Monarch butterflies, with their distinctive orange, black and white colors, are perhaps the most familiar butterflies in North America. They are also famous for making their annual late-summer migration from southern Canada and the northern and central U.S., all the way south to California, Florida and Mexico. During these migrations, the butterflies travel thousands of miles and suffer considerable risk of mortality from factors such as poor weather, predation, pesticides and vehicle strikes.
For many years, scientists have recorded decreasing numbers of monarchs in their wintering colonies in the south. In 2015, the U.S. Fish and Wildlife Service reported a study that showed nearly a billion monarchs had vanished from the overwintering sites since 1990. Various researchers have warned that the butterflies are dying off in droves and that some populations are in danger of imminent extinction. However, a new research study from the University of Georgia has found that the summer population of monarchs has remained relatively stable over the past 25 years.
“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.”
In the current study, which is the largest and most comprehensive assessment of breeding monarch butterfly populations to date, researchers analyzed more than 135,000 observations from the North American Butterfly Association. These records were made between 1993 and 2018 by citizen scientists across North America, who took part in two-day butterfly surveys every summer. Each group of observers had a defined circle to patrol that spanned about 15 miles in diameter, and the observers counted all the butterflies they saw, including monarchs.
The findings of this analysis, published today in the journal Global Change Biology, indicate an overall annual increase of 1.36 percent in the relative abundance of monarchs in their summer locations. This is contrary to the declines reported for wintering populations in Mexico and California in the past few years, and suggests that the successful survival and summer breeding in North America makes up for the winter losses.
As Davis explained, the marathon migratory journey to Mexico or California each fall 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 getting to their overwintering grounds alive.
“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.”
One of the major concerns for conservationists is that the abundance of milkweed has declined across the monarch’s range, as GMO crops are sprayed with herbicide to kill off unwanted weeds. Milkweed is the sole source of food for monarch caterpillars and its loss, due to replacement by crops and destruction by herbicides, has significantly reduced the distribution of this plant. Previous research has attributed the 90 percent decline in overwintering numbers of the eastern monarch population to the loss of breeding habitat and milkweed.
However, Davis believes this current 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.”
However, the loss of millions of acres of milkweed habitat may be bringing about some subtle changes in monarch migratory behavior. 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 be moving away from the annual migration to Mexico. San Francisco, for example, hosts monarchs year-round because people plant non-native tropical milkweed in their gardens and parks. And Florida is experiencing fewer freezes each year, making its climate more attractive 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 William Snyder, co-author of the paper and a professor in UGA’s College of Agricultural and Environmental Sciences. “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 authors warn that their findings do not necessarily mean that everything is rosy for monarch butterfly populations. Environmental conditions may change for monarchs as rising global temperatures 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 Snyder. “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.”