Multiscale Environmental Influences on Fish Assemblage Structure in Central Texas Streams


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We investigated the influences of local and landscape-scale environmental variables on fish assemblage structure among 64 stream reaches in two large river basins in central Texas. The broad spatial extent of this study region provided an opportunity to examine fish assemblage-environment relationships at multiple scales across a range of stream types in landscapes exposed to varying degrees of anthropogenic alteration. We used nonmetric multidimensional scaling (NMS) combined with permutational analysis of variance, k-means cluster analysis, and indicator species analysis to evaluate broad-scale influences of ecoregional and large river basin boundaries on fish assemblage structure. We also estimated relationships between fish assemblage structure and environmental factors with NMS and rotational vector fitting across all ecoregions and within ecoregions. Ordinations of sites based on species composition grouped stream reaches together according to ecoregion, and k-means clustering identified three groups that corresponded with ecoregional membership. Several species had high affinities with specific ecoregions, a pattern that tracked broad-scale physiographic differences in climate, topography, terrestrial vegetation, and instream habitat. Within ecoregions, we observed that local-scale stream habitat variables as well as larger-scale landscape features were significantly related to fish assemblage composition. Substrate composition was a key local-scale habitat factor, and a gradient of rocky substrate to predominance of mud and silt correlated strongly with assemblage structure within all three ecoregions. The abundance of instream woody debris was also an important local-scale correlate for fish assemblage structure. At the landscape scale, patterns of agricultural and urban land development in the surrounding watersheds were consistently associated with fish assemblage structure in each ecoregion. This study adds important information toward a better understanding of how environmental factors structure fish assemblages across scales, which should facilitate refinement of existing habitat and biological indices for conservation of stream habitats and their biota.
indicator species,biological indicator,k means clustering,analysis of variance,land development
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