MEXICO CITY — One of North America’s most dazzling natural phenomena, the annual winter migration to central Mexico by millions of monarch butterflies from the northern United States and Canada, has shrunk to record lows and is in danger of ending, environmentalists from across the continent said Wednesday.
The monarch migration has been documented in books and movies and attracts thousands of tourists to a nature preserve about 100 miles west of Mexico City. The black-and-orange butterflies hang from the trees there like shaggy beards.
In the 20 years since environmentalists began keeping detailed records of the monarch’s winter habitats, the butterflies have covered as much as 45 acres of forest in the Mexican state of Michoacan.
But the most recent winter count showed how far the migrating monarch population has fallen: As of December, they blanketed just 1.6 acres of forest, the smallest area yet.
“I am deeply saddened by the terrible news,” Karen Oberhauser, a professor at the University of Minnesota who studies the monarchs, said by telephone at a news conference here marking the release of a report by the World Wildlife Fund, Mexico’s Environment Department and the National Commission for Protected Areas. “To preserve the monarch migration, we need a groundswell of conservation.”
The butterflies face numerous threats across North America. In Michoacan, illegal logging has cut into their winter habitat in the oyamel fir trees, although government conservation efforts have slowed the rate of deforestation. In the United States and Canada, herbicides used in industrial-scale farming have destroyed the milkweed plants where they lay their eggs.
“I think it’s past time for Canada and the United States to enact measures to protect the breeding range of the monarchs, or I fear the spiral of decline will continue,” said Phil Schappert, a monarch expert in Halifax, Nova Scotia.
The butterflies’ numbers have been down before. In the drought years of the 1930s, monarch populations “were probably as low [as] or lower than they are right now, and they recovered,” Oberhauser said, attributing the rebound in part to the butterflies’ remarkable fertility. One female monarch can lay between 500 and 1,000 eggs in her lifetime.
But, she said, “the problem is that now a lot of the habitat that they had in the past is gone, due to increasing use of this land for agriculture.”
Next month, President Obama is scheduled to visit the Mexican city of Toluca, not far from the monarch preserve, to meet with his Mexican and Canadian counterparts. Omar Vidal, the director of the World Wildlife Fund’s Mexico office, said he wants North American leaders to agree on a plan to protect the monarch, saying the migration “symbolically unites our three countries.”
“The butterfly as a species isn’t in danger of extinction,” Vidal said. “What is in danger of disappearing . . . is the migration of the monarch from Canada through the United States to Mexico.”