System diversity: farm systems based on several cropping systems, cropping systems based on a mixture of crops, and crops with varietal and other genetic variability.

Intercropping is common in traditional agriculture. Rather than growing only one crop in each field, several different crops are planted simultaneously within the same area of land. For example, in Java small-scale farmers have up to 607 crop species in their gardens, equivalent to the species diversity of a deciduous tropical forest (*FAO, 1996).

Diversity of crops on a small farm

Superficially, this may seem to produce unwanted competition between the plants. In actuality, the plants grow at different heights and have different root and leaf structures. This spatial diversity promotes maximum utilization of available light and nutrients. Intercropping also ensures that plants that are structurally and functionally similar are not too close and are therefore not competing for the same resources. The different root structures allow nutrients that would normally leach from the soil to be absorbed by the deeper-rooted plants, brought to the surface and returned to nutrient cycles. Different plants, growing at different rates also make demands on available nutrients and water at different times. They are thus not directly competing with one another. This extremely efficient use of available light, nutrients and water by plants with different heights, root and leaf structures is one of the primary reasons why traditional systems are highly efficient. (Altieri 1987, Innis 1980). This system actually increases the productivity of the land and tropical farmers claim yield improvements, measured as yield per unit area, as the major advantage of intercropping. (Govinden 1984).

Traditional farming is also characterized by a high degree of genetic diversity within each crop species. Traditional Sri Lanka farmers planted 280 varieties of rice, Andean farmers planted as many as 50 different varieties of potato in the same year and in the Philippines farmers used more than 300 traditional rice varieties. (Altieri 1987, Zimmerer 1998, *FAO 1996). This practice reduces the threat of crop loss due to a strain of pathogen that is adapted to a specific plant; some of the varieties are sure to be resistant. The diversity is also insurance against losses due to unpredictable weather or changing climatic conditions. If some varieties are susceptible to mildew in wet years, others will surely thrive in those conditions. The genetic diversity within crops provides a steady harvest as different varieties mature at different rates. This ensures the harvest is a continuous process distributing labor demands over a longer time period and providing relatively constant food supplies.