Conservation and species lists: taxonomic neglect promotes the extinction of endemic birds, as exemplified by taxa from eastern Atlantic islands

Abstract

SummaryWhen formulating conservation priorities, conservation biologists often rely on published lists of species-level taxa. This paper discusses the nature and taxonomic status of “species” and “subspecies” and different ways of defining “species”. Species are here taken to be terminal and evolutionarily independent units which are qualitatively diagnosable and reproductively cohesive; genealogical biodiversity is thus taken for what it is in the first place, namely the observable result of evolutionary history, an approach which has become known as the phylogenetic species concept (PSC). In contrast to the widely applied Mayrian or “biological” species concept (BSC), no inferences are made about how the degree of morphological differentiation of allopatric but seemingly closely related taxa translates in the absence or presence of reproductive isolation. Many diagnosably distinct endemic island taxa have traditionally been treated as “subspecies” of widespread “polytypic biological” species. At the same time, the “subspecies” category is also used to name arbitrarily delimited sections of intraspecific clinal variation. Thus, the “subspecies” category subjects entirely different evolutionary phenomena to the same hierarchical level through the use of trinomials. Nevertheless, and despite the discrepancy in ontological status among its contents, “subspecies” are usually considered to be of lower evolutionary and/or conservation status than “species” and this has resulted in low conservation priorities allotted to diagnosably distinct island endemics, many of which have traditionally been considered to be “merely Mayrian subspecies”. This has been recognized by some authorities who, because of the threatened status of certain island taxa, advocated treating them binomially in order to generate appropriate conservation measures to save them from extinction, without however justifying their action by any sort of phylogenetic reasoning. Although well intended and sometimes quite successful as regards the follow-up by conservationists, this demonstrates the arbitrary manner in which “species” can be defined under the BSC. Some examples of endemic taxa from eastern Atlantic islands are discussed, demonstrating the way “list thinking” and the lack of phylogenetic reasoning among conservationists translates into the presence or absence of conservation actions. Some of the criticisms of the PSC by adherents of the BSC are discussed. It is advocated that conservationists replace “list thinking” with “lineage thinking”.