Scientists have warned that global insect populations are facing a rapidly accumulating decline in the most extensive analysis to date.
Researchers sifted through more than 166 long-term surveys carried out across 1,676 sites between 1925 and 2018. The found the number of insects is falling on average 0.92 percent every year, or about 9 percent every decade. The results have been published in Science.
First author Dr. Roel van Klink explained that though 0.92 percent might not sound like a lot, it adds up to 24 percent over 30 years—and 50 percent over 75.
"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 realise how much has changed, and all too often not for the better," said van Klink, an ecologist at the German Centre for Integrative Biodiversity Research (iDiv) and Leipzig University (UL).
While the global pattern is one of decline, the rate at which this is happening isn't heterogeneous. It fluctuates over time and geography. In some spaces, the study's authors even noticed positive trends, meaning numbers of insects were going up not down.
Specifically, the results suggest the number of midges, mayflies and other freshwater insects have been increasing on average around 1.08 percent each year. The authors cautiously attributed this result to effective water protection policies.
"These numbers show that we can reverse these negative trends," author Jonathan Chase, Professor of Biodiversity Synthesis at iDiv, said in a statement. "It makes us hopeful that we can reverse the trend for populations that are currently declining."
The authors found that populations of insects living in tree canopies appear to have remained relatively stable, while flying insects and ground-dwelling insects have experienced some of the sharpest declines.
The study suggests there can be stark differences between regions, and even between areas that are geographically close to one another. The researchers found that protected sites and those less affected by human activity had lower levels of decline than those most affected by urbanization.
In general, the steepest declines were recorded in the western and midwestern U.S. states and Europe—and in Germany, in particular, where a 2017 study published in PLOS ONE found numbers of flying insects had dropped more than 75 percent in less than 30 years.
According to the paper, had data from North America been excluded from the study, yearly declines would be 0.49 percent, not 0.92 percent. In Europe, the authors say trends have become increasingly more negative over time, with the most dramatic drop in insect numbers taking place since 2005.
"The phrase 'insect Armageddon' has captured the collective attention and shined a spotlight on one of the most numerous and diverse groups of organisms on the planet. Yet, insects are critically understudied," Maria Dornelas from the University of St Andrews, U.K., and Gergana N. Daskalova from the University of Edinburgh, U.K., said in an accompanying editorial, also published in Science.
Dornelas and Daskalova praise the nuancy of the paper, explaining that variation throws doubt to the idea that changing levels of biodiversity can be represented by "a single trend."
"The temptation to draw overly simple and sensational conclusions is understandable, because it captures the attention of the public and can potentially catalyze much needed action in policy development and research arenas. However, fear-based messages often backfire," they wrote.
"Embracing nuance allows us to balance accurate reporting of worrying losses with hopeful examples of wins. Hope is a more powerful engine of change than fear."
Tilly Collins, Senior Fellow at the Centre for Environmental Policy at Imperial College London, also praised the subtlety of the research, telling Newsweek: "Anything that adds to the detail and the subtlety underlying the "insect armageddon" is very useful."
There are, however, some limitations with the data sets available. For one, protected areas tend to be vastly overrepresented, comprising 34 percent of the sites studied but only 15 percent of landmass. North America and Europe account for the majority of data sets, leaving other areas of the world underrepresented and potentially skewing the results.
Collins, who was not involved in the research, explains that more detail, particularly in terms of different taxomony, can help paint a clearer picture in terms of which insect populations are increasing and which insect populations are decreasing across different parts of the world.
"Climate change and clean water policies are improving water quality in some areas," she said. "But there are a lot of areas around the tropics, where there is increasing eutrophication and increasing nutrient pressure into ecosystems that leads to insects declining quite dramatically."
The authors call for more comprehensive testing of human pressure and more data from underrepresented parts of the world to address these limitations and gain a broader understanding of trends in global ecosystems.
