Scientists have been warning for years that monarch butterflies are vanishing in droves due to dwindling winter colonies. However, according to recent research from the University of Georgia, monarch butterfly populations have stayed reasonably steady over the last 25 years.
The study, which was published in the journal Global Change Biology, reveals that butterfly population growth throughout the summer compensates for losses caused by migration, winter weather, and changing environmental conditions.
“There’s this perception out there that monarch populations are in dire trouble, but we found that’s not at all the case,” says Andy Davis, the study’s corresponding author. “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 authors of the study warn against complacency, noting that rising global temperatures may pose new and expanding hazards to all insects, not only monarchs.
“There are some once widespread butterfly species that now are in trouble,” adds William Snyder, co-author of the study. “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 research is the largest and most thorough examination of the breeding monarch butterfly population to date.
The researchers examined population patterns and likely drivers of population shifts, such as precipitation and widespread use of agricultural pesticides, by compiling more than 135,000 monarch observations from the North American Butterfly Association between 1993 and 2018.
During a two-day period every summer, the North American Butterfly Association uses citizen scientists to document butterfly species and numbers across North America. Each set of observers has a defined patrol area of about 15 miles in circumference, and they count all butterflies they encounter, including monarchs.
By carefully analyzing monarch observations, the researchers discovered a 1.36 percent annual rise in monarch relative abundance, showing that the monarch breeding population in North America is not diminishing on average. Although there have been recorded declines in wintering populations in Mexico in recent years, the data show that the butterflies’ summer breeding in North America compensates for those losses.
According to Davis, the butterflies’ annual marathon journey to Mexico or California is becoming more challenging as they encounter traffic, bad weather, and other hurdles along the way south. As a result, fewer butterflies make it to the finish line.
“But when they come back north in the spring, they can really compensate for those losses,” Davis adds. “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 .”
Migration patterns of monarch butterflies are changing.
The alleged countrywide drop in milkweed, the only food source for monarch caterpillars, has been a subject of concern for conservationists. However, Davis argues that breeding monarchs in North America currently have all of the habitat they require. Davis claims that if they hadn’t, the researchers would have noticed it in the data.
“Everybody thinks monarch habitat is being lost left and right, and for some insect species this might be true but not for monarchs,” adds the author. If you think about it, the monarch habitat is people’s 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.”
Monarch butterflies can be found year-round or nearly year-round in some parts of the United States, leading some scientists to assume the insects are deviating from their annual migration to Mexico. People in San Francisco, for example, cultivate non-native tropical milkweed, which attracts monarch butterflies all year. And, with fewer freezes each year, Florida’s environment is becoming a viable alternative for monarch butterflies that would otherwise migrate across the border.
“There’s this idea out there about an insect apocalypse—all the insects are going to be lost,” Snyder remarks. “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.”
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