Monarch butterfly populations are thriving in North America
Summer numbers have remained stable for 25 years despite dire warnings
Peer-Reviewed PublicationVIDEO: MONARCH BUTTERFLIES. view more
CREDIT: U.S. FISH & WILDLIFE SERVICE
For years, scientists have warned that monarch butterflies are dying off in droves because of diminishing winter colonies. But new research from the University of Georgia shows that the summer population of monarchs has remained relatively stable over the past 25 years.
Published in Global Change Biology, the study suggests that population growth during the summer compensates for butterfly losses due to migration, winter weather and changing environmental factors.
“There’s this perception out there that monarch populations are in dire trouble, but we found that’s not at all the case,” said Andy Davis, corresponding author of the study and an assistant research scientist in UGA’s Odum School of Ecology. “It goes against what everyone thinks, but we found that they’re doing quite well. In fact, monarchs are actually one of the most widespread butterflies in North America.”
The study authors caution against becoming complacent, though, because rising global temperatures may bring new and growing threats not just to monarchs but to all insects.
“There are some once widespread butterfly species that now are in trouble,” said William Snyder, co-author of the paper and a professor in UGA’s College of Agricultural and Environmental Sciences. “So much attention is being paid to monarchs instead, and they seem to be in pretty good shape overall. It seems like a missed opportunity. We don’t want to give the idea that insect conservation isn’t important because it is. It’s just that maybe this one particular insect isn’t in nearly as much trouble as we thought.”
This study represents the largest and most comprehensive assessment of breeding monarch butterfly population to date.
The researchers compiled more than 135,000 monarch observations from the North American Butterfly Association between 1993 and 2018 to examine population patterns and possible drivers of population changes, such as precipitation and widespread use of agricultural herbicides.
The North American Butterfly Association utilizes citizen-scientists to document butterfly species and counts across North America during a two-day period every summer. Each group of observers has a defined circle to patrol that spans about 15 miles in diameter, and the observers tally all butterflies they see, including monarchs.
By carefully examining the monarch observations, the team found an overall annual increase in monarch relative abundance of 1.36% per year, suggesting that the breeding population of monarchs in North America is not declining on average. Although wintering populations in Mexico have seen documented declines in past years, the findings suggest that the butterflies’ summer breeding in North America makes up for those losses.
That marathon race to Mexico or California each fall, Davis said, may be getting more difficult for the butterflies as they face traffic, bad weather and more obstacles along the way south. So fewer butterflies are reaching the finish line.
“But when they come back north in the spring, they can really compensate for those losses,” Davis said. “A single female can lay 500 eggs, so they’re capable of rebounding tremendously, given the right resources. What that means is that the winter colony declines are almost like a red herring. They’re not really representative of the entire species’ population, and they’re kind of misleading. Even the recent increase in winter colony sizes in Mexico isn’t as important as some would like to think.”
Changing monarch migration patterns
One concern for conservationists has been the supposed national decline in milkweed, the sole food source for monarch caterpillars. But Davis believes this study suggests that breeding monarchs already have all the habitat they need in North America. If they didn’t, Davis said, the researchers would have seen that in this data.
“Everybody thinks monarch habitat is being lost left and right, and for some insect species this might be true but not for monarchs,” Davis said. If you think about it, monarch habitat is people habitat. Monarchs are really good at utilizing the landscapes we’ve created for ourselves. Backyard gardens, pastures, roadsides, ditches, old fields—all of that is monarch habitat.”
In some parts of the U.S., monarchs have a year-round or nearly year-round presence, which leads some researchers to believe the insects may in part be moving away from the annual migration to Mexico. San Francisco, for example, hosts monarchs year-round because people plant non-native tropical milkweed. And Florida is experiencing fewer freezes each year, making its climate an alternative for monarchs that would normally head across the border.
“There’s this idea out there about an insect apocalypse—all the insects are going to be lost,” said Snyder. “But it’s just not that simple. Some insects probably are going to be harmed; some insects are going to benefit. You really have to take that big pig picture at a more continental scale over a relatively long time period to get the true picture of what’s happening.”
The study was funded by grants from the USDA National Institute of Food and Agriculture.
The paper was co-authored by Timothy Meehan, of the National Audubon Society; Matthew Moran, of Hendrix College; and Jeffrey Glassberg, of Rice University and the North American Butterfly Association. Michael Crossley, who worked on the study as a postdoctoral researcher in the Department of Entomology and is now at the University of Delaware, is first author of the paper.
CAPTION
Andy Davis, assistant research scientist in the University of Georgia's Odum School of Ecology
CREDIT
UGA
Video footage of monarch butterflies is available at the following link https://www.dropbox.com/s/qrv6of01skeibi7/Monarch_B_Roll.mp4?dl=0
Credit: U.S. Fish and Wildlife Service
JOURNAL
Global Change Biology
ARTICLE TITLE
Opposing global change drivers counterbalance trends in breeding North American monarch butterflies
ARTICLE PUBLICATION DATE
10-Jun-2022
Despite dire warnings, monarch butterfly numbers are solid
New study shows warmer temperatures and increases during the summer are compensating for negative factors, stabilizing breeding trends
Peer-Reviewed PublicationIMAGE: A NEW RESEARCH STUDY LED BY UD’S MICHAEL CROSSLEY TITLED “OPPOSING GLOBAL CHANGE DRIVERS COUNTERBALANCE TRENDS IN BREEDING NORTH AMERICAN MONARCH BUTTERFLIES” PUBLISHED IN GLOBAL CHANGE BIOLOGY ON JUNE 10. view more
CREDIT: PHOTO BY MICHELE WALFRED/ UNIVERSITY OF DELAWARE
Largely because of well-publicized, diminishing winter colonies in Mexico and California, monarch butterflies across North America have been long thought to be declining as a result of diminishing summer habitat. Previous butterfly research shows that the size of overwintering monarch colonies has fallen across several decades. But what is happening when monarchs breed in the summer was less clear.
In a new research study published in Global Change Biology, lead author Michael Crossley, assistant professor and agricultural entomologist at the University of Delaware, and his collaborators examined trends in breeding monarchs across their entire range, and found there are local regions of decline, but also regions of increase. When considering the entire species range, there was no overall decline, and in fact, even a slight increase in abundance.
“The whole reason that we did this research is because monarch colonies have been declining,” said Crossley. “Up until now, there were real fears that the monarch is in trouble.”
The study’s results suggest that population growth in summer is compensating for losses during the winter. Additionally, findings indicate that changing environmental variables have offsetting effects on deaths and reproduction.
North America’s three largest countries have more in common than playing the 2026 hosts of soccer’s biggest spectacle — the FIFA World Cup. The perception of the monarchs’ decline had prompted the U.S. Fish and Wildlife Service to determine if the species warrants a higher level of protection. Canada’s equivalent agency is strongly considering listing monarchs as an endangered species because of the declines in Mexico.
As the researchers analyzed the data, patterns emerged and were location specific. Portions of the U.S. Northeast and parts of the Midwest revealed population declines. The U.S. Southeast and Northwest were unchanged or increasing, yielding a slightly positive overall trend across the species range. Monarchs in Florida appear to be doing exceptionally well.
“Our question was, ‘Are monarchs declining across their breeding range?’ The key take home message was yes, monarchs are declining in some places, but increasing in other locations,” said Crossley, who conducts research and teaches in the UD Department of Entomology and Wildlife Ecology. “There is no consistent, long-term trend.”
So how is that possible? Near agricultural fields that use glyphosate, which is used in products such as Roundup, sites typically had fewer monarchs. This occurrence was especially evident in the upper Midwest — the corn and soy belt. The researchers also found positive effects of temperature, which is generally increasing faster in northern latitudes, which seems to be counterbalancing glyphosate effects.
“These two effects seem to cancel each other out, especially in the upper Midwest,” said Crossley.
Crossley contends that the different trends in these locations are related to the environment.
“For example, there are populations now that will overwinter in California, Florida or even southern Georgia because it’s become more feasible with increasing winter temperatures,” Crossley said. “We may see fewer migrants because monarchs are doing well, staying in Florida and not joining the migration.”
Since accelerating climate change may bring growing threats, Crossley cautioned against complacency.
The dataset that the researchers used was made possible by the North American Butterfly Association and thousands of citizen scientists — volunteers and butterfly enthusiasts — who collected butterfly counts for the past 25 years at more than 400 sites around the continent.
Another conclusion of the study is that other butterfly species are facing greater peril than monarchs. The authors argue that some of these butterflies — like Poweshiek skipperling (in the upper Midwest), Florida leafwing (in extreme southern Florida) and Hermes copper (in extreme southern California) — deserve more attention. Though monarchs are the most popular among the general public, they are far from the only type of butterfly. The North American Butterfly Association observes more than 450 species. Though it’s facing overall population declines, the monarch is doing comparatively well; 320 species face steeper population losses.
“It seems like breeding monarchs in North America are not in trouble despite winter colony declines,” said Crossley. “This could lead to several explanations about why a disconnect exists between numbers in the breeding range and numbers in overwintering sites.”
The grueling journey
The findings show that monarchs are breeding in strong numbers. Then the insects trek to places like Mexico to overwinter, but these colonies are generally shrinking in size. So, what is hindering the journey southward to the winter colonies? Study co-author Andy Davis of the University of Georgia has investigated parasites of monarchs and found that a protozoan parasite, Ophryocystis elektroscirrha (OE), is on the rise, and could affect the success of the fall migration. As Davis described, making that journey with parasites in tow is equivalent to running a marathon with the flu. And monarchs’ total mileage, which falls between 1,200 and 2,800 miles, far eclipses human’s 26.2-mile mark for a marathon.
“A lot of monarchs are not going to make that southward journey if they are infected,” said Crossley. “One way to reconcile the disconnect [between breeding numbers and overwinter numbers] is to say, well, maybe monarchs have an incredible ability to rebound every spring and summer from relatively small overwintering sizes. And the decline is occurring during this southward migration.”
Besides parasites, other factors are affecting monarchs’ migration. Climate change is making the journey south hotter and drier, making the extended trip more arduous for these small creatures. Additionally, many butterflies use certain valleys as their flyways, which are unfortunately along highways. Cars and trucks are simply running into the iconic insects, something Crossley points out could potentially be amended.
“For other types of species, officials reduce speed limits or reroute traffic during certain times of year, so these animals can cross the road,” said Crossley.
Finally, when you’re on a long trip, you always stop and grab a bite to eat, right? Monarchs feed on nectar. Loss of habitat and nectar resources mean more butterflies lack the energy to fly south. The average person can take action on their own and plant native, fall-blooming goldenrods, asters and perennial sunflowers to fuel the monarchs.
“We certainly can’t go wrong by making more insect habitats. Whether humans or animals, we could all use more,” Crossley said. “We can do that with native plants right in our own backyards.”
The paper was co-authored by the University of Georgia's Davis and William Snyder; Timothy Meehan, of the National Audubon Society; Matthew Moran, of Hendrix College; and Jeffrey Glassberg, of Rice University and the North American Butterfly Association.
JOURNAL
Global Change Biology
SUBJECT OF RESEARCH
Animals
ARTICLE TITLE
Opposing global change drivers counterbalance trends in breeding North American monarch butterflies
ARTICLE PUBLICATION DATE
10-Jun-2022
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