Climate Change Threatens Monarch Migration
New research indicates that climate change is threatening the mass migration of monarch butterflies in North America. Suitable milkweed habitat in the butterflies' primary overwintering sites in Mexico may shift south, potentially fracturing established migration routes and jeopardizing entire populations.
The monarch's annual journey is one of nature's great spectacles, with some populations traveling up to 3,000 miles. It's a multi-generational relay, meaning the butterflies that arrive at the overwintering sites are the great-great-grandchildren of the ones that left the previous spring. The destination for monarchs east of the Rocky Mountains is a specific, high-altitude oyamel fir forest in the Trans-Mexican Volcanic Belt, about 10,000 feet above sea level. This area, a UNESCO World Heritage Site, provides the unique microclimate the butterflies need to survive the winter. Recent population counts show a drastic decline. The area occupied by eastern monarchs in Mexico during the 2023-2024 winter dropped by 59% from the previous year, marking the second-smallest area recorded since monitoring began in 1993. The western population, wintering in California, has plummeted by over 95% since the 1980s. A primary driver of this decline, alongside climate change, is the loss of milkweed, the only plant monarch caterpillars eat. The proliferation of herbicide-resistant crops has allowed for widespread spraying that kills milkweed in the agricultural heartland of the U.S., a critical part of the monarch's breeding and migration corridor. The threats extend to their winter refuges. The forests in Mexico have been shrunk by both legal and illegal logging, while the coastal groves in California where western monarchs overwinter are threatened by real estate development. In response to the population crash, the migratory monarch butterfly is classified as Vulnerable by the International Union for Conservation of Nature. In late 2024, the U.S. Fish and Wildlife Service officially proposed listing the monarch as a threatened species under the Endangered Species Act. Conservation efforts are underway to combat the decline. Scientists in Mexico are experimenting with "assisted migration," planting oyamel fir seedlings at higher, cooler elevations where the monarch habitat is projected to be in the future. Citizen-led projects focus on planting native milkweed to restore breeding habitat.
