Land development, especially extension of farming areas, happens mostly in response to a rise in population or a rising demand for a particular crop (Plate 3.1). Pressure on land is created when an area is expected to support more than the optimal number of people. It is not only the population size that creates pressure on land but also the crowding of people in the more advantageous geographical areas within a country and the migration of less-fortunate people to marginal areas in search of agricultural land. Even countries with an overall low population may end up with high density in certain regions. For example, the concentration of the rural population of Venezuela is along the Andean foothills, not on the lower plains or the southern uplands. The population in western Malaysia is distributed along the two coastal plains, avoiding the central mountains. Pressure on land is also created by external interests, as is the case in the clearing of the Central American forest for raising hamburger-destined cattle. In addition, large-scale agricultural developments, the establishment of valley-bottom water reservoirs, the spread of industries or of urban settlements, all can create pressure on land by occupying the good agricultural land, and forcing the peasantry to migrate to other areas.
Whatever the underlying causes, agricultural expansion involves the settling and farming of marginal lands: areas which are difficult to cultivate and which would not be the first choice of most rural communities. The marginality is displayed in steep slopes, low rainfall, poor soils and so on. It is a fragile environment which is very easily disturbed. Farming of such land results in