Dispersal ability modulates macroinvertebrate metacommunity responses to environmental filtering and spatial connectivity in an Afrotropical stream network

AimTo test whether dispersal ability determines the relative importance of environmental filtering and distinct dispersal pathways in structuring stream macroinvertebrate metacommunities. LocationNzoia River catchment, Lake Victoria basin, western Kenya. TaxonBenthic macroinvertebrates (family level), grouped by dispersal ability into strong fliers, weak fliers, passive aerial dispersers, and obligate aquatics. MethodsMacroinvertebrates were sampled seasonally (2018 to 2021) at 55 sites. Using distance-based redundancy analysis and variation partitioning, we compared the influence of physicochemical water quality on community structure, representing environmental filtering, with the influence of three sets of spatial factors representing distinct dispersal pathways: Euclidean distance (overland dispersal pathway), land-use resistance (land-use mediated dispersal pathway), and Asymmetric Eigenvector Maps built on river-network connectivity (along-watercourse dispersal pathway). In addition, distance-decay relationships and turnover-nestedness partitioning were examined for the full assemblage and each dispersal group. ResultsCommunity assembly shifted predictably along the macroinvertebrate dispersal-ability gradient. Environmental filtering dominated in strong fliers and retained an independent influence in weak fliers and obligate aquatics, but contributed no independent influence in passive aerial dispersers. The overland dispersal pathway structured the three aerial groups. The land use mediated dispersal pathway constrained weak fliers and passive aerial dispersers, and the along-watercourse dispersal pathway structured passive aerial dispersers and obligate aquatics. Passive aerial dispersers showed the strongest dispersal signal, with the overland and along-watercourse pathways contributing equally, and in this group beta diversity reflected richness gradients (nestedness) rather than species replacement. Main conclusionsDispersal ability determined whether environmental filtering or a specific dispersal pathway governed community composition, and no single pathway captured assembly across the full range of dispersal abilities. Dispersal traits and connectivity must therefore be considered jointly when studying and managing aquatic biodiversity in human-modified Afrotropical catchments.