Small tributaries are increasingly recognized as important components of river networks, yet their roles in shaping regional biodiversity remain underrepresented compared to main channels. In addition, trait-based approaches in riverine biodiversity studies remain comparatively understudied. To address these gaps, this study examined how river type influences the structure of benthic macroinvertebrate communities in a gravel-bed river system by integrating taxonomic and trait-based diversity approaches.
The study was conducted in the Satsunai River, a gravel-bed river system in Hokkaido, Japan. Benthic macroinvertebrates were sampled from four sites, comprising two tributary sites and two main-channel sites. Water temperature and flow conditions were measured to characterize hydrological stability across river types. We quantified alpha (α) diversity, partitioned gamma (γ) diversity into hierarchical alpha and beta components, then examined turnover and nestedness patterns to identify the mechanisms underlying beta (β) diversity. Community composition was assessed using non-metric multidimensional scaling (NMDS) based on Bray–Curtis dissimilarities.
NMDS results showed that community composition differed significantly between tributaries and the main channel. Taxonomic alpha diversity was consistently higher in tributaries (approximately 18 ± 5 taxa) than on the main channel (approximately 16 ± 4 taxa); GLMM-Poisson error distribution p<0.001. Gamma-diversity partitioning revealed that beta diversity among microhabitats contributed roughly 25–30% of regional (γ) diversity, highlighting the importance of within-site habitat heterogeneity. Furthermore, beta diversity among microhabitats was dominated by species turnover (around 90%) rather than nestedness (about 10%), indicating that tributaries and the main channel support distinct assemblages rather than hierarchical subsets of species.
Trait-based analyses revealed patterns consistent with the taxonomic results. Alpha diversity of functional feeding groups (FFGs) was higher in tributaries than in the main channel, indicating greater functional complexity at the local scale. In addition, tributaries exhibited a higher relative abundance of shredders and collector–gatherers, both of which are strongly associated with detrital energy pathways. These functional patterns are consistent with more stable hydrological conditions and higher organic matter availability in tributaries. Together, these findings indicate that differences in community composition between river types are accompanied by coherent shifts in functional structure, linking taxonomic diversity with ecosystem processes.
These results demonstrate that tributaries function as ecologically distinct habitats that support more unique species and functional roles within river networks. Therefore, effective river management must conserve both river types and microhabitat heterogeneity to maintain regional biodiversity and ecosystem functioning.