A shadow flitted across the trail just ahead of Cynthia Brock, and she stopped suddenly to look up, one hand on top of her wide-brimmed hat. A lone monarch butterfly fluttered over her head into a tangle of branches in a stand of Eucalyptus trees on Ellwood Mesa, a coastal bluff in Goleta, California. The fall equinox had arrived the week before, and with it the first of the migratory North American monarchs—though you'd hardly know it from the weather. It was not yet 11 a.m. but the temperature was already well above 80 degrees. The creek that ran parallel to the trail was bone dry.

The Eucalyptus has had a long and, until recently, prosperous history in California's central coast. Since the rancher Ellwood Cooper first settled his family on the coastal bluffs that are his namesake and planted the first Eucalyptus nursery in 1872, the non-native trees have spread into a stand that stretches across nearly 78 acres. The eucalyptus trees on the mesa create the perfect microclimate for the roosting insects: There are sunny spots but ample shade, a thriving understory of leafy vegetation, protection from the wind and temperature fluctuations, and high humidity. The Ellwood Mesa is the southernmost in a vast network of overwintering sites along the California coast that stretch north as far as San Francisco. It's also one of only a few nesting sites on preserved land that's protected from private or commercial development.

Brock herself had a hand in ensuring that the winter site was preserved for the monarch. A former Goleta city councilwoman, she's lived two blocks from the mesa since 1980, and she still walks its meandering trails nearly every day. In the 1990s, as president of the Homeowners Association, she helped to fight a development proposal to put 161 private units on the mesa. She formed a group called Friends of the Ellwood Coast, which, together with other non-profit groups like the Environmental Defense Fund and the Trust for Public Land, managed to work out a deal with the developer, and secure the land for public use—and for the butterflies.

Every year, hundreds of thousands of monarch butterflies from all over the Western United States gather to overwinter along the California coast. The monarch is the only butterfly with a long-distance migration to rival some birds and mammals. But their numbers, along with their habitat, have been declining rapidly, which makes Ellwood Mesa all the more important for the migratory insects.

Unfortunately, the Eucalyptus grove that attracts the orange and black butterflies finds itself in a similarly dire state. Some 1,400 trees across the Eucalyptus forest on the mesa are dead or dying, thanks to the historic five-year-long drought and an onslaught of insect pests.

It's a grim problem, for both the trees and the butterflies they host—and one nearly made worse by mismanagement. Over the summer, Goleta city officials concluded that the dead trees were a threat to the entire grove, not to mention public safety; they could fall across trails, knock down healthy trees, or spread disease via pests that migrate between branches. The city closed the trails that crisscrossed the grove to the public in July, and the city council mulled over an emergency plan to remove all the trees marked as dead or dangerous first, and then create a habitat management plan, which could take years.

The Xerces Society's Conservation and Management Guidelines explicitly caution against such drastic measures. "The indiscriminate removal of one or more trees within or bordering overwintering habitat may adversely affect the sunlight and wind exposure for the roosting butterflies," the guidelines state. The butterflies are so sensitive to changes in the microclimate of their habitats that even well-meaning landowners who subtly prune the trees in their yards can destroy suitable monarch habitat, according to Charis van der Heide, a biologist who's been studying the movement of the monarchs for years.*

Now, Brock finds herself once again fighting to protect the grove.

The monarch migration is an intergenerational endeavor. In the spring, the butterflies leave California and fan out across the Western U.S. toward the Rockies. The monarchs that depart the coast make it part of the way, maybe 50 to 100 miles, before settling on some milkweed—the only plant that monarchs lay eggs on, and the only plant that monarch caterpillars eat—laying eggs, and passing away. The next generation makes it a little bit farther, and the next even farther, until, four or five generations later, the butterflies have spread out as far north as the Canadian border and as far south as Arizona or New Mexico. Each generation of summer butterflies live roughly four to five weeks. Then the milkweed emits a chemical signal that tells the butterflies when it's time to head back to California.

A monarch butterfly hangs from its cocoon. (Photo: Justin Sullivan/Getty Images)

"There's a chemical shift in the plants that triggers a chemical shift in the butterflies," says van der Heide, who is also a volunteer regional coordinator for the Xerces Society's Western Monarch Thanksgiving Count. "It triggers them to go into a sexual diapause, where they shut down their reproductive system for a time, and that helps them be able to live longer and fly." The winter monarchs—the last generation of the year—can live up to eight or nine months. "They're channeling their reproductive energy into longevity," van der Heide says.

Milkweed once grew abundantly throughout the Midwest, around the corn and soybean fields of the Corn Belt along the monarch's migratory route. But weeds, which compete with crops for nutrients and lower yields, are the enemy of any farmer. The rise of Roundup-resistant crops allowed farmers to carpet their fields in herbicides, killing off any weed that tried to take hold in or around their cropland, including the butterfly's primary food source. In 2015, the U.S. Department of Agriculture announced $4 million in funding from the department's Natural Resources Conservation Services to help agricultural producers plant more milkweed on the borders of fields or in latent pastures. "America's farmers, ranchers and forest landowners are stewards of the land, and this effort helps them make voluntary improvements that benefit working lands and monarchs," the NRCS's Jason Weller said in a statement.

It's not just breeding habitats that are at risk. Urban development has destroyed more than 50 overwintering sites in California over the last few decades, according to the Xerces Society's State of the Monarch Butterfly Overwintering Sites in California report. Of the 412 known overwintering sites in California, just over half have been confirmed active since 2010.

Researchers have been tracking the decline of the monarch population for decades, but this fall it become clear that they are much closer to extinction than anyone realized. Since 1981, the population of West Coast monarchs has declined by 97 percent, according to a study published in September in Biological Conservation. "In the 1980s, 10 million monarchs spent the winter in coastal California. Today there are barely 300,000," Cheryl Schultz, a study author and associate professor at Washington State University–Vancouver, said in a statement in September. "This study doesn't just show that there are fewer monarchs now than 35 years ago. It also tells us that, if things stay the same, western monarchs probably won't be around as we know them in another 35 years."

Cynthia Brock doesn't need a researcher to tell her about the monarch's decline; all she has to do is count the butterflies on her walks. Her worry now is that the city of Goleta's plan for the mesa might inadvertently hasten the butterfly's demise. From October 1st through the end of March, the monarchs cluster at six specific sites within the eucalyptus forest at Ellwood Mesa, but the trees that surround those clustering sites are just as important, as they provide protection from the wind.

"When I heard that they were talking about taking out hundreds of trees and with no plan to replace them or to mitigate the change to the microclimate, I was alarmed," Brock says. "When they tell me they're just going to take out these dead ones, I multiply that number by five or six." That's because of an "incident" she recalls from last December—right in the middle of the monarch season. A massive eucalyptus tree had fallen down across one of the trails that cuts through the main monarch aggregation site.

The trunk hung over the trail, held up in the air by the roots on one side and the branches on the other. City officials understandably deemed the tree a danger. "Well, in order to get that one dead tree out, they took out about six live trees. They cleared the area, including the understory," says Brock—and that's an important part of monarch habitat.

"They had workers in there with this little tractor and a chainsaw for two days, right in the middle of the butterfly season," Brock says. "There were over 2,000 butterflies in that grove the day before they did that work, and the day after they finished there were none, and they never came back the rest of the season." She paused on the trail at the point where she thought the tree had been the winter before. It was right on the edge of the forest, before it opens up into a grassy meadow, dry and yellow at the end of a long, hot summer, and a light breeze coming off the ocean infiltrated the trees. "It changed the way the wind came in through that grove," she says. "Those of us who go there almost every day for years and years and decades could feel it. You could just feel that it had changed." She decided to get involved. "We started calling ourselves Friends of the Ellwood Monarchs, instead of Friends of the Ellwood Coast."

In order to limit the number of trees to be cut down, the group identified the key trails residents use to cross through the Eucalyptus forest on the way to the beach. If the city re-opened just these trails, and left the others that cross the actual aggregation sites closed, only 29 trees would have to be removed.

With just days to spare, on September 26th, the City of Goleta obtained an emergency permit from the California Coastal Commission to clear five trees marked for immediate removal before the butterfly season. None of those trees were within the six defined sites in the forest where the monarchs cluster, but they were close enough to the aggregation sites that officials worried the sound and vibrations from the process would still disturb the butterflies. All five were removed on September 30th.

California's epic drought may be over, but its effects linger on Ellwood Mesa and across the state. A U.S. Forest Service survey last year revealed that more than 102 million trees in California's forests had been killed by the drought, and the die-off were expected to continue despite the winter rains. "The scale of die-off in California is unprecedented in our modern history," the Forest Service's Randy Moore told the Los Angeles Times.

As Goleta officials discuss how to restore Ellwood Mesa after the drought, climate change is only expected to make such dry spells more common. Average temperatures in California are climbing, and so far in 2017, there were four times as many record hot days as record cold ones. "If it gets a lot warmer, we might not see them overwintering in Goleta," van der Heide says. Despite the city's best efforts, global warming may force the monarchs north in search of the right habitat conditions.

That's if they survive: According to the same Biological Conservation study, the western monarch faces an 86 percent chance of extinction over the next 20 years. If that happens, scientists may never know for sure how the butterflies know to return to the same wintering grounds their ancestors left four or five generations ago. Beyond that, it could mean the loss of a focal species for insect conservation. Monarchs are the rare charismatic critter; most other bugs are viewed as pests, despite their essential role in ecosystems.

"If we were to lose the monarch," van der Heide says, "we would lose an avenue into insect conservation as a whole."

*Update — November 8th, 2017: This article had erroneously reported that the Xerxes Society's Conservation and Management Guidelines explicitly caution against tree removal. The organization is, in fact, the Xerces Society.