The significance of forest biotopes for insect biomass and richness in normal landscapes

Based on a stratified sample of 179 plots spread across normal landscapes of Bavaria/Southeast Germany and equipped with Malaise traps, we related insect biomass, composition and richness (based on metabarcoding) to landsacpe metrics and plant species composition and richness using mixed linear models and variance partitioning.

Compared to grasslands, arable fields and settlements, forest clearings had the highest insect biomass and richness. Plant species composition explained the major fraction of variance in arthropod composition, while land-cover composition was another important predictor. Habitat conditions as indicated by plant composition through Ellenberg-values appeared more relevant for arthropod composition than specific trophic plant-insect relationships.

Across trophic levels, arthropod richness showed a strong positive relationship with the richness of plants indicating legally protected biotopes (although those had negligible area), and butterflies and red-listed arthropods were related to the richness of red-listed insects and butterflies. Biotope-indicator plant species were more important predictors for arthropod richness than the amount of surrounding semi-natural habitat or landscape configuration. Plant species richness was mainly driven by local land use, with total species richness being highest in settlements and biotope-indicator richness being highest in forests.

The results highlight the importance of forests for the conservation of plants and arthropods as well as of preserving and restoring high-quality habitats in agricultural and urban areas.