Is There Good News for Monarch Butterflies? Scientists Disagree

Last month, government officials met in Washington, DC, for the first Monarch Butterfly Summit , just as the milkweed in the “ Monarch Waystations ” that are now ubiquitous across American lawns began to bloom. Like everyone, they were worried about the iconic insect’s fate, following decades of notable population decline in the butterfly’s winter colonies. There are two distinct (but genetically identical) populations of monarchs in the United States, and both are migratory.

Monarchs west of the Rocky Mountains spend their winters in Southern California, while those east of the range fly thousands of miles from as far north as Ontario to central Mexico, where they wait out the cold months in stands of oyamel fir trees. Since the mid-90’s, scientists have found that the number of butterflies that make it to Mexico has fallen by about 70 percent. They blame bad weather, deforestation, and automobile collisions for the decline.

In 2020 alone, 26 percent fewer eastern monarchs made it to Mexico than the year before, having been waylaid by storm and drought. Those that survived the journey found their already-tiny wintering grounds reduced by illegal logging. In 2019, researchers concluded that the western monarch was “hovering at its quasi-extinction threshold” after a 97 percent reduction in that subpopulation since the 1980s.

So it may be surprising—and perhaps controversial—that a recent study published in the journal Global Change Biology suggests that some populations of monarch butterflies are actually on the rise . “There is no monarch butterfly apocalypse,” says Andrew Davis, an ecology professor at the University of Georgia (UGA) and coauthor of the study. “Not in the United States, anyway.

” His group’s work is unusual because it focuses on the insects’ breeding grounds, not their migratory stopovers. In other words, the team looked at counts taken in the summer throughout the US, not in the winter in Mexico or Southern California. Davis and his fellow researchers relied on more than 135,000 monarch observations made on both sides of the Rockies between 1993 and 2018 during the North American Butterfly Association’s (NABA) annual count.

These events call on citizen scientists to record all the butterflies they see in a 15-mile radius over two days in early July. While the research team noted that there have been slight declines in some regions of the US, particularly the Midwest and New England, areas like the Southeast and Pacific Northwest are seeing more monarchs. Taken together, the data suggests an overall annual increase of 1.

36 percent across the species’ summer range, meaning that over the 25-year period, the summer population of monarchs in the US has increased by about 35 percent. Davis says his team’s findings demonstrate that the butterflies’ breeding in summer is making up for the losses the insects experience during winter. “They’re able to rebound and repopulate their entire breeding range every year, regardless of how many are at the winter colonies,” he says.

“It’s just mathematics. A single female can lay 500 eggs. If the conditions are right, the population explodes.

” In fact, Davis wasn’t particularly surprised by the study’s results, arguing that it’s possible the monarch population may never have been in peril in the first place. “I pointed this out years ago,” he says. “I said, ‘Maybe we should be looking at other parts of their life cycle, other time periods.

’ But the wintering story got traction. That’s the one that gets remembered and publicized very, very, very widely. ” That may be because counting the butterflies as they descend on a very small patch of land in Mexico is much simpler than counting them as they scatter across their enormous summer range, which covers all of the United States and southern Canada.

“Monarch decline is a gnarly scientific problem,” says Anurag Agrawal, a professor of environmental science at Cornell University and author of the book Monarchs and Milkweed. “It takes place over a large temporal scale and vast range of land—getting data is challenging. ” That explains why, he says, “the lion’s share of the scientific work has been on the wintering population.

” Another reason there are fewer monarchs in Mexico may be that, as Agrawal notes, “some aspect of monarch biology has been changing over the last 30 years. They’re not so migratory anymore. ” As the climate warms, many butterflies won’t need to make it all the way to Mexico; a place like Florida, which is experiencing fewer freezes each year, may be a fine stopover.

Meanwhile, on the West Coast, San Francisco now hosts monarchs year-round, thanks to butterfly-loving residents who fill their yards with non-native milkweed. But Agrawal says that the study, which he provided feedback on but did not author, will be controversial among people in the monarch community based on the reliability of the NABA dataset and its use of citizen science. “There will be those who dismiss the study, but that will be a mistake,” he says.

Still, Agarwal says the majority of scientists agree “the monarch is not going extinct. It’s not leaving us. ” Emma Pelton, a biologist at the Xerces Society for Invertebrate Conservation, isn’t one of them.

She is critical of the UGA study, calling it “a really big example of overreach. ” The biologist questions, for example, the researchers’ use of a handful of survey sites in Oregon and Idaho to extrapolate monarch population trends across the entire Pacific Northwest. “I’m concerned we’re telling the public to stop caring about the species,” Pelton says.

“It creates a lot of confusion. ” Another factor that might create additional confusion for the public: Last winter, monarch counts were up at their wintering grounds. A World Wildlife Fund survey found a 35 percent uptick in butterflies making it to Mexico over the 2021-2022 wintering season.

The annual monarch count in the Bay Area city Pacific Grove, which dubs itself “Butterfly Town, USA” rebounded in winter 2021 after years of lows. And the 2021 Xerces Society’s Western Monarch Thanksgiving Count showed a staggering hundredfold increase from numbers in 2020. But recent population bounces don’t make up for decades of losses; Pelton says it’s important to note that this last number, while positive, still represents a 95 percent decrease in population since the 1980’s.

(She was an author on the 2019 paper that concluded western monarchs were at a “quasi-extinction” level. ) While this winter’s numbers are certainly good news in the short-term, Pelton warns against normalizing a new baseline . “The modest uptick we’re seeing is not population recovery, or even evidence of an upward trajectory,” she says.

Davis and his coauthors also stop well short of declaring victory for monarch conservation. “We caution against complacency,” they wrote, “since accelerating climate change may bring growing threats. ” Longer summers, for example, could mean delayed migrations and more severe storms along the way.

At last month’s monarch summit, nobody was ready to declare the matter resolved. The Department of the Interior pledged $1 million to the National Fish and Wildlife Foundation’s efforts to enhance habitat for western monarchs on both public and private land, and the US Fish and Wildlife Service committed to establishing a Pollinator Conservation Center to address the plight of pollinating insects—including the monarch—nationwide. After all, monarchs are not the only species at risk.

Bill Snyder, a UGA entomologist and coauthor on the study, says that across the NABA data, two-thirds of butterfly species appear to be in decline. The summer population of the Melissa Blue, for example, seems to be falling by about 2 percent a year. The West Coast Lady’s annual decline is nearly 8 percent.

“That’s what we’re investigating now,” Snyder says. “We’re digging back into the data to try to get a better idea of which species are in significant decline, and is there an alternative flagship species for insect conservation—other than the monarch?”.