EDGE birds and EDGE score
What is EDGE score?
Evolutionarily Distinct and Globally Endangered (EDGE) species are threatened species that have few or no close relatives on the tree of life. EDGE species are usually extremely distinct in the way they look, live and behave as well as in their genetic make-up. If they disappear, there will be nothing like them left on the planet.
Every species in a particular taxonomic group (e.g. mammals or amphibians) is scored according to the amount of unique evolutionary history it represents (Evolutionary Distinctiveness, or ED), and its conservation status (Global Endangerment, or GE).
Species uniqueness’ can be measured as an 'Evolutionary Distinctiveness' (ED) score, using a phylogeny, or evolutionary tree. A phylogeny is a diagram showing how all the species in a particular taxonomic group are related to one another. In the example phylogeny here, species A would have a higher ED score than either species B or C - it represents a branch rather than a twig on the tree of life. If species A were to go extinct, there would be no similar species left on the planet and a disproportionate amount of unique evolutionary history would be lost forever. Assuming that resources for conservation are limited and each species is equally threatened, it is therefore reasonable to give priority to species A.
Globally Endangered (GE) scores for each species are based on the IUCN Red List Categories (Critically Endangered, Endangered, Vulnerable, Near Threatened and Least Concern). Species which are Critically Endangered receive a higher score than less threatened species, which in turn, receive a higher score than those not currently in danger of extinction.
The ED and GE scores are combined to produce an overall EDGE score for each species. EDGE scores are calculated by multiplying ED and GE together. In mathematical terms, EDGE scores are an estimate of the expected loss of evolutionary history per unit time.