Climate Change Threatens Monarch Migration

The migratory monarch butterfly is listed as Endangered by the International Union for Conservation of Nature, with its native population shrinking by 22% to 72% over the past decade. In Canada, monarchs are federally protected as an endangered species, and in December 2024, the U.S. Fish and Wildlife Service proposed listing them as threatened. The eastern monarch population, which overwinters in Mexico, has declined by over 80%. While the winter 2024-2025 count showed a 99% increase from the previous year to 1.79 hectares, this is still below the decade's average of 2.81 hectares and the 6-hectare threshold considered necessary for a sustainable population. The western population, wintering in California, is at even greater risk, having declined by more than 95%. The latest count recorded only 9,119 butterflies, a staggering 96% drop from the previous year's 233,394. The extinction probability for the western monarch within 60 years is estimated at 98% to 99%. Monarchs undertake a unique multi-generational, two-way migration, with some traveling as far as 3,000 miles. The generation that migrates south lives up to eight times longer than the preceding generations that fly north. They can cover 50-100 miles in a single day. Climate change not only shifts milkweed habitats but can also alter the plant's toxicity. Rising temperatures may cause some milkweed species to produce higher levels of cardenolides, their natural toxins, potentially making them poisonous to monarch caterpillars. Additionally, elevated CO2 levels can reduce the medicinal properties of milkweed that help monarchs fight parasites. Beyond climate change, the monarch population is threatened by habitat loss from agriculture and urban development, as well as the use of pesticides that kill both milkweed and the butterflies themselves. Severe weather events, such as winter storms and droughts, also pose a significant risk to their survival.