【正文】 s surface is mostly inert nitrogen (79%), which in itself is a good thing, as an atmosphere richer in oxygen than it already is (%) would literally fan the flames of outofcontrol 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 e from? With billions of anisms 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 plex microbes, as well as all green plants, trap solar energy by producing sugars and releasing oxygen as an incidental byproduct. 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 es from. Most of the world39。s species, on which we realize we have e to depend? No, we cannot. We have emerged at the other end of the 10,000year 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 prises 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 (spacetravel fantasies notwithstanding) where we will have to stay if we have any chance of longterm 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 don39。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 can39。 the quality and circulation of water。t move around, so they can39。t do to put all your geic eggs in a single basket allowing varieties with all sorts of as yetunexploited valuable features to disappear in the rush to concentrate on a few, geically homogeneous strains. Geic 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 pelling 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 world39。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 oldfashioned way: One must first find a geic 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 geic 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 bination of local soil and climate. The history of agriculture has itself produced, through simple artificial selection, a vast storehouse of geic 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 bination 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 geicists 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 geic variation in wild species to our collective agricultural effort is that Zea diploperennis almost certainly would have bee extinct within at most a few decades as its limited natural habitat in Mexico39。 and (3) moral, ethical, and aesthetic values. Just as most of us don39。s lists of why diversity matters. We have already encountered all three in passing. They are (1) utilitarian values (such as medicine and agriculture)。s thesis is that the survival of wild varieties of plants, or plants that have been domesticated but vary geically from small farm to small farm, is extremely important, as variety in the plant genome can provide a way of breeding diseaseresistant varieties and enriching human agriculture. Plant biodiversity, in particular, Tuxill says, is ar