The number of western monarch butterflies overwintering in California experienced a sharp decline this year, dropping to a near-record low that is far below the number that scientists consider a stable population level.
Data released by the Xerces Society for Invertebrate Conservation, which conducts a statewide count of monarchs every year, found that the number of monarchs that spent the winter in California peaked at 9,119. This number is a sharp decline from the past three years when the Society reported more than 200,000 overwintering western monarchs each year at the more than 250 parks, open spaces, and other sites monitored by volunteers.
Unlike the eastern monarch butterflies that overwinter in central Mexico, western monarch butterflies primarily overwinter in tree groves along the coasts of California and Baja California. “We know small populations are especially vulnerable to environmental fluctuations, and we think that’s what happened this year,” said Emma Pelton, an endangered species biologist with the Xerces Society.
“The record high late summer temperatures and drought in the West likely contributed to the significant drop-off we saw in the third and fourth breeding generations,” she added.
Students and volunteers clearing grass to plant Monterey cypress and understory crops in the Gill Tract's monarch butterfly grove in 2023.
But amid an otherwise dismal year for the migratory insect, the Gill Tract—a living laboratory for student- and faculty-led plant and agricultural research that is managed by Rausser College of Natural Resources—emerged as a bright spot. One hundred ninety-six butterflies were counted at the Gill Tract this winter, the largest number ever recorded at the site. The Gill Tract was also one of a few California overwintering sites that welcomed more monarchs this year compared to past seasons.
“Monarchs tend to bounce around a lot, but this year, something really attracted them to the Gill Tract,” said Stu Weiss, chief scientist for the Creekside Center for Earth Observation. “We couldn’t be more thrilled about it.”
Monarch butterflies at Gill Tract seek refuge in habitat managed by the Gill Tract Farm Coalition, Sogorea Te' Land Trust, and faculty researchers. The vegetation management practices at the site provide both favorable habitat and nectar sources during the winter months. However, concerns about the Gill Tract’s monarch habitat began to emerge in 2020, as construction of several nearby commercial and residential properties—including the newly completed xučyun ruwway Apartments—began to progress. UC Berkeley contracted Weiss, a monarch expert who has worked with cities across California, that year to study the project’s potential impact and recommend management and restoration actions to support the butterflies.
Weiss’s assessment suggested that the six-story, 761-bed housing complex would provide more shelter from the winds than before and that it would not block the sun’s rays from reaching the monarch habitat. “They didn’t hunker down in the interior of the grove to get out of the wind like they had in previous years,” he said. “Instead, they got to enjoy the sunny side of the grove because now it's sheltered, and they even stuck around during the really big wind events last December.”
Additional improvements to the site based on Weiss’ findings were made in 2023 to further revitalize the habitat for monarchs and other pollinator species. Twenty-one Monterey cypress trees were planted to complement the existing cluster of cypress, eucalyptus, and redwood trees, and two types of California native shrubs—Ceanothus and Toyon—were planted to create additional habitat for pollinator species.
“They haven't really kicked in yet as a wind shelter, but the trees have more than doubled in height since we put them in,” he said. “We’re also continuing to work out the details of how to best maintain the site when the monarchs aren't around.”
Late last year, the U.S. Fish and Wildlife Service recommended monarch butterflies be listed as a threatened species and designated critical habitat under the Endangered Species Act. The designation could provide vital support for the species through improved protection for overwintering habitat and more incentives for breeding habitat restoration.
“Protecting crucial habitats is essential for the survival of monarch butterflies,” said Professor Dennis Baldocchi, executive associate dean and director of agriculture and natural resource programs at Rausser College. “Rausser College is proud to support the ongoing stewardship of the Gill Tract, as it aligns with our goals of performing research, education, and outreach within the community.”