises, and could turn out to be more resilient than we think. Maybe species will be able to adapt to a warmer climate, disperse to newly suitable areas, or hang on in human-altered habitats. The Brazilian maroon-bellied parakeet survives in Rio de Janeiro's city parks and gardens, despite the fact that over 90 percent of its native coastal-forest habitat has been wiped out.

But most ecologists agree that while a few species here and there will be able to make a go of it in a changed world, such species will be part of a small minority. Chris Thomas points out that species trying to adapt to a warmer climate will have to compete with heat-loving species that will arrive from warmer climes. And Stuart Pimm has found that those tropical forest species able to survive in human-altered habitats like cow pastures are relatively widely distributed generalist species that are not at high risk of extinction anyway. In other words, the maroon-bellied parakeet is also a lonely bird, having once been part of a teeming avian community in the Atlantic Forest; 200 species of birds with which the parakeet shared its lost habitat are on the brink of extinction.

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If there is any real cause for optimism, it lies in the time lag before extinction. If species can hang on for 50 or 100 years, we humans may be able to organize a system of protected areas and alter our own activities to ensure their long-term survival in the wild. The ivory-billed woodpecker was decimated when the mature bottomland forests it depends on were razed in the 19th and early 20th centuries, but these forests are now coming back. If the bird has managed to survive for this long, its chances will only be better in the future as the big trees continue to grow. Similarly, Thomas says that if global temperatures peak at a relatively low level sometime late this century and then decline towards pre-industrial levels 150 or 200 years from now, about half of the extinctions predicted by his group's analysis could be avoided. Perhaps we humans are not yet fated to be lonely.

Sarah DeWeerdt is a Seattle-based science writer specializing in biology and the environment.

For more information about issues raised in this story, visit www.worldwatch.org/www/extinctions.

RELATED ARTICLE: DON'T KNOW WHAT YOU GOT TILL IT'S GONE

Of the many reasons to mourn the extinction of species, self-interest would have to be near the top of the list. We depend on plants and animals for food, clothing, shelter, and often fuel. Plants provide "ecosystem services" such as prevention of erosion, flood control, and water filtration. In many societies medicine comes directly from wild plants, and even in the industrialized world half of all pharmaceuticals are plant compounds--ranging from aspirin, originally derived from willow bark, to cutting-edge cancer drugs from the Pacific yew and rosy periwinkle.

And medical advances don't just come from the plant kingdom: cone snails, which live on tropical coral reefs, contain up to 50,000 chemicals that may prove useful in the detection and perhaps treatment of certain cancers, and in the development of better painkillers. Many of the 500 species in this group of mollusks are severely threatened by habitat loss as well as overcollecting for the international shell trade.

Does each and every one of the millions of species on the planet directly benefit humans? Perhaps not, but compounding the tragedy of extinction is the fact that we will never know exactly what we are losing--or even, in many cases, that what we've lost ever existed at all. "The vast majority of the organisms that we're losing, particularly in the tropics, have never been seen by anyone or even given a name. That's not only part of the reason for the imprecision of some of the estimates, it's also particularly damaging in a world where we will ultimately depend on organisms for building sustainable systems," says Peter Raven, president of the Missouri Botanical Garden in St. Louis.

These practical concerns are balanced by philosophical ones. Many people, religious and secular alike, consider it self-evident that our species ought not go around destroying other forms of life willynilly. British ecologist Norman Myers, who was the first to focus public attention on the extinction crisis in his 1979 book The Sinking Ark, takes the argument a step further. Since new species evolve from old ones, he points out, extinction is "depleting evolution's capacity to generate replacement species." After past mass extinction events, the biosphere has required on the order of 5 million years to bounce back, so our actions are "impoverishing the biosphere for millions of years ahead."