Historical and contemporary butterfly genomes reveal genetic erosion in response to land-use intensification.

Biodiversity loss is typically assessed at the level of species extinctions, yet many populations are declining without the species going extinct. A major global driver of such declines is the intensification of agriculture beginning in the mid-20th century. To assess the genomic consequences of this shift, we compare genome-wide sequence data from historical (1875-1955) and contemporary individuals of a once-ubiquitous European grassland butterfly. Demographic inference reveals population growth over millennia, likely associated with grassland expansion under low-intensity agriculture, followed by a population decline exceeding 99% that peaks during mid-20th century land-use intensification. This rapid population decline is accompanied by the loss of genetic diversity, increased population differentiation, and inbreeding. Overall, our results demonstrate the dual role of agricultural land use in shaping biodiversity: while low-intensity practices historically facilitated population expansion, modern agricultural intensification has driven dramatic population decline and genetic erosion, a trajectory likely shared by many grassland species worldwide.