Climate Change Threatens Monarch Butterfly Migration
New scientific findings suggest that climate change is reshaping habitats critical for the migration of monarch butterflies. Researchers predict that suitable milkweed habitat in Mexico could shift southward, potentially fracturing migration routes and further endangering the dwindling populations of the species.
The eastern monarch population, which migrates from the U.S. and Canada to Mexico, has plummeted by over 80% since the 1990s. The western population, overwintering in California, has seen an even more drastic decline of over 95% since the 1980s. In early 2024, the count of western monarchs was 9,119, a 96% decrease from the previous year. Milkweed is the only plant monarch caterpillars can eat, making it essential for their survival. The widespread use of herbicides in agriculture has significantly reduced the availability of this vital food source along their migration routes. This loss of breeding habitat is a primary driver of the population decline. Beyond the loss of milkweed, climate change introduces other threats. Warmer temperatures can delay the fall migration by as much as six weeks, leading to monarchs dying in colder temperatures further north. Extreme weather events, such as a single severe storm in 2002, can have catastrophic effects, killing up to 80% of the overwintering population in Mexico. The monarch's multi-generational, 3,000-mile migration is a unique phenomenon. It takes three to four generations of butterflies to travel from their southern overwintering grounds to the northern U.S. and Canada in the spring and summer. The final generation of the year then makes the entire journey south on its own. In response to the drastic decline, the U.S. Fish and Wildlife Service proposed listing the monarch butterfly as a threatened species under the Endangered Species Act in December 2024. This would provide legal protections for the species and its habitat. The International Union for the Conservation of Nature (IUCN) already classifies the migratory monarch as vulnerable. Conservation efforts are underway to address the decline. Organizations and government agencies are working to restore monarch habitat by planting milkweed and other nectar-rich plants. Public initiatives like the Mayors' Monarch Pledge encourage communities to create habitats and educate citizens about how they can help.
