Western monarch populations grew over 100-fold in 2021. Why?

On a brisk December day in Pacific Grove, California, 10,000 monarch butterflies hang in the crown of a Monterey pine. Where the branches cast shade, they huddle body-to-body in thick brown clusters, wings folded. But where the light touches, the boughs are bejeweled with splashes of orange. Warming monarchs flap languidly, some flitting among the trees or fluttering down to the ground. Whenever the sun shifts, the air is busy with wings.

Last year at this time, there were no monarchs here at all. In fact, there are five times more butterflies in this tiny park right now than were counted in all of California in 2020.

Western monarch populations have declined precipitously since the 1990s, when 3 million to 10 million butterflies migrated annually from the northwestern United States to spend the winter at hundreds of sites along the California coast. Last year, less than 2,000 monarchs were counted in the entire state. Butterfly researchers despaired, since the number was well below the level theorized to lead to collapse and extinction. And they rejoiced when, unexpectedly, the species made a dramatic comeback last year.

California’s Xerces Society for Invertebrate Conservation announced today that with the help of volunteers it counted nearly 250,000 butterflies in 2021, a more than hundredfold increase that society Senior Endangered Species Conservation Biologist Emma Pelton calls “magnificent.” But as Pelton and her colleagues celebrate the news, they’re also asking: Why?

The monarchs of North America are split into two populations, and two spectacular migrations, divided by the Rocky Mountains. The Eastern monarch’s fliers sweep south every fall and blanket acres of forest in central Mexico each winter. At the same time, the Western monarchs depart sites across a broad swath west of the Rockies and head southwest to California. In their winter territory, these butterflies, which usually only live a few weeks, go into a state of suspended development called diapause, which allows them to extend their lives for months. About a third survive this period and go on to mate in late winter, usually February. After that, the females head northeast toward the Sierra Nevada mountains to find milkweed, which will shelter their eggs and feed the resulting caterpillars. The population expands for three or four generations, with the final one returning to the overwintering sites, says Elisabeth Crone, an ecologist at Tufts University.

News of the Western monarch’s dramatic return is still fresh, so any ideas as to what might be driving the population surge are yet to be thoroughly researched. Still, the monarch life cycle is so complex, and their annual voyage so epic, that a panoply of factors would have to line up to cause such an enormous population jump, says University of California, Davis insect ecologist Louie Yang. He calls this scenario a “series of fortunate events.” Survival of the first-generation—those born after the long winter slog and resulting mating frenzy—is especially important among monarchs, says Crone, since healthy butterflies have more offspring, which then go on to have more offspring. And since a single female monarch can produce four daughters (15 or more in ideal lab conditions), early fortunate events could have far-reaching benefits.

Those events might include warm-but-not-too-hot weather, she says, since monarchs function poorly under too-cold and too-hot conditions and milkweed tends to do better than other plants for the first few years of a drought. They also might include the right amount of rainfall at just the right time. Research by Yang and others suggests that monarch caterpillars survive best during windows in early summer and early fall, though it’s unclear exactly why. Earlier in the year, he speculates, butterflies are often ready to lay eggs on milkweed, but that milkweed isn’t ready for their young. This may lead to hungry caterpillars feeding on too-small plants, or less protection from predators. What if, Yang asks, the levels and frequency of rain caused the milkweed to bloom at just the right time or in some other way that made it more accessible to the monarchs? “If it were to increase by twofold or fivefold or tenfold, that would have a big effect on population,” he says.

Thousands of monarch butterflies gather in eucalyptus trees in Pismo Beach, California, in January 2018. Image Credit: George Rose, Getty Images

Another possible contributing “fortunate event” could involve larger world trends, Crone suggests. Renowned UC Davis butterfly researcher Art Shapiro has speculated that monarchs have done better during past drought periods because fewer crops planted meant fewer pesticides in the environment. If the short staffing and supply chain issues of the COVID era meant fewer crops were planted, that might also mean less pesticide—and more monarchs.

And at Pacific Grove, park docent and retired entomologist Paul Meredith says his money’s on wildfires. Meredith, who worked in agriculture in Texas before moving to California, learned the timing of fire-following plants when he performed prescribed burns on ranchland. What if, he asks, 2019’s terrible fires prepared the ground for an extraordinary wildflower season that gave the first generation of migrating monarchs the extra bump they needed? Crone, Pelton, and Yang are open to the idea, but they think such a theory would be difficult to corroborate without detailed monitoring and decades of data. The trouble is that fire behavior changes every year and affects every ecosystem differently, making it something of a wildcard. “There are so many possible mechanisms that are tied to wildfires that it’s challenging to disentangle them,” Yang says.

Another possible explanation for the explosion of monarchs is a dynamic built into the ecology of many species. Biologists call a population “negative-density dependent” when low numbers or densities help its survival, for example because it’s easier to find food and harder for disease to spread between individuals—or for predators to find them. “With monarchs we were very concerned that the opposite might be true,” Yang says. He’s relieved that at least this year that didn’t seem to be the case.

And, increasingly, monarch experts are asking themselves if the extra individuals could simply be coming from somewhere else. Could newly established urban populations feeding on human-planted milkweed be joining the migration? Could some number of butterflies from Eastern populations have crossed over the Rockies or come north from Mexico? Crone and Pelton believe the latter is more likely than the former. As for urban butterflies, “even if all of them left the city and went out and joined the migratory population, it would still be an outlier,” Crone says. “It would have to be that plus something else at a minimum.”

Considering the possibility of Eastern arrivals, “I’m straddling the fence on this one,” Pelton says. She feels that when it comes to the series of “fortunate events,” the idea that last year’s butterflies could reach these population heights on their own is a stretch. “I think if we’d stayed under 200,000 it’s realistic, but over that the math gets funkier.” What if, she asks, there’s always been a few thousand butterflies that made the crossing each year? Back when Western monarchs numbered in the millions, that would have been a barely noticeable addition. “But in a year when we had 1,900, a couple thousand from the East would make a big difference.”

Volunteer monarch counters Stephanie Turcotte, Shannon Conner, and Natalie Johnston compare notes in Pacific Grove Monarch Butterfly Sanctuary in December 2021. Image Credit: Candy Sarikonda

Ultimately, all the experts interviewed for this article agreed that the answer is likely to be a mix of some or all of these mechanisms, since no one factor could produce such unprecedented growth. Instead, it could have been “somewhere fire at the right time, somewhere less pesticides being used, someone having done more habitat restoration in just the right spot,” Crone says. She and fellow monarch researcher Cheryl Schultz at Washington State University are hoping to explore the question through a new project on urban monarch behavior. And the Fish and Wildlife Service is starting a study sequencing monarch genomes that could show if some are coming from Eastern or city populations.

Ultimately, we may never know the cause of this extraordinary bounce, Crone says. For one thing, Western monarchs travel such an enormous area that it’s very difficult to track them, even if funding weren’t an issue. And, Yang stresses, he and his fellow ecologists must be comfortable acknowledging the fact that there’s a lot they don’t understand and not let that discourage them from searching for answers.

Plus, “our understanding of the population is all built on the population being a lot bigger,” Pelton points out. Population models of species approaching extinction show their numbers can take wild swings—even more so for insects that reproduce rapidly. Something similar happened recently with Eastern monarchs, whose winter population covered more than six hectares in Mexico in 2019 and then quickly shrunk again by half.

Pelton emphasizes that Western monarchs are still on a dangerous precipice. The key is to try to use the extra time this year’s bump allows to garner more attention, more funding, and real policy change, she says. (That may be especially key in California, where insects were left off of the state’s endangered species act—a fluke that has led the state to argue that, for purposes of the law, imperiled insects should be considered fish.)

The monarch’s surprising rebound is still cause for celebration, Pelton says. It’s a “ray of hope that this population can bounce back, that they’re more resilient than we thought.” But, she adds, “We need to turn it into action. We can’t just move on.”

Correction: Biologist Cheryl Schultz works at Washington State University, not the University of Washington, as stated in a previous version of this article.

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