Endemism inflation and the unit problem: biogeographical consequences of taxonomic oversplitting

Abstract

Endemism is a cornerstone concept in biogeography, yet persistent inconsistencies in how it is defined and, more importantly, what taxonomic units are counted, have made many endemism-based studies difficult to compare. In practice, “endemics” are often tallied using mixed analytical currencies: species alongside subspecies, varieties, and forms, sometimes also under species concepts that encourage splitting. Here I revisit the historical development of endemism and emphasize the species as the primary analytical unit for biogeographical inference. I formalize a related phenomenon, endemism inflation, defined as the apparent increase in endemic units produced by taxonomic oversplitting and by the uncritical inclusion of infraspecific ranks in endemism metrics. Drawing on literature on species concepts and biodiversity assessment, I argue that endemism inflation is not merely semantic; it changes maps of endemism, affects the identification of centres of endemism, and can distort conservation prioritization. Even classical comparisons show that endemism values vary across taxonomic groups and with taxonomic practice, not only biology. Finally, I outline practical recommendations for reporting endemism in ways that preserve comparability, including explicit statements of the analytical unit and parallel reporting of species-only versus mixed-rank endemism where feasible.