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New Report Documents 74% Decline in Western Overwintering Monarchs

Jul 13, 2016


  • Population Trends
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A new report by the Xerces Society found a sharp decline in western overwintering monarchs over time, on par with the dramatic population loss of the eastern population that overwinters in Mexico. The study compared the average number of butterflies overwintering along the California coast during the periods 1997-2001 and 2010-2014, and found a decline of 74 percent. Their analysis of the population trend from 1997-2015 shows an average decline of 12% per year. 

During the winter, a portion of North American monarchs cluster at hundreds of overwintering sites along a 1,000 kilometer stretch of the California coast. These sites provide pockets of the ideal microclimate for overwintering monarchs: not too hot that they become active and burn up their fat reserves, and not too cold that they freeze. Very specific conditions are required for habitat to be suitable for monarch overwintering. Groves of non-native eucalyptus, native Monterey pine and cypress protect monarchs from freezing temperatures and high wind and typically have fresh water, sunlight and fall or winter blooming nectar flowers to support monarchs.

These essential habitat sites are under pressure from development and the deterioration of groves through age, disease and pests. The report also documents the Top 50 sites for protection and active management, with the highest ranking going to sites which have seen the greatest declines and that still host the largest portions of the remaining western overwintering population. The full list, including site descriptions for the Top 25, can be found in the full report. Management and enhancement of these overwintering groves is essential to restoring and protecting the western monarch population, which intermixes with the eastern population.

“Active management of these sites is the key to ensuring that they will support monarchs in the future,” said Sarina Jepsen, director of the Xerces Society’s Endangered Species program and report coauthor. “This report can be used to help conservationists and land managers prioritize how limited resources are allocated for the greatest conservation benefit.”

The data that made this analysis possible comes from volunteers across the western overwintering range who participate in the Xerces Society’s Western Monarch Thanksgiving Count. During a three week period surrounding each Thanksgiving, citizen scientists and monarch researchers count clustered overwintering monarchs using a standard protocol. Everyone can help monitor monarchs during different parts of their annual cycle by participating in monarch citizen science projects.


Some information in this post was taken from the ‘State of the Monarch Butterfly Overwintering Sites in California’ report and press release, which can be found here.

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 by Candace Fallon/The Xerces Society.