Monarch butterfly populations are thriving in North America
For years, scientists have warned that monarch butterflies are dying off en masse as a result of declining winter colonies. But new research from the University of Georgia shows that summer monarch populations have remained relatively stable over the past 25 years.
Published in Global Change BiologyThe study suggests that population growth during the summer offsets butterfly losses due to migration, winter weather and changing environmental factors.
“There’s a perception that monarch populations are in big trouble, but we found that’s not the case at all,” said Andy Davis, corresponding author of the study and an assistant researcher at UGA’s Odum School of Ecology. “It goes against what everyone thinks, but we found that they do quite well. In fact, monarchs are one of the most common butterflies in North America.”
However, the study authors caution against becoming complacent, as rising temperatures on Earth could pose new and growing threats not just to monarchs, but to all insects.
“There were once widespread butterfly species that are now in trouble,” said William Snyder, co-author of the paper and a professor in UGA’s College of Agricultural and Environmental Sciences. “Instead there is so much focus on monarchs, 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 protecting insects isn’t important, because it is.” well, just that this one particular insect may not be as much of a problem as we thought.”
This study represents the largest and most comprehensive assessment of the monarch butterfly breeding population to date.
The researchers collected more than 135,000 monarch observations from the North American Butterfly Association between 1993 and 2018 to examine population patterns and possible causes of population change, such as precipitation and widespread use of agricultural herbicides.
The North American Butterfly Association uses citizen scientists to document butterfly species and counts in North America over a two-day period each summer. Each group of observers has a defined circle to patrol about 24 miles in diameter, and the observers count all the butterflies they see, including monarchs.
By carefully examining 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. While wintering populations in Mexico have seen documented declines in recent years, the findings suggest that the summer brood of the butterflies in North America is making up for those losses.
That marathon race to Mexico or California every fall, Davis said, could get more difficult for the butterflies as they deal with traffic, inclement weather and more obstacles along the way south. So fewer butterflies reach the finish line.
“But if they come back north in the spring, they can really make up for those losses,” Davis said. “A single female can lay 500 eggs, so they’re capable of huge recovery, with the right resources. What that means is the winter colony decline is almost like a red herring. They’re not really representative of the whole species.” “population, and they’re a bit misleading. Even the recent increase in winter colonies in Mexico isn’t as significant as some would like to think.”
Changing Migration Patterns of Monarchs
One concern for conservationists has been the alleged national decline of milkweed, the sole food source for monarch caterpillars. But Davis believes this study suggests 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.
“Everyone thinks monarch habitat is being lost left and right, and for some insect species this may be true, but not for monarchs,” Davis said. If you think about it, the monarch’s habitat is a human habitat. Monarchs are very good at taking advantage of the landscapes we have created for ourselves. Backyard gardens, meadows, roadsides, ditches, old fields – all this is a monarchical habitat.”
In some parts of the US, monarchs are present year-round or almost year-round, leading some researchers to believe that the insects are partially moving away from the annual migration to Mexico. San Francisco, for example, hosts monarchs year-round as people plant non-native tropical milkweed. And Florida experiences fewer freezes each year, making the climate an alternative to monarchs who would normally cross the border.
“There’s an idea about an insect apocalypse — all insects will be lost,” Snyder said. “But it’s just not that simple. Some insects will probably be harmed; some insects will benefit. You really have to take that big pig photo on a more continental scale over a relatively long period of time to get the true picture of what is going on.”
The paper was co-authored with 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 now works at the University of Delaware, is the lead author of the paper.
Monarch butterflies are increasingly plagued by parasites
Opposing drivers of global change offset breeding trends of North American monarch butterflies, Global Change Biology (2022).
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