Riverscape Heterogeneity Controls on Spatial Patterns in Fish Diversity and Abundance Associated with Flow and Temperature Variability in Stream Networks

Environmental heterogeneity potentially has positive effects on fish diversity and abundance, but the mechanisms driving such effects, especially the importance of riverscape configuration and other large-scale drivers of heterogeneity are not well understood. Using twelve replicate mainstem and tributary combinations, creating a gradient of riverscape heterogeneity in flow and temperature characteristics in Canterbury, New Zealand, I evaluated how the juxtaposition of streams with similar (e.g., stable –> stable) versus different (e.g., stable-> disturbed) characteristics influenced fish diversity and abundance patterns. Spatial network models incorporating confluences, flow direction, and stream size revealed fish abundances changed substantially with distance to confluence in heterogeneous riverscapes, whereas little change was detected in homogenous riverscapes. Specifically, non-native trout abundance increased with proximity to stable (i.e. low flow and temperature variability) reaches, whereas galaxiid abundance decreased with proximity to stable reaches, likely due to negative competitive and predatory interactions.  As a result, heterogeneous riverscapes facilitated coexistence between galaxiids and trout, expanding riverscape diversity. Therefore the amount and configuration of flow and temperature heterogeneity within riverscapes is an important contributor to both local and large-scale patterns in fish diversity and abundance.