Each year, clouds of monarch butterflies travel thousands of miles across North America to winter in sunny Mexico and California. Last season’s mass migration was no different, except the iconic black-and-orange insects arrived in far fewer numbers than usual, stoking fear in scientists about their potential extinction.
A 2018 census found the monarch butterfly’s overwintering population in California declined by 86 percent compared to the previous year, according to the Xerces Society for Invertebrate Conservation, a nonprofit named for the extinct California butterfly, the Xerces blue.
“No one knows exactly what caused the decline this year,” the organization said in a November statement. “We were not expecting this to be a great year because we knew it had been a rough season in the breeding and migratory range. It’s worse than anyone had anticipated, however.”
Last November, volunteers performed a butterfly count at 97 sites in an area of California that normally encompasses 77 percent of the state’s monarch overwintering population. According to the New York Times on Monday, 148,000 monarchs were tallied in 2017. But in 2018, that number plummeted to 20,456.
A lethal combination of factors could be to blame, according to the Xerces Society. While habitat loss and pesticide use are some of the largest threats to monarchs, California was plagued with a late rainy season, wildfires, and smoke last year.
The precipitous drop—when compounded with a 97 percent decline in the overall monarch population since the 1980s—may signal something “potentially catastrophic,” biologist Emma Pelton, who oversees the monarch survey, told the New York Times.
Butterflies are canaries in a coal mine when it comes to climatic and ecological changes. Their annual journey depends on a chain of interconnected environmental variables, and when one goes, so too can the butterflies.
Milkweed numbers, for example—a food source and the only plant that monarchs will lay their eggs on—dropped in the US by 21 percent between 1995 and 2013. Such declines continue to threaten future butterfly populations, National Geographic reported in 2014.
Climate change is another worry. A 2013 study revealed how finely attuned monarchs are to temperature fluctuations; their annual migration is dependent on it, with a southern spring chill indicating it’s time to fly back north. However, as regions affected by climate change are becoming increasingly warmer, the butterflies may not get this signal, Scientific American reported at the time, and could continue heading south.
A 2017 study from US researchers found a 72 percent chance of extinction for western North America’s monarchs within 20 years, and 86 percent within 50 years.
“In short, 2018 was a tough year to be a monarch butterfly in the West,’ the Xerces Society noted.
“While western monarchs are facing unprecedented challenges right now,” the organization added, “there is still hope that we can recover the population if we work quickly, strategically, and together.”