Riparian vegetation connectivity and composition jointly shape larval aquatic insect diversity across local and regional scales

Riparian vegetation is a major driver of freshwater biodiversity; however, the structural degradation of riparian vegetation due to anthropogenic pressures poses a major threat to riverine ecosystems worldwide. This study examined how riparian vegetation connectivity and composition jointly shape the taxonomic, functional and phylogenetic diversity of larval aquatic insect assemblages. We specifically asked whether connectivity and forest type have distinct effects on local and regional diversity. We conducted field surveys of larval aquatic insects inhabiting aquatic habitats, specifically those belonging to the orders Ephemeroptera, Plecoptera, and Trichoptera (EPT), at 198 sites across the Yangtze River Basin. Functional diversity was quantified using 13 traits related to life history, mobility, morphology, and ecology. Sites were classified by riparian connectivity degree, and deciduous and natural forest cover were quantified using high-resolution remote sensing data. Vegetation connectivity showed significant positive relationships with taxonomic α -and phylogenetic α -diversity of EPT assemblages, whereas taxonomic β -and functional β -diversity exhibited nonlinear patterns along the connectivity gradient. Species turnover accounted for the majority of variation in EPT assemblage composition. Deciduous and natural forest cover showed positive associations with α -diversity at medium and high connectivity levels, while their relationships with β -diversity varied across connectivity levels. Vegetation connectivity and composition distinctly affect local and regional diversity via dispersal and habitat filtering, respectively. Sustaining freshwater biodiversity thus requires preserving remnant natural forest and restoring riparian connectivity. As riparian degradation accelerates, both measures are necessary for ecosystem resilience across large basins.