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Proc Natl Acad Sci U S A. 2019 Apr 23;116(17):8609-8614. doi: 10.1073/pnas.1805114116. Epub 2019 Mar 18.

Multiscale seasonal factors drive the size of winter monarch colonies.

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Department of Integrative Biology, Michigan State University, East Lansing, MI 48824;
Department of Biology, Georgetown University, Washington, DC 20057.
Centro de Investigaciones en Geografía Ambiental, Universidad Nacional Autónoma de México, Morelia, Michoacán, México.
Monarch Fund Coordinator, Col. Heroes Ferrocarrileros, Zitacuaro, Michoacán, CP 61506 México.
World Wildlife Fund-Mexico, Programa Mariposa Monarca, Michoacán, CP 61450 México.
Department of Integrative Biology, Michigan State University, East Lansing, MI 48824.
Ecology, Evolutionary Biology, and Behavior Program, Michigan State University, East Lansing, MI 48824.


Monarch butterflies in eastern North America have declined by 84% on Mexican wintering grounds since the observed peak in 1996. However, coarse-scale population indices from northern US breeding grounds do not show a consistent downward trend. This discrepancy has led to speculation that autumn migration may be a critical limiting period. We address this hypothesis by examining the role of multiscale processes impacting monarchs during autumn, assessed using arrival abundances at all known winter colony sites over a 12-y period (2004-2015). We quantified effects of continental-scale (climate, landscape greenness, and disease) and local-scale (colony habitat quality) drivers of spatiotemporal trends in winter colony sizes. We also included effects of peak summer and migratory population indices. Our results demonstrate that higher summer abundance on northern breeding grounds led to larger winter colonies as did greener autumns, a proxy for increased nectar availability in southern US floral corridors. Colony sizes were also positively correlated with the amount of local dense forest cover and whether they were located within the Monarch Butterfly Biosphere Reserve, but were not influenced by disease rates. Although we demonstrate a demographic link between summer and fine-scale winter population sizes, we also reveal that conditions experienced during, and at the culmination of, autumn migration impact annual dynamics. Monarchs face a growing threat if floral resources and winter habitat availability diminish under climate change. Our study tackles a long-standing gap in the monarch's annual cycle and highlights the importance of evaluating migratory conditions to understand mechanisms governing long-term population trends.


Danaus plexippus; NDVI; annual cycle; gamma-hurdle model; migration route

[Available on 2019-09-18]

Conflict of interest statement

The authors declare no conflict of interest.

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