How taxonomic change influences forecasts of the Linnean shortfall (and what we can do about it)?

Abstract

The gap between the number of described species and the number of species that actually exist is known as the Linnean shortfall and is of fundamental importance for biogeography and conservation. Unsurprisingly, there have been many attempts to quantify its extent for different taxa and regions. In this Perspective, we argue that such forecasts remain highly problematic because the extent of the shortfall does depend not only on the rates of exploration (sampling undescribed taxa) on which estimates have been commonly based but also on the rates of taxonomic change (lumping and splitting). These changes highly depend on the species concepts adopted and the information and methods used to delimit species. Commonly used methods of estimating the number of unknown species (e.g. discovery curves, taxon ratios) can underestimate or overestimate the Linnean shortfall if they do not effectively account for trends and rates of taxonomic change. A further complication is that the history of taxonomic change is not well documented for most taxa and is not typically available in biodiversity databases. Moreover, wide geographic and taxonomic variation in the adoption of species concepts and delimitation methods mean that comparison of estimates of the Linnean shortfall between taxa and even for the same taxon between regions may be unreliable. Given the high likelihood of future taxonomic changes for most major taxa, we propose two main strategies to consider the influence of taxonomic change on estimates of unknown species: (i) a highly conservative approach to estimating the Linnean shortfall, restricting analysis to groups and regions where taxonomies are relatively stable and (ii) explicitly incorporating metrics of taxonomic change into biodiversity models and estimates. In short, relevant estimates of the number of known and unknown species will only be achieved by accounting for the dynamic nature of the taxonomic process itself.

Nomenclatural stability and the longevity of helminth species names

“…Our analysis provides estimates of how many helminth species names proposed in the past two-and-a-half centuries turned out to be invalid, and how long on average it took for them to become unaccepted, based on when they were first proposed and the higher helminth taxon they belong to. These estimates may help to calibrate future attempts at predicting total parasite biodiversity, by allowing one to assign a probability of validity to each species name based on when it was proposed (Lessa et al 2024). Our results also serve to illustrate in numbers the ability of taxonomic research to self-correct over time.…”

The interaction between the Linnean and Darwinian shortfalls affects our understanding of the evolutionary dynamics driving diversity patterns of New World coralsnakes

“…The second component refers to changes in the number of valid species through shifts in the species concept used, taxonomic revisions, or changes in taxon delimitation (Baselga et al, 2010;Diniz-Filho et al, 2023;Hortal et al, 2015). These changes can lump two or more species under a single name or split a species into two or more distinct taxa, or reclassify them into different genera (Lessa et al, 2024). This second component of the Linnean shortfall, thus, demonstrates a clear interdependence with other knowledge shortfalls that, despite being recognized, are still poorly considered in ecological studies (Diniz-Filho et al, 2023;Hortal et al, 2015).…”