Comparative Diversity Across Biogeographic Realms

Abstract

Biogeographic realms -- the continental-scale units of biogeographic classification (Neotropical; Afrotropical; Indo-Malayan; Nearctic; Palearctic; Australasian; Oceanian; Antarctic) defined by shared evolutionary history and characteristic faunal and floral composition -- represent the most fundamental spatial units for understanding global patterns of species diversity, endemism, and the distribution of conservation priority. While species richness patterns across realms are reasonably well-characterised for vertebrates through GBIF and IUCN range maps, comparative functional and phylogenetic diversity across realms -- and the degree to which realm boundaries structure biodiversity at these dimensions -- remains poorly quantified. This study presents the first simultaneous comparative analysis of species richness, functional diversity (FRic; FEve; FDiv), phylogenetic diversity (PD; MPD; MNTD), and beta diversity across all eight biogeographic realms for five taxonomic groups (mammals; birds; reptiles; amphibians; vascular plants) using standardised hexagonal grid sampling at 100 km2 resolution across 38,400 grid cells globally. The Neotropical realm had the highest species richness (mean 847 species per 100 km2 cell) and functional diversity but intermediate phylogenetic diversity -- suggesting recent rapid diversification. The Australasian realm showed disproportionately high phylogenetic diversity (mean PD = 184 +- 47 Myr) relative to species richness -- reflecting the ancient lineages of marsupials, monotremes, and isolated endemic plant families. Realm-level beta diversity analysis confirmed that realm boundaries are stronger barriers to compositional similarity than equivalent geographic distances within realms (between-realm Sorensen = 0.94 +- 0.02 vs. within-realm = 0.67 +- 0.08; p < 0.001)