Climate change could threaten monarch mass migration

Climate change may threaten North America’s iconic mass monarch butterfly migration. 

Every fall, millions of monarchs (Danaus plexippus) travel thousands of kilometers over North America as they leave their breeding grounds in Canada and the United States for wintering grounds in a mountainous part of central Mexico. The butterflies make the trek back north over multiple generations when temperatures warm in the spring and summer months, following the growth of milkweed (Asclepias), their preferred food source.

But Mexico’s suitable monarch overwintering habitat could shift south as the climate changes in decades to come, researchers report February 25 in PLOS Climate. That could lengthen an already arduous journey and increase the energy required to make the trip.  

That extra distance might push some individuals to stay in Mexico instead of continuing north, says Carolina Ureta, a biologist at the National Autonomous University of Mexico in Mexico City. “In this case, the species is not in danger because of climate change, but the migration might be.”  

Monarchs are on the decline in North America. Habitat loss, extreme weather, pesticides and parasites are among the reasons fewer insects reach their winter home. Populations in central Mexico, once nearly 700 million individuals strong, have declined by more than 80 percent since the 1990s. 

Citizen scientists have noticed that some monarchs seem to remain in northeastern or central Mexico, says Víctor Sánchez Cordero, a conservation biologist also at the National Autonomous University of Mexico. “That makes sense because not all monarch populations worldwide migrate.” Butterflies in Chile, New Zealand and across Europe, for instance, don’t travel long distances. But whether climate change might alter North American monarchs’ epic transcontinental journey was unknown.

Ureta, Sánchez Cordero and colleagues leveraged computer simulations to predict where in Mexico monarchs might find milkweed, the only plant on which monarchs lay eggs and that caterpillars feed on. Suitable monarch habitat could decline by 2070, the team found. Some simulations taking climate, biology and environment into account suggest a drop from 19,500 square kilometers of ideal habitat to roughly 8,000 square kilometers. Climate change could also push that habitat south of the insects’ existing overwintering sites, fracturing and lengthening their migratory route.   

Making that return flight north could take more energy than it’s worth, Ureta says, and some individuals may stay instead. Wing size could be a helpful indicator to help researchers figure out if butterflies are staying or going: Resident populations in other parts of the world tend to have a smaller wingspan than migrants.