Illustration: Eric Nyquist
When western monarch butterflies leave their milkweed-laden inland habitats in late summer to seek out winter quarters along the California coast, they are driven by a few nonnegotiables: The grove of trees has to supply the right balance of wind, shelter, and sunlight—plus a bunch of other monarchs. “I call it crowdsourcing the microclimate,” says Stu Weiss, ’82, PhD ’96, chief scientist at the Creekside Center for Earth Observation in Los Gatos. “That’s how we end up with an overwintering site by a golf course.”
On a sunny day in January, a smattering of the iconic insects flitted about a eucalyptus-lined meadow next to the third fairway of the Marina course at San Leandro’s Monarch Bay Golf Club. Last winter, the meadow hosted 1,093 of the 233,394 monarchs tallied in the Xerces Society’s 2023 Western Monarch Count. The total has rebounded from its low of 1,901 in 2020 but remains far short of a sustainable population. During the first count, in 1997, volunteers spotted 1.2 million butterflies.
A biologist with a deep knowledge of monarch behavior and of the threats the thermally sensitive insects face during their four-generation migration cycle—including habitat loss, pesticides, and extreme weather—Weiss has spent decades making their winter lodgings more hospitable. Using an assessment tool he developed that applies lidar 3D mapping from above a forested area and hemispherical photography from below the tree canopy, Weiss can determine where and how much sunlight and wind penetrate a given site. At Monarch Bay, he suggested planting trees to reinforce windbreaks; in Pacific Grove, he recommended pulling out dead trees here and adding new ones there. When a new Cal student housing complex in Albany was held up in part because of its proximity to a monarch site, Weiss concluded that the building would improve overwintering conditions by blocking wind. (The complex—enhanced by 20 monarch-sheltering cypress trees—opened in August.)
“Stu has worked tirelessly to help monarchs rebuild their population, and his work is critical,” says Mia Monroe, a recently retired park ranger who co-founded the annual count. “If they don’t have a place to spend the winter, they won’t survive in any number.”
Monarchs are just one of many species championed by Weiss, who earned the 2023 Bay Nature Conservation Action Award for his work with imperiled plants and butterflies. “My job,” he says, “is to make room for the other creatures on the planet.”
Kelli Anderson, ’84, is a writer in Sonoma, Calif. Email her at stanford.magazine@stanford.edu.
