Factors influencing monarch butterfly winter colonies

A study examines environmental factors that influence the sizes of monarch butterfly winter colonies. The eastern North American monarch butterfly population has suffered severe declines in their Mexican wintering grounds over the past two decades. However, whether autumn migration conditions contribute to these losses is unclear, given that breeding-ground populations do not exhibit a downward trend. To determine the factors affecting winter monarch colonies, Sarah P. Saunders, Elise F. Zipkin, and colleagues combined environmental data with researcher-collected and citizen-scientist monarch data from all known 19 winter colony sites within and outside the Monarch Butterfly Biosphere Reserve, a protected area in Central Mexico. The authors developed a model to assess arrival sizes at the winter sites between 2004 and 2015 and found that higher summer populations on northern breeding grounds correlated with larger Mexican winter colonies. Larger colonies were also located within the Monarch Butterfly Biosphere Reserve and were correlated with dense forest habitat at colony sites and green autumns, which provide nectar resources during the southward migration. Autumn temperatures and disease rates were not significantly associated with winter population sizes. The findings suggest that summer breeding conditions, autumn greenness, and habitat at winter colony sites influence monarch winter populations, and that climate change may affect butterfly populations if floral resources and winter habitat availability diminish.

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Article #18-05114: "Multiscale seasonal factors drive the size of winter monarch colonies," by Sarah P. Saunders et al.

MEDIA CONTACT: Sarah P. Saunders, Michigan State University, East Lansing, MI; tel: 708-220-6590; email: saund123@msu.edu; Elise F. Zipkin, Michigan State University, East Lansing, MI; tel: 513-260-4434; email: ezipkin@msu.edu

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