450 butterfly species rapidly declining due to warmer autumns in the western U.S.

Butterflies are not only ephemerally beautiful, they’re crucial pollinators for a variety of important food crops and flowers. And in the western U.S., they’re disappearing—fast.

Over the past four decades, more than 450 butterfly species have declined at an average rate of nearly 2 percent a year, according to a study published today in the journal Science. 

It’s already known that the western monarch has plummeted in population by 99.9 percent and was recently denied protection by the U.S. Endangered Species Act. But the study revealed lesser-known species, like the Boisduval’s blue and California’s state insect, the California dogface butterfly, are heading toward extinction.

“The declines across species are so ubiquitous,” says study leader Matthew Forister, a biology professor at the University of Nevada, Reno. “They’re all suffering.”

The scientists focused on what is likely butterflies’ biggest danger: climate change.

Analyzing both butterfly observations and climate data in 70 locations across the West from 1972 to 2018 revealed a big surprise: Warmer autumns in particular were the clearest culprit behind the drop in butterflies, Forister says. (Read why insects are plummeting in population worldwide, and why it matters.)

More than 200 cities across the U.S. are experiencing warmer fall seasons, with the biggest autumn temperature increases in the Southwest. In Arizona, for instance, fall temperatures have risen by 0.2 degree Fahrenheit every decade since 1895. That may be why the west coast lady, a vibrant orange-and-black butterfly, has declined at a rate of 3 percent a year in the state.

 “We’ve been really focused on the [warming of] spring for a couple of decades now,” Forister says, but “warming at the end of the season is a really negative impact.”

Falling numbers, warmer falls

To find out where butterflies are in free fall, researchers pored over four decades’ worth of academic and community science data in 70 locations, from Seattle to Santa Fe to Tucson. The data drew mostly from observational sightings of butterflies.

The team relied on three datasets: one academic; one from the crowdsourcing data site iNaturalist, a joint project of the California Academy of Arts and Sciences and the National Geographic Society; and one from the North American Butterfly Association.

The study locations, a mix of urban and wild, revealed butterflies were vanishing even in pristine areas. California’s Castle Peak, one of the more remote butterfly watching spots, is one of the homes of the anise swallowtail, which has dwindled significantly in number—though not as dramatically as other species.

That could be because “a lot of that damage is already done,” Forister says. “Fertile river valleys and riparian areas where people like to build farms and cities—they're already gone.” (Read why we haven’t seen a quarter of known bee species since the 1990s.)

As for why warming falls are so detrimental, it may be connected to butterflies’ hibernation-like diapause in the fall. Warmer temperatures could be forcing the insects, most of which live around a year, to stay awake longer and starve.

In other words, they’re “getting old and crunchy and dying sooner,” says study co-author Katy Prudic, an entomologist and assistant professor of citizen and data science at the University of Arizona School of Natural Resources and the Environment.

An elegant call to action

Conservation biologist Scott Hoffman Black, executive director of the Xerces Society for Invertebrate Conservation, called the study “elegant” in its combination of academic and community data.

“It adds to our growing understanding of what’s going on with insects and insect decline across the globe,” says Black, who wasn’t involved in the study. “The overall conclusion that climate is a major driver when it comes to butterfly decline is the important takeaway from this paper.”

There’s also a lot individual citizens can do to protect butterflies and other insects at home, such as planting native vegetation and avoiding pesticides. “It doesn’t matter whether you have a little tiny yard or you manage a national park,” he says. (Learn more ways you can help pollinators at home.)  

And, of course, people can be curious about the world around them and record what they see, contributing to citizen science websites that have become more popular during the pandemic.

“Without all those people who are interested in taking pictures and watching butterflies and spending time in nature,” Prudic says, “this study wouldn’t have been able to happen.”