Managed forests are a stronghold of non‐native beetles in Europe

Abstract The species richness of vascular plants in forests can have contrasting effects on the occurrence of non‐native insects. The establishment of non‐native insect populations may be facilitated by low plant species richness, which reflects the availability of few but easily accessible resources, or hampered by high plant species richness due to spatial dilution of resources or biotic resistance (i.e., resistance against biological invasions). The relationship between the species richness of plants and non‐native insects is likely influenced by disturbance regimes, which, in European forests, mostly consists of timber harvesting. We investigated this relationship considering two major forest attributes: (i) species richness of non‐native vascular plants and (ii) forest management. From 1101 forest plots in Europe, we gathered occurrences of 1212 vascular plant species, including 160 non‐native species, and of 2404 beetle species, including 29 non‐native species. We tested the relationship between the species richness of non‐native beetles and plants using non‐linear quantile regressions. We disentangled the effect of non‐native plant species richness from that of management on the species richness of non‐native beetles, while accounting for forest structural variables, using structural equation models. We found clear evidence of a hump‐shaped relationship between non‐native beetle and plant species richness. The general shape of the relationship persisted when considering only woody or non‐woody plants, as well as only non‐native plants. The relationship was also similar between managed and unmanaged forests. However, the proportion of non‐native beetles in managed forests was higher than in unmanaged forests at the same plant species richness. Management had a direct negative effect on non‐native beetle species richness, whereas non‐native plant species richness had a direct positive effect. When considering all direct and indirect effects, management facilitated the occurrence of non‐native beetles indirectly via non‐native plants rather than directly. Synthesis and applications . Species richness of native and non‐native vascular plants modulates the species richness of non‐native beetles through relationships with opposite signs. The interplay with management regimes and forest structures determines whether non‐native beetles are promoted. Forest management aimed at reducing the intensity of disturbance while encouraging native plant species richness could promote the dominance of dilution effects and biotic resistance and could moderate the establishment of non‐native insects.