see all of these events as having coalesced into worldwidensecular trends. The release of chlorofluorocarbons in thennorthern hemisphere has opened a hole seven thousandnmiles away in the stratospheric ozone over Antarctica,nadmitting higher levels of ultraviolet radiation. This localncollapse foreshadows the fate of the global atmosphere)nthreatening higher levels of skin cancer and disruption ofnecosystems worfdwide. The consumption of huge quantitiesnof beef and timber in the United States accelerates thendeforestation of Latin America, dries its great river basins,nruins its soil, and on a larger scale, raises carbon dioxidenlevels in the global atmosphere.nThe secular trends can be conveniently classified as thenfour horsemen of the environmental apocalypse: globalnwarming by the greenhouse effect, undeniably coming butnat a rate still disputed among atmospheric scientists; ozonendepletion, severe and not in dispute; toxic waste accumulation,nsevere and not in dispute; and mass extinction bynhabitat destruction, also severe and not in dispute.nWhy should these man-induced changes be thoughtnapocalyptic? After all,, environmental change is perpetual,nand organisms have always adjusted to it in past geological,ntimes. Over millions of years species adapted to alternatingnclimatic warming and cooling, the expansion or shrinkage ofncontinental shelves, the invasion of new competitors andnparasites. Those that could not change became extinct, butnat such a relatively slow rate ^ that other, better adaptednspecies evolved to replace them. In the midst of endlessnturnover, the balance of life was sustained. But now thenvelocity of change is too great for life to handle, and thenequilibrium is shattered. The change is the greatest experiencednsince the end of the Age of Dinosaurs 65 millionnyears ago. It has reached precipitous levels within a singlenhuman life span, a mere tick in geological time. Humanity isncreating a radical new environment too quickly to allow thenadjustment of species, which need thousands to millions ofnyears to accomplish complex genetic adaptations. Most ofnlife is consequently at risk. Our life is at risk.nLike many other environmental scientists today, I thinknthe tragedy can be averted, and in a manner that movesnus toward a safer and more pleasant existence. It is thenessence of the new environmentalism that further economicndevelopment need not harm the environment, at least notnto anywhere near the degree now prevailing. The exactnopposite is the case: intelligent economic development cannsave the natural environment, and the natural environmentncan be used to accelerate economic development. The keynlies in the preservation and use of wild species and geneticnstrains, in other words biological diversity. Before exploringnhow this step can be accomplished, let me explain whynbiological diversity is the most fragile and in one sense thenmost valuable part of the environment. The extinction ofnspecies is the index of all the other secular changes. Andnwhere these processes—global warming, ozone depletion,nand toxic pollution — can be reversed, extinction cannot.nNo species can be called back by a summit conference ornMontreal Protocol. Each one is unique, precious, andnirreplaceable. It is the terminus of an immense number ofnmutations, gene recombinations, and episodes of naturalnselection by which certain sets of genes increased as othersndeclined. Each is prescribed by a genetic code that adapts itnexquisitely to a particular part of the environment. Many ofnthe genes vary within the species, so that every individualncreated by sexual reproduction has its own unique DNAncode. A typical eukaryotic organism, such as a floweringnplant or a bird (or human being), is prescribed according tonspecies by one billion to ten billion nucleotide pairs, orngenetic “letters,” in its DNA. If we could take the DNAndouble helices from a single cell of house mouse (Musnmusculus is a well studied species), stretch them to fullnlength and enlarge them until they were as wide as a piece ofnwrapping string, they would stretch about nine hundrednkilometers, with each inch containing twenty-odd nucleotidenpairs or genetic “letters.” The amount of information innthe genes of a mouse is about equivalent to that in all 16neditions of the Encyclopaedia Britannica published sincen1768.nWithout interference from man, species live an average ofnone to ten million years. How many of these ancient entitiesnare being driven to extinction by human action? We don’tnknow the absolute number, because the total size of thenfauna and flora is not known. With the help of othernbiologists, I recenfly estimated that about 1.4 million speciesnhave been described to date, that is, given formal namesnsuch as Canis familiaris and Homo sapiens. But mostnspecialists on biodiversity today would agree that the actualnnumber is at least ten million, and it could be as high as onenhundred million. In other words, the vast majority of kindsnof organisms have not even been discovered, much lessnstudied to any extent. The size of the biodiversity is notnknown even to the nearest order of magnitude! A majoritynof the species live in tropical forests, which are beingnreduced by cutting and burning at the rate of 1 percent anyear. This destruction alone, if continued unabated, isnprojected to extinguish over one quarter of all the species ofnorganisms on earth during the next fifty years. Thisntranslates to an extinction rate at least ten thousand timesnthat before the coming of man. These estimates help tonexplain why so many biologists have grown anxious andnvocal about the fate of tropical rain forests.nEvolutionary biologists are like art curators watching thenLouvre burn down. You may ask why anyone but a fewnscientists should care about frogs and orchids in farawayntropical countries. Let me peel away the reasons, startingnwith enlightened self-interest and ending with ethics.nDestroying the habitats where life is richest pollutes thenglobal environment as a whole. Between 2 and 5 percent ofnthe land surface of the world is burned yearly, accountingnfor 95 percent of all the fires, with the rest coming mosflynfrom lightning. Gases released by the burning, includingncarbon monoxide and ozone, currently approach the levelsngenerated by the use of fossil fuels. As tropical forests arencleared, they uncover some of the poorest soils in the world,nwhere less than one part in a thousand of nutrients penetratenfive centimeters beneath the surface. The situation isnradically different from that in the temperate forests, where anlarge portion of the organic material is invested in the deepnleaf litter and humus layers. In tropical forests the material isnlocked up in the living vegetation and quickly reabsorbed bynhighly efficient root systems as it decays on the groundnsurface. When the trees are cut over a wide area, thennnAUGUST 1990/17n