Taxonomic and functional β-diversity patterns reveal random assembly rules in nearshore fish assemblages

Evaluating beta-diversity provides ecologists with a greater understanding of the processes that drive compositional variation of biological communities in ecosystems. The decomposition of taxonomic and functional beta-diversity into their components of turnover (species replacement) and nestedness (species loss) was conducted for nearshore fish assemblages in 4 areas with different habitat characteristics on the southeastern Brazilian coast. We hypothesized that the environmental filtering process is occurring along a gradient of wave exposure, anthropogenic influences, sediment types and water physicochemical variables, inducing high taxonomic and functional beta-diversity. Higher functional beta-diversity compared to taxonomic beta-diversity was found, which suggests that different species tend to play different functions axonomically and functionally distinct assemblages. Taxonomic and functional turnover presented similar values and were both comparatively higher than nestedness, indicating the predominance of species replacement over species loss. beta-diversity did not differ significantly from the null expectations of randomness, suggesting no influence of environmental filtering in the assemblages. The high nestedness in the area where anthropogenic activities are more intense suggests that only fish with the ability to tolerate harsh conditions can potentially settle there. We conclude that there is an environmental gradient that partially explains the differences in the components of functional beta-diversity. However, the large unexplained part of the variation suggests that species are functionally distinct and organized at random with no influences of environmental filtering. This study helps to assess the impact of natural and human activities on the functional facets of biodiversity at the local scale.