Small farming systems, a wealth of varieties

Most of the world's farmers have less than two hectares of cultivated land. These small family farms are home to many original local varieties, whose genetic diversity is fostered by seed exchanges between farmers. To characterize and conserve this heritage as effectively as possible, it is therefore vital to investigate the way in which seed circulates. According to Christian Leclerc, an anthropologist at CIRAD, who supervised the work, "anthropology has taught us that within a given society, not everyone exchanges with everyone else. We therefore suggested that if there are social barriers between farmers, they influence seed exchanges and thus gene flows, resulting in social structuring of crop diversity." Vanesse Labeyrie, an agronomist at CIRAD, looked at the case of sorghum, an annual cereal capable of resisting difficult environmental conditions that is widespread in Africa. By combining genetic characterization of seeds and social anthropology, she analysed the way in which social relations between farmers influence varietal diversity.

Varieties specific to the different ethnic groups

At the study site, a 15-km² zone on the slopes of Mount Kenya, some 900 metres above sea level, there are three ethnic groups: the Chuka, the Tharaka and the Mbeere. The Chuka and Tharaka are part of the same ethnolinguistic group and see each other as allies. The Mbeere, who arrived in the region later, belong to a distinct ethnolinguistic group and have, in the past, had conflictual relations with the Chuka. "In a previous study, we discovered that some sorghum varieties were only grown by certain groups. For instance, the Mbeere grow a smaller range of varieties than the Chuka and the Tharaka, although they live in the same zone and share the same environment", Vanesse Labeyrie recalls. "This suggests that seed and crop practices do not circulate freely between the three ethnic groups. If that were the case, they would all grow the same varieties." These differences can therefore be put down to the habits and customs of each group.

Relationships and social links drive diversity

To describe the relationships that exist between farmers and assess the impact on the plants they grow, Mathieu Thomas, a post-doc student at CIRAD and second author of the study, suggested using network analysis: "this methodology, imported from quantitative sociology, served to formalize and test the hypotheses about the rules governing exchanges between farmers", the researchers explains. This approach showed how seed circulates: it is exchanged via the social relations between women, who are generally in charge of work in the field. It turns out that women farmers prefer to share their seed with people who live in the same place as them, and subsequently, outside that inner circle, with other members of their ethnolinguistic group. This is due to the fact that most exchanges take place between relations: the women share their seed with their husband's family, then with their own relations once they have settled in their own home. The Chuka and Tharaka, who belong to the same dialect group, marry more among themselves than with the Mbeere, and this differentiation is also observed in terms of the crops they grow.

Consequences for conservation

The system of relationships and marriages therefore strongly influence local seed exchange networks and consequently the varieties grown on the slopes of Mount Mont Kenya. "These results have improved our understanding of the dynamics of genetic diversity in situ", Vanesse Labeyrie stresses. "They will help us perfect models, for instance those aimed at predicting the consequences of introducing a new variety, or the resilience of local agrobiodiversity in the face of disruption." In fact, the tools used currently, which were initially developed to model wild plant diversity, do not take account of human social dynamics, whereas those dynamics make a major contribution to agricultural diversity, as shown by this study. It is therefore vital to take them into account in order to conserve local agro-biodiversity as effectively as possible.

Reference

Labeyrie V. et al . (2015) "Seed exchange networks, ethnicity, and sorghum diversity", Proc Natl Acad Sci USA, doi: 10.1073/pnas.1513238112