Butterflies are Disappearing in the Western US

Climate change plays a major part, study finds.

Monarch Butterfly
Monarch butterfly on a Mexican sunflower in California. Barbara Rich / Getty Images

Fewer butterflies are flitting across the Western United States, with rising temperatures playing a role in their dramatic decline over the past four decades, a new study finds.

There’s been a gradual but serious drop in recent decades in species population. Researchers have calculated a 1.6% decrease in the number of butterflies spotted each year since 1977, according to a new report released in the journal Science.

“To put that in concrete terms, if you imagine going to a nice meadow in the middle of summer two decades ago and seeing 1,000 individual butterflies (which wouldn't be that hard to do, if you think about lots of different species), you would now expect to see around 725 individual butterflies," study lead author Matt Forister, biology professor at the University of Nevada, Reno, tells Treehugger. "So that's a loss of a bit more than 1/4.”

The population declines include the iconic monarch butterfly which has been hovering on the brink of extinction.

“Monarch populations have declined by more than 70% in the eastern U.S. and by 99.9% in the western U.S.,” Sarina Jepsen, director of Endangered Species and Aquatic Programs at The Xerces Society, told Treehugger in December.

That was when the U.S. Fish and Wildlife Service announced that monarchs would not be protected under the Endangered Species Act at the time. The FWS determined that the beloved species was “warranted but precluded,” meaning that it qualifies for federal protection but other species have higher priority.

Analyzing Butterfly Data

For the new study, researchers analyzed data from 72 sites across the Western U.S.

The U.S. West offers a wide range of locations including cities and national parks, valleys and mountains, and coastal and inland areas. This allows researchers to observe climate impacts on all these lands.

The data was collected by experts and from citizen scientists. They studied information on more than 450 butterfly species.

“Citizen science data was central to our analyses. The heart of our paper is data from 4th of July butterfly counts organized by the North American Butterfly Association (NABA). Amateur butterfly enthusiasts go to hundreds of locations across the country during a day in the summer (kind of like the Christmas bird counts) and count all the butterflies they can find in a certain area,” Forister says. 

“It's excellent data, and finds similar patterns to an expert-collected dataset that we also have from a more narrow geographic area.”

Across all the locations studied, they found a 1.6% drop in insect populations, which is consistent with declines reported for other insect species throughout the world.

Insect numbers have long been reported to be in danger. For example, a scientific review of global insect populations published in 2019 in Biological Conservation found that more than 40% of the world’s insect populations are in decline and threatened with extinction.

Undeveloped Spaces

In earlier studies, researchers showed that land development and some agricultural practices like using certain pesticides can be harmful to butterflies, Forister points out.

But this current study found that even butterflies in open, untouched spaces have been affected.

"The fact that declines are observed across the undeveloped spaces of the western U.S. means that we cannot assume that insects are okay out there far from direct human influence," Forister said. "And that's because the influence of climate change is, of course, not geographically restricted."

Fighting climate change is the top priority, Forister says. But there are more immediate, attainable steps people can take to help butterflies.

“At more local scales, we need to think about better management of lands that we can control, and these include back yards, city parks and marginal spaces around agriculture,” he says.

“We can make all of those places better for butterflies and other beneficial insects if we use fewer pesticides and do a bit of ‘re-wildling’ which in this context means planting natives or even just letting native plants recolonize.”

View Article Sources
  1. Forister, M. L., et al. "Fewer Butterflies Seen by Community Scientists Across the Warming and Drying Landscapes of the American West." Science, vol. 371, no. 6533, 2021, pp. 1042-1045, doi:10.1126/science.abe5585

  2. "Questions and Answers: 12-Month Finding on a Petition to List the Monarch Butterfly." U.S. Fish & Wildlife Service.

  3. Wolterbeek, Mike. "Fewer Butterflies Seen Across the Warming, Drying Landscapes of American West." Nevada Today, 2021.

  4. Sánchez-Bayo, Francisco, and Kris A.G. Wyckhuys. "Worldwide Decline of the Entomofauna: A Review of its Drivers." Biological Conservation, vol. 232, 2019, pp. 8-27, doi:10.1016/j.biocon.2019.01.020