Solving (sorta) the monarch butterfly migration mystery
Contrary to conventional wisdom, the east and west populations are linked
When winter’s chill descends across the nation, two distinct populations of monarch butterflies get the urge to migrate: The eastern insects head to Mexico, and the western insects flock to the California coast.
That’s the age-old dogma. And it’s wrong.
Emerging research shows that these spectacular migrations are linked by shared genes and behaviors, a finding that buoys hope for the resilience of the iconic species.
“This old ‘Berlin Wall’ model — that these butterflies are two different things, divided by the Rockies — is bankrupt,” said Robert Pyle, founder of the Xerces Society, at the first-ever Western Monarch Summit in Carmel last weekend.
It’s true that eastern monarchs fly south to Mexico in the winter — but when they reverse course in the spring, not everyone returns to the East. Some head to Arizona and California.
And not all western monarchs flock to the warm California coast. Some fly to Arizona and Mexico.
These migratory meetups likely explain another new discovery, revealed through the DNA sequencing of monarchs’ genomes: The two groups are very similar and likely belong to a single genetic population.
“A California monarch might mate with a monarch from Massachusetts and the monarch from Massachusetts goes back and lays eggs in Texas,” said insect ecologist Chip Taylor, founder and director of the education, conservation and research program Monarch Watch.
Monarchs’ spectacular long-distance migrations — an aerial dance of orange and black wings that’s part marathon, part relay race — have long fascinated scientists, who tag and study them in hopes of better understanding the biological basis of this behavior.
Unlike birds, this migration is multigenerational. Mysteriously, the insects arriving at their wintering grounds have never been there before. Rather, they are the descendents of insects that left months earlier and laid eggs along the way, breeding future generations who follow their own urge to move.
Both western and eastern populations are declining, and scientists are scrambling to learn why. Much of the overwintering habitat has disappeared. Pesticide use, climate change, fierce storms and drought also take a toll. In the Bay Area and coastal California, they overwinter in clusters on eucalyptus trees. In early spring, they
fan out to the Central Valley, Sierra Nevada, Washington, Oregon and beyond.
But for reasons no one yet fully knows, our butterflies are worse off than their eastern peers. Last year, their numbers hit a historic low, a 97% decline from the 1980s. In contrast, last year’s population of eastern monarchs grew 144% from the previous year. This year’s census has not yet been released.
This is why connectedness is so important. It creates hope that any local extinction could be reversed by the introduction of outsiders.
“We know some of the eastern ones that go to Mexico sometimes decide to shoot up our way,” said Connie Masotti of the Pacific Grove Museum of Natural History, who has been counting monarchs since 2013. Only 800 monarchs were found at Pacific Grove last year. That’s down from 3,000 in 2018 and 17,000 in 2017.
“That’s a really good thing for the West right now,” she said. “And it keeps a really healthy population by mixing up that DNA.”
The traditional notion of an East-West divide is the basis of government policy. The U.S. Department of Agriculture forbids the interstate movement of the butterflies across the Rocky Mountains for release into the environment.
While genetically very similar, it’s too soon to firmly conclude that the DNA of eastern and western monarchs is identical, or that eastern monarchs could be easily moved to the Western U.S. to replicate the migratory phenomenon here, according to Emory University biology professor Jacobus de Roode, who is completing work on a major study of the monarchs’ genomes. Even with this analysis of millions of genetic mutations, he said, there is a chance of overlooking tiny portions of the genome that differ.
But there’s good evidence that winds can blow butterflies into new habitats, expanding their ranges. That’s what happens to birds such as warblers, said scientists. Once there, they mix and mate.
Monarchs have stunning navigational tools, such as a time-compensating circadian clock, a magnetic compass and the ability to detect the angle of the sun. Additional environmental cues, such as rivers and canyons, also guide them. But these are imperfect.
“If winds push them off course, they have to use local information to get back,” said Taylor. “There’s no genetic memory. And they don’t have a brain that can say, ‘Oh, it’s shorter here than it is there.’ There’s no thinking going on here. It’s only responding to the physical factors.”
In fact, the historical record suggests that California’s butterflies, like many of its humans, are relatively new immigrants, said Pyle.
There’s no mention by early Spanish naturalists of monarch clusters, even as they described bobcats, birds and other creatures. In the 1914 book “The Butterfly Trees,” the first detailed study of Pacific Grove’s monarchs, author Lucia Shepardson wrote that the “earliest authentic information” about the region’s butterflies was in 1849.
“People have always assumed our migration was an ancient phenomena. But it may be relatively recent, happening by chance,” said Pyle. “If a lot of Mexican monarchs got blown west, they may have occupied California and then western monarchs arose from that.”
Biologists also are discovering other routes that migratory butterflies may take. For instance, it’s possible that some eastern butterflies end up in the desert Southwest. From there, they journey between Mexico and California.
In this newly discovered eastern, western and Mexican fandango, “They kind of converge together. There’s a mix. They’re going both directions,” said Gail Morris, coordinator of the Southwest Monarch Study, which has tagged 18,000 butterflies in Arizona, Nevada, New Mexico, Utah, western Colorado and California’s southern deserts.
Some western butterflies avoid California altogether. Monarchs tagged in Oregon and Washington have been found in Idaho.
A few intrepid insects have even been seen flying across the Continental Divide.
“Populations are integrated in a way that we never imagined a long time ago,” said Pyle. “Everything we do to protect habitat in Mexico, Canada and the eastern United States has to do with everything that happens in the West — and vice versa.”
“As our butterfly populations are linked,” he said, “so we should be linked in our conservation.”