Butterfly numbers have dropped by one third in the last two decades in the US, echoing declines seen in Europe. These figures raise alarm bells for the health of other insect populations, because butterflies face similar environmental changes and are used as a proxy for studying insects in general.
Much of what we currently know about declining insect populations comes from European monitoring programmes.
To find out if similar patterns were occurring in the US, Tyson Wepprich of Oregon State University and his colleagues turned to volunteers at the Ohio Lepidopterists, who have been collecting weekly data on butterfly sightings across the state over the last two decades.
“We analysed their data to estimate trends for 81 species over this time and found that many more are declining than increasing,” says Wepprich. “Overall, the number of butterflies you’d expect to see has fallen by 33%, or at a rate of 2% per year.”
As temperatures increased, Wepprich and his team found that species from the south moved north into Ohio and were growing in number, while the number of northern species shrunk.
“Insects are very sensitive to temperature, and these changes in some species suggest that they are responding to ongoing climate change,” says Wepprich.
It wasn’t only rare and vulnerable species whose numbers were decreasing.
“I was surprised that some common species that are adapted to live in human-dominated habitat, like agricultural or urban areas, were declining,” he says.
Common butterflies like the Cabbage White are not often considered to be in need of protection. “But we think this shows that the populations of some of the hardiest butterfly species may be affected by environmental changes,” says Wepprich.
The researchers believe habitat loss or fragmentation and agricultural practices have also probably made it harder for butterflies to survive. Nevertheless, some species find new ways to exploit these habitat changes.
“In Ohio, the wild indigo duskywing started using a non-native plant for food – crown vetch – which was widely planted along roadsides for erosion control,” says Wepprich. “Now, this butterfly’s population has tripled over the last 20 years.”
This is one of the most extensive insect monitoring programmes in North America, and only made possible through the “incredible effort” of the volunteers over so many years, says Wepprich. But we need to be careful of slow, gradual declines that we may not notice in the short-term, he says.
“I’m not a good naturalist, but I’ve started to use the iNaturalist app to learn insects around my yard and hope to contribute more observations that other scientists can use,” says Wepprich.
Journal reference: PLoS One, DOI: 10.1371/journal.pone.0216270
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