West Coast Monarch Butterflies Flutter Toward Extinction

The number of West Coast monarch butterflies has suffered shocking population losses in the last few decades. But a new study suggests the decline has been even greater than scientists expected. According to a new study, the number of colorful insects has declined by 97 percent in just over three decades, putting the area's population at risk of extinction.

Monarch butterflies across North America have been dissapearing at an alarming rate due in part to the loss of their primary food source, milkweed. But past focus had primarily been placed on the Eastern populations, which have declined by more than 90 percent since 1996, Peter Fimrite reports for the San Francisco Chronicle. But a new study, published this week in the journal Biological Conservation,​ suggests that the outlook for the western populations is equally as grim. 

In the past, spotty data has made it difficult to evaluate the status of the West Coast monarchs. "Like many at-risk species, systematic monitoring of this population began after dramatic declines had already been noticed," the researchers write in the paper. So for the latest study, the scientists lengthened the available historical records by using statistical models to combine spotty data from the 1980s and 1990s with the more complete records from citizen scientists collected during the last 20 years.

The results are sobering. The population of Western monarch butterflies is declining by roughly 7 percent per year, even more rapidly than the much larger population of Eastern monarch butterflies. "In the 1980s, 10 million monarchs spent the winter in coastal California. Today there are barely 300,000," Cheryl Schultz, a biologist at Washington State University Vancouver, in a statement.

Along with loss of their primary food source, increasing urbanization in California is likely to blame for the decline, writes Jason Daley for Sierra magazine. Many feeding and nesting sites have been obliterated by development, while agriculture has wiped out other areas. Unlike Eastern monarchs, which famously migrate to Mexico in the winters from as far north as Canada, Western monarchs spend their winters in nests on the coastal areas of California before dispersing up and down the West Coast for the warmer months. So the loss of these nesting sites is a blow to the populations.

Schultz and her team are now working to study the breeding times and locations for the Western monarch butterflies in hopes of better understanding how it could be protected, reports Daley. While things seem grim now, there is room for hope, the researchers stress.

"In the 20th century, we brought bald eagles back from the brink of extinction by limiting use of DDT," co-author Elizabeth Crone, an ecologist at Tufts University, says in a statement. "If we start now, we can make the 21st century the era in which monarchs return to our landscapes."

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Ben Panko is a staff writer for Smithsonian.com