Climate Change Threatens Monarch Butterfly Migration
Climate change is fracturing the annual migration of monarch butterflies, according to new research. Shifting temperatures and habitat loss are causing suitable milkweed habitat to drift southward in Mexico, which could split the butterfly populations and undermine their historic migratory routes.
A recent study in the journal *PLOS Climate* by researchers at the National Autonomous University of Mexico projects that suitable monarch breeding habitat in Mexico could shrink by 8% to 40% by the year 2070. The research, led by Francisco Botello and Carolina Ureta, used computer modeling to determine that rising temperatures will alter the distribution of essential milkweed plants. This southward shift in milkweed, the only plant monarch caterpillars eat, could make the butterflies' multi-generational journey from Canada and the U.S. more strenuous and energetically demanding. The increased distance to suitable breeding grounds may even encourage some monarch populations to become non-migratory, establishing year-round residency in parts of Mexico. The eastern monarch population that overwinters in Mexico has already seen a drastic decline. The 2023-2024 winter season saw a 59% decrease in the area they occupied compared to the previous year, marking the second-lowest population count since monitoring began in 1993. This follows a long-term trend that has seen the eastern monarch population decline by approximately 90% since the 1990s. Beyond climate change, the monarch migration is imperiled by other significant threats. The use of herbicides has led to a massive loss of milkweed in the American Midwest, with an estimated 850 million plants lost since 1999. Insecticides, particularly neonicotinoids, are also a major concern, as they are toxic to all life stages of the monarch and can contaminate the milkweed and nectar plants they rely on. The oyamel fir forests in central Mexico, the specific overwintering habitat for eastern monarchs, have also been under threat. Historically, illegal logging has led to significant degradation of these forests, which provide a crucial microclimate that protects the butterflies from extreme temperatures. While conservation efforts have slowed the rate of deforestation in recent years, the forest ecosystem remains fragile.
