(Reuters) - Monarch butterflies west of the Rocky Mountains are teetering on the edge of extinction, with the number wintering in California down more than 90 percent from the 1980s, researchers said in a study published on Thursday.

FILE PHOTO: A monarch butterfly clings to a plant at the Monarch Grove Sanctuary in Pacific Grove, California, December 30, 2014. REUTERS/Michael Fiala/File Photo

While much is known about the black-and-orange winged insects’ decadeslong population decline in the eastern United States, scientists have been unable to track the western variety accurately until the recent development of new statistical models.

The new study, published in the scientific journal Biological Conservation, was funded by the U.S. Fish and Wildlife Service, which is considering giving monarch butterflies Endangered Species Act protections.

Monarchs, which depend on a diminishing supply of milkweed plants for reproduction and food, are arguably the most popular of North America’s butterflies and have a huge international following among students and scientists. However, the western population has fallen to about 300,000 from 10 million less than four decades ago.

“If the population continues to decline at that rate, we will lose migratory monarchs in the western United States over the next several decades,” Washington State University biologist Cheryl Schultz, the study’s lead author, said in a telephone interview.

The migratory monarchs of the western United States have a 63 percent chance of extinction in 20 years and an 84 percent chance in 50 years if current trends continue, according to the study.

Scientists believe declines in U.S. monarch populations are linked to human development that has wiped out their habitats, as well as the destruction of roosting forests in California and Mexico, climate change and farmers’ increasing use of pesticides that kill milkweed plants and other native vegetation.

“The change has been so dramatic that if we don’t act to protect them, they are threatened with extinction,” said Tufts University ecologist Elizabeth Crone, a study co-author.

The western monarchs winter in coastal tree groves in California, only to fan out in spring to lay eggs on milkweed and feed on flowers there as well as in Arizona, Idaho, Nevada, Oregon, Utah and Washington.

By pairing counts of monarchs wintering in California with historical estimates, researchers determined the decline of the western population was steeper than previously believed.

The eastern and central U.S. population of monarchs, numbering in the tens of millions, is the most studied and is famed for its Mexico-to-Canada migration.