Climate Change Threatens Monarch Migration
New research suggests that climate change is threatening the mass migration of monarch butterflies by shifting their critical milkweed habitats in Mexico further south. This habitat fragmentation could disrupt the migration pattern, potentially splitting populations and putting the species under greater stress.
The migratory monarch butterfly is now listed as Endangered by the International Union for Conservation of Nature. The eastern population has shrunk by 84% between 1996 and 2014, while the western population, at greatest risk, has declined by an estimated 99.9% since the 1980s. Each fall, millions of monarchs travel up to 3,000 miles. Butterflies east of the Rocky Mountains journey to oyamel fir forests in Mexico's Sierra Madre mountains, while western populations travel to the California coast. Milkweed is the only plant monarch caterpillars will eat, making it essential for the species' life cycle. Adult butterflies lay their eggs exclusively on milkweed leaves, which contain toxins that protect the caterpillars from predators. Climate change-induced drought limits the growth of milkweed, and higher temperatures can increase the concentration of toxic compounds within the plants. This can make the sole food source for monarch larvae poisonous to them. Beyond its effect on milkweed, climate change brings other direct threats. Temperature extremes can trigger migrations before milkweed is available, and severe weather events like winter storms have killed millions of butterflies in their overwintering colonies. The monarch's decline is also accelerated by factors other than climate change. The widespread use of herbicides is destroying milkweed, while pesticides kill both caterpillars and adult butterflies. Deforestation and urban development have also destroyed significant portions of their winter shelters in Mexico and California. In response to the population crash, the U.S. Fish and Wildlife Service proposed listing the monarch as a threatened species in December 2024. Canada listed the monarch as endangered in 2023, and the butterfly's overwintering habitat is legally protected in Mexico's Monarch Butterfly Biosphere Reserve.
