A simple translation from indices of species diversity to indices of phylogenetic diversity

Measures of phylogenetic diversity have two main objectives: first disentangling the processes that drive species assemblages and second defining priorities of conservation while considering how much each species might contribute to biodiversity. A now widely used approach for measuring phylogenetic diversity consists in summing branch lengths on phylogenetic trees. Thanks to this approach, a change of perspective has been done since the 90s in the ecological literature: to measure biodiversity in sites, species in compositional indices have been replaced, often implicitly and sometimes explicitly, with phylogenetic units (units on the branch lengths of a phylogenetic tree). Here we highlight how any traditional index of species diversity, can be adapted, with this simple change of perspective, to measure phylogenetic diversity. We analyze other potentials of this simple family of indices: the ratio of phylogenetic diversity to the associated species diversity may offer a meaningful way to apprehend phylogenetic uniqueness in communities; and the phylogenetic diversity standardized by its maximum value may provide a meaningful way to apprehend phylogenetic evenness. Theoretically, the same approach could be applied to functional dendrograms to assess functional diversity, evenness and uniqueness. However this requires, somehow, to force functional data to adhere to a hierarchical structure of functional differences between species. As an illustration, we analyze phylogenetic and functional diversity in plant communities along a primary succession in Italy. Overall, this simple framework has a critical potential for future analyses of tree-based diversity as it benefits from myriads of previous researches on the measures of species diversity and species evenness.