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

The decline in insect biodiversity is a major ecological concern. While studies in agricultural landscapes have identified land use as a key driver, insect trends in forests—which cover a significant portion of terrestrial land and harbor high biodiversity—have received less attention. Forests are often considered biodiversity refuges, yet some studies report declining insect populations within them. Interpreting these trends is challenging due to multiple interacting factors: natural disturbances, forest management practices (e.g., simplification of forest structure), and climate change. These factors can alter habitat conditions and resource availability, impacting insect communities. Understanding these complex interactions requires detailed analyses considering both local site conditions and landscape-scale effects. Species traits, such as body size, trophic level, and dispersal ability, are also crucial for understanding differential responses to environmental changes. Previous research suggests that species with narrower dietary niches and specialized habitat requirements decline disproportionately compared to generalists. Larger species, often less abundant, may be more sensitive to resource availability changes. This study uses data from the Biodiversity Exploratories project in Germany to investigate the drivers of insect decline in forests, considering both site-level and landscape-scale factors, as well as species-specific traits.

Existing literature highlights the global decline of insect populations, with trends varying across taxa, regions, and ecosystems. Most studies focus on agricultural landscapes, linking land use intensification to insect decline. In contrast, forest insect trends are less well-understood, despite forests' significant role in global biodiversity. While some forests show stable or increasing insect populations, others exhibit declines across taxonomic groups. The challenges in interpreting forest insect trends stem from the interplay of natural processes (stochastic disturbances, succession) and human interventions (forest management). Forest management practices, such as simplifying forest structure through timber harvesting, can alter habitat conditions and resource availability, potentially causing long-term changes in insect communities. Climate change further complicates the picture by influencing forest structure and microclimate, both directly and indirectly through other ecological drivers. Previous work has also emphasized the role of species traits, showing that specialists with narrow dietary niches and specific habitat requirements are disproportionately affected by environmental changes. Body size, trophic level, and dispersal ability have also been linked to insect population trends, with larger, less abundant, and higher trophic level species potentially more vulnerable.

This study utilized data from the Biodiversity Exploratories project, a long-term ecological research initiative across three regions in Germany. Beetle and true bug data were collected over ten years (2008-2017) from 140 forest sites, representing a gradient of forest management intensities. A total of 120,996 individuals from 1,805 species were identified to species level. Site-level correlations between insect community responses (species richness, abundance, biomass) and sampling year were calculated using Pearson's r. Environmental variables at both local (e.g., deadwood volume, proportion of non-native trees, canopy openness) and landscape scales (landscape heterogeneity) were considered, including their temporal changes. Species-level correlations between individual counts per species and sampling year were also analyzed. Species traits such as body size, abundance, and trophic level were included to explore their influence on population trends. Linear mixed-effects models were used to examine the relationships between site-level correlations, environmental variables, and trophic groups. Similarly, linear mixed-effects models were used to analyze species-level correlations and species traits.

The study revealed widespread declines in insect populations across most sites and species, with negative correlations between insect community responses and sampling year. Negative correlations were significantly stronger in sites with a high proportion of non-native trees and in sites with a history of high timber harvesting. Species richness, abundance, and biomass correlations were all negatively related to the proportion of non-native trees. Changes in deadwood volume and canopy openness were also significantly associated with insect decline. Herbivores were an exception, showing positive correlations in species richness and abundance, but not biomass. Species-level analysis demonstrated that larger species, more abundant species, and species of higher trophic levels showed more consistent declines. These findings suggest potential shifts in forest food webs.

The findings highlight the importance of forest management practices in influencing insect population trends. The negative associations between insect decline and the proportion of non-native trees and the intensity of timber harvesting suggest that these management practices may be contributing to biodiversity loss. The contrasting trends observed in herbivores (increase) compared to other trophic groups (decline) suggest a potential restructuring of forest food webs. The finding that larger, more abundant, and higher trophic level species experienced stronger declines highlights the vulnerability of certain species to environmental changes. This underscores the importance of considering species-specific traits when assessing the impacts of environmental changes on biodiversity. The study demonstrates that targeted forest management strategies, such as promoting native tree species composition and reducing harvesting intensity, can potentially mitigate insect declines. This emphasizes the need for integrated approaches that consider both ecological and management factors for effective biodiversity conservation.

This study provides strong evidence for widespread insect decline in German forests, linked to forest management practices and species traits. The negative impact of non-native tree dominance and intensive harvesting, along with the vulnerability of larger, more abundant, and higher trophic level species, emphasizes the need for sustainable forest management that prioritizes native species diversity and reduces human interventions. Future research could focus on experimental manipulations of forest structure and composition to establish causal links and further refine management recommendations. Understanding the complex interplay between these factors is crucial for the conservation of forest insect biodiversity.

The study is observational, not experimental, limiting the ability to definitively establish causality between management practices and insect decline. The focus on two insect orders (Coleoptera and Heteroptera) might not fully represent the entire forest insect community. The study period (2008-2017) does not fully encompass recent extreme climatic events, which could impact results.