Insect decline in forests depends on species' traits and may be mitigated by management

Species richness and biomass of insects has declined on average across 140 forest sites sampled over a decade in Germany, with larger, more abundant, and carnivorous species found to be more susceptible to decline. Insects are declining, but the underlying drivers and differences in responses between species are still largely unclear. Despite the importance of forests, insect trends therein have received little attention. Using 10 years of standardized data (120,996 individuals; 1,805 species) from 140 sites in Germany, we show that declines occurred in most sites and species across trophic groups. In particular, declines (quantified as the correlation between year and the respective community response) were more consistent in sites with many non-native trees or a large amount of timber harvested before the onset of sampling. Correlations at the species level depended on species' life-history. Larger species, more abundant species, and species of higher trophic level declined most, while herbivores increased. This suggests potential shifts in food webs possibly affecting ecosystem functioning. A targeted management, including promoting more natural tree species composition and partially reduced harvesting, can contribute to mitigating declines.

Automated summary

Species richness and biomass of insects has generally declined in German forests over the past decade, with larger and carnivorous species being more vulnerable. The underlying drivers of insect decline and variations between species are not yet fully understood. This study highlights the importance of forests in understanding insect trends and suggests that non-native tree species and extensive timber harvesting may contribute to declines. Furthermore, it found that larger and more abundant species at higher trophic levels are most affected, while herbivores show an increase in population. The findings indicate potential shifts in food webs and emphasize the need for targeted management to mitigate declines.