The western monarch butterfly, a migratory population that overwinters on the California coast, has both delighted and befuddled conservationists this year with an apparent surge after last year’s low numbers. According to the Xerces Society for Invertebrate Conservation, more than 200,000 monarchs overwintered in California in 2021. The 2020 Thanksgiving count reached record lows, with a little over 1,900 monarchs found at overwintering sites. 

“We’re so grateful the numbers are up,” said Emma Pelton, a senior conservation biologist at the Xerces Society. “While we don’t know if these numbers will sustain, it does give us a reason to hope. Last year the numbers were so low, people were really depressed.” 

Yet scientists are struggling to explain the rebound, and some believe the numbers may be misleading. 

Chip Taylor, a professor emeritus of ecology and evolutionary biology at the University of Kansas and the director of Monarch Watch, believes monarchs are changing their behavior to adapt to changes in their environment. The low numbers in 2020 may have been the result of a dispersal that led the butterflies to simply not be where scientists and volunteers expected to find them. 

Monarchs are found throughout the world, but the two major populations reside in North America. The smaller of these two populations, the western monarch, spends summers from the Rockies to the Pacific Ocean and north as far as southern Canada. They overwinter along the California coast from Mendocino to Baja, sheltering in giant clusters on Monterey cypress and non-native eucalyptus trees. 

As recently as the 1980s, the western monarch migration boasted millions of individuals, and West Marin was home to huge, densely populated sites. Even with a significantly higher count this winter, the number of monarchs is a mere fraction of what it once was, with very low numbers in Marin, Sonoma and other Bay Area counties. The majority of this year’s monarchs were found between Santa Cruz and Ventura, though one site near Agate Beach in Bolinas had 105 butterflies. 

Although the monarch is a well-studied insect, the population “is now bouncing around in uncharted territory,” Ms. Pelton said. Scientists have attributed the western monarch’s decline in part to pesticide use and habitat loss due to development, but recent studies emphasize the impact of climate change and its domino effects: drought, rising temperatures, fallen trees, fires and habitat reduction. 

Dr. Taylor said monarchs took a deep dive starting in 2017, a year that brought unusual heat at critical times of the year. Prolonged periods with temperatures over 90 degrees are unfavorable to butterfly eggs. High heat means the realized fecundity—the number of eggs laid versus the potential total—drops dramatically. Maps of temperature rise across the country are correlated with declining butterfly populations, particularly on the West Coast, where temperatures have been rising 0.7 degrees per decade since 1975.

“Monarchs can be affected by a number of things, and you can track a lot of things in parallel with temperature changes, but monarchs are a species that are going to be greatly affected by temperature changes,” he said. 

Dr. Taylor said the 2020 Thanksgiving count was likely not comprehensive enough. Given the mortality that normally occurs in a population, “there’s no way the survivors of [last year’s] count—maybe 400 to 500 females—could account for 200,000 butterflies this fall,” he said. 

He suggested that the breeding population on the California coast may have moved inland, leaving them outside the range of the winter count. Larger numbers of butterflies last winter would help explain this winter’s apparent surge. 

At the same time the migratory population is collapsing, a small population of resident monarchs has been growing in the Bay Area. Typically, western monarchs, which are dependent on milkweed for reproduction, do not breed in the winter. Native milkweed dies back in winter, and the migrating population goes into reproductive diapause. 

Resident monarchs do not migrate, and typically live in parts of the United States where the weather is mild and milkweed plants can grow year-round. But in recent years Bay Area gardeners have been planting more non-native milkweed, such as tropical milkweed, which does not die back, so resident monarch populations are now breeding year-round in urban gardens. 

In California, the resident and migratory populations appear to be mingling, though to what extent is unclear. Researchers are looking into “whether that’s helpful or dangerous, or if it impacts the urge to migrate, or if it’s adding to the migratory population,” said Mia Monroe, a ranger for the Golden Gate National Recreation Area who founded the western monarch’s Thanksgiving count.

The resident population, estimated at around 12,000 in the Bay Area, is “not sufficient to make up for the loss of the migratory monarchs,” Ms. Monroe said.  

Ms. Pelton sees this year’s count as “a hopeful sign and evidence that the migration is not lost, as some feared after last year.” Yet with only one year of population increase, it’s unclear whether the western monarch is “clawing its way back from the brink, or if this is a blip in the continuation of the migration’s decline,” she said.  

For his part, Dr. Taylor is optimistic about monarchs’ abilities to adapt to a changing climate. “The species will still be here, but the migrations will end,” he said. “All of the temperature models indicate that monarch migrations are not sustainable in the long run.”

Many conservation groups are promoting beneficial actions to support the butterfly’s migration, such as making habitat improvements and establishing pockets of wild meadows in agricultural areas. These are efforts they say have made a difference. 

At the Environmental Action Committee of West Marin’s offices, a new educational display on western monarchs covers the environmental risks and the life history of the butterfly.

“In Marin County we’re fortunate to host both breeding and overwintering populations of western monarchs, so what you do to help depends on where you live,” said Leslie Adler-Ivanbrook, the nonprofit’s program director. “The monarchs are a window into the whole biodiversity crisis…a piece of the bigger picture.”

The E.A.C. has guidelines for Marin residents both on their website and at the exhibit, and the Salmon Protection and Watershed Network’s website offers a list of native nectar plants, such as coyote mint, that can be found at local nurseries. 

Overwintering monarchs fly from their clusters in sheltering trees on sunny days, searching for nectar plants and water. Gardeners on the coast can plant and protect native nectar plants to create a refuge where the butterflies can feed and build fat stores. 

By late February, after a flurry of mating, monarchs’ great migration begins, and the females seek milkweed plants to lay their eggs on. Gardeners who live five or more miles from the coast can plant native milkweed to help stabilize and increase the population. 

Ms. Monroe also suggests that Marin residents consider common spaces such as libraries, medians and city halls as locations for much-needed habitat for pollinators. Nature is messy, and butterflies will find refuge in unexpected places. 

Citizens can also contribute to researchers’ body of data by observing and noting monarchs through the app iNaturalist and at More data will help researchers understand the butterfly’s presence throughout the county: when, where, and what they’re doing throughout the year.

For Dr. Taylor, those interested in helping monarch populations should focus on actions centered around climate change. 

Ultimately, monarchs are indicators of a healthy and balanced environment. The sudden change in the population emphasizes the extent to which environmental changes can have unexpected and swift consequences. 

“If we want to sustain monarch butterflies and many, many other things, we have to make it personal,” Dr. Taylor said. “We have to connect people with the change that’s going on out there.”