Can monarchs adapt to a rapidly changing world?

Every fall, monarch butterflies escape deadly winter temperatures in the northern United States and southern Canada by flying south. Most head for the mountains of central Mexico, where they spend the season in the same fir groves visited by their grandparents and great-grandparents. As temperatures warm in the spring, new generations of monarchs return north to their breeding grounds, dispersing across an area many times the size of their winter quarters.

These multi-thousand-mile journeys are essential to the abundance of these iconic North American insects: Migration allows monarchs to breed in places too cold for them in winter, giving them more space to reproduce and, consequently, more opportunities to sustain their numbers. But as the climate becomes more extreme and less predictable, monarchs may have to change their strategy.

Pinpointing the influences of climate change on a species is notoriously difficult, especially when the species is as mobile and wide-ranging as the monarch. But scientists who study the effects of climate change on butterflies are informed by decades of data collected by volunteers across the continent.

“There’s no researcher or even team of researchers that could go out and collect those kinds of data,” says ecologist and EEB director Elise Zipkin, an associate professor at Michigan State University. “These volunteers are the only reason that we’re able to understand what’s going on at all with butterflies.”

In a 2021 study, Zipkin and several colleagues analyzed more than 18,000 volunteer surveys of monarch butterflies conducted between 2004 and 2018. During those years, they found, weather conditions in the monarch’s springtime range were nearly five times more important than other factors combined in determining the size of the monarch’s summer breeding population. More extreme temperatures and precipitation in the spring meant smaller monarch populations in the summer.

In a subsequent study, Zipkin and postdoctoral student and EEB alumna Erin Zylstra, now an ecologist with the Tucson Audubon Society in Arizona, used climate models to predict which counties in the midwestern U.S. and southern Canada are likely to offer the most tolerable breeding-season climate conditions to monarchs during the next 80 years.

Adult monarchs depend on milkweed plants to feed their larvae and on nectar-producing flowers to fuel their long migrations. Zipkin says individuals can help buffer monarchs from climate change by growing more of these types of native plants, and that larger-scale restoration efforts could be especially important in places like the potential climate refuges she and Zylstra identified. “Planting milkweeds in your garden is great,” she says. “What we also want to think about is restoring native prairies, because that can benefit not only monarchs but many different species.”

Milkweed thrives in disturbed areas, and it once grew abundantly on roadsides and in farm fields throughout the U.S. Midwest. In 1996, however, Monsanto began to introduce genetically modified “Roundup Ready” soybeans, corn, and other crops resistant to the herbicide glyphosate. Many farms began growing the strains and using glyphosate for weed control, killing benign natives such as milkweed in the process.

Karen Oberhauser, who recently retired as director of the University of Wisconsin-Madison Arboretum, worked with Iowa State University ecologist John Pleasants to analyze the consequences of the glyphosate-resistant crop revolution for monarchs. They estimated that between 1999 and 2010, milkweed availability in the Midwest declined by 58 percent, and production of monarchs in this region plummeted a shocking 81 percent. The smaller monarch population that migrates along the Pacific Coast declined more than 90 percent over a similar period, likely due to the effects of pesticides and development on milkweed availability.

Since the mid-2000s, glyphosate-driven milkweed loss has leveled off, but milkweed remains dangerously scarce in the Midwest and elsewhere. In a study published in 2017, Pleasants estimated that to reproduce at rates that will stave off extinction, North American monarchs require at least a doubling in the amount of milkweed currently available in the Midwest. “Climate is now the most important driver of the monarch population, but the amount of available [milkweed] is still setting the ceiling,” says Oberhauser. “We can help monarchs by raising that ceiling.

”Monarchs can and do survive without migrating. Southern Florida hosts a year-round monarch population, and reports suggest that more monarchs are spending the winter in the Southeast, perhaps because of rising temperatures and the increasing availability of food throughout the year. Monarch butterfly populations in Australia, New Zealand, Europe, and elsewhere travel much shorter distances than their North American ancestors. “We have plopped monarchs down in places all over the globe, and they've done just fine,” says Oberhauser.

But if climate change renders large parts of the monarch’s North American range uninhabitable, forcing the continent’s populations to significantly shorten their migrations or even stay put year-round, they will have far less room to reproduce. Such a radical reduction of their habitat could further shrink their already diminished numbers.

(Anyone can help monarch butterflies. All you need is a yard.)

Oberhauser and other scientists emphasize that despite decades of research and volunteer dedication, monarch butterflies remain full of mysteries, and their responses to climate change are only partly understood. André Green, an assistant professor of ecology and evolutionary biology at the University of Michigan and a National Geographic Explorer, has been working with colleagues to track individual monarchs as they travel south to Mexico, hoping to identify the genes and environmental cues that guide them through their life cycle—and could help them survive in a disrupted climate.

Researchers know that monarchs navigate with two compasses: one that uses the sun, and another that uses the Earth’s magnetic field. While this elaborate system keeps monarchs headed in the right direction, it doesn’t fully explain their ability to home in on the same circumscribed wintering grounds year after year. Neither does it explain how monarchs suddenly stop migrating once they reach their destinations. And that kind of behavioral flexibility interests Green as much as the butterflies’ famous perseverance.

“There are some species that are more able to adapt to rapid changes, and maybe the open question is, how adaptable are monarchs?” Green says. “It seems like they’re a lot more adaptable than we give them credit for.”