Monarch Butterflies Bounce Back 64% at Their Mexican Wintering Grounds

Each autumn, one of nature’s most astonishing journeys unfolds as monarch butterflies stream south from Canada and the United States to a handful of mountaintop forests in central Mexico. For years, the news from those wintering grounds has been worrying, as the orange-and-black clouds of insects grew thinner. This winter, though, brought a welcome turn: monarchs occupied 2.93 hectares of oyamel fir forest, a striking 64 percent increase over the previous year’s 1.79 hectares.

Because monarchs cluster so densely on the fir trees, scientists measure the population by the area of forest they cover rather than by counting individuals. The latest survey, released by the World Wildlife Fund and Mexico’s National Commission of Natural Protected Areas, also recorded a meaningful drop in forest degradation, as illegal logging in the reserves eased. Together, those trends point to a population breathing a little easier than it has in recent memory.

“For years, the news from those wintering grounds has been worrying, as the orange-and-black clouds of insects grew thinner.”

The rebound did not come from nowhere. In the mountain communities of Michoacán and the State of Mexico, people have been reforesting hillsides, patrolling the reserves to deter illegal logging, and building livelihoods around monarch ecotourism. By giving local residents a direct stake in the butterflies’ survival, these efforts have helped protect the fragile oyamel forests the monarchs depend on each winter.

Still, conservationists are careful to keep the celebration in perspective. Even after this year’s jump, the monarchs cover less than half the roughly six hectares that scientists believe the population needs to stay safely above the risk of migratory collapse, and numbers remain far below the levels of the 1990s. The greatest challenge lies along the migration corridor through the United States, where the milkweed the caterpillars need has been lost to development and herbicides. A 64 percent rise is not a finish line, but it is a hopeful sign that with reforestation, protection and cross-border cooperation, one of the planet’s great natural spectacles can still be saved.