Each fall, swarms of migrating monarchs descend on the mountains of central Mexico. It’s the final leg of a journey of thousands of miles, as the butterflies travel from their North American breeding grounds to their winter colonies, where they cover the fir trees. Visitors who time their trips correctly may find the air so laden with monarchs that they can hear the flapping wings of butterflies.
“It’s a to-do list,” said Andrew Davis, an ecologist at the University of Georgia in Athens. “Imagine being surrounded by billions of butterflies. It’s a magical experience.
It is also one that may not last. In recent decades, the number of Monarchs overwintering in Mexico and overwintering sites in Southern California droppedwhich raises fears that the species is seriously threatened with extinction.
But a new study, which focuses on the size of the summer breeding population, could complicate this picture. Although summer monarch abundance has declined in some areas, including parts of the Midwest, it has increased in others, the scientists found. Overall, across the North American range, abundance has remained stable or even increased slightly between 1993 and 2018, according to the article published Friday in Global Change Biology.
The results suggest that so far, monarch populations have been able to recover enough during the summer breeding season to offset winter declines, the researchers said.
“So it’s not really a production issue,” said Dr Davis, author of the new paper. “We don’t have fewer monarchs. We have fewer monarchs reaching overwintering colonies.
But not all scientists agree with this interpretation of the results, which seems likely to fuel an ongoing debate about the threats monarchs face and the causes of the well-documented winter declines.
“There are areas where monarch numbers are increasing a bit, but that’s not the core of the breeding range,” said Karen Oberhauser, a conservation biologist at the University of Wisconsin-Madison who doesn’t know. did not participate in the new research. .
The new study, she noted, documented a decline in monarch abundance in the Midwest’s “Corn Belt” — an agricultural region that is a critical monarch breeding ground. “The numbers are mostly going down in the corn belt,” Dr. Oberhauser said. “That’s where most of the monarchs are.”
North America is home to two monarch populations. The largest eastern population spends its summers breeding in milkweed-filled fields along the East Coast and Midwest before heading to Mexico each fall. A smaller western population typically converges on California each fall.
Numerous studies have shown that the size of these winter colonies has dropped sharply since the mid-1990s. “They’re just a shadow of their former selves,” said Jeffrey Glassberg, president of the North American Butterfly Association and author of the paper.
These declines have been attributed to a variety of factors, including climate change and logging near wintering sites.
The losses also coincided with the increasing use of an herbicide known as glyphosate across large swaths of the butterfly’s breeding range in the Midwest. The herbicide kills milkweed, which often grows in corn and soybean fields and plays a vital role in the monarch’s life cycle. adult butterflies lay their eggs on plants, which are the only food source for caterpillars. Use of the herbicide increased sharply during the 1990s and early 2000s.
Winter declines have prompted campaigns encourage the public to plant milkweed, as well as worries about the future of the monarch. In December 2020, the US Fish and Wildlife Service concluded that the butterfly qualified for federal protection but that other threatened and endangered species were a higher priority for the agency.
However, overall population trends, across the monarch’s entire range and annual migratory cycle, have been more difficult to determine.
The new study was led by Dr. Davis and Michael Crossley, an entomologist at the University of Delaware. They and their colleagues mined data collected through the North American Butterfly Association’s annual Summer Butterfly Count, in which volunteers count the number of butterflies they observe in defined areas.
The scientists analyzed more than 135,000 monarch sightings at 403 sites, limiting their analysis to locations that had data spanning at least a 10-year period between 1993 and 2018. They adjusted the counts to account for the number of hours the volunteers spent on the task, and then modeled the relative abundance of monarchs over time.
Scientists have found evidence of summer population declines in the southwest, northeast and corn belt. But in the Upper Midwest, as well as the Northwest and Southeast, monarch abundance appeared to be increasing over time. At all sites combined, relative abundance has increased by about 1.36% per year, the scientists calculated.
“Monarchs in the summer are doing well,” Dr. Davis said.
So what explains why winter monarch colonies have shrunk? It’s not yet clear, Dr Davis said, but one possibility is that global warming means fewer monarchs are migrating seasonally. It’s also possible that fewer monarchs will survive their journeys south, for reasons that could range from an increase in car strikes to increased infection rates with a parasite that reduces the success of migration.
“There are a number of natural and anthropogenic threats to fall migration that could increase over time,” Dr Davis said.
Leslie Ries, an ecologist at Georgetown University, praised the journal’s broad reach. “The geographic scope of their analysis is larger and more comprehensive than anything I’ve seen,” she said.
But she said it’s not clear that population increases in some parts of the country can completely offset declines in important breeding grounds, like the Midwest.
Dr. Oberhauser also noted that previous research had shown that during the latter part of the study period, the monarch population in Mexico essentially stabilized at a new, lower level. So at sites that only had data from the past decade, researchers might have missed the population decline, she said.
“I think it would be a mistake to conclude that what we do in the loam doesn’t matter,” Dr. Oberhauser said. “It’s important to conserve habitat through all phases of the annual cycle. We know that every phase must work for the monarch migration to survive. »
Dr. Davis and his colleagues also used federal data to estimate the amount of glyphosate used in the area around each survey site. They found that in some areas, particularly parts of the Midwest, glyphosate use was associated with lower abundance.
But they also documented a countervailing force: climate change. In the northern part of the United States, increasing temperatures were correlated with increased monarch abundance. This effect was particularly pronounced in the Midwest, suggesting that global warming may have partially offset the effects of glyphosate in that region, Dr. Davis said.
In parts of the country that generally have warmer climates, however, rising temperatures were correlated with a decline in monarch abundance, consistent with previous research.
“Warmer temperatures where it’s normally cold are good for monarchs, but warmer temperatures where it’s usually warm are bad,” Dr. Oberhauser said.
In recent years, as glyphosate use has stabilized, weather fluctuations in spring and summer were the biggest predictors of summer population size, she and her colleagues found in a 2021 study. monarchs now, continued climate change and more extreme weather could cause problems in the future, she said.
The results do not suggest that the monarchs face any threat, the researchers said.
“These overwintering colonies are having problems for a whole bunch of reasons,” Dr Glassberg said. If winter populations become small enough, the monarch population may not recover in the summer, the researchers noted.
But Dr Davis said scientists should devote more attention to understanding and tackling the threats monarchs might face as they migrate south – or even other butterfly species, including many are more endangered than the popular monarch.
With few easy answers, the debate over the threats monarchs face seems destined to continue. “It’s been a tough problem,” Dr. Ries said. “Everyone in this community cares about monarchs, and we’re all trying to figure out what’s going on.”