Ranking Mammal Species for Conservation and the Loss of Both Phylogenetic and Trait Diversity

The 'edge of existence' (EDGE) prioritisation scheme is a new approach to rank species for conservation attention that aims to identify species that are both isolated on the tree of life and at imminent risk of extinction as defined by the World Conservation Union (IUCN). The self-stated benefit of the EDGE system is that it effectively captures unusual 'unique' species, and doing so will preserve the total evolutionary history of a group into the future. Given the EDGE metric was not designed to capture total evolutionary history, we tested this claim. Our analyses show that the total evolutionary history of mammals preserved is indeed much higher if EDGE species are protected than if at-risk species are chosen randomly. More of the total tree is also protected by EDGE species than if solely threat status or solely evolutionary distinctiveness were used for prioritisation. When considering how much trait diversity is captured by IUCN and EDGE prioritisation rankings, interestingly, preserving the highest-ranked EDGE species, or indeed just the most threatened species, captures more total trait diversity compared to sets of randomly-selected at-risk species. These results suggest that, as advertised, EDGE mammal species contribute evolutionary history to the evolutionary tree of mammals non-randomly, and EDGE-style rankings among endangered species can also capture important trait diversity. If this pattern holds for other groups, the EDGE prioritisation scheme has greater potential to be an efficient method to allocate scarce conservation effort.“‘生存边缘’（EDGE）优先级划分方案是一种全新的物种保护优先级排序方法，旨在识别同时满足两项条件的物种：一是在生命之树上演化分支孤立，二是符合世界自然保护联盟（IUCN）定义的濒临灭绝紧迫风险。该方案宣称的核心优势在于，可有效捕捉到独特的非典型物种，进而在未来保全某一类群的整体演化历史。鉴于EDGE指标最初并非为捕捉整体演化历史而设计，本研究对这一主张进行了验证。分析结果显示，若优先保护EDGE排名靠前的物种，相较于随机选取受威胁物种的方案，所能保全的哺乳动物整体演化历史显著更高。相较于仅以受威胁等级或仅以演化独特性作为优先级划分依据的方案，EDGE方案所能覆盖的生命之树总分支占比也更多。在考量IUCN与EDGE优先级排序所能覆盖的性状多样性时，有趣的是，相较于随机选取的受威胁物种集合，优先保护最高排名的EDGE物种，甚至仅优先保护受威胁程度最高的物种，所能获取的整体性状多样性均更高。上述结果表明，正如该方案所宣传的那样，哺乳动物类群中的EDGE物种并非随机地为哺乳动物演化树贡献演化历史，且濒危物种中的EDGE式排序同样能够捕捉到重要的性状多样性。若这一规律在其他类群中同样成立，则EDGE优先级划分方案有望成为分配稀缺保护资源的高效手段。