It’s a common October sight in Western coastal cities like Pacific Grove, Santa Cruz and San Diego, California: clusters of thousands of black-and-orange monarch butterflies clinging to pine, cypress and eucalyptus trees after migrating from colder northern states.

According to a new study published in Biological Conservation, however, numbers of migrating monarchs are down more than 90 percent from the 1980s, and if the decline continues at its current rate, monarchs could be extinct on the West Coast in as little as 20 years.

"In the 1980s, 10 million monarchs spent the winter in coastal California," said Washington State University Vancouver associate professor Cheryl Schultz, lead author of the study. "Today, there are barely 300,000."

While the monarch butterfly’s decline on the East Coast has been well documented over the past decade — populations there have declined more than 90 percent since 1996 — this is the first time statistical models have been able to accurately paint a similar picture for the West.

"Scientists, policy makers and the public have been focused on the dramatic declines in the well-known Eastern population, yet this study reveals that Western monarchs are even more at risk of extinction," said Emma Pelton, a biologist with the Xerces Society. "We will need significant conservation action to save monarch butterflies in the West."

In the study, researchers from Washington State University Vancouver, Tufts University in Massachusetts, the University of Georgia and the nonprofit Xerces Society pooled data gathered from volunteers across California dating back to the 1980s, and used mathematical modeling to eliminate minor year-to-year differences.

The results showed a population decline even more rapid than that of the East Coast’s. If the West Coast downtrend continues at its current rate of about 7 percent per year, monarchs have a 72 percent risk of becoming extinct in 20 years — and an 86 percent chance in 50 years.  

"This study doesn't just show that there are fewer monarchs now than 35 years ago. It also tells us that, if things stay the same, western monarchs probably won't be around as we know them in another 35 years," said Schultz.

Though the exact cause of the population decline is not known, habitat loss and pesticides are strongly suspected. The monarch, one of the largest butterflies in the world, is especially sensitive to toxins and changes in habitat. The insects consume and lay their eggs on milkweed plants, whose numbers have been annihilated by both development and pesticide use, especially in heavily agricultural central California.

The WSU study was funded by the U.S. Fish and Wildlife Service, which is considering declaring monarchs an endangered species.

“The hard part of being a conservation biologist is documenting species declines,” said Tufts University professor and study co-author Elizabeth Crone. “The exciting part is figuring out how to help declining species recover. In the 20th century, we brought bald eagles back from the brink of extinction by limiting use of DDT. If we start now, we can make the 21st century the era in which monarchs return to our landscapes.”