Indicators for the Expected Loss of Phylogenetic Diversity

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PD or “phylogenetic diversity” was proposed by Faith (Biol Conserv 61:1–10, 1992) as a measure of biodiversity “option value”, justifying its importance as a target of biodiversity conservation. The threats to phylogenetic diversity can be quantified by integrating PD with IUCN Red List categories and corresponding inferred extinction probabilities, to estimate amounts of threatened or imperilled PD. Practical conservation applications face decisions about which summaries of imperilled PD best provide priority setting among species and about how to use limited available data to estimate imperilled PD. Extensions of Weitzman’s phylogenetic “expected distinctiveness” provide a unifying foundation for many useful expected PD calculations. This reinforces the utility of expected PD calculations compared to methods in the EDGE programme. However, simplistic use of expected PD also has weaknesses. A resulting priority set of species may neglect other species that also could deserve conservation action. Improved priority setting might utilise conservative estimates of the reduction in extinction probability from conservation action. The priority ordering of a species has been equated with its order of selection in priority sets, but should instead reflect the lost opportunity in averting PD loss if there is no conservation action on that species. Species priorities can be estimated, under simple assumptions, even with the simple “evolutionary distinctiveness” (ED) information that is available for many species from the EDGE programme. This provides a simple approach to priority setting in which threatened species are simply ranked by their ED score, as an estimate of their averted PD loss if the species is conserved. The approximations also provide a tabulation of current total expected PD loss for a given taxonomic group. For biodiversity assessments by the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services, this approach indicated fractional imperilled PD varying from a low 8% for squamates to a high 65% for corals among the assessments for six major taxonomic groups.