Or so we have always thought.
A study published today in the Annals of the Entomological Society of America complicates the commonly held notion that monarchs going missing by the hundreds of millions (a report from the U.S. Fish and Wildlife Service this February said that 970 million individuals have been lost since 1990).
By looking at a different set of data than the one most conservationists have always relied on, the researchers concluded that the yearly peak monarch population has not declined nearly as steeply as was previously believed.
To understand the new finding, first you have to understand monarchs.
The butterfly is a keystone species — one that has a disproportionate effect on its ecological community relative to its population size — and its migration across North America is considered one of the continent’s greatest natural treasures. Monarchs are the only butterfly known to make an annual two-way migration as birds do.
The 3,000-mile journey takes several months and multiple generations of the butterfly. It starts each year in the mountains of southern Mexico, where the monarchs that flew down the previous fall have roosted all winter. As springtime approaches, the butterflies begin to move north, stopping at some point to mate and lay eggs on milkweed plants. Those offspring pick up where their parents left off, flying for several weeks before mating and laying the next generation of travelers. It can take three to four generations for the species to make it to their summer breeding grounds in the northern U.S. and Canada.
Unlike the meandering springtime journey, the reverse trip is made in one shot by a single “migratory generation.” Though summer monarchs live and die in a matter of weeks, the south-bound butterflies must last up nine months, long enough to endure the return trip south and the several-month stay in Mexico.
Until now, the vast majority of research on the monarch population has relied on counts of their winter numbers. Each year conservationists follow the butterflies to their sanctuaries in Mexico to conduct an annual census. It is by far the easiest and most efficient way to determine their population size — in Mexico, the monarchs roost by the millions in a relatively contained area, rather than fluttering about across a vast and chaotic continent.
In recent years, the results of the winter count have been alarming. In 2014, researchers announced their lowest census yet: only 34 million, a decline of more than 90 percent since the survey began in 1993. This year’s count showed a modest improvement to 56.5 million. The numbers have gotten so bad that this winter the Fish and Wildlife Service took the first steps toward considering whether the monarch should be labeled an endangered species.
But the new study, which is part of a collection of seven papers that look specifically at non-winter numbers, suggests that the Mexico census may not show the whole picture. Though the number of wintering monarchs is undoubtedly in decline, their population seems to rebound during the warmer months.
“Spring and summer recruitment is the key to monarch resilience,” the authors wrote.
That conclusion comes from analysis of two summer censuses conducted mostly by “citizen scientists,” civilian lepidopterists who make up for their lack scientific degrees with conservationist zeal. Overseen by specialists with the North American Butterfly Association (NABA) and the Illinois Butterfly Monitoring Network (IBMN), volunteers head out each year to track down and record the number of monarchs within a given area.
Though the size of the summer population fluctuated wildly from year-to-year, the results of these censuses showed no significant downward trend, the researchers reported. What’s more, the size of the wintering generation didn’t seem to affect the summer population’s ability to bounce back.
“I think we’ve really been focused too much on Mexico,” University of Georgia ecologist Andy Davis, who organized the newly published monarch collection, told The Washington Post. “There’s no other critter on the planet where managers and conservationists track the population size by looking at numbers during the overwintering stage.”
He called the annual monarch migration a “grand marathon of the insect world,” and said that scientists have been focused on the wrong end of the race.
“If you were charged with figuring out how many people are participating in 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 said. “We’ve been doing it backwards.”
Whatever dangers are devastating the monarchs during the fall migration and overwintering season, they seem to be rebounding during the spring and summer months. Which means conservationists have been worried about the wrong problem.
“What is declining is the migration. It’s not the monarch itself,” Davis said.
Lincoln Brower, a biologist at Sweet Briar College in Virginia and the “grandfather” of monarch conservation, has a more skeptical take.
“There’s a lot of interesting research here that advances a lot of monarch knowledge,” he said, looking at the studies. “But I don’t agree with their conclusion.”
While the authors of the summer census study conclude that there has been no statistically significant decline in the monarch’s summer numbers, Brower says that he sees a downward trend in the same data starting in 2007 — the same year that the winter census started to drop off precipitously.
Brower, who was not involved in the collection, defended the significance of the Mexico census and cast some doubt on the summer counts. He said that the data could be biased to make it seem like there were more monarchs because the summer counts are conducted in areas where the butterflies are known to congregate. It would be like judging the abundance of politicians in Washington by sitting on the steps of the Capitol building.
But both scientists agree that, regardless of the summer numbers, the threat to the monarch migration is cause for concern.
One explanation for the precipitous decline of the overwintering numbers comes from a second paper from the collection, written by a citizen scientist in Pennsylvania who has been tagging and monitoring monarchs for the past 18 years. The author, Gayle Steffy, found that monarchs who started their fall migration late in the year were far less likely to make it to Mexico.
Brower and Davis both fear that global warming has encouraged monarchs to fly farther and farther north during the summer months, venturing into areas of Canada that they normally wouldn’t reach but which now offer more suitable climates for them. That means that the fall migrants have to cover a lot more distance to reach their overwintering grounds. Those who start late, as Steffy’s study showed, aren’t making it in time to beat the cold.
“If they continue to have problems making it to Mexico what it’s going to mean is they’re going to lose their migration,” Davis warned. “At some point there’ll be a tipping point and they simply won’t get there, or they will shift their behavior and simply won’t migrate any more.”
“We may not lose the monarchs,” he continued, “but losing the migration would be tragic.”