To Many, Out of Sight...
Biologists and the public see the world differently in regard to environmental threats, recent polls show.
Biologists overwhelmingly view the loss of biodiversity -- that is, the loss of different species of plants and animals and their habitats -- as the most serious environmental problem, but the public is mostly unfamiliar with the concept of biodiversity and considers pollution the biggest threat.
Most biologists say the world is experiencing a mass extinction of plant and animal life, mainly a result of human activity, according to the poll, which was conducted for the American Museum of Natural History by Louis Harris & Associates. The nationwide survey of 400 biologists taken from the membership of the American Institute of Biological Sciences was conducted in January and February.
The majority of biologists agree that if trends continue, the loss of plant and animal species will have a very negative effect on the earth's ability to recover from both natural and man-made disasters. "When a species is driven to extinction, that's it," said Dr. Stuart Pimm, a professor at the University of Tennessee who specializes in endangered species. "The potential of that species for our benefit is lost."
The museum also commissioned a Harris poll of the public, asking some of the same questions of 1,000 adults nationwide in January. Most say they do not believe a mass extinction is occurring. Mass extinction is defined by some scientists as a rate of extinction 100 to 1,000 times the natural rate.
Pollution is seen as the single most important environmental threat by 62 percent of the adults surveyed.
Biologists consider the earth more threatened by population growth (39 percent) and loss of biodiversity (22 percent).
"Pollution is obvious -- you can see it and smell it," Dr. Pimm said. "But pollution can be reversed, while extinction is irreversible."
It is difficult to know for sure, but there may be more kinds of creatures living on earth now than ever before in the planet's 4.6-billion-year history. Or at least there were that many before human beings appeared very late in the game and began changing the rules.
At first, life consisted of microscopic, single-cell balls, rods and filaments.
For perhaps three billion years, these blue, green, purple and white microbes were all the life there was, all the decoration available to a wet, stark and rocky world.
Not until about 600 million years ago, after the earth emerged from its deepest ice age ever, did bigger species evolve. Then life's variety exploded.
The expansion of biological variety has been on an upward curve since then.
From sequoias and nematodes to fungi and elephants, from poison ivy and jellyfish to eagles, cockroaches, spiders and algae, millions of species -- the vast majority undiscovered and unnamed by humans and most of them very small -- populate the world.
The curve has not been a smooth one.
At least five times in the last 600 million years, planetwide environmental cataclysms, like drastic climatic change and colliding asteroids, have wiped out whole families of organisms. Such events have threatened to erase life, but succeeded only in partly clearing the stage for new and different families.
Between extinction spasms, in the background hum of evolution, single species winked out here and there to be replaced by new ones.
Because of this continuous turnover, scientists believe, more than 95 percent of all species that have ever existed are extinct today.
Now one species, Homo sapiens, has become so dominant on the planet and so powerful an influence on the rest of the biosphere that many experts fear it is perpetrating, willy-nilly, a sixth major extinction.
If so, it is happening simply because people are acting naturally -- harvesting wild species to burn, eat or sell; expropriating and destroying wild habitat to make way for human works like farms, cities and suburbs; moving plants and animals around the globe in a mix-and-match game in which a relatively few, superadaptable plant and animal species are crowding out a larger number of less hardy ones.
The pressure is likely to intensify as the earth's human population, now about six billion, approaches a doubling before leveling off in perhaps a century.
The impact of the numbers, now and in the future, is magnified greatly by technology's power to shape the landscape and exploit the oceans.
This combination has been especially worrisome for the rain forests, where most of the world's species are found and large-scale clearing is under way.
No one knows exactly how many species live on earth.
About 1.4 million have been described and named, but biologists believe there are at least 10 times that many.
And no one can put precise numbers on the rate of present-day extinction, although several scientists have made rough stabs.
One relatively conservative estimate says that for some species found only in one locality, the rate may be 100 to 1,000 times the normal "background" rate between mass extinctions of the past.
Despite the difficulty of making exact or even reasonably approximate measurements, few scientists who study the situation doubt that humans, by proceeding with business as usual, are driving other creatures toward extinction at a growing rate.
To whatever extent they are doing so, people are reducing the variety of life on earth -- what conservation biologists now call biological diversity, or just biodiversity.
Biodiversity is not merely a matter of different kinds of species.
It applies to many layers of nature on different scales.
On one level, it is expressed by the number of genetic variations within a species, the raw material of evolution; destroy the wrong genes, and the species might not be able to adapt and survive in the long run.
On a broader scale, biodiversity is expressed in the bewildering variety of types of ecosystems, the tightly interwoven communities of plants, animals and microbes that are nature's working units.
Forests, wetlands and grasslands are examples of one level of ecosystem variety, and each comprises numerous interconnected subunits, right down to the microscopic.
It is these interconnected communities that constitute the fabric of life on earth.
To whatever degree human activity is narrowing the variety of life, scientists say, it is doing so on all levels.
Why does this matter? Conservationists and biologists offer a number of answers.
On the practical level, wild species provide the raw material for medicine and food. Who knows what undiscovered plant might provide a cure for our era's killer diseases? Acting together, wild species support the human economy by providing an array of services like water purification, soil formation, pollination, flood control and, in our age, outdoor recreation.
On a more fundamental level, they support human life: without humans, the biosphere would not miss a beat.
But without insects and microbes, humans would disappear quickly.
Some experts say natural systems are intrinsic to psychological well-being. Finally, many conservationists assert a moral imperative: it is simply wrong to destroy large slices of creation.
Moral arguments aside, are all species equally valuable? Scientists believe that in many ecosystems, a few keystone species hold everything together, and that if they are removed, the system will collapse and all the other species in it will disappear.
The problem is that in most instances, no one is sure which are the crucial species.
What's more, scientists have found, the greater the variety of plant species in an ecosystem, the more productive it is over all.
So, conservationists argue, it is necessary to save as many species as possible.
They have their work cut out for them, since the catalogue of imperilment is long.
In April, the World Conservation Union, in Gland, Switzerland, added nearly 34,000 plant species -- 1 of every 8 known plant species in the world, and nearly 1 of 3 in the United States -- to its growing "red list" of imperiled organisms. Previously, the group placed nearly a quarter of all mammals and more than 10 percent of bird species on the list.
In possibly an even more important gauge of overall biological vitality, an earlier assessment, by United States Federal Government scientists, found that vast stretches of the natural landscape, amounting to at least half the area of the contiguous 48 states, had declined to the point of endangerment.
This report, the first ever done on the state of health of the nation's individual ecosystems, found that 30 of them, some of which dominated vast regions before the arrival of Europeans, have been destroyed or degraded by 98 percent.
If they were to disappear, entire assemblages of species would vanish.
The individual species most at risk are those existing only in one small area, like on an island or in a particular river in the United States or, in some cases, even on a single tropical mountainside.
These endemic species, as they are called, account for most of whatever extinctions are in progress now, scientists believe.
Not even the trackless ocean is immune. Overfishing has pushed some marine species -- the cod and the blue-fin tuna, for example -- to the brink of commercial extinction.
Coral reefs in some parts of the world are under severe assault; in parts of Asia, fishermen squirt sodium cyanide into the water to stun fish, in doses strong enough to kill the corals that maintain the reef ecosystem.
The abundant evidence for the threat to biodiversity should not be overinterpreted. It does not necessarily mean that many of the species thought to be imperiled will disappear immediately, or even within many years.
The recent red-listing of plants includes many simply because they are rare.
While many may be vulnerable, humans do not threaten them just yet.
The red list of plants can be read another way, too: while 1 in 8 plants was judged imperiled, almost 90 percent were not.
Some conservation biologists, fearing that the sixth extinction is in its early stages, see no cause for comfort here. But others say there is still time for action.
On the action front, there are bright spots. Over the last century, a worldwide conservation movement has struggled to stem the human assault on nature.
The United States' Endangered Species Act is regarded as the strongest protection law in the world.
In the last six years, more than 160 countries, not including the United States, have ratified a treaty obligating governments to to protect plant and animal species.
A growing restoration movement has begun to reverse the trend toward biotic impoverishment in some places.
Restoration projects have achieved eye-catching results: in the resurgence of Midwestern prairies and oak savannas; in Yellowstone National Park, where wolves are thriving again, and on the southern Plains, where buffalo roam once more.
But victories are scattered, hard won and subject to reversal or cancellation.
Recently, a Federal court, for instance, invalidated the Yellowstone wolf restoration project, and a legal battle is under way.
Meanwhile, habitat continues to disappear at an "unprecedented" rate, according to the United Nations.
The magnitude of the sixth extinction, if that is what is occurring, will become clear in time.
If the extinction turns out to be in the class of the five big ones of the past, that should be made abundantly evident in the decades ahead.
If so, the world can expect to recover its lost biological diversity slowly, in 5 million to 10 million years -- assuming the extinction spasm can be controlled.
The 21st century may reveal whether the clever brain bequeathed to Homo sapiens by the evolution of biological diversity can be used to save diversity as well as destroy it.
William K. Stevens covers environmental sciences for The New York Times.