US butterfly populations drop by 22% in two decades, study says

Butterfly populations in the US shrank by more than a fifth within the space of two decades, according to a new study.

Numbers fell by 22% between 2000 and 2020, according to research by Binghamton University in New York.

A third of species saw serious decline, with some, like Julia’s Skipper, losing more than 90% of their populations.

However, the researchers say butterflies may be able to recover if urgent conservation measures are taken.

The study published in the journal Science measured butterfly “abundance” – the number of individuals of a species within a specific area. It analysed 12.6 million butterfly sightings from 76,000 surveys across 35 monitoring programmes.

This included data from citizen science programmes like the North American Butterfly Association’s Fourth of July counts.

Using statistical models, they estimated population trends for 342 species.

The results showed that 33% were in significant decline, with many exhibiting extreme losses – 107 species declined by more than 50%.

“While the results aligned with global trends, seeing the extent of the decline at such a large spatial scale was sobering,” said Prof Eliza Grames, an assistant professor of biological sciences at Binghamton University.

Some of the most affected species include the Florida white, Hermes copper, tailed orange, Mitchell’s satyr, and West Virginia white, all of which have declined in abundance by more than 98% within the US.

The West Coast lady, once a common backyard butterfly, has declined by 80%, raising alarm as even this highly adaptable species struggled.

“That’s alarming because it suggests even common butterflies aren’t safe,” Prof Grames said.

Habitat loss, pesticides, and climate change are key causes of this decline, according to the researchers.

Butterflies are crucial pollinators, supporting plants and crops. Experts say their decline could disrupt food production and entire ecosystems.

They also serve as indicators of environmental health – when butterfly numbers fall, it signals trouble for other species.

Species are declining most severely in the US Southwest, one of the hottest and driest regions, researchers say. They believe drought may be a major contributor to these losses.

“Drought is a double threat – it harms butterflies directly and also affects their food and host plants,” Prof Grames explained.

The results could help drive important conservation efforts, such as prioritising species for the International Union for Conservation of Nature (IUCN) Red List of Threatened Species and Endangered Species Act protection.

Despite the decline, there is hope for recovery.

“Butterflies can recover quickly because they have short generation times. Small actions like planting wildflowers, reducing pesticide use, or even leaving part of a backyard unmowed can significantly improve their chances,” Prof Grames said.

She also stressed the need for government action.

“Insects are fundamental to life on earth, and we need conservation actions and policies that support insects.”