A note on functional diversity measures

Functional diversity (FD) has been seen as the key to understanding ecosystem processes, such as productivity, nutrient cycling and storage, carbon sequestration, and stability to perturbations. Yet it is still unclear how FD should be measured. In this paper, I propose a set of fundamental criteria that a meaningful index of FD should satisfy to reasonably behave in ecological research. If FD is computed from the pairwise functional distances among the species of a given assemblage, the candidate measures should be set monotone, monotone in distance, and should conform to the twinning property. On the other hand, if FD is computed taking into account both the pairwise functional distances among species and their relative abundances, the candidate measure should be concave, thus allowing additive diversity decomposition into a- b- and g-terms. Conformity to the above requirements may be beneficial for selecting a family of measures that are most appropriate for a correct evaluation of the relations between biological diversity and ecosystem functioning.