Climate Change Responsible for Recent Decline of Eastern Monarch Butterflies

A monarch butterfly feeding on a groundsel tree on Poplar Island, Maryland.

A monarch butterfly feeding on a groundsel tree on Poplar Island, Maryland. Will Parson / Chesapeake Bay Program via Flickr

The population of eastern monarch butterflies has been dropping steadily for decades, with some estimates suggesting their numbers are just 20 percent of what they once were. According to a new study, climate change is the biggest driver of recent declines.

Researchers developed a model based on more than 18,000 surveys of monarchs in Mexico, the eastern United States and Canada, undertaken between 1994 and 2018, most of which were conducted by volunteers. They determined that between 2004 and 2018, climate change at the monarchs’ summer breeding grounds was nearly seven times more significant than other drivers of population loss. The findings were published in the journal Nature Ecology & Evolution.

From the mid-1990s to the mid-2000s, glyphosate weed killers spurred declines by decimating milkweed plants, the sole source of food for monarch caterpillars. But as glyphosate use leveled off, climate change emerged as the primary factor in the disappearance of eastern monarchs.

“With monarchs, everything is nuanced, and everything is tricky,” said Erin Zylstra, a postdoctoral researcher in Michigan State University’s College of Natural Science and lead author of the study. “But in recent years, as glyphosate applications have remained more stable, although still very high, there is strong evidence that population changes are driven by climate on the spring and summer breeding grounds.”

The decline of eastern monarchs has consequences for numerous species, from the birds and other small mammals that feed on them, to the wildflowers that depend on them for pollination.

“A lot of it is not good news,” Zylstra said. “But in understanding the reasons why a species is declining, there is also a message of hope: There’s something we can do about it.”