Local–regional relationships for taxonomic and functional richness complementarily reveal community assembly

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The classic local–regional richness paradigm is widely used to evaluate the relative influence of regional and local processes, but it overlooks the role of functional traits. Following the Species-Pool Functional Diversity (SPFD) framework, we propose to investigate community assembly by integrating taxonomic and functional richness into the classic paradigm. We applied this integrating strategy to detect the relative contributions of regional and local processes in structuring aquatic insect communities in tropical headwater streams across multiple mountains (regions) in the Pearl River Basin, China. Unsaturated local–regional relationships for taxonomic and functional richness indicated dominant regional effects but limited local constraints for all insects as a whole. These unsaturated patterns also worked for three major functional feeding groups, highlighting the importance of regional processes. Structural equation modeling that incorporated regional and local environmental variables further revealed that annual mean temperature directly affected regional richness and indirectly influenced local richness via environmental factors (aluminum and dissolved oxygen). Trophic interactions played an important role, with prey taxonomic and functional richness positively associated with predator richness. Ultimately, integrating taxonomic and functional diversity provides a powerful approach for disentangling multi-scale drivers of community assembly.