The population of butterflies - the beautiful insects that play a vital role in pollination and the health of ecosystems - has fallen in the United States by more than a fifth this century. According to research spanning hundreds of species from the red admiral butterfly to the American lady to the cabbage white.

Data from about 76,000 butterfly surveys conducted by various groups documenting millions of the insects representing 554 species showed that their numbers dropped by 22% from 2000 to 2020 in the contiguous United States, researchers said. The scientists attributed the decrease to factors including habitat loss, pesticide use and climate change.

Population declines were largest in the southwestern region spanning Arizona, New Mexico, Oklahoma and Texas.

Among the 342 butterfly species documented in the study that had sufficient data to analyze their numerical trends, 114 of them - about a third of the total - sustained losses, including 107 down by more than 50% and 22 off by more than 90%. The numbers for four butterflies - the Florida white, Hermes copper, tailed orange and Mitchell's satyr - plummeted by more than 99%. Nine species - about 3% of the total - showed gains.

Among some common species, the red admiral declined by 58%, the cabbage white slipped by 50% and the American lady dropped by 44%.

"The results of this study are pretty depressing. But butterflies have the capacity to recover if we can improve things for them," said ecologist Collin Edwards, formerly of Washington State University and now with the Washington state Department of Fish and Wildlife, lead author of the study published on Thursday in the journal Science.

"Butterflies have fast life cycles - at least one generation per year, often two or three. And each of those generations lays a ton of eggs. This means that if we make the world a more hospitable place for butterflies, butterfly species have the capacity to respond very quickly and take advantage of all our efforts," Edwards added.

The study included monarch butterflies. But because the clearest evidence on their numerical declines comes from their overwintering grounds in Mexico and the researchers used only U.S. monitoring data, the study did not identify a clear trend for these butterflies.

"We've lost one out of every five butterflies in just 20 years. That means if you went out to watch butterflies in 2000 and you saw 100 butterflies, in 2020 you would only see 80. That's an astounding loss over such a short time span," said conservation biologist and study co-author Eliza Grames of Binghamton University in New York.

"There are a lot of different stressors affecting butterfly populations, and it is hard to pinpoint just one. In the Southeast, for example, drought is likely a big threat. In the Midwest, insecticides are the number one driver of butterfly biodiversity loss. In other regions, the story is not so clear, and it is likely a combination of anthropogenic stressors that is leading to the drastic declines we're seeing," Grames said.

While reliable information on global butterfly population trends is lacking, studies in some other countries have documented declines at roughly the same rate as in the U.S. data.

Butterflies are the most systematically monitored insects in the United States. The researchers calculated that there are 650 butterfly species whose range intersects with the contiguous United States, with at least some monitoring data on 554 of them.

The drop in butterflies, which have inhabited Earth for more than 100 million years, is one piece of the ongoing global biodiversity decline, with losses among insects particularly worrisome given their crucial roles in many ecological processes.

"Ecologists use butterflies like a canary in a coal mine. The results of this study suggest that there may be declines in the many insect species for which we don't have good data," Edwards said.

"For me, butterflies are important because they are beautiful and inspire us. They deserve to exist simply for the sake of existing," Grames said. "In terms of ecosystem function, butterflies are really important pollinators, herbivores and also serve as prey for insectivores," especially birds.