On a cool winter morning in the town of Pacific Grove, California, you may think the trees are dangling last summer’s browned leaves. But if you look closely, says Sarina Jepsen, director of endangered species and aquatic conservation for the Xerces Society for Invertebrate Conservation, you’ll realize that those “leaves” are living animals: individual monarch butterflies nestled close together “like paper files in a folder.” The town is an overwintering site for the migratory monarch; only as the day warms will the black and orange insects float into the sun in search of nectar.

Monarchs are beloved North American creatures: photographed in meadows, welcomed in butterfly gardens, grown in classrooms, celebrated in art. They also are rapidly dwindling in number. Yet until recently, no one knew just how badly the monarch’s western population was faring. Even a 2014 petition to add the butterfly to the Endangered Species List – which the U.S. Fish and Wildlife Service still is considering – focused on the eastern monarch population, found east of the Rocky Mountains. Partly this was because researchers lacked estimates of the historic western monarch population.

Now, a new analysis has quantified the western monarch’s historic population as far back as the early 1980s by combining long-term data sets collected by researchers and citizen scientists. The results both surprised and alarmed butterfly experts. “We knew the population was declining,” says Cheryl Schultz, a conservation biologist at Washington State University and lead author on the study. What surprised her was realizing that the West held, on average, 10 million monarchs in the 1980s – a far cry from recent annual counts of approximately 300,000. If current trends continue, Schultz says, western monarchs could stop migrating in as few as 35 years.

Unlike eastern monarchs, famous for their winter travels to Mexico, the western population overwinters along the California coast, taking refuge from cold and wind in groves from Mendocino County to Baja California. “When given a choice, they are choosing Monterey pine and Monterey cypress. They also cluster in giant native redwoods on the coast,” Jepsen says, although they’ll shelter in introduced eucalyptus if that is all that’s available.

Overwintering monarchs take a break from reproducing. When summer comes, the butterflies travel across the West in search of mates and milkweed plants – where females lay eggs – rarely mixing with their relatives to the east of the Rocky Mountains.

Unlike migratory birds, monarchs migrate over generations. Somehow, come the following winter, the descendants of the past winter’s butterflies will return to the coastal regions where their ancestors huddled out the previous year’s cold weather. But without enough monarchs, says Schultz, the butterflies won’t be able to complete the journey. “We have to have enough butterflies at the overwintering sites to persist over the winter, then fan out across the West to breed, persist over several generations, then come back for the next winter,” she says.

So what will monarchs do instead? Schultz isn’t sure. “It’s possible we could lose the migratory phenomenon if we don’t do something,” she says. In some spots that are warm year-round, such as southern California, people have planted tropical milkweeds that support non-migratory populations of monarchs. “The monarch itself isn’t going to go extinct,” Schultz says. “Could it go the way of the California condor with intense captive breeding because there are so few? I hope not.”

Researchers haven’t yet pinpointed the main causes of the western monarch’s decline, though they have some leads. For eastern monarchs, one of the main culprits has been the rise of genetically modified soy and corn farming, which induces farmers to spray fields with glyphosate (brand name Roundup). The chemical kills off weeds – including the essential milkweed. Increasingly, California farmers are also spraying glyphosate on crops including almonds, wine grapes and genetically modified cotton and alfalfa.

Jepsen points out that in California’s overwintering grounds, development may also be responsible for the monarch’s tailspin. “Some of the places they choose to overwinter are our highest value real estate,” she says. “It’s the California coast. Some of it is multi-million dollar properties.” Because there are no federal protections for monarchs, any local protections are voluntary. Jepson also points to increased drought, caused by climate change. Although the state received record rainfall this past year, host trees at overwintering sites died off from previous years of drought.

One unusual feature about the decades of western monarch data sets used in this new analysis is that citizen scientists collected them. “There’s been a lot of crowdsourcing that’s gone into our understanding of monarch movement and biology,” Jepsen says. “It’s amazing the role the public’s played in understanding and conserving this species.” And these updated numbers have given conservation managers pause. “We used to think 1.2 million should be our recovery target, because that was there in 1997,” Jepsen says. “(The new data) has made us really reconsider what recovery should look like.”

Maya L. Kapoor is an associate editor with High Country News.