Dan Fagin is a professor of science journalism at New York University and the author of “Toms River: A Story of Science and Salvation,” winner of the 2014 Pulitzer Prize for general nonfiction.

On a damp January afternoon 46 years ago in rural Mexico, a wayfaring American retiree, his adventurous young Mexican wife, and a local villager, along with a skinny pack horse and a dog named Kola, trudged up a muddy mountainside in search of a waking dream. At 11,000 feet, improbably, they found it.

Ken and Catalina Brugger, searching on behalf of Canadian butterfly scientist Fred Urquhart, had heard hard-to-believe stories about patches of a high-altitude forest teeming with millions of butterflies.

Now, suddenly, they knew those stories were true. They had found the hidden winter refuge of the eastern monarch butterfly, an iconic species whose epic annual migration had preoccupied Urquhart and other butterfly chasers for decades.

Today, the overwintering colonies in Mexico are still visually stunning, and the story of monarch migration retains much of its alluring mystery. But both the colonies and the 3,000-mile journey that sustains them are far more fragile than they seem — especially right now, as the social disruption and chaos of the global coronavirus pandemic reaches even to the isolated mountaintops where the Bruggers made their historic find. The potential losses — to monarchs, to science, and to all of us — are beyond calculation.

There is nothing else in the world like the migration of North America’s best-known butterfly. Despite the intensive efforts of Urquhart’s successors, who have made the monarch the world’s most studied butterfly, the experts still haven’t worked out precisely why and how an insect that spreads out widely enough during the summertime to occupy most of eastern North America — including your neighborhood, probably — consolidates every winter on the same half-dozen or so Mexican mountainsides. Why would tens of millions of finger-length creatures propelled by gossamer wings — imagine orange stained-glass panels made of tissue paper — undertake such a perilous journey? How could they possibly find their way?

It turns out that migration has been so vital to the monarch’s survival as a species that the best science now suggests it has evolved two extremely sophisticated navigation systems: a light-sensitive “sun compass” that compensates as the sun moves across the sky, and an “inclination compass” that can detect Earth’s magnetic field. (The second compass is especially important on days when clouds block the sun.) Both compasses are essential for the “Methuselah” generation of monarchs that does most of the round-trip migrating and lives about eight times longer than the summer generations that gambol in backyard gardens in the United States and Canada.

There’s a lot more mystery still left to solve, though, because the Methuselahs have an uncanny ability to find the very same mountaintop redoubts their great-great-grandparents occupied 12 months earlier. How they do that, no one knows — yet.

The overwintering sites are not only a compelling puzzle of evolutionary biology, they are also a phantasmagoric spectacle. I have visited twice, most recently in January 2020, as part of my research for a book on monarchs and the future of life on a changing planet, and can affirm that it is impossible to adequately describe in words, photographs or even video what it feels like to be surrounded by millions of butterflies. This is especially true in midmorning, when the warmth of the rising sun filters through the oyamel fir trees and the resting monarchs blanketing almost every inch of every branch begin to stir and take flight, swooping and whirling overhead. (The collective noun for a group of butterflies is “kaleidoscope,” which is perfect.)

On both of my visits, I spent time on Cerro Pelón, the imposing mountain where the greatest secret of animal migration was first revealed to the world (though local villagers almost certainly knew about it already).

You might think there would be a plaque to mark the spot, or maybe even a statue, but there isn’t. In fact, no one is sure exactly where the Bruggers made their discovery, because while monarchs come to Cerro Pelón each November, they have several preferred roosting spots there, not just one. Lincoln Brower, the preeminent monarch scientist, who died in 2018, believed the historic spot was probably a grove on the mountain’s northwestern slope known as “La Lagunita,” or “the Pond” (monarchs like to be near water).

If Brower was right — and on the subject of monarchs, he almost always was — then you and I will never again see La Lagunita as the Bruggers saw it. More consequentially, neither will the monarchs, because during these past 15 months of pandemic-induced deprivation and desperation, La Lagunita has been trashed. Last year, someone — likely impoverished young men from a nearby community — illegally cut down several dozen oyamel firs, hauling them away for lumber. In December, the arriving monarchs tried to form a colony at La Lagunita but failed, according to Ellen Sharp, who runs a monarch-centric hotel at the foot of the mountain.

“It was chilly and cloudy, the kind of weather conditions that make them cling to the trees, but instead of clustering they were flying everywhere, restless when they should have been resting,” she told me.

More than the usual number of dead monarchs littered the forest floor, probably because the surviving trees were now spread too thin to offer much protection from orioles and other predators, as well as from winter storms. “Usually, I like to linger at the colony,” Sharp said, “but that day, I hurried home, trying to escape the panicked feeling that I’d just witnessed the beginning of the end of the eastern monarch migration.”

A few weeks later, the monarchs gave up and abandoned La Lagunita altogether, shifting to a different location on the mountain. These migrants had become refugees.

Sharp and her husband, Joel Moreno Rojas, have gone to great lengths to try to protect Cerro Pelón from loggers, even forming a nonprofit organization that has hired local “forest guardians” to patrol the mountain and report what they see to the Mexican authorities. But their reports are usually ignored, Sharp says.

Last month, the guardians found another six trees felled at La Lagunita. Eight more were cut down a few days later. These latest wounds make it even less likely that La Lagunita will ever again successfully host roosting monarchs.

The motivation behind these destructive incursions is sadly obvious. The pandemic has had a devastating effect on the struggling, tourist-dependent communities bordering the 52-square-mile core zone of the Monarch Butterfly Biosphere Reserve, where logging is nominally banned but is now on the rise, reversing years of progress. Reserve officials recently acknowledged that 33 acres in the core zone had been illegally logged last year, up from just one acre the year before. The toll this year will surely be worse.

Violence is on the upswing, too, including the unsolved killings of two men who worked at the most heavily visited monarch colony, El Rosario. Now that the pandemic has driven away international tourists, gangs are filling the vacuum.

A scientist who has worked in the area for many years told me that, for the first time, their research group was recently forced to turn back after being halted on the road by a makeshift barricade manned by a gang of young men openly identifying themselves as members of a drug cartel.

It will take years to identify all of the ways this global pandemic has frayed the social fabric that binds us together, and many more years to repair it. But the monarch migration may not have years. Monarchs are not close to extinction, thanks to robust nonmigratory populations established over the past 200 years in warm climates around the world (wherever humans wittingly or unwittingly plant milkweed, monarchs are likely to follow — even if they have to cross oceans). But in the place where the species likely first evolved, the mountains of central Mexico, the monarch’s long-term future is very much in doubt, as is the record-breaking migration that is its most distinctive feature (no butterfly flies farther).

Deforestation at the overwintering sites is the most immediate threat but is hardly the only one. In the U.S. corn belt, the widespread use of pesticide-tolerant, genetically modified crops has drastically reduced the supply of milkweed available to summering monarchs. That’s a huge problem because monarchs won’t lay their eggs on anything else.

Over the longer term, climate change is an even bigger threat. Drought along the monarch flyway is now commonplace, and the resulting lack of nectar sources worsens the odds that monarchs will survive the trip. Meanwhile, warmer winters are gradually pushing monarchs to higher elevations at Cerro Pelón, El Rosario and the other sites because the butterflies can roost successfully only if temperatures are, like Goldilocks’s porridge, neither too hot nor too cold. But the colonies are already near the mountaintops; in another couple of decades, there will be nowhere for them to move up to, and it’s unclear whether they can be induced to go elsewhere.

Don’t be fooled by their good looks and delicate features; monarchs are tough little bugs. They have dealt with global change before during their roughly 1.5 million years on this planet. Today, though, the barrage of extreme challenges we humans are throwing at them is overwhelming the much slower pace of adaptive evolution.

Fortunately, monarchs still have a crucial advantage: The same all-powerful species that poses the greatest threat to their future has also fallen hard for them, beguiled by their flashy coloring and evocative backstory of metamorphosis and migration. Hundreds of thousands of people in North America have resolved to help monarchs and other beneficial insects (monarchs are woefully inefficient pollinators, but they do pollinate) by planting native milkweeds and nectar plants in gardens and parks. Can you imagine a similarly broad social movement to protect, say, kangaroo rats, even though several species in that subfamily are much closer to extinction than monarchs? I can’t either.

With enough positive intervention by humans, the monarch migration may yet survive, though it’s very much an open question. The proliferation of butterfly gardens hasn’t halted the terrifyingly steep decline of the much smaller western monarch population; fewer than 2,000 were counted this year at their California overwintering sites, down from 1.2 million in 1997. For the eastern monarchs, this year’s population estimate from Cerro Pelón, El Rosario and the rest of the colonies in Central Mexico was roughly 45 million, down from 250 million 25 years earlier. Next year’s count is anyone’s guess, but the signs are not encouraging.

The monarchs have given up on La Lagunita. The increasingly urgent question is, are we giving up on the monarchs?

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