Ole Schell remembers seeing giant clusters of monarchs in a eucalyptus grove on his family’s ranch in Bolinas. As a child, he said, “It used to be you could shake a tree and thousands would come off.” But in recent years, Mr. Schell has found only two or three monarchs in an entire season. He began searching for answers. Western monarchs have undergone a significant decline along the Pacific coast, losing more than 95 percent of their population since the 1980s. Researchers say multiple factors are contributing to the dwindling population, including habitat loss, intense drought, severe wildfires and warmer temperatures, all intensified by climate change. This winter’s count tallied nearly 250,000 butterflies, more than a hundred-fold increase from the previous year. And although most overwintering western monarchs are typically found on the central coast, this year they skewed south, with over 90,000 in San Luis Obispo County and over 95,000 in Santa Barbara County. Fewer than 600 were counted at 54 surveyed sites between Mendocino and San Mateo Counties. Locally, there were about 170 monarchs in Bolinas, 20 in Stinson and 20 at Fort Baker. The reasons for the overall uptick in this year’s count, and the shifts in geographic prevalence, remain a mystery. Emma Pelton, an endangered species conservation biologist with the Xerces Society, said insect populations are known to ebb and flow due to weather and other conditions. The current consensus, she says, is that “many environmental factors are at play across the range and there’s no single cause or definitive answer.” An understanding that the species’ migration is not a closed system is leading researchers to examine both the western and eastern migratory populations to understand the extent to which they may be linked or comingling. With the encouraging 2021 tallies, researchers say now is the time to focus on conservation efforts. The availability of plants needed by western monarchs to breed and fuel their journeys has been greatly reduced. Conservationists are urging the protection of existing habitat and overwintering sites, a reduction of pesticide use and the creation of new habitat by planting nectar plants and native milkweed in the appropriate locations. In Bolinas, Mr. Schell is converting parts of the 200-acre ranch he lives on into a monarch sanctuary. He began work last fall with help from the Xerces Society, the National Park Service and the Salmon Protection and Watershed Network. “I started this project before I really had a complete understanding of monarchs and their migration patterns,” he said. “I’ve really had to educate myself.” After an environmental survey, the Xerces Society crafted a plan, and a visit from Audrey Fusco, a restoration ecologist and nursery manager at SPAWN, generated ideas for plants native to Bolinas. Mr. Schell said Ms. Fusco helped him think long-term, which in ecological terms meant providing host plants for more than 20 species of butterflies. The sanctuary will help fulfill SPAWN’s plan to create an educational nectar garden, and third and fourth graders from the Bolinas-Stinson School recently visited. Mr. Schell partnered with another nonprofit, Guardian Grange, which helps veterans with post-traumatic stress disorder by reconnecting them with nature, on the project. Last fall, participants helped with the initial planting and built the protective fence that surrounds the sanctuary. “We honored Sept. 11 by breaking ground,” Mr. Schell said. To learn more about the Bolinas monarch sanctuary, visit www.westmarinmonarchs.org.