Diversity exists at many levels. Crops have been cultivated for millennia, and there are currently about 7000 crops; the genetic diversity of these crops can be conserved in gene banks or by farmers in situ. The germplasm has been released back to countries after natural disasters and conflict to preserve diversity. In a comparison of cornfields in Mexico and the Midwest, the Midwestern corn is highly uniform and suited to mechanization, while the Mexican field has much more diversity and ecological knowledge of the farmer. Low diversity systems are very vulnerable. Examples of benefits of diversity are included, such as nitrogen-fixing crops and mixed farming systems. Monoculture is reliant on pesticides to control pests, which are becoming useless. Diet diversity is important for health. Species diversity is important for soil fertility. Mechanisms that contribute to the effectiveness of diversity include complementarity and synergies; complementary is when different components of a system draw on the resources of the soil or the sunlight in different ways at different times to be more efficient in land use, and synergies are when the chemistries of different plants actually facilitate each other.
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