Climate Models Show Monarch Butterfly Migration Routes Fragmenting by 2070

The monarch butterfly migration is one of the most quantified wildlife spectacles on the planet - ecologists count the hectares of forest occupied at Mexican overwintering sites each winter with enough precision to track year-to-year changes in the hundreds of millions of animals involved. For decades, those counts have been trending downward. A new modeling study published in PLOS Climate adds a climate dimension to the picture that conservation planners have so far largely underestimated.

Francisco Botello and Carolina Ureta at the National Autonomous University of Mexico, along with colleagues, used species distribution models to project suitable habitat across Mexico for monarch eggs, caterpillars, and their essential food plant - tropical milkweed in the genus Asclepias - under climate scenarios for 2030, 2050, and 2070. The projections show a net decline in suitable monarch habitat of between 8% and 40% by 2070, depending on emissions trajectory, with the remaining habitat concentrating in southern Mexico and moving away from the U.S. border.

Milkweed and the Timing of Migration

Monarch caterpillars are obligate milkweed feeders. The plant provides not just nutrition but toxic cardenolide compounds that make monarchs unpalatable to most predators - a defense the butterflies retain into adulthood. The geographic distribution of milkweed therefore sets hard limits on where monarchs can breed successfully during their northward migration in spring.

Climate change affects this relationship from multiple directions simultaneously. Rising temperatures shift the range of Asclepias species, potentially pushing available milkweed further from the routes monarchs have used for thousands of years. Temperature and photoperiod cues also trigger the hormonal shifts that drive migration timing, and when those cues change, the extraordinary synchronization between monarch movements and milkweed availability can break down.

The models in this study projected that climate-driven changes in milkweed distribution were a primary driver of the southward habitat shift - more significant than direct temperature effects on the butterflies themselves.

Fragmentation, Not Just Loss

The headline decline figures do not fully capture the mechanism of concern. The more consequential finding is geographic fragmentation. The modeling shows that egg-laying sites and food plant availability become concentrated in southern Mexico under future climate scenarios, while the connectivity between wintering sites in central Mexico and breeding habitat further north diminishes.

This creates a corridor problem. Monarch migration is not a direct flight; it is a relay across generations, with butterflies breeding and dying along a route that connects Mexico to Canada over two to three generations. Interrupt the route's ecology at any point and the relay cannot complete. Habitat concentration in the south could cause butterflies to establish resident, non-migratory populations in northeastern and central Mexico rather than continuing north.

"Although the monarch butterfly species itself may not be directly threatened under climate change scenarios, its migratory process is," the authors write. "Under future climate change scenarios, areas of highest suitability are projected to shift farther from the Mexico-U.S. border, making migration energetically more demanding and potentially promoting population residency rather than long-distance migration."

Limitations of the Approach

Species distribution models project habitat suitability based on climate variables and current species ranges, but they cannot directly predict behavioral outcomes. Whether monarchs would actually establish resident populations, and how quickly such a transition would occur, involves evolutionary and behavioral dynamics that correlate with but are not determined by habitat suitability maps alone.

The study also did not incorporate the effects of habitat loss from land-use change independent of climate - a major driver of milkweed scarcity in the U.S. corn belt, where agricultural intensification has eliminated much of the milkweed that once grew around crop fields. No specific funding was received for this work. Author affiliations are in Mexico.

For a species whose survival depends on a precisely timed relay spanning thousands of kilometers, a fragmented southern refuge is not a substitute for an intact migration corridor. The study's authors conclude that international collaboration across the full range of the migration will be essential for any conservation strategy to have lasting effect.