The relative influence of spatial context and catchment- and site-scale environmental factors on stream fish assemblages in a human-modified landscape

Environmental factors act in a hierarchical manner at multiple spatial scales to influence the organisation of ecological assemblages; however, the relative influence of the different scale-related factor groups is poorly known. We evaluated the importance of catchment-scale and site-scale environmental variables, as well as the spatial context of the sampling sites, in shaping stream fish assemblages in an agriculture-dominated landscape in Hungary. Beside the variables describing spatial context (principal coordinates of a truncated distance matrix among sites), altogether 60 environmental variables were used to predict variability using a variance-partitioning procedure in redundancy analysis. Presence-absence- and relative abundance-based data were examined at two assemblage levels (entire assemblage and native assemblage) at 54 stream sites. Incorporation of spatial variables increased largely the total explained variability in case of relative abundance, but not for presence-absence data. Of the environmentally explained variance, catchment-scale variables (e.g., land cover types, patch density) were relatively more influential for the native assemblage-level analyses, than for analyses at the entire assemblage level, where site-scale variables (e.g., altitude, depth) proved to be more influential. In addition, pure catchment- and pure site-scale variables have the primary role in determining fish assemblage patterns, whereas the influence of shared variance and that of site-scale riparian variables proved to be less important. Our findings demonstrate the importance of incorporating the spatial context of the sampling sites in predicting fish assemblage patterns and the effects of channelisation (dikes) in shaping assemblage-environment relationships in this human-influenced landscape.