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.