Studies have repeatedly shown a sharp decline in insects – but mostly only for individual countries. Now researchers have reviewed this trend on a global scale. The most extensive meta-study on this topic so far confirms that the frequency of insects worldwide is decreasing by almost one percent per year. The losses are greatest in parts of North America and in Europe, especially in Germany. In contrast, the amount of freshwater insects has increased by a good one percent a year, as the researchers report. Overall, however, the trends vary greatly depending on the region and habitat.
Insects are indispensable for ecosystems and the pollination of many crop plants would not be possible without these animal helpers. All the more worrying are the results of several studies that have seen drastic declines in insect numbers in recent years. In Germany, the biomass of flying insects has decreased by 76 percent in the last 27 years, and the number of all arthropods has even dropped by 78 percent in ten years. “Nevertheless, it remains unclear whether these declines also exist across geographical regions and habitats,” explain Roel van Klink from the German Center for Integrative Biodiversity Research (iDiv) and the University of Leipzig and his colleagues. After all, studies from other countries and regions have shown similar insect losses, but also less severe or even slight increases.
Land insects decrease by around one percent per year
To create more clarity, van Klink and his international team have now carried out the most extensive meta-study on this topic. To do this, they analyzed data from 166 long-term studies at 1,676 locations worldwide, in which frequencies and biomass were collected from insects and spiders. Some studies went back to 1925, but the average duration of the study was around 20 years. The researchers evaluated insect numbers separately for land insects and freshwater insects, and also recorded the values separately for countries and regions.
The evaluation confirms that the number of insects has also decreased worldwide – on average by 0.92 percent per year. However, the global shrinkage is significantly less than previously determined for example for flying insects in Germany. Nevertheless, the researchers see little reason to give the all-clear: “0.92 percent may not sound like much, but it means 24 percent fewer insects over 30 years and even a halving over 75 years,” explains van Klink. “Given the crucial role insects play in food webs and ecosystems, such a decline is worrying and could also be related to other changes, such as the decline in insectivorous bird populations.”
On the other hand, there is good news from rivers, lakes and pools, because the number of insects that spend their lives temporarily in the water, such as dragonflies, sandpipers and caddis flies, has increased on average by 1.08 percent per year, as the researchers report. However, since the frequency and not the diversity of the insects was also determined here, this says nothing about whether the biodiversity of these water inhabitants has also increased – in theory, a mass increase in mosquito larvae could also cause such increases.
Geographic and habitat-specific differences
At the same time, the results showed that there are significant geographic and habitat-specific differences – even between nearby locations. The frequency of flying insects and insects on the ground, for example in meadows, has decreased more. In contrast, the number of insects living in tree tops has remained almost the same. Insect loss was also comparatively low in areas with high agricultural use. In view of the often extensive monocultures, this may come as a surprise, but there is an explanation as the ecologist Christoph Scherber from the University of Münster, who was not involved in the study, explains: “This is a completely normal phenomenon: an increase in the mass of insects can mean that certain Insects in the agricultural landscape – or even pests – have increased. For example, the more cereals and oilseed rape are cultivated, the more hoverflies and rapeseed beetles there are. ”The meta study therefore does not reveal what role agriculture plays in the development of insect numbers and biodiversity.
The scientists also found significant differences between regions worldwide: The greatest decline is the decline in land-based insects in the west and midwest of the USA and in Europe. Germany is also one of the countries with particularly large losses. For Europe, the researchers also noticed a worsening trend: “Here, the slope for terrestrial insects became more and more negative over time and the decline was steepest from 2005,” they report. Insect loss has therefore increased measurably with us in recent years. In North America, on the other hand, it has rather declined over time. Based on their study, van Klink and his team cannot say why. In Asia and Africa, by contrast, the number of insects appears to be increasing or remaining the same.
However: “Our sources were not representative of the world,” say the researchers. “And even in North America and Europe, where most of the data came from, intensely changed locations were underrepresented. Protected areas were disproportionately well represented. ”But that means that the areas with the greatest human intervention have so far hardly been recorded – and this is where the insect decline could be particularly pronounced, as van Klink and his colleagues explain. In their assessment, the destruction of natural habitats in particular – especially through urbanization – seems to be pushing back land insects.
Source: Roel van Klink (German Center for Integrative Biodiversity Research, Leipzig) et al., Science, doi: 10.1126 / science.aax9931