In recent years, the population of the much-loved butterflies had plummeted to dangerously low levels. Experts speculate on what’s fueling their astonishing comeback.
10,000 monarch butterflies flutter on the summit of a Monterey pine on a chilly December day in Pacific Grove, California. They cuddle body-to-body in dense brown clumps where the trees offer shade, wings curled. The boughs, however, are bejeweled with orange splashes where the light catches them. Warming monarch butterflies fly about aimlessly, some hovering among the branches and others dropping to the ground. The air is bustling with wings as the sun moves.
There were no monarchs present at this time last year. In fact, there are already five times as many butterflies in one little park than there were in all of California in 2020.
Since the 1990s, when 3 million to 10 million butterflies travelled yearly from the northwest to spend the winter at hundreds of locations along the California coast, monarch numbers have plummeted. Only about 2,000 monarchs were counted throughout the whole state last year. The number of butterflies was much below the amount predicted to cause collapse and extinction, which made biologists despair. They were ecstatic when the species suddenly made a stunning return last year.
The Xerces Society for Invertebrate Conservation in California reported today that it counted approximately 250,000 butterflies in 2021 with the aid of volunteers, a more than 100fold increase that society Senior Endangered Species Conservation Biologist Emma Pelton called “magnificent.” Pelton and her coworkers are ecstatic at the news, but they’re also wondering why.
The Rocky Mountains separate the monarchs of North America into two populations and two remarkable migrations. Every fall, the Eastern monarch’s flyers march south, blanketing acres of woodland in central Mexico. At the same time, Western monarch butterflies leave a wide swath of territory west of the Rockies and fly southwest to California. These butterflies, which ordinarily only survive a few weeks, enter a condition of halted development termed diapause in their winter region, allowing them to live for months.
About a third of the birds make it through this phase and mate in late winter, generally around February. The females then go northeast into the Sierra Nevada mountains in search of milkweed, which will protect their eggs and nourish the caterpillars that develop. According to Elisabeth Crone, an ecologist at Tufts University, the population increases for three or four generations before returning to the overwintering places.
A succession of lucky circumstances
Because the news of the Western monarch’s spectacular comeback is still recent, any theories about what is causing the population explosion have yet to be properly investigated. Still, according to University of California, Davis insect ecologist Louie Yang, the monarch life cycle is so complicated, and their yearly journey so epic, that a slew of conditions would have to align to induce such a massive population increase. This is what he refers to as a “chain of fortuitous circumstances.” According to Crone, the survival of the first generation—those born after the long winter slog and subsequent mating frenzy—is especially essential among monarch butterflies because healthy butterflies produce more progeny, who in turn have more offspring.
Early lucky events might have far-reaching effects because a single female monarch can generate four daughters (15 or more in optimal lab settings).
Warm-but-not-too-hot weather might be one of those occurrences, she adds, because monarchs fare poorly in too-cold and too-hot temperatures, and milkweed does better than other plants during the first few years of a drought. They might also include just the right quantity of rain at the correct moment. According to Yang and others’ research, monarch caterpillars thrive in early summer and early fall when they are exposed to windows, however the reason for this is unknown. Butterflies are typically ready to lay eggs on milkweed earlier in the year, but the milkweed isn’t ready for their young, he believes. This might result in hungry caterpillars munching on too-small plants or less predator protection.
What if, Yang speculates, rainfall levels and frequency led the milkweed to blossom at at the right moment or in some other way, making it more accessible to monarch butterflies? “It would have a tremendous influence on population if it increased by twice, fivefold, or tenfold,” he argues.
According to Crone, another contributing “lucky occurrence” might be wider global tendencies. Art Shapiro, a renowned UC Davis butterfly researcher, believes that monarch butterflies fared better during previous droughts because less crops were produced, resulting in fewer toxins in the environment. If the COVID era’s low workforce and supply chain concerns resulted in less crops being planted, it may also imply less pesticide—and more monarch butterflies.
Paul Meredith, a park docent and retired entomologist in Pacific Grove, predicts flames. When he did programmed burns on ranchland, Meredith, who worked in agriculture in Texas before relocating to California, studied the timing of fire-following plants. What if, he wonders, the devastating fires of 2019 paved the way for an exceptional wildflower season, giving the first generation of migratory monarchs the boost they needed? Crone, Pelton, and Yang are receptive to the notion, but they believe it would be impossible to verify without extensive data collection and decades of observation. The problem is that fire behavior varies from year to year and has a distinct impact on each ecosystem, making it a bit of a wildcard.
“It’s difficult to untangle all of the various pathways that are linked to wildfires,” Yang explains.
Forces of greater magnitude
A dynamic inherent into the ecology of many species might be another reason for the monarch butterfly’s growth. When low numbers or densities help a population survive, biologists term it “negative-density dependent.” This is because it’s simpler to obtain food and difficult for illness to transmit between individuals—or for predators to discover them. “We were really afraid that the opposite would be true with monarchs,” Yang explains. He’s glad that this didn’t appear to be the case this year.
And monarch experts are now wondering if the extra people may just be arriving from somewhere. Could new urban populations relying on milkweed seeded by humans be joining the migration? Could a group of butterflies from the East have crossed the Rockies or traveled north from Mexico? The latter, according to Crone and Pelton, is more likely than the former. “Even if all of them left the city and went out and joined the migratory population, it would still be an anomaly,” Crone says of urban butterflies. “At the very least, it would have to be that plus something more.”
“I’m riding the fence on this one,” Pelton adds, referring to the likelihood of Eastern arrivals. When it comes to the chain of “lucky circumstances,” she believes it’s a stretch to believe that last year’s butterflies could have reached these population levels on their own. “I think it’s doable if we keep around 200,000, but beyond that, the arithmetic becomes a little wacky.” What if, she speculates, a few thousand butterflies always make the journey each year? That would have been a minor addition back when Western monarchs were in the millions. “However, in a year when we only had 1,900, a few thousand from the East would make a significant impact.”
Action and celebration
In the end, all of the experts consulted for this piece agreed that the explanation is likely to be a combination of some or all of these mechanisms, because no one reason could account for such enormous development. Instead, “somewhere fire at the right time, less chemicals being used, someone doing more habitat restoration in precisely the right area,” according to Crone. She and Washington State University’s Cheryl Schultz, a fellow monarch researcher, hope to investigate the topic through a new project on urban monarch behavior. The Fish and Wildlife Service is also undertaking a project to sequence monarch genomes to see if some come from Eastern or urban populations.
Plus, as Pelton points out, “our concept of the population is entirely constructed on the population being a lot greater.” Population models of species on the verge of extinction demonstrate that their numbers can fluctuate dramatically, especially for insects that reproduce quickly. Something similar happened recently with Eastern monarch butterflies, whose winter population in Mexico grew to almost six hectares in 2019 before rapidly shrinking by half.
Pelton argues that Western monarchs are still perilously close to falling down the cliff. The trick, she argues, is to attempt to exploit the extra time provided by this year’s spike to gain greater attention, financing, and genuine legislative change. (This is especially important in California, where insects were left off of the state’s endangered species act—a mistake that has prompted the state to claim that endangered insects should be deemed fish for the purposes of the legislation.)
The monarch’s unexpected comeback is nevertheless reason for joy, according to Pelton. It’s a “ray of optimism that this group may recover, that they’re more robust than we expected,” according to the report. “However, we must put it into effect,” she continues. “We can’t just go on with it.”
Cheryl Schultz, a biologist at Washington State University, does not work at the University of Washington, as reported in an earlier version of this article.