invasive and directly or indirectly caused the loss of native biodiversity as well. In short, agriculture is the human activity that has most affected the earth’s environment and that has caused the most direct and indirect biodiversity loss.

Deforestation. The annual expansion in cultivated area in Latin America from 1961 to 1997 was 1.26% per year, far greater than any other region (Dixon et al., 2001). Since 1961, cultivated land has expanded by 47%, while cropping intensity has only increased by 1% (Dixon et al., 2001), meaning that most of the increase in agricultural production has been due to the expansion in cultivated area.
     Expansion of the agricultural frontier in Latin America has commonly been ascribed to a set of key drivers: tax and credit policies and agricultural subsidies; agricultural colonization schemes; international and national markets; clearing for establishing land ownership; and technological factors (White et al., 2001). Frontier expansion in Latin America often starts with the cutting of logging roads into primary forest. Logging by itself deforests relatively minor areas of land. But logging roads allow colonists, usually small farmers using traditional production methods, to enter into hitherto impenetrable areas and slash and burn the forest, cultivating primarily subsistence crops for one to three years, until the soil begins to lose its fertility. Then they sell the land they have cleared to others, often large landowners, for conversion to pasture (Nations, 1992; Vandermeer and Perfecto, 2005). Cattle production is usually extensive, with low levels of inputs. Because of the characteristics of soils in tropical rain forests and grazing practices on the recently cleared land, pastures often quickly become degraded. When this happens, it can be very expensive to recuperate them and since land at the frontier is cheap, pastures are simply abandoned for newly cleared areas. In the Amazon, pastures are often abandoned within ten years and more than 50% of the area cleared is estimated to have been abandoned by the early 1990s (Hecht, 1992). Some research, however, indicates that soil fertility does not decline as markedly as widely believed and that agriculture in the Amazon may continue to be profitable over time if appropriate cultivation techniques are used (Schneider, 1995; Vosti et al., 2002).
     The relative contribution of small-scale, traditional agriculture to deforestation is a matter of some dispute (Vosti et al., 2002; Sanchez et al., 2005). While small farmers using traditional cultivation methods are certainly part of the phenomenon of the expanding frontier, large-scale clearing may ultimately be responsible for a larger absolute area of deforestation (Partridge, 1989). Nevertheless, spontaneous or state-sponsored agricultural colonization, which uses the frontier as a safety valve to address the problems of land tenure, has certainly played an important role in deforestation throughout the region. In some cases, such as immigration from traditional farming areas in Guatemala to the Petén (Barraclough and Ghimire, 2000), small-scale farmers are displaced by the intensification of agriculture in the sending areas. In other cases, farmers from marginal agricultural areas move away in hope of better opportunities. This has been one reason for internal migration in Brazil and elsewhere, where farmers from the poor, drought-prone northeast of the country were among the most likely to migrate

to the Amazonian agricultural frontier (Mahar, 1989; Lisansky, 1990). Typically the farming techniques that migrant farmers learned in their areas of origin are inappropriate for the fragile soils and vastly different climatic conditions of the frontier they have colonized, leading to even quicker degradation of the areas they have cleared and greater need to continually clear new areas.
     The two most active agricultural frontiers in Latin America over the last few decades of the 20th century have been in the rainforests of Central America and Brazil, both areas of high biodiversity. Central America, for example, has only around 0.5% of the world’s land area, but represents around 7% of the world’s biodiversity. It is considered a biological hotspot and has many endemic and threatened species. Much of the original forest has already been cleared, with only 20% of the isthmus still covered in dense forest. Nevertheless, a significant swath of tropical moist broadleaf forest remains along the Atlantic Coast, stretching from southern Mexico to Panama (Dinerstein et al., 1995).
     The expansion of the agricultural frontier has been linked to export cycles of commodity crops in Central America, but the ultimate use of cleared lands has been predominantly for pasture, generally using extensive systems with low levels of inputs. The total area in pasture has almost quadrupled from approximately 3.5 million ha in 1950 to over 13 million ha in 2001 (Harvey et al., 2005). Much of the cattle production was export-oriented. The decline in forest cover across the peninsula since the mid-20th century has been precipitous. Nicaragua, for example, lost 50% of its forest cover from 1963 to 1992 (Barraclough and Ghimire, 2000). The agricultural frontier has disappeared in El Salvador and Costa Rica, where most forest has already been cleared or, in the case of Costa Rica, designated as protected, but there is still an active agricultural frontier along the Atlantic Coast of the remaining countries of Central America (Harvey et al., 2005).
     Government policies also provided incentives for colonization of the agricultural frontier. In both Brazil and Central America, those seeking titled land were required to show “productive” use of the land by clearing it. This has been documented as a major factor in agricultural conversion at the frontier in Costa Rica, Honduras and Panama in Central America (Barbier, 2004). Government policies that subsidized credit for certain activities have also had a big impact. In the 1960s and 1970s, Costa Rica embarked on a program of diversification of agro-exports, supported by government credits, which pushed cattle exports up to become the third largest agro-export earner (Lehnmann, 1992). By 1973, a third of the land area of Costa Rica was in pasture. Statesponsored colonization schemes, in the Guatemalan Petén, for instance, also directly added to deforestation (Barraclough and Ghimire, 2000).
     Export-oriented production of commodities using conventional production systems has led to extensive clearing of native vegetation outside the rain forest in many parts of Latin America, as exemplified by the recent expansion of soybean cultivation throughout the Brazilian cerrado and the forests of Argentina. The cerrado is a mosaic of savannah and woodlands on Brazil’s vast central plateau. It is one of the world’s biodiversity hotspots and is home to the most diverse savannah flora in the world (UNEP, 1999a), an