Biodiversity is the variability among living organisms from all sources, including terrestrial, marine and other aquatic ecosystems, and the ecological complexes of which they are a part. The interaction of various forms of biodiversity creates and shapes the environment in which we live; it also creates and sustains the agroeco-systems on which we depend for food and other basic needs. Diversity within an ecosystem enables that ecosystem to survive and be productive and to produce an enormous range of products and services. Agrobiodiversity is that component of biodiversity that is important to agriculture and agroecosystems. It helps ensure sustainability, stability, and productivity of production systems regardless of the level of complexity of the ecosystem in which it occurs, and, in the final analysis, it contributes to social welfare of the population through its contributions to poverty alleviation and sustainable food security.

Diversity at the agroecosystem level contributes to greater food security, helps increase employment opportunities, and increases local or national self-reliance by allowing a variety of enterprises, based on products and services, to develop on a national, regional, or community scale. A diversity of crop and animal species, at the community, farm, or field level adds to social and economic stability through reducing reliance on a single enterprise. Such diversity can also lead to a more efficient use of natural resources, for example, through providing greater opportunities for nutrient recycling (Carroll et al., 1990). Species diversity can also provide a buffering effect against losses to diseases and pests or adverse weather conditions. Diseases and insects that are major problems in large, single-crop species plantings become less of a problem and cause less damage when additional crop species are added to the system (Alexander and Bramel-Cox, 1991). Even at the field level, diversity generated through planting crop mixtures can reduce losses to pests and diseases. The net result of these types of utilization of crop diversity is resilience and sustainability of agroecosystems.

Within any particular species, genetic diversity is the variation which is most important. It is the variation that enables that species to adapt to new ecosystems and environments through natural and/or human selection. Genetic diversity within a cultivated crop species at the field or farm level helps diminish the risk of losses through diseases or pests, and provides opportunities to exploit different features of the microenvironment through, for example, the presence of diverse growth habits and rooting patterns (Smith and Zobel, 1991). Such factors can contribute both to greater stability and, in many circumstances, greater productivity. Both multilines and variety mixtures have been used effectively for this purpose in grain crops (Matson et al., 1997).

Modern agroecosystems often rely on crop species which are more uniform (i.e., less genetically diverse) than those in traditional agroecosystems. However, the conservation of the existing genetic diversity of species is critical to the modern farmer as well as the traditional farmer. Genetic diversity provides the reservoir of genes for future crop improvement by farmers and professional plant breeders. The ability to continue to rely on uniform, high-yielding crop species in modern agro-ecosystems depends on the constant new identification and use of genes that are, or have been, found in the genetically diverse crops of the traditional agroecosystems. Similarly, useful genetic diversity can be found in wild relatives of crop species and this diversity must also be conserved and appropriately used for improving crop performance.

Thus, diversity is important to agriculture at all levels and in all agroecosystems. While recognizing the importance to agriculture of diversity at all these levels, this chapter will focus on genetic diversity within crops — the genetic resources that lie at the heart of sustainable agricultural development and provide the basis for the continued evolution and adaptation of crops.

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