Climate Change Is Killing Butterflies in the American West

Butterflies play essential role in pollinating wild plants and crops. Which makes a new study published in Science on Thursday such bad news. It shows the climate crisis is posing an existential threat to their survival, particularly in the American West.

The researchers—who hail from Tennessee, Arizona, California, and Texas—combined three datasets on butterfly observations. The first, from a University of California professor, included 45 years of data from California. The second, from the North American Butterfly Association, contained 27 years of data compiled by experts and citizen scientists across America. And the third was from the the global iNaturalist web platform in which volunteers can log butterfly observations on an app.

The research focused on 450 species of butterfly populations from Washington down to California and stretching as far east as Montana and New Mexico. That area, the scientists write, is “particularly useful for understanding the effects of climate change on insects” because it’s been ground-zero for warming and drying trends. It also has a variety of ecosystems and elevations, and includes all kinds of land uses from cities to protected parks to farms.

Across that region, the team observed a precipitous decline of 1.6% in the number of butterflies every year over the past four decades. Previous research has documented the decline of butterfly populations as well, but scientists have had difficulty determining how great a factor the changing climate has been. It’s been hard to separate global warming’s role from that of other stressors, like deforestation as well as pollution from chemical pesticide use and extractive industry.

To begin to uncover the role of climate change, the authors of the new study overlaid their data with temperature records from across the 11 states they observed. They found that warmer summer months have actually resulted in increased butterfly populations, likely due to increased availability of nectar from plants and larval bugs that caterpillars eat.

Yet warming in the autumn—which has been more dramatic on average—led to population drops in butterfly populations, likely because many plants can’t withstand high temperatures outside of the summer, and because of increased populations of predators like spiders, fire ants, and wasps during fall.

Crucially, this was true across all western areas, not just ones near agriculture or urban development. The drop in butterfly populations on protected, open lands suggests that proximity to industry and pesticide use are not the sole causes of the overall decline. Instead, the climate crisis is likely at play.

“Out there, removed from those factors, we see a shifting climate as the main driver of declining butterfly numbers,” Matthew Forister, a biologist at the University of Texas in Reno who was the report’s lead author, wrote in an email.

The study suggests that increasing fall temperatures play a large part in butterflies’ decline, which in turn suggests that butterfly conservation plans can’t just focus on conservation of public lands and reducing pesticide use. They must also include efforts to curb global warming. Doing so isn’t just crucial for butterflies, it’s crucial for every species on Earth (including us).

“Protected natural areas are of course great, but they’re not enough,” Forister said.