It is important that conservation priority-setting approaches are able to satisfy two conditions: they capture biodiversity and are robust to uncertainty. The method described herein satisfies the first condition because EDGE scores incorporate species value (in terms of originality, or irreplaceability) weighted by urgency of action (i.e. risk of extinction). Our approach satisfies the second condition because the scores are also robust to clade size, missing species and poor phylogenetic resolution. EDGE scores are also easy to calculate, as all that is required is a set of Red List assessments and a near-complete phylogeny containing at least 100 species.
In particular, EDGE priorities are much less sensitive to taxonomic uncertainty than alternate methods. The current trend towards the adoption of the phylogenetic species concept among biologists [27] is likely to produce a large number of ‘new’ threatened and endemic species [37], potentially altering the distribution of hotspots [38] and distorting other biodiversity patterns [27]. The EDGE approach is robust to such distortion because any increase in extinction risk due to splitting is balanced by a decrease in ED. A good example is that of the ruffed lemurs (Varecia spp.), which consist of one Endangered biological species (ED = 19.8; EDGE = 5.11) or two phylogenetic species (Endangered and Critically Endangered; ED = 10.3; EDGE = 4.50 and 5.20). Using the same approach, we estimate that the long-beaked echidna (Zaglossus bruijni) would fall from the second-ranked priority to the 20th after the addition of two new congeners [suggested by 39]. Thus, EDGE scores for existing species are robust to the ongoing discovery of new species.
EDGE priorities are also robust to several other forms of uncertainty. Like all phylogenetic methods, the precise EDGE scores are dependent on the topology and branch lengths of the phylogeny. However, errors in the phylogeny are unlikely to alter the identity of high-ranking species, particularly for clades of several hundred species. Topological uncertainty is usually expressed in supertrees as polytomies, which are accounted for using simple correction factors. Likewise, branch length uncertainty has been incorporated into the scoring system to down-weight the priority of species descended from nodes with imprecisely estimated ages (see Materials and Methods). These developments make it possible to estimate robustly the contribution to phylogenetic diversity of species in poorly known clades. The other major source of uncertainty is in estimating extinction risk: most recent changes in Red List category have come about through improvements in knowledge, rather than genuine changes in status [32]. EDGE scores will inevitably be affected by future changes in extinction risk, although no more so than other approaches using the Red List categories.
A minority of mammal species could not be assigned EDGE scores. Around 300 species are classified as Data Deficient and could not be meaningfully included, although in reality they may have a high risk of extinction [17]. By far the most likely candidate for high EDGE status following future Red List re-assessment is the franciscana or La Plata River dolphin Pontoporia blainvillei (ED = 36.3 MY). In addition, fifty extant species are missing from the phylogeny. The highest ranked of these are probably a pair of Critically Endangered shrews (Sorex cansulus and S. kizlovi); median and maximum ED scores for the genus are 4.55 and 14.6 MY, giving potential respective EDGE scores of 4.49 and 5.52 for these species (cf. figure 3). A further 260 species have been described since the chosen taxonomy was published [40]. Of these, the recently described Annamite striped rabbit Nesolagus timminsi [41] is the sister species to the tenth-ranked Sumatran rabbit N. netscheri, so would be a high priority if similarly threatened.
Extract only the main points for the information given above mentioned position above
1 answer