Drivers of the Functional and Taxonomic Dark Diversity of Regional Flea Assemblages in Four Biogeographic Realms

ABSTRACT Dark diversity (DD) represents species that could occur in a community due to suitable abiotic conditions but are absent. Taxonomic dark diversity (TDD) is measured via species co‐occurrences, whereas functional dark diversity (FDD) is measured via the co‐occurrences of functional groups (species possessing similar trait combinations). In this study, FDD and TDD approaches were used to investigate the environmental and host‐associated factors affecting regional FDD and TDD size of fleas parasitic on small mammals in four biogeographic realms (the Afrotropics, the Nearctic, the Neotropics, and the Palearctic), asking whether environmental and host‐associated effects on DD and the community completeness index (CCI; a ratio between observed and dark diversity) differed between (a) FDD and TDD and (b) biogeographic realms. The FDD sizes and the FCCI (= functional community completeness), in all realms, were consistently lower and higher than TDD and TCCI (= taxonomic community completeness), respectively. The FDD sizes and FCCI of regional flea assemblages in the realms of the Northern Hemisphere were mostly associated with host functional diversity, whereas in the Southern Hemisphere realms, they were mostly associated with environmental variables. In contrast, the main predictor of TDD was host species richness in the Southern Hemisphere and environment in the Northern Hemisphere. The TCCI in all realms was mainly associated with host species richness, increasing with an increased host fauna richness. In conclusion, the functional and taxonomic facets of dark diversity complement each other, despite being different. The application of both approaches results in a more comprehensive characterization of potential biological diversity.