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Monarch butterfly populations may be more stable than previously thought

New research found that a successful summer breeding season can help make up for losses over the winter, alarming conservationists and causing concern about the insects' future.
Monarch butterflies share a flower
Monarch butterflies share a flower in the Cole Family Monarch Conservation Center and Butterfly House at the Greensboro Science Center in N.C.Woody Marshall / News & Record via AP

Monarch butterfly populations in North America may be more stable than scientists previously thought — at least during the summer breeding months, according to new research.

A study published Friday in the journal Global Change Biology found that monarch populations in some parts of North America in the summer are actually increasing, which may be helping to offset well-documented declines attributed to the insects’ winter migration and environmental threats.

"There's this perception out there that monarch populations are in dire trouble, but we found that's not at all the case," Andy Davis, an assistant research scientist at the University of Georgia's Odum School of Ecology and one of the study's authors, said in a statement.

The research, based on observations collected from 1993 to 2018, is potentially good news for the overall health of monarch butterflies, but the findings are likely to be controversial among conservationists who have spent years drawing attention to the plight of monarchs and their dwindling winter colonies.

There are two types of migratory monarchs in North America: the Eastern monarch butterflies and the Western monarch butterflies.

Eastern monarchs typically breed over the summer across a wide swath of the United States and Canada east of the Rocky Mountains. The insects then travel south to overwinter in parts of central Mexico. Western monarchs usually spend the summers breeding within a much narrower corridor, in California, Nevada, Oregon and Washington state. These insects typically overwinter along the California coast, at sites stretching from Marin County in the north to Baja California.

Both populations have seen steep declines in their winter colonies in recent decades, owing in part to the overuse of pesticides, habitat loss from agriculture and urban development, and climate change.

The new study found, however, that a successful summer breeding season can help make up for losses over the winter.

"A single female can lay 500 eggs, so they're capable of rebounding tremendously, given the right resources," Davis said in the statement. "What that means is that the winter colony declines are almost like a red herring. They're not really representative of the entire species' population, and they're kind of misleading."

The researchers found an overall increase in monarch abundance, relative to other butterflies observed at various sites around the country, of about 1.4 percent per year, according to the study.

The population increases were not uniform across breeding sites. While upticks were detected in the Northwest, Southeast and Upper Midwest, declines were seen across Illinois, Indiana, Ohio, southern Wisconsin and parts of the Northeast, the researchers reported.

The study was based on more than 135,000 observations gathered by citizen scientists for the North American Butterfly Association. The data, collected over a two-day period every summer from 1993 to 2018, documented sightings of monarchs and other butterfly species.

The findings could help provide a more complete understanding of how monarchs and other insect populations are faring in North America, said William Snyder, an entomologist at the University of Georgia and a co-author of the study.

"There's this idea out there about an insect apocalypse — all the insects are going to be lost," Snyder said in a statement. "But it's just not that simple."