In the past year, the eastern population of monarch butterflies that overwinters in Mexico boosted the area it occupies during hibernation by 144 percent, as revealed by an annual survey performed by the World Wildlife Fund-Mexico and partner organizations. 

The survey measures the area monarchs occupy because of the difficulty in counting individual butterflies. This year monarchs took up almost 15 acres of forest, up from about six acres last year, making it the largest increase in 12 years, according to a statement.

Favorable weather contributed to the population’s uptick with warm temperatures that helped migrations, according The San Francisco Chronicle. “This was a Cinderella year,” Tierra Curry, a senior scientist at the Center for Biological Diversity in Tucson, Arizona, tells the Chronicle.

Populations fluctuate from year to year and experts caution that these good numbers may not repeat in years to come....

The western population of monarchs that overwinters along the Pacific Coast has fared much worse. An annual count by the Xerces Society, a nonprofit conservation organization, found a record low number of roughly 28,000 butterflies in 2018—an 86 percent drop from the previous year, according to a statement. The numbers now represent a staggering 99.4% decline from the estimated 4.5 million that overwintered in areas of California and Baja, Mexico in the 1980s.  

The two populations are genetically similar and the eastern butterflies’ surge may benefit their western brethren. “It’s possible that some of those monarchs could migrate into the western population,” Curry tells The Chronicle.

In other monarch news, researchers in Mexico are working to bolster the eastern population against habitat loss due to a changing climate by moving hundreds of oyamel fir trees up higher on a mountain in the Monarch Butterfly Biosphere Reserve, Scientific American reports. The butterflies overwinter in the trees.

The researchers conducting the project estimate that rising temperatures will cut suitable habitat for the trees by about 70 percent between 2025 and 2035. The team calculated that it could move fir seedlings up the mountainside by roughly 350 meters to reach temperatures that would be more ideal under the changing climate. The researchers have shifted over 750 seedlings higher up on the mountain, according to Scientific American

A journal article on the work is currently undergoing peer review. While there’s some controversy over assisted migration to save species from climate change as opponents argue that introduced species can threaten those already in an area, some scientists are onboard. “This is an example of a good experiment,” Sally Aitken, a forest ecologist at the University of British, tells Scientific American.

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