Climate change threatens monarch butterfly migration
A new scientific study warns that climate change is likely to disrupt the mass migration of monarch butterflies. Models predict that rising global temperatures could shift suitable milkweed habitats in Mexico southward, potentially fracturing migration routes and threatening the species' survival.
The eastern monarch population, which migrates to Mexico, has plummeted by over 80% since the mid-1990s. The western population, overwintering in California, has seen an even more drastic decline of over 95% since the 1980s. The latest count for the western migratory population recorded only 9,119 butterflies, a 96% drop from the previous year. This iconic migration spans up to 3,000 miles and can take 3-4 generations of butterflies to complete the journey north from Mexico into the United States and Canada. The generation that flies south to the overwintering sites in Mexico and California is a "super-generation" that lives much longer than the others to complete the strenuous flight. Rising temperatures not only shift milkweed habitats but can also make the plant itself more toxic to monarch caterpillars. Some species of milkweed produce higher levels of toxic compounds called cardenolides at warmer temperatures, which can be lethal to the monarch larvae that feed on them. Climate change also disrupts the environmental cues that trigger the migration. Warmer autumn temperatures can delay the butterflies' departure from their northern breeding grounds by up to six weeks, increasing the risk of them perishing in cold weather before reaching their overwintering sites. The International Union for Conservation of Nature (IUCN) has classified the migratory monarch butterfly as vulnerable. In December 2024, the U.S. Fish and Wildlife Service proposed listing the monarch as a threatened species under the Endangered Species Act. Canada listed the monarch as endangered under its Species at Risk Act in 2023. Beyond climate change, the monarch faces significant threats from habitat loss. The widespread use of herbicides in agriculture has eradicated milkweed, the only host plant for monarch eggs and caterpillars, from vast areas of the butterfly's breeding grounds. Conservation efforts are underway to protect and restore monarch habitats. Organizations like the Xerces Society and government agencies are working to plant native milkweed, manage overwintering groves, and educate the public on how to create monarch-friendly gardens.
