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IF PEOPLE KEEP MULTIPLYING AND FARMERS keep farming as they do now, farmers will soon need to grow their crops on twice as large an area as what they use today.1 Doubling the population without changing the way we farm would expand the cropland from its present tenth of the world's land to about a fifth. More than any other factor, the success farmers have in feeding more people per hectare (ha) will govern what humanity is able to spare for Nature. I capitalize Nature here and throughout to indicate a specific definition, namely, the features and products of the earth itself, as contrasted with those of human civilization.
My essay presumes a population of ten billion people because that seems to be the round number in sight. The billions may level off at ten, or they may grow further.2 In either case, we must contemplate ten billion.
I presume also that humanity should spare lots of land for Nature. Proponents of the sparing of land reason about portfolio, money, and ethics. They argue that sparing land for Nature brings security by assuring a portfolio of biological diversity. They assert that Nature saves our money through her free ecosystem services.3 At bottom, however, is the ethical argument that survives quibbling over the utility of genes in a jungle or whether a marsh purifies water more cheaply than does a sewage plant. Although most religions emphasize humanity, even Genesis declares, "Let the waters bring forth swarms of living creatures, and let birds fly above the earth. . . . And God saw that it was good." My title can presume, therefore, that humanity should spare land for Nature without further justification.4
The following example shows that expecting farmers to spare land is not a futile wish. From 1961 to 1966, Indian farmers on average grew 0.83 tons of wheat per hectare on 13 million hectares of land. Then, applying the technology of the Green Revolution, they raised production more than fivefold and used only 80 percent more land. Looking back from 1994 to 1961-1966 one can see that Indian farmers spared 44 million ha, about the area of California, by growing more per hectare (see Figure 1). "How much land can ten billion people spare for...