Should Global Conservation Initiatives Prioritize Phylogenetic Diversity?

Some recent conservation proposals – including the Zoological Society of London’s (ZSL) EDGE of Existence programme – have focused on the value of protecting species with high evolutionary distinctiveness, a dimension of biodiversity conservation that’s not been much emphasized in conservation practice. In this paper we critically examine this strategy, investigating whether there are good reasons for prioritizing evolutionarily distinctive species, and the phylogenetic diversity to which they contribute, over other forms of biodiversity. We first discuss evolutionary distinctiveness, its relationship to phylogenetic diversity, how phylogenetic diversity can be measured, and intuitive thoughts about its value. Then we consider five kinds of arguments about the value of phylogenetic diversity that might be made to support prioritizing it, given its current lack of emphasis in conservation practice. These are: arguments based on protecting biodiversity, arguments based on option value, arguments based on ecological resilience, arguments based on historical value, and arguments based on aesthetic value. We maintain that these arguments, taken individually, offer varying degrees of fairly weak support for valuing species with high evolutionary distinctiveness. Taken together, however, these arguments seem sufficiently strong to justify programs such as EDGE, insofar as such programs are framed as correctives to a past lack of emphasis on the protection of phylogenetic diversity. We suggest, however, that these arguments are not sufficiently strong to support an absolute prioritization of phylogenetic diversity over protecting other forms of biodiversity.