California’s population of monarch butterflies declined 86 percent in 2018 compared to the previous year, according to the Xerces Society for Invertebrate Conservation, a nonprofit group that conducts an annual survey of the species in the western United States. Overall, the state’s monarch population has declined 97 percent since the 1980s.
While many North American monarchs travel to an overwintering site in Mexico, tens of thousands of others spend the cold season in California, hunkered down in eucalyptus trees. The Xerces Society estimates that fewer than 30,000 butterflies will overwinter in the state this year. This is compared to more than 192,000 butterflies in 2017, more than 1 million in 1997, and at least 4.5 million in the 1980s.
This year’s decline could be “potentially catastrophic,” biologist Emma Pelton, who oversees the Xerces Society survey, told The New York Times. “We think this is a huge wake-up call.”
Western monarch populations have been threatened in recent years by habitat loss, pesticides, and climate change — and 2018 was a particularly hard year for the butterflies. “There were late rainy season storms that swept across California in March,” the Xerces Society wrote in a blog post. “There was a severe and extended wildfire season in California and, to a lesser extent, in other areas of the butterfly’s breeding and migratory range. Smoke and bad air quality was widespread in parts of the West at times. California is still recovering from a historic drought.”
The Xerces Society has analyzed results from 97 of its survey sites, which accounted for 77 percent of the state’s total overwintering population. In 2017, these 97 sites hosted approximately 148,000 monarchs. In 2018, scientists and volunteers counted just 20,456.