Functional and phylogenetic β diversities and their link with clustering/overdispersion and uniqueness/redundancy

Abstract

In the recent decades, research on biodiversity in community ecology has been marked by the consideration of species' evolutionary histories and biological traits, including those said functional as they relate to the functions a species has in ecosystems. Among the different spatial levels at which FP diversity can be quantified, less attention has been given to the definition of the general concept of between-communities (β) FP diversity than to those of local, within-communities (α) and regional, merged-communities (γ) FP diversities. Here we develop a new way for partitioning FP β diversity into elementary components to underline how and why FP β diversity differs from species β diversity, the latter reflecting only differences in species' abundances between communities. As a reference example, we consider two distinct measures of FP β diversity: Rao's dissimilarity coefficient (Qβ) that expresses an average dissimilarity between communities, and its transformation into an equivalent number of communities (Dβ). Thanks to analytical partitioning and simulations, we show that Qβ and Dβ are connected differently with typical patterns of community structure. The search for the ecological and evolutionary processes that drive community assembly and the assessment of community resilience and stability have indeed revealed typical community structures: the local clustering of species with similar traits or shared evolutionary histories; and the local (α) or regional (γ) presence of functionally or phylogenetically redundant versus unique species. While Qβ and Dβ are both increasing functions of species β diversity and γ FP uniqueness, Qβ increases with FP clustering while Dβ increases with α FP redundancy. The selection of an index of β diversity for a given study depends on its objective, such as communication to conservation policy makers or theoretical study of the ecological and evolutionary processes of community assembly. To facilitate and secure this selection, we call, through our study, for the development of formal and precise definition(s) for FP β diversity in light of the concepts of clustering versus overdispersion, and redundancy versus uniqueness. In particular, we call for further research on when and why FP β diversity should increase with FP clustering.