A puzzle of biological diversity is why numerous species persist in the same habitat. Tilman and Pacala (1993) observe that multiple species are found in places where two or more environmental factors are binding constraints and where unavoidable trade-offs exist among the responses to these constraints. Our research on the wheat agro-ecosystem in Turkey suggests that similar principles hold. Numerous environmental factors constrain the fitness of a particular wheat variety (e.g. soil, water availability and altitude), and farmers appear to confront unavoidable trade-offs as they select for one trait (e.g. yield, stability of yield, taste). Variety selection involves a complex interaction of factors at several levels (plot, household, region, nation), but our analysis suggests a positive outlook for the maintenance of landraces in Turkish agriculture. Turkey may alter its wheat breeding and promotion programmes to achieve a greater effect in marginal agro-economic zones where landraces prevail, but improving yield, reducing yield variance, and meeting quality expectations is a daunting and expensive task with uncertain returns for public investment.

The private value of landraces for farmers remains positive, leading to their continued cultivation in many places in the world. The social value of landraces is also positive, because of their contribution to the pool of genetic resources. Moreover, local knowledge about landraces and their production in centres of crop origins and evolution has positive social value because of its use to crop breeders and agricultural scientists in the formal sector. Although a high percentage of the genetic diversity of landraces of major crops is reported in ex situ collections, landraces and the knowledge and production practices associated with them continue to possess in situ value. This value derives from the landraces, both as a source of new diversity and for new collection, should there be a failure in the ex situ system. For science, peasant farming systems with landraces in centres of crop origins are an important crop evolutionary laboratory. Investing in on-farm conservation to maintain the comparative advantage of landraces in selected areas is a way to integrate the private and social values of landraces. The future of landraces is unknown, but we currently have the opportunity to understand the biological and social dynamics of these crop populations, to forecast the demise of landraces, and to devise methods to increase their private value.

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