The Use of EDGE (Evolutionary Distinct Globally Endangered) and EDGE-Like Metrics to Evaluate Taxa for Conservation

Abstract

The idea of setting conservation priorities using phylogenetic information has been in the literature for decades. Since 2007 it has been implemented by the EDGE of Existence program at the Zoological Society of London. There are now “EDGE lists” of mammals, birds, amphibians, sharks, and corals: all these lists prioritize species that are both highly threatened and evolutionarily distinct (i.e., they have few close relatives). In this chapter, we review the metrics used to calculate priority scores for species on EDGE lists. As the EDGE of Existence program enters its second decade, this is an appropriate moment to reflect on how the process of selecting species could be improved or streamlined. A suite of metrics have been proposed as alternatives or improvements to the original EDGE metric, each differing in a number of important ways, such as how they handle uncertainty in their underlying data. Perhaps the most profound differences among these metrics reflect differences in how threat status and phylogenetic information are scaled to calculate the prioritization metric and in whether (and how) they incorporate the principle of complementarity. We discuss these choices of metric in the context of the properties of each metric and what they have to say about balance between how much we value the existence of each species (its phylogenetic position) and the risk that it would be lost due to extinction.