Using the relationship between taxonomic and functional diversity to assess functional redundancy in streams of an altered tropical watershed

The relationship between taxonomic and functional diversity indices has been used to better describe and understand the structure of biological communities. Functional diversity is expected to have an asymptotic relationship with species richness because at some point, the addition of new species will increase some of the already established functional groups (functional redundancy). However, the asymptotic relationship may not be reached in intermediately disturbed systems once many intolerant species that would have played a redundant role or even represented some functional groups have been lost. This study aimed to address such a relationship (taxonomic and functional indices) and to evaluate the functional redundancy in intermediately disturbed streams in the Atlantic Rainforest domain. We expected a positive linear relationship between taxonomic and functional diversity; however, we did not expect to find an asymptotic relationship between richness and functional diversity because of the loss of many intolerant species caused by anthropogenic uses. The taxonomic diversity indices were Species Richness (SR) and Simpson’s Diversity (SD), while the functional diversity indices were the Functional Richness (FRic) and Functional Dispersion (FDisp). The two taxonomic and two functional diversity indices showed a significant positive relationship that never reached an asymptote, suggesting low functional redundancy in the fish communities. Our results indicate that care is needed in the management of the studied streams because assemblies with low functional redundancy are more susceptible to loss of functions in the case of species loss.