Alien Plant Invasions in Himalayan Riparian Ecosystems: Decoupled Taxonomic and Phylogenetic Patterns

Biological invasions can restructure community assembly by altering phylogenetic diversity and the evolutionary relationships between alien and native species, yet how these processes vary across freshwater habitats remains unclear. In this study, we addressed three related questions: (i) whether phylogenetic diversity and community phylogenetic structure differ between stream and river habitats, (ii) whether alien species richness exerts habitat-specific effects on community phylogenetic metrics, and (iii) whether alien species are phylogenetically related to native assemblages across habitats.We quantified phylogenetic diversity (PD), net relatedness index (NRI), nearest taxon index (NTI), and standardized effect sizes of mean pairwise and nearest taxon distances (SES MPD and SES MNTD). Using Bayesian hierarchical models, we found that rivers exhibited consistently stronger phylogenetic clustering than streams, as reflected by higher NRI and NTI values, whereas PD showed little evidence of habitat-specific differences. The effects of alien species richness on PD, NRI, and NTI were weak and uncertain in both habitats, indicating limited support for richness-driven restructuring of phylogenetic diversity. Analysis of alien–native phylogenetic relatedness revealed predominantly negative SES MPD and SES MNTD values, particularly in rivers, demonstrating that alien species tend to be more closely related to native species than expected under random assembly. However, direct habitat contrasts in alien–native phylogenetic relatedness were centered near zero, suggesting broadly similar strengths of alien–native phylogenetic clustering in streams and rivers.Overall, our findings indicate that alien species are non-randomly integrated into native riverine communities, consistent with phylogenetic niche conservatism and environmental filtering. While habitat type strongly shapes baseline community phylogenetic structure, it plays a comparatively minor role in mediating the evolutionary relatedness between alien and native species across freshwater ecosystems.