The Role of Physiography in the Relationships Between Land Cover and Stream Fish Assemblages

Human alteration of the landscape for agricultural and urban land use has been linked to the degradation of streams and stream biota. Biophysical and climatic characteristics are important determinants of natural land cover and constraints to human land use, and are strongly related to stream habitat and biotic assemblages. Because the physiographic template differs among watersheds and is an important determinant of landscape processes, it is important to account for these natural differences among watersheds so that the mechanistic relationship between land cover and stream biota can be properly understood. In this study we develop Land Cover Distinguished Physiographic Regions (LDPR), a regional framework that classifies watersheds based on physical and climatic predictors of land cover. We then utilize generalized linear mixed models to investigate whether relations between land cover and stream fish assemblages vary across LDPR classes. Five commonly used metrics covering trophic, reproductive and taxonomic groupings of fish assemblages showed significant variation in their response to agricultural land use across LDPRs, while variation in their response to urban land use was less pronounced. The results suggest that the physiographic differences among LDPRs can result in different pathways by which land cover alterations impact stream fish communities.