WASHINGTON — America’s butterflies are disappearing because of insecticides, climate change and habitat loss, with the number of the winged beauties down 22% since 2000, a new study finds.
The first countrywide systematic analysis of butterfly abundance found that the number of butterflies in the Lower 48 states has been falling on average 1.3% a year since the turn of the century, with 114 species showing significant declines and only nine increasing, according to a study in Thursday’s journal Science.
“Butterflies have been declining the last 20 years,” said study co-author Nick Haddad, an entomologist at Michigan State University. “And we don’t see any sign that that’s going to end.”
A team of scientists combined 76,957 surveys from 35 monitoring programs and blended them for an apples-to-apples comparison and ended up counting 12.6 million butterflies over the decades. Last month an annual survey that looked just at monarch butterflies, which federal officials plan to put on the threatened species list, counted a nearly all-time low of fewer than 10,000, down from 1.2 million in 1997.
Many of the species in decline fell by 40% or more.
David Wagner, a University of Connecticut entomologist who wasn’t part of the study, praised its scope. And he said while the annual rate of decline may not sound significant, it is “catastrophic and saddening” when compounded over time.
“In just 30 or 40 years we are talking about losing half the butterflies (and other insect life) over a continent!” Wagner said in an email. “The tree of life is being denuded at unprecedented rates.”
The United States has 650 butterfly species, but 96 species were so sparse they didn’t show up in the data and another 212 species weren’t found in sufficient number to calculate trends, said study lead author Collin Edwards, an ecologist and data scientist at the Washington Department of Fish and Wildlife.
“I’m probably most worried about the species that couldn’t even be included in the analyses” because they were so rare, said University of Wisconsin-Madison entomologist Karen Oberhauser, who wasn’t part of the research.
Haddad, who specializes in rare butterflies, said in recent years he has seen just two endangered St. Francis Satyr butterflies — which only live on a bomb range at Fort Bragg in North Carolina — “so it could be extinct.”
Some well-known species had large drops. The red admiral, which is so calm it lands on people, is down 44% and the American lady butterfly, with two large eyespots on its back wings, decreased by 58%, Edwards said.
Even the invasive white cabbage butterfly, “a species that is well adapted to invade the world,” according to Haddad, fell by 50%.
“How can that be?” Haddad wondered.
Cornell University butterfly expert Anurag Agrawal said he worries most about the future of a different species: Humans.
“The loss of butterflies, parrots and porpoises is undoubtedly a bad sign for us, the ecosystems we need and the nature we enjoy,” Agrawal, who wasn’t part of the study, said in an email. “They are telling us that our continent’s health is not doing so well … Butterflies are an ambassador for nature’s beauty, fragility and the interdependence of species. They have something to teach us.”
Oberhauser said butterflies connect people with nature and that “calms us down, makes us healthier and happier and promotes learning.”
What’s happening to butterflies in the United States is probably happening to other, less-studied insects across the continent and world, Wagner said. He said not only is this the most comprehensive butterfly study, but the most data-rich for any insect.
Butterflies are also pollinators, though not as prominent as bees, and are a major source of pollination of the Texas cotton crop, Haddad said.
The biggest decrease in butterflies was in the Southwest — Arizona, New Mexico, Texas and Oklahoma — where the number of butterflies dropped by more than half in the 20 years.
“It looks like the butterflies that are in dry and warm areas are doing particularly poorly,” Edwards said. “And that kind of captures a lot of the Southwest.”
Edwards said when they looked at butterfly species that lived both in the hotter South and cooler North, the ones that did better were in the cooler areas.
Climate change, habitat loss and insecticides tend to work together to weaken butterfly populations, Edwards and Haddad said. Of the three, it seems that insecticides are the biggest cause, based on previous research from the U.S. Midwest, Haddad said.
“It makes sense because insecticide use has changed in dramatic ways in the time since our study started,” Haddad said.
Habitats can be restored and so can butterflies, so there’s hope, Haddad said.
“You can make changes in your backyard and in your neighborhood and in your state,” Haddad said. “That could really improve the situation for a lot of species.”
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