Monarch butterflies in eastern North America have declined by 84% on their Mexican wintering grounds since 1996. However, coarse-scale population indices from their Midwestern U.S. breeding grounds do not show a downward trend. This discrepancy has led to speculation that autumn migration may be the critical limiting period. We address this controversy by examining the role of multi-scale processes impacting monarchs during autumn, assessed using arrival abundances at all known wintering colony sites over a 12-year period. Using a hierarchical gamma-hurdle modeling framework, we quantified effects of continental-scale (climate, nectar availability, disease) and local-scale (forest habitat availability, winter colony location) drivers of spatiotemporal trends in wintering abundances. We also considered impacts of peak summer and migratory population sizes on subsequent winter colony sizes.
Results/Conclusions
Our results demonstrate that higher summer abundance of monarchs in the Midwestern U.S. led to larger winter colonies, as did greener autumns, a proxy for increased nectar availability in southern U.S. floral corridors. Colony sizes were also larger within the Monarch Butterfly Biosphere Reserve compared to areas outside the reserve, and increased with the amount of surrounding dense forest cover. Given the strong link we reveal between summer and fine-scale winter population sizes, and contrary to prior coarse-scale analyses, we show that autumn mortality is unlikely to be the primary source of population decline. Yet, monarchs may face a growing threat if nectar resources diminish under climate change. Our study tackles a long-standing gap in the monarch’s annual cycle and highlights the importance of incorporating conditions experienced by animals during migration to fully understand mechanisms governing long-term trends.