How do we decide which species should be given conservation priority? The application of the EDGE score

With so many species under threat of extinction, how can we decide which species to prioritise? The EDGE score uses a scientific framework to identify the world’s most Evolutionarily Distinct and Globally Endangered species that can then be targeted for conservation intervention.

Staggeringly, of the 93,577 species whose conservation status has been assessed to date by the International Union for Conservation of Nature (IUCN), 26,197 are under threat in 2018. That’s roughly 28%. And as 95% of known species are yet to be assessed, the number of threatened species is only going to increase, and is likely to increase massively. Current estimates from the IUCN suggest that up to 10,000 species are being driven to extinction each year. Depressing stats for sure.

Unfortunately, despite the best efforts of conservation organisations and governments worldwide, resources for conservation remain comparatively scarce in the face of the enormous challenges facing our wildlife. This raises the question of how do we prioritise the species that should be receiving the greatest level of resources?

There’s plenty of different thoughts on which species should be prioritised and how these species should be selected. For example, ‘umbrella species’ could be prioritised for conservation efforts because their preservation indirectly results in the preservation of other species that share the same habitat. Prioritising ‘indicator species’ that can diagnose the health of an ecosystem could be another way to go. Or maybe ‘keystone species’ that play a unique and crucial role in the way an ecosystem functions should be prioritised.

To me, one particularly attractive method to identify species of conservation priority is the EDGE score, developed by the EDGE of Existence Programme at the Zoological Society of London (ZSL). This score provides a robust scientific framework to identify threatened species that represent a significant amount of unique evolutionary history. Its built on the premise that different species represent different amounts of evolutionary history. A species that has very few close relatives will represent a greater amount of the living planet’s evolutionary history than will a species with lots of close relatives. Species that represent a large amount of unique evolutionary history (are Evolutionarily Distinct) and are threatened with extinction (are Globally Endangered) have a high EDGE score and are identified as priorities for conservation (the exact maths of how this is done is at the end of the article for anyone interested). If the species with high EDGE scores were to disappear, there would be nothing quite like them left and the World would have lost a sizeable chunk of its evolutionary history.

A hypothetical phylogenetic tree representing a set of animal species. The base of the tree is called the root and represents the common ancestor of all of the species in the tree. Each modern day or fossil species is represented by a tip in the tree. The path from root to tip therefore represents the evolutionary history of that species since the root ancestor. Species A is highly evolutionarily distinct as it has no close relatives. In contrast, species B and C are less evolutionarily distinct as they have several closely related species.

After the EDGE score is calculated for each species, they are ranked to form a list of priority species. The EDGE of Existence Programme then acts to raise awareness of these species and to implement conservation action to secure their future. EDGE priority lists (consisting of the 100 species with the highest EDGE scores) have been developed for mammals, corals, amphibians, birds and reptiles. Each year, EDGE selects the most poorly known and neglected species on these lists for conservation attention. If a species is virtually unknown, expeditions led by ZSL determine whether and where the species still survives. EDGE Fellows are supported by the Programme to implement longer-term surveillance and conservation projects in situ.

So what are the top species identified by EDGE that need conservation attention? For mammals, the number one species on the list is the Western long-beaked Echidna, a small monotreme mammal largely restricted to the far west of New Guinea. Monotremes are an ancient order of mammals that diverge from close to the root of the mammalian phylogenetic tree. There are only 5 monotreme species left (including the duck-billed platypus), meaning this species scores highly for evolutionary distinctness. This echidna is also critically endangered.

Western long-beaked echidna, the number one EDGE mammal, is a species of monotreme found in western New Guinea. Image source EDGE

The number one EDGE coral is Siderastrea glynni, a critically endangered species that is the only member of its genus found in the Eastern Pacific. The only known colonies of this species currently reside in the Smithsonian Tropical Research Institute in Panama, although there is the possibility that additional wild populations remain to be discovered.

Siderastrea glynni the number one EDGE coral, only known from populations now housed in the Smithsonian Tropical Research Institute. Image source EDGE

For amphibians, Archey’s frog sits in the number one spot. Endemic to small pockets in New Zealand, this critically endangered frog has been described as a ‘living fossil’ as it is almost indistinguishable from fossilised frogs that lived 150 million years ago.

Archey's frog, endemic to New Zealand and the number one EDGE amphibian. Image source EDGE

In birds, the plains-wanderer has the number one spot. Found in restricted areas in Australia, this species is the sole representative of the Pedionomidae family. Its critically endangered classification means that this wanderer epitomises the two facets that make up the EDGE score.

The plains-wanderer, the number one EDGE bird, is found in pockets of Australia. Image source EDGE

The Madagascar Big-headed Turtle is not only the number one EDGE reptile but has the highest EDGE score of any terrestrial vertebrate. This turtle is the only surviving member of a genus that can be traced back to the days of the dinosaurs more than 80 million years ago. Critically endangered, this species is only found in a small area in Madagascar.

The Madagascar Big-headed Turtle with its golden burnished shell is the number one EDGE reptile. Image source EDGE

There are still plenty of groups of species to be evaluated. The EDGE scores for fish, invertebrates, plants and fungi haven’t yet been calculated.

I really like the fact that the EDGE score considers all members of a particular group of interest. It doesn’t matter how little known a species is, it is still included in the comparison. Therefore the EDGE score identifies the really endangered species, not just the sexy species often favoured by conservation efforts. Clearly, there are benefits to focusing on sexy species due to the benefits this brings to entire ecosystems and therefore many other species. But its good to stand up for the little species that are in dire need of help as well. The fact that the EDGE score is objective and data-driven is another major positive.

Another major benefit of the method is that additional facets can be included in the calculation of the EDGE score. Two facets (the species Evolutionary Distinctness and Global Endangerment) are currently included but there’s no reason that other factors couldn't be included. For example, the number of connections a species exhibits in the web of life could be incorporated to boost the score of keystone species that ecosystems rely on.

There will be more on the EDGE species in future blog posts, but the fact that this Programme not only employs a quantitative method to identify species that need urgent conservation intervention, but also provides the resources to enact this intervention, means that this Programme provides real hope that some of the World's rarest species can be saved from extinction.

Read more

The EDGE of Existence Programme website - https://www.edgeofexistence.org/

The maths behind the EDGE score

The maths used to calculate the EDGE score is actually relatively simple. There’s two parts to it: ED (Evolutionary Distinctness) and GE (Globally Endangered). Let’s look at the ED part first. This is calculated from a phylogenetic tree where each of the species are clustered by their ancestral relationships (Figure 1). Therefore species that are closely related (e.g. humans and chimpanzees) are close together in the tree while species that are less closely related (e.g. humans and rats) are further apart in the tree. The path is traced from each species at the tips of the tree back to the root and a score is calculated on each branch by calculating the length of the branch divided by the number of species that descend from that branch. Therefore species on their own branches that diverge near the base of the tree obtain a high score while the branch length leading to clusters of closely related species is divided between these species, resulting in a lower score.

Figure 1. Calculation of the ED part of the EDGE score. Shown here is a hypothetical phylogenetic tree containing 5 species A-E. The tree shows the evolutionary history of these species in units of time. Species A and B, for example, last shared a common ancestor species 1 million years before the present, while the last common ancestor of all 5 species occurred roughly 5 million years ago. The length of each branch in the tree (in millions of years) is shown in blue. The number of species that are found downstream of that branch is shown in red. The ED score is calculated for each branch by dividing the branch length (in blue) by the number of downstream species (in red). The path is then traced backwards from each species to the root and the ED score of each branch is added together to form the ED score for the species.

If we look at species A in Figure 1. This species diverged from species B 1 million years ago. Therefore the terminal branch leading to this species is 1.0 unit in length and is unique to this species so the score of this branch is 1.0/1=1.0. Species A then shares a branch with species B that is 1.5 million years in length, therefore this branch length is shared between these 2 species resulting in a score of 1.5/2=0.75. Species A then shares a branch with species B, C and D that is 2.5 million years in length and connects to the root of the tree. Therefore this branch scores 2.5/4=0.625. To find the total score for species A, we add up the score of each of the branches connecting it to the root: 1+0.75+0.625=2.375. This is the ED score for this species. The highest ED score in this figure is that of species E, which branches from the root of the tree and therefore does not share its 5 million year evolutionary history with any other species in the dataset.

That's how the ED score is calculated. The GE score is calculated from the IUCN red list classification and simply converts each classification into a number:

·      Least Concern = 0

·      Near Threatened = 1

·      Vulnerable = 2

·      Endangered = 3

·      Critically Endangered = 4

The ED and GE components are then brought together to form the EDGE score using this formula:

EDGE = ln(1 + ED) + GE*ln(2)

In technical terms, the EDGE score is a log_e-transformation of the species-specific expected loss of evolutionary history. Which translates as a scaled score to quantify how much evolutionary history would be lost if this species was to become extinct.