Amid recent reports of declining insect populations that have raised alarm, a meta-analysis published today (April 24) in Science finds that the number of individual freshwater insects has increased, even as their terrestrial counterparts have declined. The authors point to “local-scale drivers,” such as changes in land use and water quality, that may be responsible for these trends, and state that more data from diverse geographical locations are needed to better understand insect population dynamics.
Previous studies have depicted insects in a dramatic downswing, including a 2017 study that found a 75 percent decrease in flying insect populations in western Germany over a period of 27 years, and a 2019 review that suggested insect biomass may drop by 2.5 percent per year, CNN reports.
“Our work included vastly more studies than others had,” says coauthor of the new study Roel van Klink of Leipzig University in comments to CNN. “It’s a much more nuanced picture.”
Van Klink and colleagues analyzed data from 166 studies conducted in 41 countries from 1925 to 2018. They determined that the number of terrestrial insects had decreased by roughly 9 percent per decade and freshwater insects increased by roughly 11 percent per decade. This increase doesn’t balance out the decline. The authors write in the paper that “fresh water represents only 2.4% of the earth’s terrestrial surface.”
“It is great news that some aquatic insects seem to be increasing, probably from a very low level,” Dave Goulson, an ecologist at the University of Sussex, tells The Guardian. “But the bulk of insects are terrestrial and this new study confirms what was already clear: they have been declining for many decades.”
Overall, terrestrial insects dropped by a rate of 0.92 percent per year, according to the study.
Van Klink tells CNN that “0.92% may not sound like much, but in fact it means 24% fewer insects in 30 years’ time and 50% fewer over 75 years.” He adds, “Insect declines happen in a quiet way, and we don’t take notice from one year to the next. It’s like going back to the place where you grew up. It’s only because you haven’t been there for years that you suddenly realize how much has changed, and all too often not for the better.”
“The decline across insect orders on land is jaw dropping,” Nick Haddad, a butterfly expert at Michigan State University who wasn’t part of the study, tells the Associated Press. “Ongoing decline on land at this rate will be catastrophic for ecological systems and for humans. Insects are pollinators, natural enemies of pests, decomposers and besides that, are critical to functioning of all Earth’s ecosystems.”
Alison Johnston, a quantitative ecologist at the Cornell Lab of Ornithology, tells Science News that the mix of data from diverse sites makes the study “much more thorough and analytically thoughtful” than earlier studies, but also points out that a large proportion of the data come from North America, a fact that the authors also note in the paper.
Van Klink says in remarks to The Guardian the despite these new findings, he doesn’t think it’s too late. “The increase in freshwater species makes us at least hopeful that if we put the right legislation in place, we can reverse these trends.”