For crop species and varieties, including plants used for forestry, as well as for domestic animal breeds, there is a need to avoid genetic erosion and maintain a diverse genetic pool to ensure the future viability and improvement of the qualities of the varieties and breeds involved.
Genetic diversity refers to the variation of genes within species, including the genetic variation between distinct populations of the same species It also covers genetic variation within a population. Genetic diversity can be measured using a variety of DNA-based and other techniques.
Genetic diversity in agricultural crops began to erode as the political and social changes of the industrial era transformed agriculture. As a result genetic conservation began early this century when foresighted agriculturists saw that the loss of primitive varieties or "landraces" meant loss of genetic variation essential for sustained crop improvement.
One approach to the preservation of crop genetic diversity is the establishment of in situ reserves. These are preserved habitats harboring wild populations of crop species or related species. They are also regions where cultivation of indigenous landraces is continued by custodian farmers in exchange for subsides. In situ genetic reserves can help conserve wider genetic variation less expensively than ex situ conservation because the costs of germplasm storage are eliminated. The genetic variation of in situ populations is maintained by natural processes or indigenous cultural practices. Genetic and ecological patterns in reserve populations such as pest resistance or microclimate tolerance can be identified in situ and used to extract useful traits.
For plants, the best ex situ gene bank is an arboretum or an experimental garden, farm, or forest, in any of which the growing conditions resemble as closely as possible the various species' natural habitats.