Climate Change Threatens Monarch Butterfly Migration in North America
The iconic multi-generational migration of monarch butterflies across North America may be at risk, not from extinction of the species itself, but from a disruption of the migratory pattern due to climate change, according to research published on in PLOS Climate.
Every fall, millions of monarchs (Danaus plexippus) undertake a remarkable journey, traveling thousands of kilometers from breeding grounds in Canada and the United States to overwintering sites in a mountainous region of central Mexico. In the spring and summer, subsequent generations complete the return trip north, following the growth of milkweed (Asclepias), the sole food source for monarch caterpillars.
The study suggests that as the climate changes, the suitable habitat for monarchs to overwinter in Mexico could shift southward. This shift would effectively lengthen the already challenging migration, increasing the energy expenditure required for the butterflies to complete it. Researchers used computer simulations to predict where milkweed, essential for the monarch lifecycle, might be found in the future.
“That extra distance might push some individuals to stay in Mexico instead of continuing north,” explains 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.” This phenomenon isn’t unprecedented; some butterfly populations in other parts of the world, such as those in Chile, New Zealand, and Europe, do not undertake long-distance migrations.
The monarch butterfly population has been in decline for decades. Since the 1990s, populations in central Mexico have decreased by more than 80 percent, from nearly 700 million individuals to significantly lower numbers. This decline is attributed to a combination of factors including habitat loss, extreme weather events, pesticide use, and parasitic infections.
Citizen scientists have observed an increasing number of monarchs remaining in northeastern and central Mexico, a trend consistent with the potential for a shift in migratory behavior. Víctor Sánchez Cordero, a conservation biologist also at the National Autonomous University of Mexico, notes that this observation aligns with the understanding that not all monarch populations are migratory.
The simulations conducted by Ureta, Sánchez Cordero, and their colleagues indicate a potential decline in suitable monarch habitat by 2070. Currently, approximately 19,500 square kilometers of habitat are considered ideal. However, projections suggest this could decrease to around 8,000 square kilometers. Climate change could push this remaining habitat further south, fragmenting the migratory route and making the journey even more demanding.
The increased energy demands of a longer, more fragmented migration could lead some monarchs to forgo the northward journey altogether. Researchers suggest that wing size could serve as an indicator of this behavioral change. Resident monarch populations, which do not migrate, typically have smaller wingspans than those that do.
The study highlights the complex interplay between climate change, habitat availability, and animal behavior. While the monarch butterfly species itself may not be immediately threatened with extinction, the potential loss of its iconic migratory pattern represents a significant ecological shift. The disruption of this long-standing migration could have cascading effects on the ecosystems that rely on the butterflies’ pollination services.
The findings underscore the importance of continued monitoring of monarch populations and their habitat, as well as efforts to mitigate the effects of climate change and protect vital milkweed resources. Further research is needed to fully understand the long-term consequences of these changes and to develop effective conservation strategies.