Trade-offs in the conservation of phylogenetically distinctive species

Many phylogenetically distinctive species are the last surviving member of a once diverse clade. The loss of these species would thus represent the last stage in the loss of the clade, which likely occurred over a long evolutionary history. However, the implementation of conservation actions targeting threatened phylogenetically distinctive species remains limited and has relied on a single view of how phylogenetic distinctiveness should be measured. Here, we develop a simple mathematical framework to connect different points of view of the measurement of a species' distinctiveness. The framework allows controlling the depth of the phylogenetic tree that most influences the definition of phylogenetic distinctiveness and considers the risk of extinction of the target species and other species in a given timeframe. As an example, we analyze a small dataset on perissodactyls to illustrate how the proposed framework works in practice. Notably, we highlight that based on the particular biological criterion of threat and distinctiveness, conservation measures for the critically endangered wild African ass (Equus africanus) must be bolstered since the population size of this species is still declining despite current conservation efforts. We hope that our framework will help guide the use of phylogenetic distinctiveness as one of the main criteria to consider when designing conservation actions on a global scale.