The San people of the Kalahari have no trouble whatever understanding the value of biodiversity: Until fairly recently, the San had been living in small bands wholly within their local ecosystems. All their food, their clothing, their shelter, their medicines, their cosmetics, their playthings, their musical instruments, their hunting weapons, everything came from the productivity of their surroundings, the plants and animals on which they completely depended for a living. Why, then, is it so difficult for most of us in the industrialized nations -- urban dwellers but also rural farming folk -- to grasp the significance of biodiversity? The answer, I truly believe, is that we have simply forgotten what the San and all other hunter-gathering peoples still know. We have forgotten because of a profound and radical change in our relation to the natural world that came as a direct consequence of the invention of agriculture. We need to understand how people fit into the natural world -- both as hunter-gatherers and as agriculturally based industrialized societies before we can assess realistically what biodiversity means to human life.

For the first time in the entire history of life, one species, our species, Homo sapiens, has stepped outside of the local ecosystem. Agriculture changes the entire relation between humans and everything else living in the vicinity. To plow a field, to cultivate a handful of crop species, means the destruction of the dozens of native plant species that would otherwise be there naturally. No one ever heard of a "weed" until we began dictating what limited number of plant types we wanted to be growing on a given plot of land.

What is the difference between living off the natural fruits of the land and living off those we grow ourselves? The answer is simple: To live off our own cultivars, we must disassemble original ecosystems. There is very little native North American prairie left in the Midwest, and therein lies the gist of the dilemma. Most of us genuinely think we don't need prairie at all. We see prairie as simply underutilized terrain. We even tend to look at marshes like the Hackensack meadowlands and see instead Giants Stadium and the potential for still more entertainment and business complexes rather than a New Jersey tidal wetland full of cattails, migrating birds, and larval marine life vital to the restocking of the marine fisheries on which we still so heavily depend. The Botswanan cattle industry looks at the grasslands of the Kalahari -- and increasingly, the greener pastures of the Okavango Delta itself -- as underutilized rangeland. We have come by this outlook honestly: Having stepped outside local ecosystems so successfully, starting 10,000 years ago, we have come to think that we no longer need prairies, wetlands, or any other kind of natural habitat.

Agriculture has been a stunningly successful ecological strategy. Though famine has stalked the enterprise from its inception (there really is no such thing as complete control over food supplies, or anything else for that matter), the best indicator of ecological success is growth in population numbers. Estimates vary, but it seems likely that there were no more than 5 million humans on the planet 10,000 years ago. We had recently completed our spread throughout the globe by then, but we were still organized into relatively small groups as hunter-gatherers, still utterly dependent on the productivity of the local ecosystems in which we all continued to live.

The upper limit on human population numbers back then was set by the same rules that govern the numbers of all other species: Each population is limited by the environmental carrying capacity, the number of individuals that, on average, a local habitat can support, taking into account available food and nutrient resources, and other important factors, such as prevalence of predators and disease-causing microbes, and even more general factors, such as climate and rainfall. Each local population is fixed at some fluctuating number, usually 30 or 40 individuals maximum, as was usually the case with San and other hunter-gathering human beings. Thus, the total number of individuals of any species is the average size of its local populations multiplied by the number of those existing populations.

Agriculture popped the lid off natural regulation of human population size. No longer limited by the inherent productivity of local ecosystems, human agricultural societies began to expand immediately. Agriculture enables a settled existence -- and as populations began to grow, as patterns of political control and the division of labor began to emerge, human life rather quickly took on a semblance basically familiar to those of us living in even the most advanced of modern societies. Nor is this, of course, a "bad" thing: All of the great accomplishments of human civilization spring from our forsaking the local ecosystem and adopting agriculture as perhaps the pinnacle of our culture-dominated mode of making a living in the world.

If high culture is one signal of our success, so too, in the time-honored measure of ecological success, is our geometric increase in the number of individual living humans at any given moment. If there were perhaps as many as 5 million people alive 10,000 years ago, there are now nearly 6 billion [thousand million] of us. We are engaged in a perpetual race to feed ourselves, and every time we come up with a clever expansion of agricultural technology -- whether it be crop rotation and efficient plowing techniques a few centuries ago, or biotechnological manipulation of the genetics of crop plants today -- human population numbers expand right along, so that there are always people on the brink of starvation somewhere.

The sheer bulk of human numbers -- this 6 billion and ever-expanding, probably nearly doubling to over 10 billion by mid-21st century -- is wreaking havoc on Earth, on its species, ecosystems, soils, waters, and atmosphere. We are the current cause of this great environmental crisis, this threat to the global system that looms even as we approach the Second Millennium. We have created the biodiversity crisis, the next great wave of mass extinction that promises to rival the five greatest extinctions of the geologic past -- The Sixth Extinction….

THE VALUES OF BIODIVERSITY

Three themes crop up in everybody's lists of why diversity matters. We have already encountered all three in passing. They are (1) utilitarian values (such as medicine and agriculture); (2) ecosystem services (vital functions such as the continued production of atmospheric oxygen); and (3) moral, ethical, and aesthetic values.

Just as most of us don't know how our telephones, TV sets and computers work, we really have only the vaguest idea of where our foods and medicines come from. Harder yet to understand is the significance for our very existence of species and ecosystems which seem to just sit there and provide no obvious product for us to eat, use as fuel, or stock our medicine chests. Vaguer still is the calm sense of joy and simple belonging most urbanites experience with a simple walk in a woodlot, through a meadow, or along a clean shoreline. Yet these three categories of the effects of the living world on human life are absolutely crucial to modern and future human life on planet Earth.

UTILITARIAN VALUES OF BIODIVERSITY

When asked how many species humans routinely utilize in their daily life, most people (including most professional biologists) say, at most, perhaps one or two hundred. The correct answer is at least 40,000: Globally, each day we depend on over 40,000 species of plants, animals, fungi, and microbes. I am counting here only those species that we are deliberately exploiting. Still others, such as the microbe Escherichia coli, which lives by the millions in our intestines and is absolutely vital for normal digestion, are, fortunately, simply there.

Many of us think that food comes from the grocery store, and have little idea of its ultimate provenance. If some of us realize that spaghetti comes, not from trees but from wheat flour, we still tend to think that the Agricultural Revolution is long since complete, that we have already abstracted from nature all the plant and animal species that we are ever going to farm. We think that whatever improvements in crop yield and disease resistance -- two critically important factors in the ongoing race to feed the 250,000 extra mouths we are currently adding each day -- can come strictly from improved breeding techniques, and especially from the seeming magic wrought by the recently developed techniques of biotechnology.

Nothing could be farther from the truth. Here, a direct analogy with the natural world is apt: Evolution works through natural selection, the process Darwin (and Alfred Russell Wallace) discovered. On average, the organisms that thrive best will survive and reproduce, passing to their offspring the very traits that allowed them to flourish. Breeders do the same thing, allowing only those sheep, say, that have the woolliest coats to reproduce in the hopes of producing future sheep with even thicker coats than their forerunners had.

But selection alone -- whether natural or artificial -- will not do the trick. Another ingredient is required: the presence of genetic variation. You can only select from an assortment of different traits. Once you have gone as far as you can in selecting from the available range of genetic traits, the process, inevitably, comes to a halt. The reason why evolution did not stop billions of years ago is that spontaneous genetic changes -- mutations -- occur each generation, renewing and increasing genetic variation.

Biotechnology allows us to inject genes directly into domesticated plants and animals. At first glance, it seems that we have co-opted nature, once again substituting a clever bit of technology over a chancier and slower natural process. But the genes we insert to produce, say, frost-resistant strawberries, have to come from somewhere. You can't just go to a molecular biology facility and ask them to invent a gene that will make strawberry plants hardier. No one has the faintest idea what that gene would be, what its precise instructional coding would be, or where it might be inserted into the chromosomes of the strawberry cells.

Biotechnology works the old-fashioned way: One must first find a genetic feature that performs the desired function, before it can be extracted, manipulated, and inserted with the marvels of modern biotechnological technique into the stock where you would like to see that desired effect expressed. That means we must find genetic variation in the usual place: in nature, in wild versions of domesticated species, and in their nearest relatives. For many crop plants, there is an additional ace in the hole: The centuries, indeed the millennia, that farmers have been patiently tilling the land, sowing seeds, and harvesting crops that are bountiful one year, skimpy the next, have seen the emergence of countless landraces, local varieties of corn, wheat, tomatoes, etc. that seem to do best in a particular combination of local soil and climate. The history of agriculture has itself produced, through simple artificial selection, a vast storehouse of genetic variation.

All that variation is under serious threat. As science reporter Paul Raeburn recounts in his book The Last Harvest (1995), destruction of ecosystems in the wild threatens to obliterate countless species that are close kin to vital agricultural crops. He tells of the combination of skill, persistence, and luck that has enabled botanists from the United States and Mexico to locate a previously unknown species of wild corn, Zea diploperennis. This rare and rather unprepossessing plant promises to enable agricultural geneticists to abstract its genes, which convey resistance to a wide assortment of corn diseases.

The alarming coda to an otherwise encouraging story of the importance of natural genetic variation in wild species to our collective agricultural effort is that Zea diploperennis almost certainly would have become extinct within at most a few decades as its limited natural habitat in Mexico's Sierra de Monantlan was suffering precisely the same sort of conversion (for agricultural use!) that we are witnessing around the entire globe. How many other wild relatives of domesticated species have we already lost, and what will the effects of that loss be as we struggle to feed increasing billions of people over the next several decades?

Raeburn notes that a similar fate is meeting thousands of landraces. We lose species in the wild as we convert land for agricultural and other uses. We are losing landraces for a different reason: The switch from small single-family farming to large-scale agribusiness, coupled with recent dramatic advances in biotechnology, means we have begun to plant only a few "super" varieties of crop plants, apparently for good reason, as the crop yields have risen, and the quality remains high all the way to the dining room table.

There is a downside to all this success. Big agribusiness has seen huge increases in both fertilizers and pesticides, with their ongoing deleterious effects on soils, rivers, and adjoining ecosystems. The loss of hard-won genetic diversity of these landraces also poses a deep threat to continued agricultural success in the future. It just doesn't do to put all your genetic eggs in a single basket -- allowing varieties with all sorts of as yet-unexploited valuable features to disappear in the rush to concentrate on a few, genetically homogeneous strains.

Genetic diversity is the key to past, present, and assuredly future agricultural success. It is the key, as well, to our utilization of virtually all natural products. The medicines in our pharmacopoeia are as compelling an example as the agriculture story. Although we might be aware, in a vague way, that aspirin was originally extracted from the bark of willow trees, and that Europeans first learned of the drug through contact with native Americans, few of us have any idea of the extent to which indigenous healing practices, and the most sophisticated research and development efforts of the world's biggest pharmaceutical companies, rely on the genetic diversity of wild species.

Graphic examples of the importance of wild plants to the development of new drugs have recently become famous: The Madagascar periwinkle, a wildflower and close relative of a common decorative horticultural variety, has yielded a drug that has proven effective against two forms of childhood leukemia. Taxol, extracted from the bark of the Pacific yew, is now an important part of the chemical arsenal marshaled against ovarian cancer. We have already encountered the drug extracted from the seed pods of the sausage tree in the Okavango Delta and other African locales, a drug recently shown to be an effective agent against skin cancer. True, taxol and other potent drugs can be made synthetically in the laboratory. The point is, though, that we first have to know about the existence of these compounds before we can make them. It would be folly -- and horrendously expensive and time-consuming -- to sit around a laboratory randomly cooking up compounds in the hope that one of them might prove useful to combat a particular disease.

As you might expect, local peoples who are closely tied to the land, and have not forsaken the old hunter-gathering mode for agriculture, have a vast storehouse of knowledge about the natural world around them. Scientific explorers have been repeatedly struck by the detailed knowledge of local peoples concerning the plant and animal species in their environment. For example, a new species, the golden bamboo lemur (lemurs are primitive primate relatives of monkeys and apes), was discovered by Western-world primatologists living in a section of western rain forest in Madagascar in the 1980s. The local peoples had known of its existence and the fact that it was different from its close relative, the greater bamboo lemur, which the Western biologists had confused it with. They could tell just by listening to its nighttime cries coming from the forest that the golden lemur is a distinct thing -- what we westerners call a distinct "species."

More graphic still are the accounts from earlier expeditions. While collecting birds in the 1930s in New Guinea, the famous ornithologist Ernst Mayr found that the local tribesmen knew all the species that he could locate and actually could point to two confusingly similar species and tell him they were different. Nowadays, ethnobotany and ethnozoology are important areas of research -- not least for what they reveal of the utility of plant and animal species already well-known to indigenous people. Local expertise about native plants and animals has other implications, as well. When we compare lists of plants and animals drawn up by local peoples with those of professional biologists, it confirms our notion that species are real entities in the natural world, not just figments of Western-world classificatory imaginations. Local expertise can also dovetail conservation efforts with the economic needs of indigenous peoples: for example, by paying locals to act as guides in conservation reserves, or to serve as parataxonomists helping in the sorting and identification of species -- the "elemental particles" of biodiversity -- in biologically poorly known regions.

The world holds far more than the 40,000 or so species currently being utilized on a daily basis. That is why the exploratory research efforts of the chemical and pharmaceutical industries must go beyond simply cataloguing the experiences of local peoples. Although we have no precise idea of how many plant, animal, fungal, and microbial species populate the planet, there are at least 10 million of them. The living world is a vast cauldron of genetic variation: Most of it remains entirely unknown to us, yet much is undoubtedly of great potential use.

For good reason, much of the exploratory research has been focused on the tropical rain forests. Most of the terrestrial species of our planet reside in the Tropics, and tropical forests are disappearing at a frightening clip. Estimates vary, but 30 hectares per minute now seems, if anything, to be an underestimate. More recently, however, some attention has been shifted to the sea, the last great earthly frontier. We are, of course, ourselves a terrestrial species, having abandoned the sea to take up life on land some 350 million years ago. Until recently, our direct utilization of sea life has been restricted to fishing and to hunting marine mammals. This last great vestige of a hunting-gathering mode of existence until recently threatened to extirpate many whale and seal species and, as we have already seen, now threatens to collapse the most productive fisheries in the world.

Corals and sponges are but two of the major groups of marine invertebrate animals that live firmly rooted to the sea floor. They don't move around, so they can't escape when a predatory fish or crab comes by and tries to bite off a piece. These sessile creatures have evolved a stunning array of chemical defenses against such attacks -- defenses that have recently begun to attract a lot of attention from the chemical and pharmaceutical industries.

The case for the great diversity of living species as a storehouse of vital genetic variation is crystal clear. We have relied upon that variation increasingly since we developed agriculture, even as it has indeed seemed that we were abandoning nature. That reliance on the natural genetic storehouse will only increase as time goes on, a compelling reason why we must arrest the destruction of ecosystems and species that right now is systematically dismantling and destroying this vital resource.

Specific utilitarian uses are only part of the story. Ultimately what might prove even more crucial is the simple overall health of the global system: the purity of the air, the balance among carbon dioxide, oxygen, nitrogen, and other gases of the atmosphere; the quality and circulation of water; the vital cycles of carbon, nitrogen, phosphorous, and other atomic constituents of our bodies. In short, in fouling our nests, in destroying ecosystems, and driving many species to extinction, we are beginning to approach a limit on how much of the global living system -- and we ourselves -- can actually survive. In the long run, the most valuable aspect of diversity may well be the ability of our species to continue to live on the planet.

EARTH'S ECOSYSTEM SERVICES

Why, one might reasonably ask, need we worry about the health of local ecosystems if we ourselves in large measure no longer live within them? Why can't we continue our 10,000-year course of habitat conversion and ecosystem destruction now that most of us no longer look to local renewable resources in our daily lives? Can we not live in a world wholly of our own cultural devising -- without all but a few of the world's species, on which we realize we have come to depend?

No, we cannot. We have emerged at the other end of the 10,000-year honeymoon with agriculture -- and the consequent explosion in our population numbers -- and have begun to see that we are part of the global system, after all. Earth comprises a global system of interacting elements: the atmosphere, the lithosphere (soils and rocks), the hydrosphere (oceans, lakes, streams), and the Biosphere -- all of life. That global system is the summation of all those local systems interlocked across the entire face of Earth. Earth is our home -- where we were born, where we live now, and (space-travel fantasies notwithstanding) where we will have to stay if we have any chance of long-term survival.

What effect does the Biosphere have on us? What does the Biosphere do for us? Simple, essential, and downright fundamental things -- things that we mostly don't see, appreciate, or fully understand -- without which, life on Earth for all species, including ourselves, would be completely impossible.

Take the air we breathe. The atmosphere close to Earth's surface is mostly inert nitrogen (79%), which in itself is a good thing, as an atmosphere richer in oxygen than it already is (20.9%) would literally fan the flames of out-of-control wildfires. When we talk about the air we breathe, most of us mean oxygen. Oxygen is absolutely essential to all but a very few forms of microbial life. Some bacterial species use alternative chemical pathways to break down the nutrients on which they live, but all the rest -- most microbes, plants, fungi, and animals, including human beings -- require a constant supply of oxygen just to exist.

Where does atmospheric oxygen come from? With billions of organisms taking in oxygen, and expelling carbon dioxide, surely we would soon deplete this essential resource. The answer, of course, is photosynthesis, the process whereby some bacterial and other, more complex microbes, as well as all green plants, trap solar energy by producing sugars and releasing oxygen as an incidental by-product.

Though no one seriously thinks that our supply of oxygen is in imminent danger of collapse, it is important to realize just where the daily replenishment of this most precious resource comes from. Most of the world's fresh supplies of oxygen are produced by single-celled, microscopic plantlike organisms floating near the surface of the oceans, supplemented, of course, by the photosynthesizing activities of terrestrial plants. The mighty oceans are the last great frontier of relatively un-despoiled natural habitat, but land-based human activity is beginning to sicken even them. Pollutants reaching the sea through streams and via the atmosphere (as gases are dissolved in water droplets), direct oceanic dumping, and the degradation of natural marine ecosystems through overfishing and mining operations are beginning to have their cumulative effects.

Consider, for the moment, the net effect of increased erosion from denuded lands on the entire Biospheric system. Coral reefs which typically fringe the shorelines in tropical oceans are beginning to show worldwide distress. One factor in their decline is increased erosional runoff of silts from the cutting of tropical rain forests. For all their massive structure, corals are actually delicate colonial animals that are extremely sensitive to silt content in water. They quickly die when clear tropical waters become clouded with particles of clay and quartz, as has happened recently in Belize.

Coral reefs themselves provide a protective barrier to other delicate ecosystems, such as the mangroves fringing the southern tip of Florida. As we have already seen, coastal wetlands are the breeding grounds of countless species of fish and shellfish. Without viable, functional coastal wetlands, our fisheries -- already on the verge of collapse in many places, overtaxed as they are by incessant and often ruinous fishing practices -- would soon be in even worse shape.

Everything is linked. We are accustomed to hearing that a complex web of energy flow -- who eats whom -- links all creatures in a local ecosystem. This is equally true of the Biosphere itself: What happens in the Amazonian rain forest ultimately affects not only the conspicuous mammals and birds of that forest, but also its fishes; runoff from the river affects the oceans, and all of its life. Decline of the fishing productivity on the Georges Banks off the Newfoundland coast can be traced in part to the cutting of tropical rain forests, as breeding of migratory fishes is disrupted far from the point where they are eventually hauled in by fishermen. The world truly is a complex system, and we are a part of it, still dependent on its renewable productivity, which we ourselves are beginning to stifle.

We have by no means exhausted the list of ecosystem services rendered by plants. Several absolutely essential elements -- nitrogen and phosphorous to name but two -- are derived from the plant world. Even though nitrogen is an essential element of all proteins, only a few forms of bacteria (aptly called nitrogen-fixing bacteria) can extract nitrogen from the atmosphere and incorporate it directly into their bodies. Some plants such as the legumes (peas and their relatives) harbor nitrogen-fixing bacteria amid their roots -- a form of farming, in a sense. All the nitrogen in our bodies comes from eating plants or other animals that have eaten plants. The cycle is complete when decay organisms decompose dead plants and animals, producing ammonia, which is then converted by other bacteria to nitrates that plants can pick back up directly from the soil and to free atmospheric nitrogen.

There can be no doubt that the Biospheric system -- in particular, the vast range of organisms, from microbes to plants, fungi, and animals -- plays a far greater role in our everyday lives than we think. We take them for granted -- as we do our agriculturally produced foods, our cars, and our TV sets. That's fine -- so long as we don't tip the applecart, by destroying so many of the world's local ecosystems that we compromise the Biosphere's cycles and our very existence.

A MORAL AND AESTHETIC IMPERATIVE

Although we need the Biosphere's species for our own uses, and we rely on those species for the basic supplies of food, water, and chemical compounds on which all life depends, there is still a third category of concerns for the natural world -- a third set of reasons to cherish the natural world and to resist its wanton destruction. Harder to define with precision, the aesthetic appeal and moral challenge posed by Nature are to some the most compelling reasons to ward off the impending Sixth Extinction.

Not all of the world's major religions adopt the basic premise of the Judeo-Christian tradition, that the world and its living creatures were placed there by the Creator for our own human use. Genesis specifically exhorts humans to seek dominion over the beasts of the field. Recently, however, some theologians of the Judeo-Christian tradition have come to see the biblical exhortation as a call to stewardship; that is, our role ought to be as caretakers rather than as masters, to safe-guard the richness of the natural world, rather than to plunder it. Just as the Genesis role strikes me as an accurate assessment of the newly established order after the advent of the Agricultural Revolution (and Genesis was written not long thereafter), this newer theological interpretation, in my view, dovetails very nicely with the present condition of humanity vis-a-vis the natural world.

Other religious traditions, of course, espouse radically different views of nature and the place that humans take in the natural world. The oneness of humans with the rest of animate nature is perhaps especially apparent in the Hindu tradition, where the doctrine of reincarnation sees a continuity between humans and other species only matched in the Western world by the intellectual concept of organic evolution. Other traditions of the Far East -- Buddhism and Shintoism, for example -- also, in their different ways, locate humans as part of the natural firmament, and not especially exalted above the beasts of the field. These religions, it has been suggested, may make it easier for those raised in their tradition to see the urgent necessity of halting the blind, errant destruction of the natural world surrounding us all -- easier, that is, than it is proving to be for those raised in Euro-centered, Western-world traditions.

Religion, of course, is not the sole source of moral suasion, and ethical concerns of what is right, and what we ought and ought not to do, have been arising on their own with increasing frequency as the early stages of the Sixth Extinction intensify. Cries for adopting a conservation ethic -- one thinks here as much of Theodore Roosevelt and other far-sighted people of his generation as of the more recent (yet before their time) American environmentalists Aldo Leopold and Rachel Carson -- are often tied to a sense of belonging to the natural world. A feeling, not just an intellectual grasp, of somehow still belonging to the natural world pervades the words of these early prophets -- as when Rachel Carson, in the very title of her most famous book, Silent Spring, asked us to consider what spring would be like without the songs of birds and the hum of insects.

These are essentially aesthetic feelings -- the notion that human beings just cannot expect to live completely successfully and happily strictly in the steel, concrete, and plastic world that increasingly appears to lie in the future. Famed Harvard biologist and biodiversity spokesman E.0. Wilson speaks of biophilia, his term for what he considers to be an innate sense of belonging to the natural world that, though subdued, is still present in all of humanity. Though we have proven to be mighty adaptable, I have a feeling he is basically right. The old saying "You can take the boy out of the country, but you can't take the country out of the boy" has a powerful analogue that encompasses us all. You can take people out of nature (local ecosystems, at least), but you cannot take nature out of people. That may well be the best reason for us to confront the Sixth Extinction.

SOME GLOBAL ISSUES

"Study Finds Sperm Counts Are Declining." So reads a recent headline, not in the Inquirer, but in the New York Times. There is evidence -- convincing to some, although not all, medical researchers -- that the amount of sperm produced by human males in both Europe and North America has, on average, declined in the years since World War II. The story is dramatic, but the idea that someone could suggest such a far-flung effect on a biological function so fundamental to human existence is indicative of a much more general set of signals that the global system is hurting.

Scientists, thankfully, are a conservative lot, and all recent claims of global phenomena -- including purely physical changes such as global warming, holes in the ozone layer, and increase in frequency and intensity of the El Nino climatic effect -- have nearly as many doubters as proponents. It is always difficult to establish with absolute certainty that a recent trend -- say, increase in global temperatures -- represents the predicted effect of increase in carbon dioxide and other gases in the atmosphere that have been accumulating since the advent of the Industrial Revolution. After all, we have direct recordings of global temperature for at best only the last century. More importantly, we also know that Earth has been sporadically, but on the whole steadily, warming up naturally for the past 12,000 years, when the last ice age ended and the huge sheets of continental glacial ice began to retreat toward the North Pole.

It could be that the warming that has occurred during the twentieth century -- a rise in global average temperature of 0.5C causing a 2-centimeter rise in sea level along the Eastern seaboard of the United States -- is merely the continuation of a 12,000-year-old natural trend. On the other hand, carbon dioxide helps trap solar energy, keeping reflected light from bouncing entirely back into the voids of space. The equations are there: The more carbon dioxide in the atmosphere, the more solar energy will be trapped, and the higher the world's average temperatures will become. The recent dramatic rise in global temperature may well be, at least in part, a side effect of human activity: The progressive rise in burning of fossil fuels over the past several centuries and the more recent, but no less devastating, burning of forests, primarily in the world's tropical belts.

We may be uncertain and even skeptical, but we cannot afford to ignore the signs. The data are scant, so far, on the possible decline in human sperm counts, but there are several other apparent cases of negative global effects on life that seem almost certain to be caused by changes in the atmosphere -- changes wrought as a purely unintended result of human activity.

One now well-studied example is the worldwide, and often rather precipitous, decline in frog and salamander populations over the last decade. I had noticed a sharp reduction in the half-dozen species of frogs and toads in and around my favorite little pond in the Adirondack Mountains of New York State. In the early 1970s, when I first started looking, there were green and bull frogs galore down in the pond, in populations so dense that the bulls were keeping us awake at night with their calls, and restaurateurs were shooting them for their bills of fare. American toads and wood and pickerel frogs were leaping all over the place along the pond's edge. Since the 1980s, it has been hard to see any of these species. But that is just one little spot, and for all I knew, I was just witnessing a natural fluctuation of population numbers -- though it did strike me as odd that all the frogs and salamanders in my one little spot seemed to be declining at the same time.

My Adirondack amphibian experiences are decidedly anecdotal -- just casual impressions. Imagine my surprise when I learned that professional herpetologists had begun to notice an apparent worldwide decline in frog and salamander populations. More recently, frog populations all over the United States are producing many deformed individuals, a phenomenon first noted by Midwestern schoolchildren. Just as in the case of global warming, experts on frogs and salamanders are divided on this apparent pattern. Is the decline real? If so, is it somehow just normal fluctuation, or is the decline at least in part caused by human-engendered environmental change? If human environmental disruption is the culprit, is the reduction of frog and salamander populations more a matter of alteration of local habitats, or is there some truly global factor at work? All herpetologists seem to agree that there is an urgent need for serious, long term studies to get a firm handle on what precisely is happening to the world's frogs and salamanders.

There is already enough evidence to convince some serious biologists that the amphibian decline is indeed real, and is affecting the great majority of the more than 5,000 frog and salamander species known to exist in the modern world. Severe alteration of local habitats lies at the very heart of extinction, and many frog populations seem to have been reduced through human conversion of their habitats for our own purposes. More subtle, but still in keeping with the theme of human interference on the local level, is the use of pesticides and the introduction of other toxicants. Back to the anecdotal, I do note that my observation of the frog and salamander decline in the Adirondacks in the 1980s coincided with a determined spraying program aimed at reducing the numbers of mosquitoes and black flies that attack humans and depress the flow of summer tourist dollars. Pesticides -- or the human-induced drop in edible insects -- may well underlie the reduction of amphibian numbers in many places.

Yet the tantalizing possibility remains that the worldwide decline may actually represent a truly global cause. In other words, the global pattern is not just the summation of isolated local effects but is actually caused by some factor that itself acts on a global basis. Frog and salamanders must return to the water to reproduce, and the skins of many species are delicate and more permeable to various substances than is the typical case for reptiles, birds, and mammals. Many species of frogs and salamanders are known to be sensitive environmental indicators; frogs, for example, are often good indicators of rising acid content in their native waters, as their numbers typically decline as levels of acidity rise.

What global factors could conceivably underlie the amphibian decline? Oregon State University herpetologist Andrew Blaustein -- a cautious student of the problem -- has suggested at least two possibilities. For one, there is a fungus known to attack, and to reduce the viability, of frog eggs. The fungus is found around the world, and perhaps a recent expansion of its range, or its potency, has contributed to the amphibian decline. More suggestive is the more recent hypothesis that an increase in the level of intensity of ultraviolet (UV) radiation is causing the decline. Blaustein and his colleagues have found that the developing eggs of frog species known to be declining are more sensitive to -- more damaged by -- exposure to a given level of UV radiation than are the eggs of frogs whose population numbers have remained relatively stable in recent years.

Ultraviolet radiation. That rings a bell -- as anyone who has read of the recent alarming rise in incidence of human skin cancer well knows. The medical literature has established a firm link between exposure of human skin to the sun and the occurrence of skin cancer -- a correlation that, like that between lung cancer and smoking, implies actual cause and effect to most medical professionals. Dermatologists are in virtually unanimous agreement that exposure to UV radiation is one of the causes of skin cancer.

Careful measurements by atmospheric physicists indicate that UV radiation has definitely been on the rise. The increase is especially apparent in the higher latitudes, closer to the North and South Poles than to the equator. Much of the UV component of solar radiation is absorbed by ozone, a molecule consisting of three atoms of oxygen. Each winter, the natural layer of ozone in the atmosphere thins, at places disappearing entirely. In recent decades, satellite imagery and balloon probes alike have revealed increasingly large and persistent rents -- holes -- in the ozone layer. Damage to Earth's protective ozone layer was actually predicted: Chlorofluorocarbons -- organic compounds widely and routinely used in aerosol cans after World War II -- were known to interact with ozone, destroying large quantities of it in a one-way chemical reaction. That, it is widely agreed, is precisely what has happened. Spray cans, in this roundabout yet deadly way, are responsible for the epidemic increase in human skin cancers and perhaps the decline of many amphibian populations as well.

There are several critical lessons here. First, we see that humans are indeed capable of altering the global system -- in this instance, the atmosphere. We also see that such changes can have profound effects on living things -- in this particular example, most clearly and convincingly documented in the rise of skin cancer in humans. We also see that negative effects -- amphibian population declines -- occurring globally may result from a single global cause, or may be the simple, yet nonetheless devastating cumulative effect of local habitat disruption and destruction.

There are other lessons as well. Though some biologists point to the economic importance to humans represented by frogs (by keeping insect pest populations at bay, for example), and though many of us feel the noisy booming of bull frogs is a welcome nighttime experience, many citizens of the modern world more than likely would maintain that the world (meaning, of course, the human world) could get along just fine without those 5,000-odd species of frogs and salamanders. They would be missing the main lesson here: The real significance of the global decline of frogs and salamanders is that the amphibians are telling us something about the state of the atmosphere and, thus, of the global system as a whole. Human skin and frogs eggs just happen to be at the higher end of sensitivity to UV radiation. Amphibians -- and human skin for that matter -- are global analogues of miner's canaries, early warning systems that all is not right with the global system.

The emerging story of the increase of atmospheric UV radiation through human agency -- and its boomerang effects on us and quite likely all other living systems -- is stark, dramatic, and fairly easy to comprehend. More subtle are the negative effects on human life of human-engendered destruction of natural ecosystems and the consequent loss of species. It has proven difficult to explain what the loss of the northern spotted owl of the old-growth forests of the Pacific Northwest really means to local people, especially those engaged in the lumber industry in Oregon and Washington, to those of us living elsewhere in the United States, and ultimately to the people of the world. It simply doesn't make much intuitive sense to claim that the loss of a few hundred pairs of owls -- which hardly anyone actually ever sees -- will have a profound negative effect on human existence. Yet it does matter. Those owls are a litmus test of the health of the ecosystem in which they live, and we are now beginning to understand that we depend on those systems far more than we imagined since we invented agriculture and stepped away from the local ecosystem.

THE SIXTH EXTINCTION

Biologists do not know exactly how many species are currently on the planet. Science has recorded and named some 1.6 million species, but we know this can only be some small fraction -- no more than 10% to 15% -- of the true number. Some biologists believe we have identified only 1% to 3%, and that there may be as many as 100 million species on the planet. Because concern over the accelerating loss of species has been mounting, biologists have turned in earnest to the key question, How many species are on Earth? They have begun to converge on an estimate of some 14 million species, but opinions still sharply differ on this vital issue.

Why are we so ignorant of the biotic riches of Earth? Scientific survey of the world's species began in the seventeenth century, but did not switch into high gear until the mid-nineteenth century, when the heyday of European colonialism mixed with the Industrial Revolution, producing a blossoming of exploration and scientific inquiry. Naturalists like Alfred Russell Wallace and Henry Walter Bates traveled to then-exotic destinations such as the Amazon Basin and the Spice Islands (part of present-day Indonesia) to amass vast collections of plants, insects, spiders, aquatic invertebrates, fish, amphibians, reptiles, birds, and mammals. The collections of such intrepid explorers found their way at first into privately held "cabinets" of natural history and increasingly into the large natural history museums that were founded in the mid-nineteenth century -- museums such as the British Museum of Natural History in London, the Natural History Museum of the Smithsonian Institution in Washington, D.C., and my own favorite treasure trove, the American Museum of Natural History in New York. Natural history museums are libraries of biodiversity, storehouses of the world's biological riches, where scientists can compare specimens and assess the identities and evolutionary relationships of the world's species.

Though research biologists at major universities have historically contributed to the effort of finding, describing, and naming the world's species, increasingly this role has concentrated in the hands of the scientific staff of major natural history museums. Systematics is the branch of biology devoted to describing Earth's species, analyzing their evolutionary relationships, and producing biological classifications. Because biology keeps expanding (most recently into the realm of biomolecules), and because vast collections of specimens are needed as part of the routine work of systematists, museums have become the focal point for systematics research.

Needless to say, there are far more species than experts to identify them. For some groups, there are few (sometimes no) experts actively working to inventory the world's stock. One way biologists frame accurate guesstimates of how many species probably exist in the world is to assess what we know we have found already, observe the rate that new species are turning up, evaluate how concerted the effort is to find new species for a given group, and derive some sense of what might still be out there, as yet unidentified.

Ornithologists think that they have found most of the world's bird species, as the number has begun to level off at around 9,000, and mammalogists also think they have described and named well over 90% of the world's mammal species. As we have seen, even large species of mammals still turn up on a regular basis, such as the large antelope and deer recently discovered in recently war-ravaged Vietnam and the several species of lemur discovered over the past decade on Madagascar. New bird species also turn up regularly. Because so many systematists have focused on birds and mammals -- big and obvious, the charismatic megafauna -- and because much more numerous groups, such as insects, have received relatively much less attention, the ratio of named to as-yet-undiscovered species varies widely from group to group.

There is yet another major source of inference for assessing the actual number of species on Earth, one that is tied into the very critical question, How do we know that we are in the midst of a sixth, major global mass extinction? The connecting link is habitat, by now familiar as the essential ingredient in species loss, but one which, quite obviously, underlies the sheer existence of species.

In an elegant series of studies, Smithsonian coelopterist Terry Erwin came to his by-now famous -- and still controversial -- estimate that there are some 30 million species of insects in the Tropics alone. Erwin carefully sampled the insect(especially beetle) faunas of various forest canopies in Panama, Brazil, and Amazonian Peru. Erwin's goal was to determine how limited beetle species were, on average, to particular types of trees. Then, taking into account the total aerial extent and canopy diversity of the tropical rain forest, he was able to derive an estimate of the total number of beetle species currently in existence, and from that estimate extrapolated an estimate for the total number of insects.

Coming as it did when most of the world's biologists were still thinking that there are at most only a few million species on Earth, Erwin's analysis shocked a lot of biologists into attention. More recent work, including similar in-depth, total assaying of a region's biotic riches, have tempered his estimates somewhat, but we now are accustomed to thinking that there are at least 10 million species of insects, rather than 1 to 2 million species, a 10-fold increase in our estimates of Earth's living species diversity. If so rich a percentage of the world's species is yet undiscovered, then their loss, their undocumented extinction, becomes more critical. Loss of unstudied genetic diversity means never to have that knowledge and never to be able to utilize whatever potential those species might have had, one of the major reasons humanity has become alarmed at the growing rate of destruction of the world's species.

How do we measure that rate of disappearance? One simple and obvious way is to record the number of species -- such as the passenger pigeon, the dodo, and the great auk -- that are known to have become extinct in historic times. A related approach is to evaluate how well species that have been placed on threatened and endangered lists have fared. Appendix I [of the book] shows the documented species extinctions since 1600. Although they are considerable, they seem hardly the stuff of catastrophic mass extinction. Is the current loss of biodiversity overestimated? Are we really in the throes of a major mass extinction, the Sixth Extinction?

Other approaches to measuring extinction rates yield more alarming results. Many species in museum collections simply can no longer be found in the wild. Many of my colleagues at the American Museum of Natural History have told me of returning to locales where, a few years earlier, they had found new species -- in one case, a new species of spider in Chile -- only to discover that the species' habitat had disappeared.

Thus, we can use the very same sort of reasoning that gives us one way of estimating the number of species that exist (complete sampling of species richness in finite areas) to yield an estimate of the rate of species loss. We can measure the rate of habitat destruction. Aerial and satellite photography have revealed that tropical rain forests are being destroyed at a rate of some 12 million hectares a year.

Combining known rates of habitat destruction with accurate assessment of numbers of species occurring per hectare in tropical rain forests, and the average size of areas they occupy, yields rough but credible estimates of current rates of species loss. We have already encountered the most famous of these: E.0. Wilson's figure of 27,000 species a year, which boils down to 3 species an hour lost forever. Some biologists think Wilson's figure is too high, but plenty more think he has underestimated the situation.

How can we reconcile the rather low rates of extinction revealed from actual documented examples with the vastly more impressive -- and troubling -- estimates based on loss of habitat acreage? Most of the documented examples of extinction in the past few hundred years involve large, easily observed species (such as the quagga, a zebralike horse species); species from islands, where diversity is most easily monitored (the dodo was a giant, flightless pigeon living on the island of Mauritius in the Indian Ocean); and the well-studied north temperate latitudes, where the vast majority of the world's biologists have lived and worked (the passenger pigeon was a North American species). We know about these species simply because we have had the opportunity to study them and observe their loss.

There is more to the story than simple convenience of study. The Tropics harbor vastly more species per acre than the higher latitudes, where species are physiologically broadly adapted, and consequently distributed over far greater ranges. Thus, a species with huge populations spread out over a large area can suffer huge reductions in numbers, yet still be nowhere near total annihilation. Tropical species are much more narrowly adapted and narrowly restricted in geographic range so that the same amount of habitat destruction in the Tropics accounts for many times more species extinctions than would occur in the higher latitudes.

The conclusion seems clear: Ongoing, unrelenting habitat destruction is driving thousands of species to extinction each year -- species that for the most part we haven't even come to know as yet. Direct destruction of ecosystems is having a mounting effect on ecosystem services, which are vital to our quality of life and our continued ability to elude extinction.