The annual overwintering count of monarch butterflies released on February 9 confirms Monarch numbers fell by nearly one-third from last year’s count, indicating an ongoing risk of extinction for America’s most well-known butterfly. Scientists report that this year’s population is down by 27 percent from last year’s count, and down by more than 80 percent from the mid-1990s. Additionally, a survey of monarch butterflies overwintering in California shows that the Western population has not rebounded.

A recent study by the U.S. Geological Survey concluded that there is a substantial probability that monarch butterflies east of the Rockies could decline to such low levels that they face extinction. Researchers estimate the probability that the monarch migration could collapse within the next 20 years is between 11 percent and 57 percent. This year’s drastic decline is attributed in part to more extreme winter storms that killed millions of monarchs last March in Mexico’s mountain forests, where 99 percent of the world’s monarchs migrate for the winter.

On the West Coast of California, volunteers with the Xerces Society’s Western Monarch Thanksgiving Count visited 253 sites and tallied a total of 298,464 monarchs—a fraction of the 1.2 million monarchs recorded in the late 1990s. Key sites such as Pismo Beach and Natural Bridges saw lower populations this year, and all but one of the 15 sites which have been continuously monitored since 1997 had lower counts than in the prior year. Monarchs from as far away as Idaho, Utah and Arizona converge to spend the winter in tree groves along the Pacific coast from Mendocino County, California, to northern Baja, Mexico. This gathering of monarchs provides a unique opportunity to gauge the health of the monarch population from the western United States.

Read More: Monarch Butterfly Population Drops by Nearly One-third | Number of Monarchs Overwintering in California Remains Low

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