Beyond mean trait distance. An integrated approach to functional distinctiveness

Functional distinctiveness is a key concept in ecology, reflecting the extent to which a species exhibits unique functional traits relative to other species in the assemblage. While functionally distinct species often contribute disproportionately to ecosystem processes, their role in shaping community functioning remains underexplored. In this study, we propose an integrated framework that links functional distinctiveness to rarity functions, a fundamental component of diversity measures, thereby generalizing distinctiveness metrics beyond the simple calculation of mean trait distance. The main advantage of the proposed framework lies in its ability to adjust the sensitivity of distinctiveness to changes in the abundance of rare or common species, thus providing a more accurate representation of community structure. We apply this approach to an Alpine plant succession, examining changes in functional distinctiveness across six successional stages. Our results reveal a gradual decline in functional diversity along the succession, driven by a progressive reduction in the functional distinctiveness of the most abundant species within plots. Additionally, we demonstrate that measures of functional distinctiveness that are more sensitive to rare species provide a more refined understanding of changes in the functional structure of communities during succession. This study highlights the importance of integrating distinctiveness and rarity functions to better capture the ecological roles of functionally distinct species and inform biodiversity conservation strategies.