Multiscale seasonal factors drive the size of winter monarch colonies

A significant decline of 84% in eastern North American monarch butterfly populations on Mexican wintering grounds since 1996 contrasts with stable population indices from northern breeding areas, suggesting autumn migration as a critical bottleneck. This study reveals that multiscale factors—including higher summer abundance on breeding grounds, greener autumns indicating increased nectar availability, and local dense forest cover within the Monarch Butterfly Biosphere Reserve—positively influence winter colony sizes. Unlike previously assumed, disease rates did not affect colony sizes. The research highlights the importance of evaluating migratory conditions to understand long-term population trends, emphasizing the vulnerability of monarchs under climate change due to diminished floral resources and winter habitat availability.

Saunders, S. P., Ries, L., Neupane, N., Ramírez, M. I., García-Serrano, E., Rendón-Salinas, E., & Zipkin, E. F. (2019). Multiscale seasonal factors drive the size of winter monarch colonies. Proceedings of the National Academy of Sciences, 116(17), 8609–8614. https://doi.org/10.1073/pnas.1805114116