An emerging view in community ecology is that a focus on functional traits as they relate to environmental gradients can reveal general patterns and improve our ability to predict responses of communities to environmental change. In this study, we applied such an approach to examine fish assemblages across a range of stream environments in a large river basin in southern Mexico. Morphological traits with known functional relationships to feeding, locomotion, and habitat use were measured along with a suite of environmental variables in communities throughout the watershed. Communities were sampled within a variety of stream types, from high-elevation tributaries to lowland streams in the coastal plains.
The most important environmental influences on community structure and trait diversity were factors related to food resource availability and habitat stability. Functional trait diversity was highest in lowland communities, where habitats are more stable and high productivity and spatial heterogeneity provide for a broad prey resource base. This diversity decreased in communities further upstream in the basin, where the flow regime is harsher and fewer favorable habitat types are available for fishes or their prey resources. Knowledge of functional trait responses across environmental gradients may be applied across geographical scales, and it provides vital information for conservation and management.