In twelve research articles and two opinion pieces, researchers from Europe, America and Australia make clear that globally, both the number of species and individuals and the biomass of insects are decreasing and the communities are becoming more uniform overall. "Just as landscapes are becoming more similar, for example in agricultural land, insect communities are also getting more alike," says co-editor of the special issue Martin Gossner of the Swiss Federal Institute for Forest, Snow and Landscape Research WSL. This trend is occurring in numerous ecosystems worldwide, on land and in water.
A consequence of human activities
Based on the studies collected, the main drivers are land use change, climate change and the spread of non-native invasive species. In addition, these drivers often interact with each other and thus amplify negative effects. For example, ecosystems damaged by intensive land use along with their insect communities are more sensitive to climate change. Furthermore, invasive species can gain a foothold in ecosystems that have been damaged by land use and displace native species.
"It looks as if it is the specialized insect species that suffer most, while the more generalized species tend to survive. This is why we are now finding more insects capable of living nearly anywhere while those species that need specific habitats are on the wane," says Florian Menzel from the Johannes Gutenberg University Mainz. The consequences of this development are numerous and generally detrimental for the ecosystems.
Among the examples mentioned in the special issue is that the decline in bumblebee species diversity has led to a decrease in plants that rely on certain bumblebee species for pollination. Dwindling species diversity reduces the stability of ecosystems: Fewer species means fewer species that pollinate plants or keep pests at bay. And there is simply less food available for insect-eating birds and other animals. Thus, a decline in insects can also lead to their decline.
More hedges and deadwood
Even if the changes vary greatly locally and are not yet understood in detail, Gossner is certain: "We know enough about the insects' condition to take action." This means: "The landscapes need to be revitalised and more diversity needs to be brought back into the habitats." In the forest, more structural elements such as deadwood, old trees with hiding places and areas with lots of light are needed, as well as a greater variety of landscapes in general. In agricultural land, hedges and woody plants are a central element for the diversity of insects, but also the birds and bats that depend on them.
Furthermore, the authors of the special issue advise establishing interconnected nature reserves. They allow species to migrate from one habitat to another, for example from areas that have become inhospitable due to global warming to higher or more northerly, cooler regions. More attention also needs to be paid to reducing the spread of invasive species through the global movement of goods and travel. For example, introduced insectivorous fish in Brazil have led to a sharp decline in freshwater insects.