Impoverished flora in grasslands and forests threatens insects

The intensification of land use poses a major threat to biodiversity, especially herbivorous insects and their host plants. If beetles, Orthoptera (grasshoppers / crickets), Heteroptera (true bugs) and Auchenorrhyncha (cicadas / leafhoppers / leafhoppers / grasshoppers / sunfish) specialize in only one or a few plant species, they must migrate or else they do. become extinct locally when their host plants disappear. On the other hand, if an insect can feed on a wide range of species, it is able to survive even if the number of plant species decreases. The interaction of species from different groups of organisms is ultimately a decisive factor in the stability of ecosystems.

Researchers from several research institutes in Germany and Switzerland, led by the WSL, have studied these interactions between plants and insects. For the study, funded by the Swiss National Science Foundation (SNSF) as part of the German Research Foundation’s (DFG) Biodiversity Exploration Priority Program, they examined the diversity of plants and insects, and the interactions between them, in three natural landscapes in Germany. : the Swabian Alps (Baden-Württemberg), Hainich (Thuringia) and Schorfheide (Brandenburg). The study covered a range of intensively to non-intensively exploited meadows and pastures (meadows), as well as beech and conifer forests managed in different ways, to learn more about the interaction between plant species and insect species that make up networks. Landscapes, each covering an area of ​​up to 1,300 km², encompass enough differently managed spaces to provide statistically reliable results.

The researchers speculated that they would find widely diverse insect communities in the areas examined, which include a mix of near-natural ecosystems and those heavily used by humans. “Since some of these areas also contain insect species specializing in a few food plants, we expected to obtain new information on the consequences of intensive land use for the ecological stability of grasslands and forests,” says WSL entomologist Martin Gossner, who led the study project. In total, the researchers recorded 531 species of plants and 1,053 species of insects and their abundance on 289 long-term sample plots.

More plant diversity means more stable insect communities

The study found that plant-insect networks in lightly grazed grasslands included at least 70 species of plants and 80 species of herbivorous beetles, Orthoptera, Heteroptera and Auchenorrhyncha. For example, the wild carrot, a plant typical of moderately grazed pastures, provides food for many specialized beetle species. In meadows and pastures that are frequently mown or fertilized, an average of only 40 species of plants and 60-70 species of insects have been detected.

In newly unmanaged forests with dense tree cover, biodiversity was significantly lower than in open forests, with an average of 25 plant species and 30 of the above-mentioned insect species. Insects that depend on only a few species of trees or grasses for food cannot survive there. In contrast, in forests with many gaps in the canopy, a lot of light penetrates down to the ground, and as a result up to 80 species of plants and 50 species of herbivorous insects from the groups studied have been found there. “The light promotes a diverse range of plants, which in turn provide food for more insect species. At the same time, insect species are less likely to become extinct locally, so the system is more stable “, explains Felix Neff, researcher at WSL, principal author of an article which has just been published in the journal ScienceAdvances. An example of the stabilizing effect of light-flooded forests is nettle, which promotes such environments and is a food source for many specialized caterpillars, weevils, Auchenorrhyncha and Heteroptera. “As a doctoral student, I found this research fascinating, and not just because of the large areas rich in species that we studied. For me, the collaboration with the many interdisciplinary research groups has also been rewarding, ”says Neff. .

Results must be transferable to Switzerland

Promoting more open forests improves the diversity not only of ground plants, shrubs and trees, but also of insect species that benefit from the diversity of plants. Mixed stands made up of different deciduous and coniferous trees are also beneficial, and should also prove to be more stable in the face of ongoing climate change. On the other hand, where plant diversity decreases, the recorded insect diversity, and therefore overall biodiversity, also decreases. Such ecosystems are being depleted, in other words.

For grasslands, researchers recommend moderate grazing rather than intensive mowing to promote diverse and stable insect communities. “These discoveries can also be transferred to Switzerland, for example on the Swiss plateau, the Jura or the low altitudes of the Alpine foothills”, explains Martin Gossner. “From the outset, the aim of the Exploratories project has been to draw conclusions that apply to different regions of Europe.”

Provided by Swiss Federal Institute for Forest, Snow and Landscape Research WSL