The feared fatal decline of western monarch butterflies in California paused at the precipice this winter, when almost the same number as last year showed up along the coast, giving biologists a granule of hope that the colorful insects can be saved.
The striking orange-and-black butterflies have declined more than 99% since the 1980s along the Pacific Coast, where an estimated 10 million monarchs once blanketed trees from Marin County to Baja California.
Only 29,418 clustering monarchs were counted in the trees along the coast around Thanksgiving, when the winter population is believed to be at its peak. That’s 2,200 more than during the winter of 2018-19, but volunteers surveyed more sites this year, so the numbers are considered statistically identical. The count last year was an all-time low and 86% below 2017-18.
Emma Pelton, a conservation biologist for the Xerces Society for Invertebrate Conservation, said the population is now close to the point at which the species can no longer recover, but the halt in their decline is a hopeful sign.
“This is still a crisis, but there is still reason to think that if we work hard enough we could bounce back,” said Pelton, leader of western monarch research for the Xerces Society, an international nonprofit whose mission is to protect invertebrates and their habitats.
“It’s not all bad, but I think it’s a pretty good wake-up call,” she said. “We need to be dealing with this because we don’t know how low this population can go before it can’t bounce back.”
Western monarchs spend the winter in more than 300 forested groves of eucalyptus, cypress and pine trees along the California coast, including large populations in Riverside and Los Angeles counties, Monterey, Pacific Grove (Monterey County) and at Natural Bridges State Beach in Santa Cruz.
The California butterflies, first observed by a Russian expedition looking for a passage across the Arctic Ocean in 1816, cluster together in trees from November to March, providing viewers with a spectacular winter display of color before they head back north to the Pacific Northwest and Canada for the summer, mating as they go.
Pelton said the overwintering population is considered the “canary in a coal mine” for ecosystem health because declines in pollinating insects and songbirds usually go hand-in-hand. Losing monarchs could send bees, insect-eating birds and the plants and animals that rely on them into similar declines.
There are two major migrations of monarch butterflies — the eastern and western populations — which scientists believe divide themselves at the Rocky Mountains when they head south for the winter from their summer homes to the north.
The more abundant eastern monarchs, which spend their winters in Mexico, are famous for covering whole sections of forest in a kaleidoscope of color. It is the largest insect migration in the world but it, too, is in trouble. The eastern monarchs have declined more than 90% since 1996, when scientists estimated there were 1 billion nesting in the trees.
The journeys of both populations are remarkable in that it takes several generations of butterflies — eggs for each successive generation are left on milkweed plants along the way — to make the six- to nine-month journey north after wintering in California and Mexico.
The California population has been declining at an average of about 7% a year, according to a 2017 study by the U.S. Fish and Wildlife Service. That study calculated the point of no return would likely come when there are fewer than 30,000 butterflies.
The die-off has been blamed on a variety of factors, including urban sprawl, the spraying of pesticides and herbicides on corn and soybean crops, and the plowing under of the monarch’s milkweed habitat along their migratory route.
Xerces Society officials are urging governments to identify butterfly habitat and adopt plans to protect monarch overwintering sites from logging or inappropriate trimming. They have identified 20 sites — including stands of trees on Treasure Island and Yerba Buena Island in San Francisco — actively used by monarchs over the past five years that were damaged or destroyed by home builders, utility companies worried about fire danger, park officials or open space managers.
“We must protect all remaining overwintering sites in order to save our monarchs,” said Sarina Jepsen, director of the Xerces endangered species and aquatics program. “This is an urgent issue that needs to be addressed by federal, as well as state and local government in California.”
The group also recommends that Californians begin growing milkweed and other native plants that produce nectar, especially flowers that bloom in early spring. They recommend woolly pod, known scientifically as Asclepias eriocarpa; California milkweed, called A. californica; and heartleaf milkweed, or A. cordifolia.
Recent studies have shown that tropical milkweed, like Asclepias curassavica, can interrupt monarch migration and spread disease, effects that are bound to get worse as the climate warms. A recent University of Michigan experiment found that higher carbon dioxide levels from car and factory emissions reduce a natural toxin in milkweed that feeding monarch caterpillars use to fight off parasites.
“Without immediate action, I fear we will lose these animals from the Western landscape,” said Scott Black, executive director of the Xerces Society. “Without them ... we will lose the ability for our children to experience the majesty of the monarch migration.”
Peter Fimrite is a San Francisco Chronicle staff writer. Email: [email protected] Twitter: @pfimrite