Multiple dimensions of functional diversity affect stream fish taxonomic beta-diversity

When investigating metacommunity dynamics, functional differences among species are often assumed to be as important as environmental differences between sites in determining beta-diversity. However, few studies have examined the influence of functional diversity on beta-diversity. We examine the relative importance of regional functional diversity partitioned by niche dimensions and environmental variation in structuring taxonomic beta-diversity of stream fishes using a large dataset of stream fish assemblages (hereafter, simply beta-diversity). We predicted that both functional diversity and environmental variation play a role in determining beta-diversity. We tested this prediction by modelling the patterns of stream fish beta-diversity as a function of environmental variation, functional diversity and gamma-richness across 10,220 sites for 329 fish species using a series of conceptual path models. Environmental variation consistently affected beta-diversity across all models, whereas functional diversity and gamma-richness influenced beta-diversity only in some models. We show that including relevant trait differences among species in path models can improve their ability to explain beta-diversity, suggesting that functional traits influence beta-diversity. The ability of path models to explain beta-diversity varied depending on the trait grouping included in the model, demonstrating that specific path models representing different niche dimensions can improve the ability of a model to explain beta-diversity. In addition, parsing traits into different niche dimensions revealed alternative patterns of functional diversity-beta-diversity relationships that otherwise would have been missed. The selection of relevant traits and linked niche dimensions is critical for detecting relationships between functional diversity and beta-diversity. Using traits associated with different niche dimensions allows for the identification of niche dimensions most strongly associated with species sorting and the detection of patterns missed by focusing on a single niche dimension. Determining the niche dimensions that influence beta-diversity could provide insights into the processes driving biodiversity and metacommunity dynamics, improving our ability to conserve or restore aquatic communities.