On the EDGE
Many EDGE species under high threat are largely unknown, poorly researched, and commonly overlooked by existing conservation frameworks. But these genetically unique species are actually particularly worthy of protection.
Changing conservation priorities
Given the huge number of endangered species and limited financial resources, prioritizing conservation efforts is crucial. The EDGE method was developed for this purpose. It indicates how irreplaceable and globally threatened a species is.
By focusing practical conservation on species that are evolutionarily distinct (ED) and globally endangered (GE), it is possible to prevent a disproportionately large part of evolutionary history from becoming extinct in the near future.​​
The EDGE score, first published in 2007*, essentially measures the contribution made by a species to phylogenetic diversity and combines it with the global threat to this species as per IUCN Red List category. The score may be used to rank EDGE species per group, the top rank being 1. The score is independent of clade size in phylogenies of more than 100 species, suggesting that scores from unrelated taxonomic groups are likely comparable, and robust to taxonomic changes. *Isaac NJB et al., Mammals on the EDGE: Conservation priorities based on threat and phylogeny (https://doi.org/10.1371/journal.pone.0000296).
Evolutionary uniqueness is a consequence of independent development, e.g. during 0.06 million years for 19 mouse species compared to 97.6 million years (!) for the (evolutionarily unique) platypus. There are more than a thousand species of mouse-like rodents such as the wood mouse on Earth, but only one species of platypus. It is also much older than the wood mouse, which as a genetically independent species may only have existed for less than a hundred thousand years. Platypuses are therefore more worthy of protection, assuming similar risk of extinction.
A rational basis for choosing conservation targets
​​Evolutionary distinctiveness is significantly lower in Least Concern species (per IUCN Red List category) than in species under higher threat (Near-Threatened and conservation-dependent; Vulnerable; Endangered; and Critically Endangered). In the EDGE score, each increment of Red List category doubles extinction risk.
Low EDGE scores (Evolutionary Distinctiveness and Global Endangerment) indicate lower conservation priority than higher ones.
In contrast, threat status alone does not guarantee high conservation priority. Thus, 10 Critically Endangered mammal species in the genera Gerbillus, Peromyscus and Crocidura as well as 32 Endangered mammal species fail to make the top 1000 conservation priorities, whereas 130 Near-Threatened species do.
Plotting an EDGE score
©edgeofexistence.org, Zoological Society of London.​
EDGE score of Western Long-beaked Echidna, Zaglossus brujinii The EDGE score (yellow horizontal bar) combines an assessment of ED (evolutionary distinctiveness) and GE (global endangerment). This helps prioritize conservation attention. Within a group of species, higher scores mean higher priority. The right end of the yellow bar shows how high this species' EDGE score is compared to the median of all species within its taxonomic group (vertical black line). The ED score (black horizontal bar) represents the unique evolutionary history surviving in this species, expressed in millions of years. The vertical black line in this score shows the median ED for the rest of this taxonomic group. The GE (global endangerment) rating or IUCN Red List category shows how close this species is to becoming extinct. Species that are Vulnerable (VU), Endangered (EN), or Critically Endangered (CR) are considered to be at significant risk of extinction.
Discovering orphan EDGE species
The EDGE concept is very useful for overruling the common mammal-and-bird bias, which makes us prefer creatures that are familiar to us. Some charismatic, well known EDGE species (think of a Giant Panda, African Elephant or Orang Utan) receive much attention. In contrast, other EDGE species under high threat are largely unknown, poorly researched, and commonly overlooked by existing conservation frameworks. Yet these — the not-so-awesome — are just as important. Indeed, it's mostly the creepy-crawly critters which keep the living world alive and kicking.
​​Whilst the 100 highest EDGE priority mammal species include several large, charismatic ones, many smaller and less appreciated species also warrant high priority, including 16 rodents, 13 eulipotyphlas (hedgehogs, moles and shrew-like species) 12 bats, 4 lagomorphs (hare family) and 1 elephant shrew.
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After edgeofexistence.org, Zoological Society of London.​
Some clades (taxonomic groups) contain a large proportion of EDGE species, others much fewer. Whilst almost a third of crocodile and turtle species are on the EDGE, this applies to 25% of conifers and cycads, but only about 3% of flowering plants. Such data help prioritize conservation efforts. This graph displays the number of EDGE species per group (left to right): amphibians, birds, crocodylians, ray-finned fish, lepidosaurs (snakes, lizards and the tuatara), mammals, sharks and rays, turtles & tortoises, conifers & cycads, flowering plants.
After Gumbs et al., nature communications (2024)15:1101​
This plot shows the proportion of threatened species in a clade which are also among its top 1% EDGE species. This puts crocodylians, turtles & tortoises, rays & sharks and amphibians as top conservation priorities among jawed vertebrates. These species, however, tend to be neglected by conservation.
Conservationists are familiar with biodiversity hotspots (see map), i.e. the most biologically rich and at the same time most endangered terrestrial ecoregions worldwide. More than half of the world’s plant species and nearly 43% of amphibian, reptile, bird and mammal species are endemic to areas totalling just 2.5% of Earth’s land surface.
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Similarly, EDGE species cluster in EDGE zones, of which 25 have been identified worldwide. Together, these cover 0.723 % of the world's surface, but contain a third of the global evolutionary history of the endangered tetrapods. Half of them are endemic to the respective zone (i.e. they only occur there worldwide).
© Pipins et al 2023, Advancing EDGE Zones spatial priorities, Biorxiv 2023, https://doi.org/10.1101/2023.08.15.553233
EDGE2: progress on EDGE
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​In 2024 Gumbs et al. published the interdisciplinary consensus-based EDGE2 protocol* to reflect important research advances since 2007 so they can be adopted in conservation practice.
EDGE2 retains the key elements of phylogenetic irreplaceability (ED) and global vulnerability (GE). However, unlike the original EDGE protocol, it allows for quantifying extinction risk, includes methods for dealing with uncertainty (e.g. data deficient or non-evaluable species, or uncertainty in phylogenetic data) and accounts for the extinction risk of closely related species. *The EDGE2 protocol: Advancing the prioritisation of Evolutionarily Distinct and Globally Endangered species for practical conservation action. https://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.3001991 EDGE2 conceptually separates components of the protocol for future consensus-based updating without needing to revisit the entire protocol. Key advances of the EDGE2 metric are the incorporation of phylogenetic diversity complementarity and adopting the probabilistic framework for calculating expected losses in phylogenetic diversity in case of a species’ extinction. This allows for estimating both the minimum unique phylogenetic diversity lost and the relative importance of internal phylogenetic branches for a species’ evolutionary distinctiveness. It also allows for quantifying the impacts of conservation actions. Whereas the 2007 EDGE protocol prioritized species with above-median ED (evolutionary distinctiveness) score, priority EDGE2 species must be noteworthy both in EDGE2 score and risk of extinction (GE2). They must be a Red List species, but now also include Extinct in the Wild species, and they must be above the median EDGE2 score with 95% confidence. EDGE2 also proposes an EDGE Research List for species that are credibly above-median EDGE2, but currently not evaluable or listed as data deficient by IUCN Red List. Similarly, EDGE2 defines an EDGE Watch List for least-concern and near-threatened species that rank above-median EDGE2 with 95% confidence. Maintaining such species (e.g. the Aardvark and the Duck-billed Platypus) is critical for conserving deep branches of the Tree of Life.
1wild priorities beyond EDGE
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Beyond the EDGE focus, we also pay special attention to keystone species (those pivotal for an ecosystem, e.g. ecosystem engineers) and to ecosystems or biomes of high importance (biodiversity and or EDGE hotspots). And we prefer supporting programmes which receive inadequate conservation funding.
Overall, our approach is pragmatic. We feel free to support any conservation project that we deem important.