Unlike germplasm banks, on-farm conservation allows crops to evolve continuously in response to changing conditions. Agricultural adaptation to climate change, emerging pests, and diseases thus depends on conserving crop genetic diversity in situ. However, increasing awareness of these issues has not translated into effective conservation policies. We find that previous assessments of on-farm maize diversity in Mexico are flawed and conceal widespread genetic erosion that could thwart current food security strategies for climate adaptation. Unable to mitigate declining yields by recourse to diversity, farmers might abandon agriculture, leading to a vicious cycle of yield and diversity losses. A reassessment of the conservation status in other centers of crop diversity is similarly urgent but could take a decade given data requirements.
Crop genetic diversity is an indispensable resource for farmers and professional breeders responding to changing climate, pests, and diseases. Anecdotal appraisals in centers of crop origin have suggested serious threats to this diversity for over half a century. However, a nationwide inventory recently found all maize races previously described for Mexico, including some formerly considered nearly extinct. A flurry of social studies seems to confirm that farmers maintain considerable diversity. Here, we compare estimates of maize diversity from case studies over the past 15 y with nationally and regionally representative matched longitudinal data from farmers across rural Mexico. Our findings reveal an increasing bias in inferences based on case study results and widespread loss of diversity. Cross-sectional, case study data suggest that farm-level richness has increased by 0.04 y−1 nationwide; however, direct estimates using matched longitudinal data reveal that richness dropped −0.04 y−1 between 2002 and 2007, from 1.43 to 1.22 varieties per farm. Varietal losses occurred across regions and altitudinal zones, and regardless of farm turnover within the sector. Extinction of local maize populations may not have resulted in an immediate loss of alleles, but low varietal richness and changes in maize’s metapopulation dynamics may prevent farmers from accessing germplasm suitable to a rapidly changing climate. Declining yields could then lead farmers to leave the sector and result in a further loss of diversity. Similarities in research approaches across crops suggest that methodological biases could conceal a loss of diversity at other centers of crop origin.