Walking around my family’s property in upstate New York last summer, I noticed something I hadn’t seen in years: scores of monarch butterflies flying around the milkweed that rings the perimeter of the yard. Six months later, at the end of January, biologists attending the Trinational Monarch Science Meeting, in Mexico City, confirmed what I and many others had been seeing throughout the summer and fall: the eastern monarch population was a hundred and forty-four per cent larger than it had been a year earlier. The announcement offered a modicum of hope amid dire warnings of mass extinctions and ecological catastrophe. Just two weeks earlier, the Xerces Society for Invertebrate Conservation had issued a finding that the monarch population west of the Rockies dropped by around ninety per cent in the past year and is on the verge of collapse.

Monarch butterflies migrate. Though they weigh less than a gram, they travel thousands of miles each fall to overwintering sites that provide the right microclimate to enable them to survive for months with little food or water. To track these migration patterns, citizen scientists have been gluing small tags on monarchs’ wings since the nineteen-fifties. The data from recovered tags is continually overlaid on maps that show where the butterflies are going and where they are coming from. That’s how we now know that monarchs east of the Rockies typically start the journey in Canada and the upper Midwest, aiming for the Oyamel-fir forests in Mexico’s Transverse Neovolcanic Mountains. In the spring, the butterflies lay eggs in Texas before dying; successive generations move northward in a kind of relay race that follows the proliferation of milkweed, their host plant. Monarchs that begin their journey west of the Rockies do something similar: after wintering on the coast of California, shielded by stands of eucalyptus or Monterey pine, they move inland to the Central Valley, but also north to Washington State and southern British Columbia, and to Nevada, Arizona, Idaho, and possibly Montana.

One day in July, walking around our pond as my dog hunted for frogs, I watched two monarchs mate overhead, and, when they separated, I followed the female as she laid an egg on the underside of a nearby milkweed leaf. Milkweed is the monarch’s host plant because it contains a toxin that is poisonous to the butterfly’s predators. Monarch caterpillars chew the leaves and ingest the poison, which will protect the butterflies that they eventually become. I took the leaf with the egg and brought it inside.

Four days later, the egg, which was white and approximately the size of a grain of sand, was gone, and in its place, barely visible, was the tiniest yellow-black-and-white caterpillar, so small that my husband had no idea where it was when I showed him. A few days, a few molts, and a few milkweed leaves later, the caterpillar was about a half-inch long and fattening up, following a timetable nearly as reliable as a Swiss watch. Another molt followed, and the caterpillar unzipped, then discarded, its striped jacket. Underneath, there was an identical one, one size larger. This phase of the metamorphosis happened to coincide with the death of Lincoln Brower, an emeritus professor of zoology who the Times, in its obituary, rightly called the “champion of the Monarch butterfly.” I attached a small sign, which read “The Lincoln Bedroom,” to the enclosure that I’d made for the caterpillar. I started to refer to the caterpillar as Lincoln, too—an anthropomorphism that I hoped Professor Brower, with whom I’d travelled to Mexico a number of times to see the butterflies, would have forgiven.

In 1995, Brower, who was then at the University of Florida, published a paper in the Journal of the Lepidopterists’ Society, explaining that while the monarch butterfly was not an endangered species—it can also be found in Hawaii, the Caribbean, Spain, Portugal, Australia and New Zealand—its North American migration patterns east of the Rockies were at grave risk of disappearing. In that paper, he called it an “endangered biological phenomenon.” Less than two decades later, in 2013, that danger appeared to have come to fruition, when that winter’s monarch population covered less than three acres of the Mexican forest, down from around twenty acres ten years earlier.

At the recent Monarch Science meeting, in Mexico City, when researchers reported that this year’s migration covered six hectares, or nearly fifteen acres, “people were cheering,” Chip Taylor, an emeritus professor of ecology and evolutionary biology at the University of Kansas, said. This year was not only a banner year for eastern monarchs, it was one for tagging as well. According to Taylor, who, since 1992, has run Monarch Watch, the most prolific monarch-tagging program in the world, four hundred thousand tags were distributed in 2018, one and a half times as many as the year before. Lincoln Brower’s great friend and occasional collaborator, the Mexican poet Homero Aridjis, another monarch champion, told me in an e-mail that he was seeing aggregations of monarchs in the mountains near his boyhood home of Contepec for the first time in a decade.

Though this is all encouraging, a single year is not a trend, and scientists remain concerned about the viability of the migration. “We should celebrate the fact that we go up to this six-hectare number, and people who are living in areas where monarchs breed really noticed them this summer,” Karen Oberhauser, the director of the University of Wisconsin-Madison Arboretum, who researches monarchs, told me. “It illustrates the fact that they have this incredible potential.” But a population swing in the opposite direction can happen, too, she said. Oberhauser and her research associates have found that there is an underlying threat from habitat loss. (One study estimated that two hundred and twenty three million milkweed stems were lost in the Midwest between 2008 and 2016.) According to the World Wildlife Fund, six hectares is just enough to sustain a viable population in North America.

Taylor has a one-word explanation for why 2018 was an especially good year for eastern monarchs, and why that good year likely portends bad ones to come: temperature. In March, when the monarchs arrived in Texas from Mexico, the mean temperatures were 5.4 degrees Fahrenheit above normal, which should have blown the butterflies all the way to Kansas.The butterflies couldn’t go farther north, though, because it was too cold in northern Texas. “I looked at the weather every day and I’d say, ‘damn, that’s good. Keep your babies in Texas,’” Taylor said. In May, the temperatures were optimal as the butterflies moved north, and they were ideal in the summer as monarchs spread out across their breeding range. Then, in the fall, as the butterflies headed south, those numbers were favorable, too. “What we had this year is everything was positive in every one of those stages,” Taylor told me. “This is why I say this is not likely to happen again—because you’re not likely to see a pattern like this again.”

West of the Rockies, the number of monarchs overwintering in California is estimated to be around twenty thousand, down eighty-six per cent since last year, which “isn’t completely unprecedented,” Emma Pelton, a researcher with the Xerces Society, told me. “I’ve looked back at data between 1980 and today, and there have been other instances where we’ve had a single-year drop of this magnitude. But the difference is that this is all in the context of a ninety-seven per cent decline since the nineteen-eighties.” The current situation, Pelton added, is considered “a quasi-extinction.” There may not be enough butterflies to repopulate the range. If that happens, there will still be western monarchs, but they will be increasingly hard to find. “That would be like losing all the rhinos, except the rhinos in the zoos,” Pelton said. “Really, the threat is that we’re going to lose migration.”