Phylogenetic diversity is a better measure of biodiversity than taxon counting

Biodiversity is most commonly measured in taxonomic richness. For example, it is common to describe how diverse a genus or a geographic area is by counting the number of species within them. Phylogenetic diversity (PD), a measurement of the branch lengths in a phylogenetic tree, is a better measure of biodiversity that provides a comparable, evolutionary measure of biodiversity not possible with species counts. Despite its advantages, PD is rarely used as the primary measure of biodiversity. We developed a genus-level phylogeny for nearly 90% of taxonomically described Australian land plants and compared PD to genus richness in multiple clades. The proportion of PD per genera was skewed among clades. Non-angiosperm clades had more PD than expected given the number of genera while angiosperm clades had less PD than expected. For example, ferns comprised only 4.7% of the genera yet 13.0% of the PD, while the angiosperms as a whole comprised 78.9% of the genera but only 62.7% of the PD. It is likely that cultural reasons, such as taxonomic biases, are more important than methodological and biological phenomena in explaining these discrepancies. Regardless of reasons for the observed results, we conclude that a shift towards the use of PD as the primary descriptor of biodiversity will promote an important conceptual shift in biodiversity studies as a quantitative science.