Monarch butterflies at their wintering ground near Angangueo, Mexico
Monarch butterfly (Danaus plexippus) wintering in Oyamel pine forests (Abies religiosa) Monarch Butterfly Biosphere Reserve
Last month, the U.S. Fish and Wildlife Service (FWS) declared that one of North America’s best known butterflies, the monarch, might be in trouble. But the agency put off protecting the insect under the federal Endangered Species Act. In the meantime, researchers are continuing to debate how best to gauge the health of monarch populations.
In recent months dueling preprints and publications have intensified the debate. In one camp: researchers who have documented drastic declines in the number of monarchs in Mexico and other areas where some butterflies spend the winter. They believe the species needs immediate help, particularly by protecting and expanding the milkweed-filled meadows where its larvae feed. In another: scientists who have tallied butterfly numbers in areas they occupy during the warmer months and concluded there is less cause for alarm. As a species, monarchs “don’t really need saving,” says Andrew Davis, an ecologist at the University of Georgia, Athens.
Monarchs are found around the world, but North America is home to the two native migratory populations. A larger group, which accounts for an estimated 90% of all North American monarchs, occupies the eastern half of the continent. Once, hundreds of millions of these butterflies completed a 9600-kilometer migration loop that takes them to Mexico and back as far north as Canada each year. Monarchs in a second, smaller population in western North America travel only as far as Southern California to overwinter.
The insects face threats including habitat loss and pesticides, and there is broad consensus that the western population has suffered deep declines, with wintering numbers dropping as low as 2000 in 2020. But it is the condition of the larger eastern population that will determine whether monarchs get federal protection. And those numbers have proved difficult to pin down.
One obstacle is the monarch’s complex life cycle, which can involve lengthy migrations made relay race style by individuals from four different generations, each completing just one leg of the journey. As a result, population estimates can vary depending on “how and where you count the monarchs, and over what time period you look,” says ecologist Joshua Puzey of the College of William & Mary.
Some researchers focus on the number of monarchs that arrive each year at wintering grounds in a small region in the highlands of central Mexico. The butterflies congregate on fir trees in such dense clusters that researchers estimate numbers by measuring the area of trees they cover. That area has shrunk over the past 2 decades. During the 2019–20 season, it dropped to 2.83 hectares, half of the previous winter and down from 18 hectares in 1996–97. The U.S. Geological Survey (USGS) has estimated 6 hectares are required for monarchs to stay afloat in the long run. “Current monarch numbers are not sustainable,” says ecologist Karen Oberhauser of the University of Wisconsin, Madison.
But a different picture emerges from data collected north of Mexico during other parts of the year, Davis and others say. Each season, scores of scientists and thousands of volunteers tally butterflies as they fly past recording stations, and count monarch eggs and caterpillars they find on milkweed, the larvae’s only food source. When Davis and colleagues examined 20 of these data sets, covering time spans from 15 years to more than 100 years, they saw little evidence of drastic declines. Winter and spring populations had shrunk a little but summer and fall surveys showed few losses, they reported in October 2020 in a preprint posted on Preprints.org, which has not yet been peer reviewed.
Davis acknowledges his view that monarchs are not in danger of extinction is unpopular: “No one wants [it], ironically,” he says. “They want to keep saving them.”
And the preprint has drawn mixed reviews. Insect ecologist Anurag Agrawal of Cornell University calls it “important. … Some of these data have never been put together this way.” But Leslie Ries, an ecologist at Georgetown University, rejects its conclusions. “The picture painted … includes many faulty conclusions based on the studies or data sets cited,” she says.
Volunteer surveys, Ries says, “are invaluable,” but “there are some issues that need to be addressed.” For example, volunteers tend to count monarchs in places that are easy to reach and where they are likely to encounter butterflies, potentially skewing the numbers, notes ecologist Laura López-Hoffman of the University of Arizona. “If you don’t look for butterflies where they used to be … you could wrongly conclude that monarchs aren’t declining,” she says.
Agrawal points to another source of confusion. Based on volunteer tallies of monarch numbers across their life cycle, he has found that each of the four generations in a year can expand or decline independently, presenting conflicting trends. Like Davis, he does not think the species faces extinction, but he does believe the North American population is declining. That “does not bode well,” he says.
If the numbers are dropping, some researchers have argued for years that more milkweed could help. A number of studies have concluded that, since the 1990s, shifts in farming practices and the broad use of weed-killing chemicals have driven monarch declines by causing precipitous declines in the tall, weedy plant. By attaching tracking tags to butterflies, for example, the nonprofit Monarch Watch found that the size of the summer populations, which require milkweed to reproduce, determines the size of the overwintering populations. That suggests “increasing milkweed habitat, which has the potential of increasing the summer monarch population, is the conservation measure that will have the greatest impact,” insect ecologist and Monarch Watch founder Orley Taylor of the University of Kansas, Lawrence, and colleagues concluded in their August 2020 study, which appeared in Frontiers in Ecology and Evolution.
Wayne Thogmartin, a research ecologist with USGS’s Upper Midwest Environmental Sciences Center, agrees, noting unpublished data by researchers that evaluated 18,000 surveys from six monitoring programs over 25 years. They, too, found a strong correlation between late summer and winter populations.
But some doubt the milkweed limitation hypothesis. “While more milkweed won’t hurt the monarch … there is very compelling evidence … that milkweed is not necessarily limiting,” Puzey says. For example, if milkweed was in short supply, monarch caterpillars should be defoliating the few plants that are left, and that has not happened, Ries says. Indeed, sometimes volunteers have to look hard to find monarch larvae even in fields full of milkweed.
Ries believes other factors, such as weather and long-term climate change, play a bigger role in determining monarch numbers. She notes that some studies have found the number of butterflies that head north in the spring to breed “is the biggest factor in how big the populations grow each year.” So, a spring freeze or storm that kills many butterflies can have a yearlong ripple effect.
Given such uncertainties and the fact that conservation efforts are already underway, U.S. officials say the monarch is not yet a prime candidate for federal protection, especially because resources are limited and other species are in greater need of help. But they plan to decide on the monarch’s status in 2024, FWS ecologist Lori Nordstrom explained last month at a virtual press conference.
The delay worries Tyler Flockhart, an adjunct ecologist at the University of Maryland Center for Environmental Science’s Appalachian Laboratory who has been modeling monarch population dynamics. “We run the risk of studying this problem to death by not taking actions until we are completely certain.”