Climate Change Threatens Monarch Migration

The annual journey of the eastern monarch population covers up to 3,000 miles from the U.S. and Canada to central Mexico. It is the only butterfly to make such a long, two-way migration, but the butterflies that fly south are the great-great-grandchildren of the ones that left Mexico the previous spring. Their destination is a small, specific area within the Monarch Butterfly Biosphere Reserve, a UNESCO World Heritage site located in the mountains of Michoacán and the State of Mexico. The monarchs cluster on oyamel fir trees at elevations of nearly 10,000 feet, where the cool, humid conditions are essential for their winter survival. The population of eastern monarchs that reached these overwintering sites plummeted by 59% in the 2023-2024 season compared to the previous year. This marked the second-lowest measurement since monitoring began in 1993, with the butterflies occupying an area of only 2.2 acres. North America has two distinct migratory populations. While the larger eastern population winters in Mexico, a smaller western population migrates from west of the Rocky Mountains to specific groves of eucalyptus and Monterey pine trees along the coast of California. The overall eastern monarch population has fallen by approximately 80% since the 1980s. A primary cause for this long-term decline is the widespread loss of milkweed plants across the United States due to increased agriculture; milkweed is the sole food source for monarch caterpillars. Illegal logging and deforestation in the Mexican mountain sanctuaries also pose a significant threat. The loss of the forest canopy exposes the butterflies to more extreme temperatures and disrupts the delicate microclimate that has protected them for millennia.