Climate Change Threatens Monarch Migration
New research indicates that climate change could soon fracture the monarch butterfly's mass migration. Scientists have found that suitable milkweed habitat, crucial for the butterflies' wintering grounds in Mexico, is shifting south, which could disrupt the entire migration route.
The annual monarch migration is a multi-generational journey spanning up to 3,000 miles. Butterflies starting the trip south are the great-grandchildren of those that left the wintering grounds the previous spring, yet they instinctively navigate to the same locations. This migrating generation is also biologically different, living up to nine months longer than the two-to-six-week lifespan of other generations. Two distinct populations of monarch butterflies undertake this incredible feat. The eastern population, which is larger, travels from the northern U.S. and Canada to the oyamel fir forests of central Mexico. The western population migrates from west of the Rocky Mountains to specific groves of trees along the California coast. The eastern monarch population has declined by more than 80% since the 1990s. In the winter of 2023-2024, the area they occupied in Mexico was the second smallest ever recorded, a 59% decrease from the previous year. The western population's decline has been even more stark, collapsing by over 99% from its size in the 1980s. The 2024 Western Monarch Count recorded just 9,119 butterflies, a massive drop from over 233,000 the year before. Beyond the threat to milkweed, rising temperatures create other dangers. Warmer autumns can delay the instinct to migrate south by as much as six weeks, causing monarchs to be caught in deadly cold when they finally depart. Extreme weather events, like a single severe storm in 2002, can have catastrophic effects, killing up to 80% of the overwintering population. Monarch caterpillars are entirely dependent on milkweed plants; it is the only food they eat. The toxins in milkweed are stored in the caterpillars' bodies, making them unpalatable to predators. The widespread loss of this vital plant due to herbicide use and land development is a major contributor to the butterfly's decline. Conservation efforts are underway to protect this iconic species. In 2023, Canada listed monarchs as endangered under its Species at Risk Act. In the United States, a proposal to list the monarch as threatened under the Endangered Species Act was put forth in late 2024. Organizations like the National Wildlife Federation are working to restore milkweed habitat and create networks to support conservation.
