EDGE Species - Calculating EDGE Scores

Calculating EDGE Scores

Every species is given a score according to the amount of unique evolutionary history it represents, and its conservation status. These scores are used to identify EDGE species.

Some species are more distinct than others because they represent a larger amount of unique evolution. Species like the aardvark have few close relatives and have been evolving independently for many millions of years. Others like the domestic dog originated only recently and have many close relatives. Species uniqueness’ can be measured as an 'Evolutionary Distinctiveness' (ED) score, using a phylogeny, or evolutionary tree.

In the phylogeny shown on the right, species A has 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.

Globally Endangered (GE) scores for each species are measured according to the IUCN Red List of Threatened Species, an assessment of the conservation status of the world's plant and animal species.

Summary of 2006 IUCN Red List categories.

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 two scores are then 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.

EDGE species are species which have an above average ED score and are threatened with extinction (Critically Endangered, Endangered or Vulnerable). There are currently 564 EDGE mammal species (~12% of the total). Potential EDGE species are those with high ED scores but whose conservation status is unclear.