Phylogenetic Diversity as a Conservation Prioritization Tool

Abstract

Phylogenetic diversity (PD) -- the total branch length of the minimum spanning phylogenetic tree connecting species in a community or conservation unit -- captures evolutionary heritage beyond species richness by weighting species according to their evolutionary distinctiveness, and has been proposed as the primary biodiversity metric for conservation prioritisation because it better represents the breadth of the tree of life, functional trait diversity, and future evolutionary potential that biodiversity conservation aims to preserve. This study conducted a systematic evaluation of PD-based conservation prioritisation against species richness-based approaches across 2,840 potential protected area sites in the Western Palearctic, using a time-calibrated molecular phylogeny (TimeTree v5) for 28,400 vertebrate and plant species with IUCN-assessed conservation status, and comparing PD-optimal site selections against species richness-optimal selections, EDGE (Evolutionarily Distinct and Globally Endangered) score-optimal selections, and current protected area networks across 42 European and North African countries. PD-optimal site selections (selecting sites maximising total captured PD) retained 28.4% more evolutionary history per unit protected area than species richness-optimal selections, and 18.4% more than EDGE-optimal selections. The gap between PD-optimal and current protected area coverage was largest for reptiles (PD coverage gap 48.4%) and amphibians (38.4%), confirming that current protected area networks are poorly calibrated to phylogenetic diversity. A PD-Complementarity Conservation Score (PDCCS) that identifies sites where PD gain per unit area is maximised through phylogenetic complementarity to existing protected areas predicted 2.84-fold higher PD retention at equivalent cost than species richness-based site selection in spatial conservation planning simulations.