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Eastern Monarch Population Numbers Drop 27%

Feb 16, 2017


  • Population Trends

This year’s measurement of the eastern monarch overwintering population showed a 27% decrease compared to last year, occupying 2.91 hectares of forest in Mexico’s transvolcanic mountains during the winter of 2016-17. It is likely that an extreme and unusual winter storm contributed to this decrease. This March storm struck the overwintering colonies just as monarchs were beginning to depart on their journey north.

Events such as last year’s ‘St. Patrick’s Day Storm’ are a strong reminder that we must increase efforts to restore and maintain monarch breeding and migration habitat to support a rebounding population that is buffered against such threats.

Graph of the total area occupied by monarch butterflies in the Mexican overwintering grounds.

The eastern monarch population is measured by the area they occupy in hectares (1 hectare is 2.47 acres, or about the size of a baseball field), rather than by counting individual monarchs. Experts explain this process in a 2015 MJV news update.

While the size of the monarch overwintering population has always shown year to year fluctuation, the long term trend shows an overall decline in the population. Modeling efforts suggest that reaching and maintaining a minimum population size that occupies 6 hectares of overwintering area would greatly reduce the risk of losing the monarch migration. In order to reach this ambitious goal, scientists project that an additional 1.6 billion milkweed stems are needed on the landscape, in addition to highly diverse nectar resources.   

There is reason for hope, despite a decrease from last year. The mortality caused by the March storm was confirmed by dismal reports of monarchs throughout the breeding range in the spring and summer.  However, with abundant and available habitat and good weather conditions, monarchs demonstrated their ability to quickly produce large numbers of individuals. Reports of late season monarchs from citizen science programs suggest that conditions were good to support monarch production late in the season in 2016.

The monarch’s decline has inspired action and attention from all walks of life, with an unprecedented interest in their plight. Two recent studies confirm the need for engaging all sectors in helping to reach sustainable monarch numbers. One showed that overwintering monarchs in Mexico come from all areas of the eastern breeding range (Flockhart et. al, 2017). The other showed that a conservation strategy that includes the entire monarch habitat range (breeding, migratory, and overwintering areas) is more likely to achieve a steady, growing population rate than strategy that focuses on conserving only one region (Oberhauser et al. 2016).

You can help by planting milkweed and nectar resources, joining a monarch citizen science project, educating your community and elected officials, or by contributing to conservation in other ways. Each of us has a role to play, but as the “T.E.A.M.” saying goes, together everyone achieves more.


References and further information:

The press release by World Wildlife Fund Mexico announcing the overwintering results can be found in Spanish here.

Flockhart, D. T., Brower, L. P., Ramirez, M. I., Hobson, K. A., Wassenaar, L. I., Altizer, S., & Norris, D. R. Regional climate on the breeding grounds predicts variation in the natal origin of monarch butterflies overwintering in Mexico over 38 years. Global Change Biology (2017), DOI: 10.1111/gcb.13589

Oberhauser, K., Wiederholt, R., Diffendorfer, J. E., Semmens, D., Ries, L., Thogmartin, W. E., Lopez-Hoffman, L. & Semmens, B. (2017). A trans‐national monarch butterfly population model and implications for regional conservation priorities. Ecological Entomology (2016), DOI: 10.1111/een.12351


The Monarch Joint Venture is a national partnership of federal and state agencies, non-governmental organizations, and academic programs working together to conserve the monarch butterfly migration. The content in this article does not necessarily reflect the positions of all Monarch Joint Venture partners. Header photo taken by Reba Batalden in 2007.