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Know the West

Butterflies in the West are disappearing

The climate crisis has caused a steep decline in butterfly sightings in the Rocky Mountain range.


Populations of the Mourning Cloak butterfly have collapsed in recent years.

This story was originally published by the Guardian and is reproduced here as part of the Climate Desk collaboration. 

The varied and beautiful butterfly species that dot the West are being cut down by the climate crisis, new research has found, with rising temperatures helping cause a steep decline in butterfly numbers over the past 40 years.

There has been a 1.6% reduction in the total number of butterflies observed west of the Rocky Mountain range each year since 1977, researchers calculated, which amounts to a staggering loss of butterflies over the timespan of the study period.

“Certainly many butterfly species are becoming so rare it’s hard for some people to see what were once widespread, common species.”

“With the monarch it seems we are on the verge of losing the migration, if not the species itself.”

The declines are winnowing away much-loved species such as the monarch butterfly, which is known for is spectacular mass migrations to California each year but has lost 99% of its population compared with 40 years ago. “With the monarch it seems we are on the verge of losing the migration, if not the species itself,” Forister said.

The research, published in Science, analyzed citizen-gathered sightings of butterflies in 72 locations spanning all of the western U.S. states. In all, more than 450 butterfly species were included in the study.

Across all of these sightings, the researchers found an annual 1.6% drop in butterfly numbers in the West, which is consistent with the rate of decline of other insects found by researchers in different places around the world, fueling concerns of a deep crisis among the creatures that help supply much of our food, break down waste and form crucial foundations to the web of life.

While butterflies, like other insects, are being negatively affected by habitat loss and toxic pesticide use, the researchers accounted for these factors in their study and found that the heating of the planet, even without those other pressures, is causing the steady decline of butterflies.

This could be because plants are drying up more rapidly at the end of summer, meaning nectar resources are more scarce for butterflies, or that warming winters are interfering with the stasis-like state butterflies enter during colder months, meaning they are in worse condition when spring arrives.

“We have a lot of wide open land in the West and people often struggle to understand that a few degrees in temperature can make a big difference, but they can,” Forister said. “We are seeing these climate change impacts even in nice, natural areas and my feeling is that areas damaged by agriculture or urbanization are already lost to the butterflies.”

Forister said while temperatures will continue to rise, people can provide butterflies with some breathing room by conserving areas rife with wildflowers and cutting back on certain chemicals.


“The declines are extremely concerning ecologically, said Dara Satterfield, a butterfly researcher at the Smithsonian Conservation Biology Institute, who was not involved in the study. “We know butterflies and moths act as pollinators, decomposers, nutrient-transport vessels, and food sources for birds and other wildlife.

“This study is consistent with other large datasets from around the world, showing us that recent decades have presented new hurdles to survival for numerous butterfly species.”

Oliver Milman is an environment reporter for Guardian U.S. Email High Country News at [email protected] or submit a letter to the editor.