Fragmentation Alters Ecological Gradients and Headwater Fish Assemblage Composition Relative to Land Use in a Dendritic River System

The spatial and hydrological properties of headwaters allow dendritic systems to contribute to patterns of regional diversity. However, such ecological gradients may be disrupted as a result of habitat fragmentation. We tested the hypothesis that coarse-scale anthropogenic disturbances such as upstream land use and proximity to reservoirs can alter ecological gradients, thus influencing instream habitat, headwater fish assemblage composition, and species turnover in the Little Tallahatchie River system in north-central Mississippi. To test this hypothesis, we calculated species turnover coefficients, ordinated samples, and examined the correlations between assemblage composition and environmental and anthropogenic variables. Assemblage composition was strongly correlated with instream habitat and river system connectivity, and instream habitat was strongly associated with land use. Gradients in assemblage composition associated with ecological factors were altered due to land use. Our research highlights the importance of headwaters as distinctive habitat patches driving species turnover and the influence of land use in disrupting the ecological gradients that allow for the formation of these distinctive habitat types.