Even for people who aren't big fans of insects in general, it's hard to deny the appeal of butterflies. They tend to be colorful, graceful and charming, flitting serenely among flowers as they provide valuable pollination services.
Yet the popularity of butterflies still hasn't spared them from the potential extinction event currently threatening insects around the world. The decline of migratory monarch butterflies is well-known, but many other butterfly species are also among the estimated 40% of insect populations now in decline, a trend widely attributed to habitat loss, pesticides and climate change.
This could be a disaster, given the outsized importance of insects, but we still barely understand how bad it is. We don't have a lot of systematic, long-term data for most insect species, and in many cases we have none. Concerns about an insect apocalypse began largely with anecdotal evidence — people seeing fewer fireflies and butterflies than they did years ago, for example, or noticing insect-free windshields on road trips — and have only recently been backed up by more rigorous research.
Scientists are still hindered by a shortage of historical data, though, inspiring some entomologists and ecologists to pore over whatever long-term records do exist, hoping for any insight into what's happening today. That recently led a team of researchers to Ohio, where trained volunteers have spent the past two decades counting butterflies for "the most extensive, systematic insect survey in North America," as the researchers describe it in a new study.
Pollinators in peril
Their study, published this month in the journal PLOS One, examined more than 24,000 butterfly surveys conducted by citizen scientists across Ohio from 1996 to 2016. This deep dataset let the researchers estimate population trends for 81 species, and the results were alarming: The state's overall butterfly abundance fell by 2% per year, leading to a decline of 33% in just 21 years.
Although this study is limited to one group of insects in one U.S. state, its relevance extends beyond just butterflies in Ohio. It provides a baseline to inform research on butterflies elsewhere, as well as the broader plight of other insects that face a similar mix of threats. Plus, by casting such a wide net, it shows this is not only a problem for species that are already endangered.
"These declines in abundance are happening in common species," says Oregon State University researcher Tyson Wepprich, who led the study, in a statement. "Declines in common species concern me because it shows that there are widespread environmental causes for the declines affecting species we thought were well-adapted to share a landscape with humans. Common species are also the ones that contribute the bulk of the pollination or bird food to the ecosystem, so their slow, consistent decline is likely having ripple effects beyond butterfly numbers."
The butterfly crash is widespread across Ohio, but population trends also vary widely from species to species. Butterflies with more northern distributions and fewer annual generations are suffering the steepest declines, the researchers found, noting those species are adapted to cooler climates and might face an especially bleak future as Ohio warms up due to climate change. Some species are stable or increasing, yet even their fortunes seem heavily influenced by human activities.
The wild indigo duskywing, for example, "is doing really well in Ohio because a plant it eats is used as erosion control on construction sites," Wepprich tells PBS Newshour. "It's three times more numerous now than it was 20 years ago."
Among the species with a clear up or down trend, however, the study found about 75% declined during the monitoring period.
A wing and a prayer
The hackberry emperor butterfly, pictured here at Ellis Lake Wetlands in Ohio, is named after the hackberry trees where it lays its eggs. (Photo: Andrew Cannizzaro/Flickr)
These findings are bad enough on their own. Butterflies not only help pollinate many native plants, but both adults and caterpillars are also key food sources for birds, amphibians, mammals and even other insects. On top of what it tells us about butterflies, however, the study also sheds light on the larger insect crisis. That's because, as Wepprich explains, the prominence and popularity of butterflies make them useful proxies to study what's going on with insects in general.
"Because it's easier to monitor butterflies than other insects — lots of people like butterflies and enjoy keeping track of them — butterflies tend to be the best source of abundance data for tracking insect population declines and increases," Wepprich says. "Environmental assessments use them as an indicator for the general trajectory of biodiversity since they experience the same types of pressures from land-use changes, climate change and habitat degradation as other insect groups."
This study also stands out because of its wide scope. Long-term studies of butterfly abundance often focus on individual species, Wepprich says, especially rare or iconic ones thought to be at risk of collapse. That's how we know eastern North American monarchs have declined by more than 85% over the past two decades, for example, and that western North American monarchs have fallen by more than 95%. In this study, however, citizen scientists helped reveal a much broader butterfly crisis.
"Monarchs and rare species were monitored because people are worried about them going extinct," Wepprich says. "In Ohio, they monitored every species they could and found declines in species previously not on the radar for conservation."
That may be discouraging, but it also illustrates how everyone can help protect butterflies and other insects. This 21-year dataset only exists thanks to citizen scientists who volunteered their time to monitor butterflies, and their findings suggest the insects' fates are closely linked to what humans are doing in their habitats. And while none of us can solve the problem on our own, we can do things like help monitor insect populations, avoid insecticides and grow plants that support beneficial insects.