countries of the humid tropics vary widely as a function of their cultures, rates of population growth, economic circumstances, and environmental conditions. As a result, the degree to which different causal factors contribute to forest conversion varies from country to country and even within countries. Furthermore, the influence of these factors relative to one another changes over time. For example, in Côte d'Ivoire the expansion of the agricultural frontier has been the leading direct cause of forest conversion and is primarily responsible for a two-thirds reduction in the area of forest between 1965 and 1985. The deforested area is often in sloping uplands with marginal soils that cannot support intensive permanent cropping (Ehui, Part Two, this volume). In the Philippines, a combination of intensified commercial logging, agricultural expansion, increased use of fuelwood and other wood products, and a lack of alternative means of livelihood has greatly accelerated the rate of forest conversion since World War II (Garrity et al., Part Two, this volume). In Brazil, the formerly extensive Atlantic coast forest has been reduced to remnants through conversion to agricultural use over the centuries. Large-scale conversion of forestlands to cattle pastures and the opening of access roads was the leading cause of deforestation in the Amazon Basin (Serrão and Homma, Part Two, this volume). The removal of incentives to clear forestlands appears to have slowed the conversion to cattle ranching, but the migration of people to establish small-scale farms in forest areas has increased.

Subsistence farmers and forest dwellers have modified forestlands in the humid tropics for hundreds and even thousands of years (G ómez-Pompa, 1987a; Gómez-Pompa and Kaus, 1992). The scale of these modifications, however, was generally small, and the rate at which they occurred allowed time for forests to adapt and regenerate. As a result, their effects on the total area of forest cover and on nutrient cycling, watershed stability, biological diversity, and other ecosystem characteristics were limited.

Although forest conversion has expanded steadily over the past five centuries, the three continental expanses of humid tropic forest remained largely intact prior to the late nineteenth century (Tucker, 1990). Extraction of woods, spices, nuts, and other commercial products, although widespread, seldom exceeded the forests' productive capacities. The expansion of sugarcane, coffee, cacao, and other plantation systems was confined primarily to lowlands and adjacent uplands