Butterflies are disappearing in the western United States, but not as scientists imagined

People on Earth are in the midst of insect revelations, and thousands of species are dwindling over the past few decades. Scientists often blame it on habitat loss or pesticide use. But a new study of butterflies in the western United States has found that warming weather in the fall can cause just as much damage.

The findings are a wake-up call — not just butterflies, but all insects — and Jessica Ware, an entomologist at the American Museum of Natural History, said: "If humans don't take dramatic steps to curb global warming, "entire ecosystems could disappear, with incalculable impacts on biodiversity and human health."

Scientists already know that some butterflies are in trouble. Recent studies have shown that the population of the dominant species, the monarch butterfly, is declining dramatically, while surveys of insects in general show a shrinking number. While most of the data from these studies comes from densely populated or intensively farmed areas, butterflies are also at risk in open places. Art Shapiro, an insect ecologist at the University of California, Davis, and colleagues have shown that over the past 35 years, butterflies have disappeared even in pristine protected areas such as the Sierra Nevada Mountains in the western United States. Insect ecologists at the University of Nevada at Reno, who collected data from the North American Butterfly Society, which has coordinated butterfly counts across the United States for more than 42 years, and the duo also included 15 years of data from the iNaturalist website, a portal that collects sightings of plants and animals, including butterflies. In total, the researchers tracked the fate of 450 species of butterflies from 70 locations in the western United States.

The team reported today in the journal Science that butterfly populations fell by an average of 1.6 percent per year between 1977 and 2018. In the dataset used, at least two species declined by 50 species, including the Edith Lattice Butterfly (Euphydryas editha), the Ochlodes agricola and the Large Copper Butterfly (Lycaena xanthoides). Forister said some species could disappear entirely from part of their habitat in the coming decades.

Decline does not seem to be related to human development or pesticide use, although such activities can still pose problems. Instead, insects appear to be disappearing in areas where autumn temperatures are significantly higher than summer temperatures, in the past few decades, such as in the southwestern United States. Forister speculates that warm weather can disrupt the butterfly's reproductive cycle or negatively affect the plants on which they depend.

"[The new study] handles the inherently chaotic data well," said Scott Hoffman Black, an ecologist and executive director of the Xerces Invertebrate Conservancy. Given that butterflies are key pollinators, this decline bodes well for larger problems for plants and even entire ecosystems, Forister added. "Climate effects will almost certainly affect many other insects, including bees."

And these effects will "undermine" efforts to protect and restore butterfly habitats, added Dave Goulson, an ecologist at the University of Sussex.

Combined with other studies, including a 2019 study that found that Ohio's butterflies were declining, the new work was "very disturbing," said Leslie Rees, an ecologist at Georgetown University. This is "further proof that these declines appear to be universal and universal".