Patterns in Functional Structure and Diversity of Stream Fish Assemblages Related to Environmental Factors at Multiple Scales

The distribution and abundance of stream fishes are influenced by many factors operating at multiple scales. Understanding how environmental variables influence the structure of stream fish assemblages is important for habitat assessment, stream restoration, and for predicting responses to environmental change. An emerging view in community ecology is that a focus on the functional structure of species assemblages in relation to environmental gradients may reveal more general patterns applicable across geographic regions. In this study, I used functional traits related to the trophic ecology, habitat use, and life-history strategies of fishes to examine the influences of environmental factors on stream fish assemblages. The research was carried out in two large regions: the Río Grijalva basin in southern Mexico and the Brazos and Trinity basins in Central Texas. In both regions, relationships between functional structure of stream fish assemblages and environmental factors at local, landscape, and regional scales were examined. Environmental characteristics at all three scales influenced the functional attributes of assemblages studied here. At the local reach scale, stream size, substrate characteristics, the availability of riffle and pool habitats, and abundance of in-stream cover structures were related to the functional trait composition of fish assemblages in the Río Grijalva Basin and in Central Texas streams. Landscape features most strongly related to functional structure in both regions were the extent of forested area in the watershed and the amount of land developed for urban and agricultural uses. At the regional scale, broad physiographic differences between ecoregions had a large influence on the taxonomic and functional composition of stream fish assemblages in Central Texas. Along the broad-scale longitudinal fluvial gradient of the Grijalva region, pronounced changes in the species composition, functional trait diversity, and trophic structure of fish assemblages were observed