Monarchs on coast still face population crash

03/11/2020

The recent counts of overwintering monarch butterflies on California’s coast, including at long-surveyed sites in Bolinas, Stinson Beach and Muir Beach, show that the western population of the species did not rebound from a crash last winter. Surveys conducted last Thanksgiving and in the first weeks of 2020—an annual volunteer effort orchestrated by the Portland-based Xerces Society for Invertebrate Conservation—show that overall numbers were on par with those from last year. Yet last year, monarch populations reached an all-time low, a shocking 86 percent drop from the previous year. “I try to see the positive,” said Mia Monroe, a National Park Service ranger who started the counts in 1997 and coordinates efforts in Marin as a volunteer. “The population didn’t decline further. I call it ‘holding steady.’ We continue to have a chance to help them to rebound.” The western population of monarchs—a species under evaluation by Fish and Wildlife for a new endangered listing—has shrunk to less than 1 percent of the numbers reported in the 1980s. Scientists attribute the decline to unseasonably warm weather, habitat loss and increased pesticide use. The recent plummet, from nearly 200,000 in 2016 to 27,218 in 2017, may have been the effect of particularly heavy storms in the spring of 2017. Last Thanksgiving, surveys at 240 sites showed 29,418 monarchs. Though that number is slightly higher than last winter’s, around 30 more sites were surveyed this year; Emma Pelton, a conservation biologist at the Xerces Society, said it is fair to assume the population remained level. (There are around 400 sites total in California.) “We are definitely really concerned,” Ms. Pelton said. “We think 30,000 is the tipping point. If it falls below that, [the population] could enter a downward spiral and not recover. We are hovering right around that 30,000 tipping point.” The Rocky Mountains divide monarchs nationally into two populations; the western group overwinters on California’s coast, where they roost in eucalyptus and Monterey pines and cypresses before heading inland in January to breed. Less is known about preferred breeding sites and habits. The eastern population, which overwinters in Mexico, has suffered similar declines. Since counting started in 1997, monarchs have been found in Marin at five overwintering sites in Bolinas—primarily on private property—in addition to one at Chapman Ravine in Stinson Beach, two in Muir Beach, one at Fort Baker and the last at the Marin Headlands. Across the Bolinas sites, 22,253 monarchs were counted in 2015; that number dropped to 18,455 in 2016; to 7,706 in 2017; to 2,431 in 2018; and to 328 in 2019. The Stinson site saw one monarch in 2018 and none this year; in 2015, there were 8,200. Even more disheartening, the number of monarchs that survive the winter is typically far less than the number counted around Thanksgiving. In the early 2020 count, the Xerces Society reported a 46 percent decline in numbers from Thanksgiving. Ms. Pelton said the drop may have been caused by poor habitat. Ms. Monroe explained, “The trees get old, and possibly the lower branches that buffer the monarchs from the wind and weather break. Or people trim them because they fear fire threat, or want to preserve their view. They don’t plant new trees. PG&E is cutting trees. Those are some of the big threats.” Historically, she added, landowners—including federal, state and county agencies—have collaborated with conservation efforts. Should the Fish and Wildlife department list the species as endangered, the sites could wind up with legal protections.