Every fall, a flock of migrating monarchs descends into the mountains of central Mexico. This is the final stop of a thousands of miles of travel, as butterflies move from breeding grounds in North America to winter colonies, where they cover fir trees. Visitors who time their journey correctly may hear the monarch’s air very dense and the wings of the butterflies beating.
“It’s from the bucketlist,” said Andrew Davis, an ecologist at the University of Georgia at Athens. “Imagine being surrounded by hundreds of millions of butterflies. It’s a magical experience.”
It’s also something that may not last long.The number of monarchs who have spent the winter in Mexico and overwintered in Southern California in recent decades It plummetedRaises concerns that the species may be at serious risk of extinction.
However New research, The situation can be complicated by the focus on the size of the breeding population in the summer. Scientists have found that the number of summer monarchs is declining in some areas, including parts of the Midwest, but increasing in others. Overall, abundance was stable or slightly increased between 1993 and 2018 across North America, according to a paper published Friday at Global Change Biology.
Researchers say the findings suggest that the monarch population has so far recovered enough to compensate for the winter decline during the summer breeder season.
“So it’s not really a production issue,” said Dr. Davis, the author of the new paper. “The number of monarchs is not small. Fewer monarchs reach the winter colonies.”
However, not all scientists agree with that interpretation of the findings. This is likely to fuel an ongoing debate about the threats the monarch faces and the causes of the well-documented winter decline.
“In some areas, the number of monarchs has increased a bit, but they are not the center of breeding range,” said Karen Oberhauser, a conservation biologist at the University of Wisconsin-Madison who was not involved in the new study. increase. ..
She said the new study recorded a decline in the number of monarchs in the Midwestern “Cornbelt,” an agricultural area that constitutes an important breeding ground for monarchs. “The numbers are declining mainly in the corn belt,” said Dr. Oberhauser. “Most of the monarchs are there.”
There are two monarchs living in North America. More eastern populations breed in summer in dairy-filled fields along the East Coast and Midwest, and then fly to Mexico each fall. The small western population is generally concentrated in California each fall.
Many studies have shown that the size of these winter colonies has declined sharply since the mid-1990s. “They are just their shadows,” said Jeffrey Glasberg, president of the North American Butterfly Society and author of the paper.
These declines are due to a variety of factors. Includes climate change and logging near wintering areas.
The loss is also consistent with increasing use of a herbicide known as glyphosate over a wide range of midwestern breeding of butterflies. Herbicides kill milkweed. Milkweed is common in corn and soybean fields and plays an important role in Monarch’s life cycle. Adult butterflies lay eggs on plants. Plants serve as the sole food source for caterpillars. The use of herbicides increased sharply from the 1990s to the early 2000s.
The decline of winter prompted the campaign Encourage the public to plant milkweed, And concerns about the future of the monarch. In December 2020, the US Fish and Wildlife Service concluded: Butterflies with federal protection qualifications However, other endangered and endangered species were a higher priority for agencies.
However, it was more difficult to determine the overall population trends over the entire range of monarchs and the annual migration cycle.
The new study was led by Dr. Davis and Michael Crosley, entomologists at the University of Delaware. They and their colleagues leveraged the data collected as part of the North American Butterfly Association’s annual summer butterfly number. There, the number of butterflies observed by volunteers in the defined area is aggregated.
Scientists analyzed the observations of more than 135,000 monarchs on 403 sites and limited the analysis to locations with data for at least 10 years from 1993 to 2018. They adjusted the aggregate to take into account the amount of time the volunteers spent on the task. Next, we modeled the relative abundance of monarchs over time.
Scientists have found evidence of summer depopulation in the southwest, northeast, and corn belts. But in the Upper Midwest, northwest, and southeast, the number of monarchs seemed to increase over time. Scientists have calculated that the relative abundance of all sites combined increased by about 1.36 percent each year.
“The summer monarch is fine,” said Dr. Davis.
So what explains why the winter monarchy’s colony shrank? According to Dr. Davis, it’s not clear yet, but warming climate may mean fewer monarchs migrating seasonally.From the increase in car strikes Increased infection rate With parasites that reduce the success of migration.
“Autumn migration has many natural and human threats that can increase over time,” said Dr. Davis.
Leslie Ries, an ecologist at Georgetown University, praised the widespread spread of this paper. “The geographic extent of their analysis is larger and more comprehensive than I’ve seen,” she said.
However, she said it is not clear whether population growth in some parts of the country can completely offset the decline in important breeding grounds such as the Midwest.
Dr. Oberhauser also said earlier studies showed that the population of Mexican monarchs was new, lower and inherently stable later in the study period. Therefore, on sites that had only data for the last decade, researchers may have missed a declining population, she said.
“I think it’s a mistake to conclude that it doesn’t matter what we do in the breeding grounds,” said Dr. Overhauser. “It is important to protect habitat at all stages of the annual cycle. We know that all phases need to function in order for Monarch’s migration to survive.”
Enemy troops
Dr. Davis and his colleagues also used federal data to estimate the amount of glyphosate used in the area around each research site. They found that glyphosate use was associated with reduced doses in some areas, especially in the Midwest.
But they also recorded a countervailing force, climate change. In the northern part of the United States, rising temperatures correlated with increased monarchy abundance. This effect is particularly pronounced in the Midwest, suggesting that the warming climate may have partially offset the effects of glyphosate in the region, Dr. Davis said.
However, in some countries, which usually have warm climates, rising temperatures correlate with lower abundance of monarchs, consistent with previous studies.
“Usually the warm temperature in cold places is good for the monarch, but usually the warm temperature in warm places is bad,” said Dr. Overhauser.
Due to the recent flattening of glyphosate use, Fluctuations in spring and summer weather She and her colleagues were the largest predictors of summer population size in the 2021 study. Moderate temperatures in the northern part of the Monarch Mountains may be good for Monarch now, but continuous climate change and more extreme weather conditions could cause problems in the future, she said.
Researchers say the findings do not suggest that the monarch is not facing a threat.
“These wintering colonies have problems for a variety of reasons,” said Dr. Glasberg. Researchers said that if winter populations are low enough, monarch populations may not recover in summer.
But Dr. Davis understands and deals with the threats that scientists may face on their journey south, or even other butterfly species that are more threatened than popular monarchs. He said more attention should be paid to that.
There are few simple answers, but the debate about the threats the monarch faces seems destined to continue. “This was a difficult problem,” said Dr. Reese. “Everyone in this community cares about the monarch and we are all trying to understand what is happening.”