Functional trait diversity and trait-environment relationships in Central Texas stream-fish assemblages: Implications for biomonitoring

Background/Question/Methods

Understanding how environmental variables at multiple scales influence the structure of stream fish assemblages is important for habitat assessment, stream restoration, and for predicting responses to environmental change.  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, we 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 in the Brazos and Trinity river basins in Central Texas.  Relationships between functional structure of assemblages and environmental factors at local, landscape, and regional scales were examined.  We also identified associations between functional diversity of fish assemblages and established measures of biotic integrity and habitat quality.

Results/Conclusions

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.  Landscape features most strongly related to functional structure 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.  In general, the functional trait-space occupied by fish assemblages was greatest in streams with high habitat quality scores located within landscapes with less alteration for agriculture and urban development.  Currently used indices of biotic integrity incorporate some functional aspects of fish assemblage structure (e.g., trophic guilds, pollution tolerance categories), but identifying specific functional traits or suites of traits related to stream habitat quality could potentially improve on these approaches and provide more sensitive and broadly applicable metrics.