Habitat fragmentation impact on insect diversity: opposing forces at patch and landscape levels.

Context: Habitat loss is widely recognized as a major threat to biodiversity, but the effects of habitat fragmentation, whether positive or negative, remain controversial. It has been suggested that these effects vary depending on the spatial scale studied (patch vs. landscape) and the biodiversity metric considered (α-, β-, or γ-diversity). Objectives: We aimed to test the contrasting effects of habitat fragmentation on insect diversity across different scales. Specifically, we tested whether habitat fragmentation negatively affect α-diversity at the patch scale, while having positive effects on β- and γ-diversity at the landscape scale. Methods: We conducted surveys of Lepidoptera and Orthoptera in 18 dry meadows of varying size and isolation in Switzerland. We assessed the effects of patch size and connectivity on species diversity (α-diversity), analyzed species turnover (β-diversity) between patches, and performed SLOSS analyses to compare cumulative species richness (γ-diversity) between patches. Results: Patch size and connectivity positively influenced α-diversity for both Lepidoptera and Orthoptera. However, at the landscape scale, multiple small patches supported equal or even higher γ-diversity than a single large patch of equivalent area. β-diversity increased with geographical distance between patches, indicating greater species turnover between more distant patches. Conclusion: Our results highlight that the effects of habitat fragmentation, whether positive or negative, are scale-dependent. While habitat fragmentation negatively affects α-diversity at the patch scale, it can enhance overall β- and γ-diversity at the landscape scale. These findings suggest that conservation strategies should consider both large and small habitat patches to maximize biodiversity. Supplementary Information: The online version contains supplementary material available at 10.1007/s10980-025-02133-w.