selective pressure from the cropping, fertilization, and pest control practices of modern agriculture. Global markets, global values, global social organizations, and global technologies have resulted in global criteria for environmental fitness. Diversity of all kinds has been lost.

The economic way of thinking sustains the global exchange economy. The concepts of comparative advantage, specialization, and the gains from exchange are central to the neoclassical economic model. Comparative advantage stems from differences in the productivities of people, tools, and land in various economic activities. It immediately follows that total output can be increased through specialization of people, tools, and land in those activities for which they have a comparative advantage. Specialization in particular activities leaves each producer with lots of one product. Producers then exchange with each other until they have a mix of goods, which makes each of them as happy as possible given the willingness of others to exchange. Comparative advantage, the efficiency of specialization, and the gains through exchange are basic to our understanding of economic systems and to our understanding of the development process.

The gain from trade arguments underlies many development policies and justifies many specific projects. Road construction, much of it financed by international lending agencies, has encouraged traditional farmers to switch to cash crop agriculture, specializing in only a few crops according to market prices rather than to criteria of sustainable environmental management. Farmers who once planted diverse crops for subsistence thus have become connected with the global exchange. Other subsistence farmers were simply bought out or moved out by larger commercial agricultural ventures. Since labor with specific skills as well as capital equipment can be purchased in the market, the pattern of agriculture tends to be determined by the physical environment. For this reason, large, physically homogeneous regions now specialize in only a few crops.

The reduction in the number of crop species grown results in an even larger reduction in the number of supporting species. The locally specific nitrogen-fixing bacteria, fungi that facilitate nutrient intake through mycorrhizal association, predators of pests, pollinators and seed dispersers, and other species that coevolved over centuries to provide environmental services to traditional agroecosystems have become extinct or their genetic base has been dramatically narrowed. Deprived of the flora with which they coevolved, soil microbes disappear. Specialization, exchange, and the consequent regional homogeneity of crop species have reduced biological diversity.

Participation in the global exchange economy also transforms local agroecosystems because it forces farmers to stay competitive with other farmers who have been put in the same bind. This encourages use of inputs common to modern agriculture worldwide—fertilizers, pesticides, and high-yielding seed varieties—thereby eliminating many of the remaining regional differences in selective pressure (Ehrlich and Ehrlich, 1981). The adoption of modern technologies, however, must be understood in context of the complementary change in social organization (de Janvry, 1981).

The global exchange economy also induces temporal variation for which species have not evolved the strategies needed to cope. Crop failures, new technologies,

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