When less is more and more is less: the impact of sampling effort on species delineation

Abstract

Taxonomy is the very first step of most biodiversity studies, but how confident can we be in the taxa delineation? One may hypothesize that the more abundant the material, the more accurate the description of morphological variability and hence the better the taxonomic delineation. Yet, as we shall see, in the case of numerous transitional forms, this hypothesis may prove wrong. Similarly to rarefaction curves that assess the degree of knowledge on taxonomic diversity through sampling effort, we aim to test the impact of sampling effort on species delineation by subsampling a given assemblage. To do so, we use an abundant and morphologically diverse conodont fossil assemblage from the Smithian of Oman. We first recognize four well established morphospecies but about 80% of the specimens are transitional forms. We quantify the diagnostic characters in a sample of 159 P1 elements using geometric morphometrics and assess, via gradually subsampling the assemblage, the number of morphometric groups (i.e. morphospecies) using ordination and clustering analyses. Four morphospecies were detected when less than 20% of the specimens were considered. The number of detected clusters dropped to two when including more than 30% of the specimens. Such influence of sampling effort on species delineation highlights the complexity of taxonomic work, especially when transitional forms are more abundant than typical specimens. These results should encourage researchers to extensively illustrate, measure and quantitatively compare their material to better constrain the morphological variability and delineation of taxa.

Nested singletons in molecular trees: Utility of adding morphological and geographical data from digitized herbarium specimens to test taxon concepts at species level in the case of Casearia (Salicaceae)

“…Therefore, the various lines of evidence suggest that C. selloana and C. zizyphoides represent distinct taxa, separate from C. sylvestris , even if not obvious from the limited molecular data. Therefore, in all five tested cases, we argue to maintain the hitherto used classification rather than sinking the respective accepted names into synonymy, while highlighting the need for additional investigation (Scherz et al, 2017 ; Guenser et al, 2022 ).…”

“…Proper species delimitation is crucial not only for accurate biodiversity assessments and biodiversity monitoring but also for downstream studies, such as ecology and conservation (Agapow et al, 2004 Therefore, in all five tested cases, we argue to maintain the hitherto used classification rather than sinking the respective accepted names into synonymy, while highlighting the need for additional investigation (Scherz et al, 2017;Guenser et al, 2022).…”

Potential of closed contour analysis for species differentiation and holotype designation: a case study on lower Norian (Upper Triassic) conodonts

“…The analysis involved a total of 136 lower Norian conodont P 1 elements (Table 1) that can be considered an average size statistical sample in the Triassic in comparison with the studies by Guenser et al . (2019, 2022) and Souquet et al . (2022).…”

“…Landmark analysis was combined recently with more traditional (angle and linear distance) measurements on aboral and lateral views of P 1 elements (including the holotypes) of the Olenekian (Early Triassic) Neospathodus by Guenser et al . (2022). They tested the effect of the sample size on the observed morphological variability of a group and taxonomic determination.…”

The Endemic Marine Fish Fauna From the Eastern Paratethys Reconstructed From Otoliths From the Miocene (Middle Sarmatian S.L.; Bessarabian) of Jurkine (Kerch Peninsula, Crimea)

“…However, in our opinion, subdivision in species or subspecies reflecting such morphotypes would only result in problematic definitions, and we, therefore, refrained from such action. In a recent study by Guenser et al (2022), the merits of lumping taxa versus the loss of resolution in species with about 80% of morphological intermediate forms between them was discussed. In our case of A. kerichensis and the subsequently described A. scitulus, we are of the opinion that further taxonomic subdivision would only result in diagnostic ambiguities rather than enhancing resolution.…”