Silent changes in functionally stable bird communities of a large protected tropical forest monitored over 10 years

Temporal taxonomic changes have been noted in bird communities in protected tropical forests. However, it is not known whether these changes compromise the conservation of functional diversity. Here we monitored bird communities in an Atlantic Forest Protected Area in southern Brazil annually during 10 years. We assess temporal trends in taxonomic and functional diversity metrics and compare the observed functional diversity values with expected values to identify the mechanisms that drive community assembly. We also verified temporal beta diversity and the prevalence of its components. We found variations in species richness over time when assessing the entire community, passerines, non-passerines, and specialized insectivorous passerines. Taxonomic turnover was predominant in the temporal beta diversity of these groups. Nonetheless, no significant variations in functional richness were recognized, indicating functional redundancy between the changing species. However, functional nestedness was predominant in the group of specialist insectivorous passerines, which indicates functional erosion in this group. There were also increases in the dispersion of the entire community and functional divergence in non-passerines. These alterations seem to reflect the occurrence of rare species with unique functional traits and the optimization of the use of available resources. Our data suggest that the communities had random functional structures. Consequently, taxonomic alterations are possible due to random substitution generated by dispersal processes however, the redundancy between species allows overall functional stability of community. We also highlight that different groups of species can present different temporal trajectories and their evaluations are necessary to unveil silent alterations in their functional roles.