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Every autumn, herds of migratory monarchs descend into the mountains of central Mexico. Butterflies are the last stop on a journey of thousands of miles on their way from their North American breeding grounds to their winter colonies where they cover fir trees. Visitors who time their travels right may find the air so dense with kings that they can hear butterflies flapping their wings.

“This is the stuff on the to-do list,” said ecologist Andrew Davis of the University of Georgia in Athens. “Imagine that you are surrounded by gillions of butterflies. It’s a magical experience.”

It also may not take long. In recent years, the number of monarchs wintering in Mexico and Southern California has fellraises concerns that the species may be at serious risk of extinction.

But a new study Focusing on the size of the summer breeding population can complicate this picture. The scientists found that while summer monarch abundance decreased in some areas, including parts of the Midwest, it increased in others. Overall, over the entire North American range, abundance remained stable or even slightly increased between 1993 and 2018, according to the article published Friday in Global Change Biology.

So far, the researchers show that monarch populations have been able to recover sufficiently during the summer breeding season to compensate for the winter declines.

Author of the new article, Dr. “So it’s not really a production issue,” Davis said. “We have no less rulers. We have fewer monarchs reaching their wintering colonies.”

But not all scholars agree with this interpretation of the findings, which seems to fuel an ongoing debate about the threats facing monarchs and the causes of well-documented winter declines.

“There are some areas where monarch numbers have increased slightly, but they are not the heart of the breeding range,” said Karen Oberhauser, a conservation biologist at the University of Wisconsin-Madison who was not involved in the new research. .

He noted that the new study documents a decline in monarch abundance in the Midwestern “Corn Belt,” an agricultural region that constitutes a critical monarch breeding ground. Dr. “The numbers mostly fall in the Corn Belt,” Oberhauser said. “Most of the monarchs are there.”

North America is home to two monarch societies. The larger Eastern population spends their summers breeding in milkweed-filled fields along the East Coast and Midwest before landing in Mexico each fall. A smaller, Western population usually converges in California each fall.

Numerous studies have found that the sizes of these winter colonies have declined rapidly 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 decreases were attributed to several factors. including climate change and logging near wintering grounds.

The losses also coincided with the increased use of an herbicide known as glyphosate in large areas of the butterfly’s Midwestern breeding range. The herbicide kills milkweed, which often occurs in corn and soybean fields and plays a critical role in the monarch life cycle; Adult butterflies lay their eggs on plants that are the caterpillars’ sole food source. Fertilizer use increased sharply in the 1990s and early 2000s.

Winter drops trigger campaigns encouraging the public to plant milkweed, as well as concerns about the future of the monarch. In December 2020, the U.S. Fish and Wildlife Service concluded: butterfly qualifies for federal protection however, other threatened and endangered species were a higher priority for the agency.

However, it has been more difficult to determine general population trends over the monarch’s entire range and annual migration cycle.

The new study, an entomologist at the University of Delaware, Dr. It was directed by Davis and Michael Crossley. They and their colleagues drew on data collected as part of the annual summer butterfly count, in which the North American Butterfly Association calculates the number of butterflies that volunteers observe in certain areas.

The scientists analyzed more than 135,000 observations of monarchs at 403 sites and limited their analysis to locations with data spanning at least a 10-year period from 1993 to 2018. They adjusted the numbers to take into account how many hours volunteers spent on duty, and then modeled the relative abundance of monarchs over time.

Scientists found evidence of summer population declines in the Southwest, Northeast and Corn Belt. But in the Upper Midwest as well as in the Northwest and Southeast, monarch affluence has increased over time. At all sites together, relative abundance increased by about 1.36 percent per year, according to the scientists’ calculations.

Dr. “The monarchs are doing well in the summer,” Davis said.

So what explains why the winter monarchy colonies are shrinking? It’s not clear yet, said Dr. But one possibility is that the warming climate means fewer monarchs are making seasonal migrations, Davis said. It’s also possible that fewer monarchs survived their southward journey from the increase in car crashes. increased infection rates With a parasite that reduces migration success.

Dr. “There are a number of natural and anthropogenic threats to autumn migration that may increase over time,” Davis said.

Leslie Ries, an ecologist at Georgetown University, praised the paper’s extensive scan. “The geographic scope of their analysis is greater and more comprehensive than anything I’ve seen,” he said.

But he said it’s not clear whether population increases in parts of the country can completely offset declines in key breeding areas like the Midwest.

Dr. Oberhauser also noted that previous research has shown that during the second part of the study period, the monarch population in Mexico essentially stabilized at a new, lower level. So, on sites with only data for the past decade, the researchers may have overlooked the population decline, he said.

Dr. “I think it would be a mistake to conclude that it doesn’t matter what we do in the breeding ground,” Oberhauser said. “It is important to protect habitat at all stages of the annual cycle. We know that every stage must work in order for monarch migration to survive.”

opposing forces

Dr. Davis and colleagues also used federal data to estimate how much glyphosate was used in the area around each research site. They found that in some areas, particularly parts of the Midwest, glyphosate use was associated with a decline in abundance.

But they also documented a stabilizing force: climate change. In the northern part of the United States, rising temperatures have been associated with increases in monarch abundance. Dr. This effect is particularly pronounced in the Midwest, suggesting that warming climate may have partially offset the effects of glyphosate in that region, Davis said.

However, in parts of the country that typically have warmer climates, increased temperatures have been associated with decreases in monarch abundance, consistent with previous research.

Dr. “In places that are normally cold, higher temperatures are good for monarchs, but higher temperatures are bad in places that are generally warm,” Oberhauser said.

In recent years, as glyphosate use has flattened, weather fluctuations in spring and summer He and colleagues found that in a 2021 study, it was the largest predictor of summer population size. While modest increases in temperatures in the northern parts of the monarch belt are good for monarchs right now, continued climate change and more extreme weather could pose problems in the future, he said.

The findings do not suggest that the monarchs faced no threats, the researchers say.

Dr. “The heads of these overwintering colonies are in distress for a number of reasons,” Glassberg said. If winter populations shrink enough, the monarch population won’t be able to recover during the summer months, the researchers said.

However, Dr. Davis said scientists need to pay more attention to understanding and addressing the threats monarchs may face during their southward migration—even other butterfly species that are more threatened than most popular monarchs.

With a few easy answers, the debate about the threats facing monarchs looks set to continue. Dr. “This has been a difficult subject,” Ries said. “Everyone in this community cares about monarchs, and we’re all trying to understand what’s going on.”