Part 4 of the TED Radio Hour episode Migration
Monarch butterflies fly the longest two-way migration of any insect species. Ecologist Sonia Altizer shares how these intrepid butterflies make the journey — and how it's being threatened.
About The Monarch Migration
Each year, millions of monarch butterflies fly some 3,000 miles, from their summer breeding grounds as far north as Canada to their overwintering sites in the oyamel fir forests of central Mexico. It is one of the Earth's great migrations, and it's also one of the best studied. In recent years, ecologists like Sonia Altizer have been able to better answer how and why these intrepid butterflies make the journey.
The how is owed to a complex suit of navigational and physiological adaptations. Migrating monarchs have internal sun and geomagnetic compasses, and the monarchs that make the longest trek actually grow to be bigger and more powerful fliers, particularly the females. They can even read the skies like a meteorologist, finding and following air currents to quicken their travels. Peering into their genes has also revealed more clues into how they evolved this migratory behavior.
In terms of why they migrate, ecologists think there are three main reasons. The first two have to do with milkweed and temperature. To breed, monarchs need milkweed, and in North America native milkweed dies back with cold winter temperatures. Those same temperatures would kill the monarchs. The fir forests in Mexico offer the ideal shelter and perfect goldilocks temperature while they wait for milkweed to grow back in the spring. The forests in Mexico are warm enough that the butterflies don't freeze but cool enough that their metabolism drops and they can enter a kind of dormant state, called diapause. Altizer has helped establish the third reason why monarchs migrate: to escape a parasite that affects them in their breeding grounds. Infected monarchs are less likely to survive the journey, so the long migration weeds out those who are infected and keeps the population safer. For more on monarchs, check out the Forest Service's monarch FAQs.
How You Can Help Protect Monarchs And Their Migration
First, help protect existing monarch breeding habitat. The Corn Belt produces more than half of the monarchs that migrate to Mexico. As Altizer presents in her TED talk, GMO crops, by design, leave no room in agricultural fields for milkweed and nectar plants to grow. Altizer asks us to see agriculture from an ecosystem perspective and consider using GMOs more judiciously.
Second, help create new habitat for monarchs. Right now, organizations are creating alternative breeding habitat for monarchs and you can support those wide-scale efforts. You can also make monarch and butterfly-friendly habitats in your neighborhood or backyard. If you do, Altizer stresses the importance of planting native milkweed species. Many plant nurseries carry what's called tropical milkweed, and unlike native species it doesn't die off in winter. While that might sound good, it actually increases the spread of the parasite and it sends signals to the monarchs to stop migrating. If you do choose to create native monarch habitat, it's best to plant more than just milkweed. Variety ensures you'll help all kinds of pollinators and wildlife.
Third, consider joining a community or citizen science project. There are many that help track the migration. Dr. Altizer also founded the citizen science group Monarch Health to track the prevalence of the protozoan parasite Ophryocystis elektroscirrha (OE) in monarch butterflies.
About Sonia Altizer
Sonia Altizer is a Professor and Associate Dean of Academic Affairs in the Odum School of Ecology at the University of Georgia. Her main research focuses on how pathogens affecting wildlife are shifting in response to human environmental change.
At UGA, her students run a citizen science project called Monarch Health. In 2015, Altizer co-edited a book titled Monarchs in a Changing World: Biology and Conservation of an Iconic Insect, and participated in high-level task forces dedicated to monarch butterfly conservation.
Altizer received a B.S. from Duke University and a Ph.D. from the University of Minnesota. She did her postdoctoral work at Princeton University and Cornell University.