Conservation groups have lobbied the federal government to protect monarch butterflies for over a decade. Advocates say declining populations and habitat loss are driving the species toward extinction, and a federal listing under the Endangered Species Act is one of the best ways to safeguard them. Now, a paper by a group of researchers from the University of Georgia casts doubt on that theory. What the researchers found after analyzing citizen science data is that rather than blinking out, monarchs are struggling with a very specific part of their life cycle: their southern migration. These findings could influence how the US Fish and Wildlife Service, the federal agency that manages threatened and endangered species, protects the species going forward. The agency is expected to make a listing decision by December 4.

Researchers used to count monarch populations in Mexico to assess overall population trends. From these numbers, they determined that the butterflies are in steep decline. However, more recent studies show that monarchs are hatching at about the same or only slightly lower rates in their northern breeding grounds. That realization led Andy Davis, a research scientist at the University of Georgia and lead author of the study, to hypothesize that the issue with the declining numbers in Mexico may be that the monarchs are not surviving the migration south.

“There’s been a lot of debate over whether the monarch butterfly population has been declining. That’s because the numbers that we see in the Mexico overwintering colonies have indeed been going down. That much is true, and it’s unquestionable,” Davis said. “However, at the same time, the number of monarchs you see during the summer in the US and Canada hasn’t really changed that much. And so if the numbers at the start of the journey haven't really changed, but the numbers at the finish line have changed, then a lot of scientists, including myself, have speculated that perhaps the real problem is that the monarchs are failing to reach the wintering destinations.”

Traditionally, monarchs lay eggs in southern Canada and the northern United States. When it gets cold, they fly thousands of miles south to Mexico. During the journey, the monarchs will settle into a tree to rest at night, making up a roost. Anywhere from a few dozen to a few thousand butterflies congregate together in these roosts. Once they finally arrive in Mexico, they overwinter for several months until they begin another migration to their breeding grounds back north. 

Researchers analyzed about 2,600 observations of monarch roosts reported to Journey North, an organization that crowd-tracks wildlife migrations, from 2006 to 2023.  But in recent years, these enthusiasts have logged smaller and smaller roosts along the monarch’s typical migration route. This dataset is valuable, according to Bill Snyder, another one of the study’s authors and an agroecology professor at the University of Georgia, since it would be difficult for scientists to collect the data over such a large spatial scale, which ranges the continent, and time scale, which ranges the past couple of decades.

Davis, Snyder, and Jordan Croy, a postdoctoral researcher at the University of Georgia, found that roost sizes have declined by as much as 80 percent at the southernmost part of the butterfly’s migration route, with roost sizes progressively declining the farther south you go. In other words, 20 years ago, monarch roosts in Texas might number in the thousands. Today, it’s in the hundreds, Davis said. 

In addition, the researchers attempted to rule out other possible explanations. For example, they found that the monarchs haven’t changed the timing of their migration. They also showed that climate change probably isn’t directly responsible for the issue since the migration route has become warmer and greener, which is usually associated with larger roost sizes. However, this does not rule out indirect climate effects since global warming can cause non-native milkweeds to flourish and produce more extreme weather.

Jaap de Roode, a biology professor at Emory University who researches monarchs and parasites and is unaffiliated with Davis’s study, said there’s always the possibility that monarchs might have started roosting in different locations, although the study did a nice job addressing this potentially confounding variable. “We see the monarch roosts that are reported. But we don’t know how many others there are,” de Roode said. “There’s always the question, ‘Have the monarchs changed their routes of migration?’ and maybe we find them in different places.” That’s one of the unanswered questions here that we cannot completely disprove based on the data we have.”

The study doesn’t answer why the monarchs are struggling to migrate. However, it does offer a few hypotheses. The first is that there is an increasing number of diseases caused by parasites, which hinder monarchs’ ability to migrate. This is linked to two other possible explanations. Increasing levels of non-native milkweeds along the migration route may be causing increased parasite levels. Another reason is that large numbers of captive-reared monarchs, where people capture caterpillars, raise them until they become butterflies, and then release them, may hinder their ability to migrate long distances by making them weaker and more susceptible to parasites. 

Davis said that these well-intentioned people may be getting misguided advice. Nurseries that promote Save the Monarchs campaigns sell non-native milkweeds to unassuming customers. Internet chatrooms and Facebook groups encourage people to capture and rear monarchs to help preserve the species, but the monarchs they release back into the wild are weaker. Businesses raise monarchs in greenhouses to then release at weddings and funerals. Monarch enthusiasts who want to plant the right kind of milkweed might turn to Monarch Watch, Monarch Joint Venture, or the Xerces Society to find plants native to their area, de Roode said.

This maladaptive behavior, where people try to help the monarchs but actually end up harming them, is one of the reasons Davis strongly believes that the monarchs shouldn’t be listed as an endangered species. “I think these decisions to list the monarchs as endangered aren’t really based on much evidence at all. They’re based on emotions,” he said. “Here’s the problem with listing them: Everybody freaks out, and then they increase their efforts to ‘save the monarchs,’ which are actually causing more problems. . . . When people hear that monarchs are in trouble, it leads them to do bad things for the monarchs. What people really need to do is leave the monarchs alone. That would help them a lot.”

Some of the researchers believe an Endangered Species Act designation wouldn’t do much to help the monarchs. It wouldn’t prevent people from planting non-native milkweed, for example. “The emphasis should be shifted (to the migration), which I think the ESA designation kind of misses,” Jordan Croy, the postdoctoral researcher, said. 

This is in contrast to what some conservation groups have advocated. For example, the Center for Biological Diversity has lobbied for monarchs to be listed under the Endangered Species Act. And in May this year, before the publication of Davis’s study, 22 conservation groups wrote a letter to Congress asking for $100 million toward monarch conservation, citing the overwintering counts in Mexico. They called attention to the depletion of native milkweed due to pesticides and forest reduction from wildfires in Mexico, worsened by climate change. 

Still, Davis is skeptical that USFWS will list the monarchs as endangered. An endangered species designation could mean significant regulatory and legal repercussions. It would mean that the agency would have to protect the monarch’s habitat, which is “basically everywhere—roadside ditches, power line cuts, farmer’s fields, backyards … you basically have to protect the entire eastern North American seaboard as critical habitat for this endangered butterfly,” Davis said. “It’s sort of an endless list of craziness that will occur because no one will really know what’s protected and what’s not. It would be logistically and legally a minefield.”

Davis said that what might be more helpful is some sort of rule that prevents people from touching monarchs. De Roode said that USFWS should focus on monarch habitat protection, which would benefit other species as well. “By protecting the habitat, re-creating pollinator gardens, re-creating nectar sources,” de Roode said, “we help monarchs but also help a lot of other native species that need our help as well.”