Global Patterns of Taxonomic Uncertainty and its Impacts on Biodiversity Research

Over two million species have been named so far, but many will be invalidated due to redundant descriptions. Undetected invalid species (i.e., synonyms) can impair inferences we make in biodiversity research and hamper the implementation of effective conservation strategies. However, the processes leading to the accumulation of invalid names remain largely unknown. Using multi-model inferences, we investigated the patterns and potential drivers of species- and assemblage-level variation in synonym counts across terrestrial vertebrates globally. We also explored how taxonomic uncertainty (i.e., instability in species identities) can affect latitudinal variation of diversification rates. The average number of synonyms was higher for species described earlier, better represented in scientific collections, with larger geographic ranges, occurring in temperate regions, and in areas of high biodiversity attention. In assemblage-level models, a higher average number of synonyms was associated with temperate regions harbouring more early-described species. Areas of high endemism richness showed fewer synonyms across amphibians and reptiles but had an inverse effect for birds and mammals. Other predictor-response relationships varied across taxonomic groups, biogeographical realm, and spatial grain. Assuming that more synonyms indicate more stable species that have been thoroughly studied and reviewed, high synonym numbers in temperate species and assemblages support claims of a potential latitudinal taxonomy gradient, where geographic variation in taxonomic practice could hinder the proper recognition of tropical species. We show that the accumulation of invalid names is not random and discuss how invalid hidden names can affect biodiversity inferences. A potential approach to address this problem would be developing a taxonomic uncertainty metric that could be incorporated into models (i.e., as weights to account for varying degrees of uncertainty during the fitting process). Our study provides an initial approximation and highlights the often-neglected issue of uncertainty and instability in species identities from a macroecological perspective.