Within-basin fish assemblage structure: the relative influence of habitat versus stream spatial position on local species richness

Local species richness (LSR) in stream fish assemblages is related to both local habitat conditions and stream spatial position. In this study, we examined the within-basin distribution of LSR in the Upper Saone River (France). We describe the longitudinal patterns in LSR and assess the relative effects of both local habitat and spatial (upstream and downstream) influences on species richness. We showed that among the different local habitat variables, only stream width and gradient significantly influenced LSR. Spatial factors also influenced the within-basin distribution of LSR and resulted in spatial autocorrelation, highlighting contagious biotic processes in structuring stream fish assemblages. However, we did not confirm other published predictions that headwater streams entering large rivers directly should have greater species richness. The spatial autocorrelation was only significant downstream (from 4th- to 7th-order streams), suggesting that the relative importance of local habitat and biotic processes may depend on the position along the longitudinal gradient.