Small farms play a crucial role in conserving the agricultural biodiversity that underpins longterm food security worldwide. Particularly in centers of crop genetic diversity – such as Mesoamerica in the case of maize (corn) and the Andean region in the case of potatoes – small farmers are the ‘keystone species’ in agricultural ecosystems of great value to humankind. Today, however, a formidable nexus of market forces and political forces threatens both small farmers and the biodiversity they sustain. Countervailing public policies are urgently needed. These should include the removal of existing policy biases against small farmers; social recognition of the contribution of in situ conservation to human well-being; development of markets for ‘traditional’ varieties of crops and livestock; the provision of local public goods in areas where farmers cultivate diversity; payments for the environmental service of on-farm conservation; and support for part-time farming as an element of diversified household livelihood strategies.