Confluence configuration of river networks controls spatial patterns in fish communities

Abstract

Given the importance of spatial heterogeneity in altering dispersal, interspecific interactions, and population persistence, high rates of habitat homogenisation across the globe are a concern. In river ecosystems, confluences likely act as heterogeneity ‘hotspots’ by creating discontinuities in longitudinal processes and influences that are propagated both up and down stream networks. We predicted the layout of abiotic conditions around confluences would influence the presence and configuration of spatial heterogeneity, and strongly influence spatial patterns in fish abundance and evenness. Twelve replicate mainstem and tributary stream combinations in Canterbury, New Zealand, were electrofished to evaluate how the configuration of flow disturbance (i.e. flooding) characteristics around a confluence (i.e. stable mainstem and disturbed tributary versus disturbed mainstem and stable tributary) influenced fish communities. The configuration of abiotic conditions around confluences, and position of a sampled reach with respect to the confluence, significantly interacted to create configuration-specific patterns in fish abundance and evenness. Fish abundances were particularly high in disturbed tributaries juxtaposed with stable mainstems, suggesting certain confluence configurations are disproportionately ecologically important. Evenness scores differed significantly downstream of confluences, depending on configuration, with highest fish evenness in reaches downstream of confluences between a stable and a disturbed stream. These results reveal strongly context-dependent spatial patterns in communities and demonstrate the role of spatially transferred influence in river systems. Understanding importance of not just the presence of heterogeneity, but its configuration and context-dependence in the landscape will assist in identifying sites of ecological significance for management and conservation purposes.