For years, researchers and hobbyists have watched monarch butterfly populations plummet.
Millions of monarchs fluttered around California’s overwintering sites in the 1980s, but in November of 2020, scientists and volunteers counted less than 2,000 in the state. Pacific Grove, sometimes called “Butterfly Town USA,” recorded none.
This drop was particularly dramatic, but the population has declined for decades.
This past winter offered some hope, with over 200,000 monarchs recorded in the state, but most researchers remain concerned.
The authors of a new, controversial study in the journal Global Change Biology, however, suggest that even though monarch numbers are declining at overwintering sites, the overall population might actually be increasing—a conclusion that has been met with skepticism by other scientists.
The debate comes at a critical time for monarch populations, as the U.S. Fish and Wildlife Service considers the butterfly for protection under the Endangered Species Act. The conversation shows just how complicated monitoring a species can be—even for something as well-documented and beloved as monarch butterflies.
North America has two main monarch populations—eastern butterflies migrate from the northeast to Mexico, and the western population settles in central and southern California for the winter.
But these migrations don’t happen all within one lifetime. A full annual migration cycle includes four generations of monarchs. The first three generations emerge from their pupae, mate and die within a few weeks. But the fourth generation is different. This migrating generation flies south, overwinters and begins the journey back north in the early spring to lay eggs.
“It used to be that we all thought we could just sample them during the wintering phase,” says Andrew Davis, an ecologist at the University of Georgia, and co-author of the study.
But Davis disagrees with that approach now, and instead turned to summer breeding numbers. Increased parasites, car strikes, pesticides and other threats prevent many monarchs from reaching the final overwintering site, he says.
“So it’s not like there’s fewer monarchs being produced. It’s more like there’s fewer monarchs reaching the end goal,” he says.
The researchers pulled community science data from the North American Butterfly Association (NABA). Each summer, volunteers count butterfly numbers at specified sites.
The scientists modeled monarch numbers across the U.S. using 135,705 observations from 403 sites over time periods that ranged from 10-26 years.
They concluded that although some summer areas saw declines, all the sites together showed a slow annual increase.
“The statistics that were involved in reaching our conclusions were absolutely mind-boggling,” says Davis. “We had to account for how many people were counting, and for how long and so forth. We also had to account for the effects of weather and climate.”
The researchers concluded that in some parts of the country—particularly in the midwest—higher temperatures had a positive effect on the summer population of monarchs.
“But on the other hand, there’s other places in the country where the reverse is true. So it seems to vary widely depending on where you are,” he says.
Grounds For Skepticism
Davis sees the conclusions as good news, but other researchers disagree with the findings.
“The breeding grounds for the monarch have always been complicated, with some areas going up and some going down in different stretches of years,” says Matthew Forister, a professor and insect ecologist at the University of Nevada Reno. “We’ve known that for a while. But the overwintering grounds continue to go down, which was not part of this paper.”
Forister says although the data set is useful, it cannot provide the complete picture.
“It’s that once-per-year survey point,” he says. And in a species that goes through multiple generations a year, it can make things look better than they are. “A couple of generations of insect reproduction, and you can bounce back up to pretty high numbers. But that doesn’t reduce the concern for having low overwintering numbers.”
The reason for continued concern, he explains, is a process called the “bottle neck effect,” which occurs when a population experiences an extreme decrease. Losing a large chunk of a population removes the diversity that species need to stay healthy and adapt to changing conditions.
“One of the few things that we know with certainty in conservation biology is that bottlenecks are bad,” says Forister. “You go through a constriction in population numbers, you’re vulnerable to extreme events like storms. You’re vulnerable to inbreeding depression. It’s bad.”
Other researchers take issue with the data itself.
“I think the way it was applied for this particular study is somewhat problematic,” says Emma Pelton, the western monarch lead for the Xerces Society for Invertebrate Conservation.
She calls the data set a “snapshot of some usually pretty high-quality butterfly habitat.”
“The sites where people are going out to look for butterflies as part of this community science project are not chosen randomly over the landscape,” she says.
Davis pushes back on that critique.
“These survey sites had dozens of different types of habitats, including good ones and bad ones,” he says.
Davis believes some of the skepticism is “political.”
“The monarchs-are-in-trouble narrative is very powerful. It motivates people to go out and plant a butterfly garden, to join an organization, to donate money to a cause,” he says.
Pelton argues that the criticisms are backed by other studies.
“The fact that they conclude that this means the populations are okay—it really goes against the grain of what a much larger body of work shows and what the vast majority of monarch researchers believe is happening,” she says.
One thing all the scientists agree on is that habitat destruction, pesticides and climate change pose serious threats to insects around the world.
“Everything is not fine for any insect in North America—or just about anywhere else—because habitat is decreasing, and the remaining habitat is being fried by climate change and contaminated,” says Forister.
Another source of agreement is the benefits of community science. Tracking population numbers is clearly a complicated process, and the more observations involved, the more opportunities researchers have to piece together the puzzle.
“Community science and reporting what you see really can make a difference,” says Pelton.
Interested volunteers can participate in projects like the Western Monarch Milkweed Mapper, visit local sanctuaries like Natural Bridges or find a local NABA chapter.