Scientists Are Still Baffled by Monarch Migration

When it comes to declining winter butterfly populations, something just doesn’t add up

Monarch butterflies
Overwintering monarch butterflies skeeze via Pixabay (Public Domain)

Experts have known it for years: The mighty monarch butterfly’s migration is out of whack. The surest sign is the precipitous decline in monarchs that make it to a large overwintering area in the mountaintop fir forests of Mexico. Habitat loss has gotten the blame for this distressing trend, but the latest evidence hints that the picture could be more complicated.

Not one but seven new studies on monarch butterflies, published in the Annals of the Entomological Society of America, have called into question how well researchers understand the reasons for monarchs' declining overwinter populations, reports Emily DeMarco for Science. The data from the studies relies in part on tallies of the striking butterflies taken over the past 18 years.

So what's the problem? It turns out that researchers haven’t noticed a correspondingly dramatic decline in monarchs fluttering in their summer ranges across the midwestern U.S. and southern Canada. Fall migration counts, likewise, indicate that butterflies are flying to the right place. Researchers have even collected evidence that shows populations rebounding and recovering in spring and summer areas.

But other evidence does show that something is awry: Butterflies might be laying fewer eggs than before. And monarchs’ complex cycle of migration could be clouding the picture, too.

One monarch may start the journey in Mexico, move north as the spring turns, and stop to mate and lay eggs. Her offspring then continue the migration north for several weeks before they stop and create the next generation.

It can take three to four generations for the butterflies to make it the 3,000 miles from Mexico to the northern U.S. — but the return trip happens in one "migratory generation," explains Sarah Kaplan for The Washington Post. Tracking butterfly populations along that migratory route is tricky, and that difficulty could explain the confusion.

Also, volunteers who track monarchs might be looking for them in places where they expect to find them, like parks and protected areas. Survey results might not include agricultural areas where herbicides are used, and data sets may not be large enough, writes DeMarco.

Andy Davis from the University of Georgia, who organized the collection of seven papers, offers an explanation to The Washington Post: The migration is like a marathon. “If you were charged with figuring out how many people are participating in the Boston Marathon each year, you wouldn’t count the number of people who cross the finish line. But for many years we’ve been counting the finishing monarchs in Mexico,” he says. “We’ve been doing it backwards.”

Still, Davis' point doesn’t reveal what happens to those who don’t finish. Are fewer butterflies making the trip because of climate change? Are more dying along the way because of some danger? Regardless of what they see along the way, there's still something seriously amiss with monarch migration. If it halts altogether, the Earth will have lost one of its most incredible animal journeys.

Get the latest stories in your inbox every weekday.