Role of network positions in structuring fish metacommunities in subtropical streams

Spatial position within stream networks can determine the strengths of different metacommunity processes. The dendritic structure of stream networks has facilitated the development of the network position hypothesis; however, its generality remains controversial among studies. During the wet season, standardized surveys were conducted at 41 sampling sites along the Xin’an River. We evaluated the effect of network positions on taxonomic and functional β-diversity patterns of fish assemblages, comprising a broad range of habitats in stream networks such as the mainstem rivers, headwaters, and adventitious streams (which flow directly into mainstems). We observed the significant differences in fish taxonomic and functional β-diversity patterns and the influence of dispersal mechanisms and environmental filtering processes on fish communities across different network positions. Specifically, fish assemblages in mainstem rivers were influenced by both environmental and spatial variables. Spatial variables exerted stronger effects on taxonomic β-diversity, while environmental variables dominated functional β-diversity. In headwater streams, environmental and spatial variables accounted for the functional β-diversity, while only environmental variables explained their taxonomic aspects. Additionally, taxonomic β-diversity in adventitious streams was explained solely by spatial variables, whereas functional β-diversity was more strongly explained by spatial variables than by environmental factors. Different metacommunity processes between adventitious and headwater streams suggested that isolation of similarly ordered streams varies with their network position. These results broaden our understanding regarding the generality of the network position hypothesis under high‑connectivity conditions and highlight the importance of conserving fish assemblages by maintaining habitat heterogeneity in headwater streams and ensuring connectivity in mainstem rivers.