Species are the currency of most biodiversity studies. However, many shortfalls and biases remain in our biodiversity estimates, preventing a comprehensive understanding of the eco-evolutionary processes that have shaped the biodiversity currently available on Earth. Biased biodiversity estimates also jeopardize the effective implementation of data-driven conservation strategies, ultimately leading to biodiversity loss. Here, we delve into the concept of the Latitudinal Taxonomy Gradient (LTG) and show how this new idea provides an interesting conceptual link between the Linnean (i.e., our ignorance of how many species there are on Earth), Darwinian (i.e., our ignorance of species evolutionary relationships), and Wallacean (i.e., our ignorance on species distribution) shortfalls. More specifically, we contribute to an improved understanding of LTGs and establish the basis for the development of new methods that allow us to: (i) better account for the integration between different shortfalls and, (ii) estimate how these interactions may affect our understanding about the evolutionary components of richness gradients at macroecological scales.

More than two million species have been described so far, but our knowledge on most taxa remains scarce or inexistent, and the available biodiversity data is often taxonomically, phylogenetically and spatially biased. Unevenness in research effort across species or regions can interact with data biases and compromise our ability to properly study and conserve biodiversity. Herein, we assess the influence of biological, conservation, geographic and socioeconomic correlates of reptile research effort globally and across six biogeographic realms. We combine bibliometric data from the Scopus database with trait‐based approaches and provide research effort information for 10 531 reptile species, modelling it as a function of 10 putative correlates of species‐level variation in research effort through negative binomial generalised mixed effect models. We show that reptile research effort is highly skewed toward certain taxa and regions, such as turtles, crocodiles, tuatara, viperids, pythons and some anguimorph lizards, as well as for temperate compared to tropical regions. Our findings indicate that greater research attention is directed towards large‐sized and early described reptile species, particularly those whose geographic range overlap with biodiversity institutions. Although we demonstrate that biological and socioeconomic factors more strongly affect reptile research effort variation, geography and conservation‐related factors also matter. Global patterns are mostly consistent, but variation across realms were observed and likely reflects differences in socioeconomic attributes as well as in the amount of species to be studied in each realm. Directing researchers and citizen scientists' attention toward understudied taxa will contribute to alleviate this biased biodiversity knowledge, although the sheer amount of species in tropical regions inevitably makes it a long‐term solution. Performing comparative studies across species with similar levels of research attention could represent a more immediate and feasible alternative.

Most species remain unknown to science and might go extinct before we recognize their existence. Although specimens belonging to many of these unknown taxa may already be housed in scientific collections, they can remain ‘shelved’ for years bearing the wrong name or without a formal name. We investigate factors underlying variation in time lag between collection and description dates for 2356 reptile species described worldwide between 1992 and 2017. We modelled the time to description using biological and sociological variables in a time-to-event analysis. Time lag between collection and description varied from zero to 155 years (median = 5). More than one-quarter of species involved specimens ‘shelved’ for 12 years or more. The time lag was shorter when the collector of the holotype – specimen serving as the name-bearer of the species – was an author of the description, while taxonomic revisions uncovered species with longer time lags. Unknown species collected by non-taxonomists and ‘shelved’ in scientific collections remained incorrectly identified for a much longer time. Taxonomic revisions are crucial to reverse this trend and improve benefits of the collecting performed by non-taxonomists. Our findings reveal the kinds of preserved reptile specimens that most likely represent unknown species in scientific collections.

Over 2 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 harboring more early-described species. Areas of high endemism richness showed fewer synonyms across amphibians and reptiles but had an inverse effect on 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 taxonomic uncertainty metrics 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.