A compilation of hundreds of international studies confirms that the population of terrestrial insects is in decline, with a decrease in biomass of the order of 24% over the past thirty years. However, in the same time frame, the number of insects living underwater (at least in part) increased by 36%.
These results come from the largest meta-analysis on changes in insect population carried out to date, and published in Science end of April,. It includes 1,670 analysis sites around the world. Researchers from the German Research Center for Integrative Biodiversity (iDiv), the University of Leipzig (UL) and the Martin Luther University of Halle-Wittenberg (MLU) are working with this gigantic database to fill knowledge gaps regarding the ” insect decline “
A meta-analysis to bring together all the studies
Many studies have been published in recent years, showing a dramatic decline in the insect population. The most impactful was that carried out in the West German nature reserves, announcing in 2017 a real slaughter: a reduction in biomass by 75% in 27 years. The study was then the subject of an important media treatment suggesting a “ generalized apocalypse For insects.
Other studies done in different places of the planet were also published before and after, sometimes showing decreases (more or less strong) and sometimes increases in the biomass of insects. However, to date, no one has combined all of the available data. Today it is done with this meta-analysis!
More varied and complex results than announced
The results, which include 166 surveys carried out in forty countries from 1925 to 2018, paint a more complex and nuanced picture than expected, illustrating, of course, an obvious decline in terrestrial insects, but also a net increase in freshwater insects . The analysis reveals a very large variation in trends, including in very close sites.
For example, in countries where multiple insect surveys have taken place (Germany, the United States, and the United Kingdom), some places have experienced sharp declines while others, fairly close, have reported no change, even recording an increase in the number of individuals.
When all the trends in the world were combined, an estimate of the average abundance of insects over time could be made. Overall, the population of terrestrial insects (butterflies, grasshoppers, or ants) decreases by about 0.92% per year, while that of freshwater insects (midges or mayflies) increases by 1, 08% per year. In other words, this implies a drop of 24% for the former on the three-decade scale, and a simultaneous increase of 36% for the latter.
What lessons for researchers?
In an attempt to explain this surprising good fortune of freshwater insects, German researchers have established several hypotheses: better protection of watercourses (with the passing of laws to this effect in many countries), a possible effect of the climate crisis (the rise in temperatures making living conditions more favorable for these species), or an increase in nutrients flowing in watercourses (due to agricultural discharges) could have contributed to this phenomenon.
Scientists also show in their meta-analysis that populations of terrestrial insects in protected areas are declining in the same way as those living in urbanized areas, but a little more slowly. On this basis, they believe that better protection or restoration of their habitat would be an effective strategy to preserve their population.