Behind the projections of species loss lurk a number of crucial but hard-to-plot variables, among which two are especially weighty; continuing landscape conversion and the growth curve of human population.

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　　Landscape conversion can mean many things: draining wetlands to build roads and airports, turning tall grass(高秆草) prairies under the plow, fencing savanna(热带大草原) and overgrazing it with domestic stock, cutting second- growth forest Vermont and consigning the land to ski resorts or vacation suburbs, slash-and-bum clearing of Madagascar’s(马达加斯加) rain forest to grow rice on wet hillsides, industrial logging in Borneo(婆罗洲) to meet Japanese plywood demands.

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　　The ecologist John Trough and a colleague Card P.van Schaech have described a four-stage process of landscape conversion that they call the land-use cascade.The successive stages are: (1) wild lands, encompassing native floral and faunal communities altered little or not at all by human impact; (2) extensively used areas, such as natural grasslands lightly grazed, savanna kept open for prey animals by infrequent human-set fires, or forests sparsely worked by slash-and-bum farmers at low density; (3) intensively used areas, meaning crop fields, plantations, village commons, travel corridors, urban and industrial zones; and finally; (4) degraded land, formerly useful but now abused beyond value to anybody.

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　　Among all forms of landscape conversion, pushing tropical forest from the wild lands category to the intensively used category has the greatest impact on biological diversity.You can see it in the central Amazon, where big tracts of rain forest have been felled and burned, in a largely futile attempt to pasture cattle on sun-hardened clay.By the middle of the next century, if trend continue, tropical forest will exist virtually nowhere outside of protected areas—that is, national parks, wildlife refuges, and other official reserves.

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　　Human population growth will make a bad situation worse by putting ever more pressure on all available land.The annual increase is now 80 million people, with most of that increment coming in less developed countries.According to the UN’s middle estimate, human population will rise from the present 5.9 billion to 9.4 billion by the year 2050, then to 10.8 billion by 2150, before leveling off there at the end of the twenty-second century.Anyone interested in the future of biological diversity needs to think about the pressures these people will face, and the pressures they will exert in return.

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　　That direction, necessarily, will be toward ever more desperate exploitation of landscape.Even Noah’s Ark only manages to rescue paired animals, not large parcels of habitat.The jeopardy of the ecological fragments that we presently cherish as parks, refuges, and reserves is already severe, due to internal and external forces: internal, because insularity itself leads to ecological unraveling; and extremely, because those areas are still under siege by needy and covetous people.We shouldn’t take comfort in assuming that at least Yellowstone National Park will still harbor bears in the year 2150 that at least Royal Chowan in Nepal will still harbor tigers and Girt in India will still harbor lions.Those predator populations, and other species down the cascade, are likely to disappear.“Wildness” will be a word applicable only to urban turmoil.Lions, tigers, and bears will exist in zoos.Nature won’t come to an end, but it will look very different.
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