Climate Change Threatens Monarchs
New research indicates that climate change threatens the mass migration of monarch butterflies. Scientists warn that rising temperatures may shift suitable milkweed habitats in Mexico, the primary destination for migrating monarchs, farther south. This could potentially fracture established migration routes and lead to further population decline.
The migratory monarch butterfly is classified as Endangered on the IUCN Red List, with its population having shrunk by between 22% and 72% over the last decade. The eastern monarch population, known for its migration to Mexico, has declined by 84% between 1996 and 2014. The western population, which overwinters in California, is at an even greater risk, having declined by an estimated 99.9% between the 1980s and 2021. The monarch's annual migration is a multi-generational relay. The generation that flies south to Mexico can live up to nine months, surviving the winter before starting the journey north in the spring. Subsequent generations, each living only two to five weeks, continue the journey northward, breeding and laying eggs along the way. It takes three to four generations to complete the entire migration cycle. Milkweed is the only plant that monarch caterpillars will eat, and female monarchs lay their eggs exclusively on it. Climate change can alter the growth and distribution of milkweed, creating a mismatch between the butterflies' arrival and the availability of their food source. Additionally, rising temperatures can cause milkweed to produce higher levels of a toxic compound called cardenolide, which can be harmful to the caterpillars. Beyond the disruption of milkweed, climate change poses other direct threats. Monarchs are highly sensitive to temperature variations, which act as cues for migration and reproduction. Extreme weather events, such as severe storms and droughts, can also lead to significant monarch mortality during their long journeys and in their overwintering grounds. The overwintering sites in Mexico's oyamel fir forests are also under threat. These high-altitude forests provide a specific microclimate that protects the butterflies. However, climate change is predicted to make these areas unsuitable for the monarchs within the next 40 to 50 years. A single winter storm in 2002, marked by extreme cold, resulted in a 70-80% mortality rate in two major overwintering colonies. Habitat loss from deforestation and the widespread use of herbicides are also major contributors to the monarch's decline. The use of herbicides on agricultural crops has led to a massive loss of milkweed plants, a critical food source for monarch larvae. Conservation efforts are focused on restoring milkweed and nectar plant habitats along the migratory routes.
