Do cryptic species matter in macroecology? Sequencing European groundwater crustaceans yields smaller ranges but does not challenge biodiversity determinants

Abstract

Ecologists increasingly rely on molecular delimitation methods (MMs) to identify species boundaries, thereby potentially increasing the number of putative species because of the presence of morphologically cryptic species. It has been argued that cryptic species could challenge our understanding of what determine large‐scale biodiversity patterns which have traditionally been documented from morphology alone. Here, we used morphology and three MMs to derive four different sets of putative species among the European groundwater crustaceans. Then, we used regression models to compare the relative importance of spatial heterogeneity, productivity and historical climates, in shaping species richness and range size patterns across sets of putative species. We tested three predictions. First, MMs would yield many more putative species than morphology because groundwater is a constraining environment allowing little morphological changes. Second, for species richness, MMs would increase the importance of spatial heterogeneity because cryptic species are more likely along physical barriers separating ecologically similar regions than along resource gradients promoting ecologically‐based divergent selection. Third, for range size, MMs would increase the importance of historical climates because of reduced and asymmetrical fragmentation of large morphological species ranges at northern latitudes. MMs yielded twice more putative species than morphology and decreased by 10‐fold the average species range size. Yet, MMs strengthened the mid‐latitude ridge of high species richness and the Rapoport effect of increasing range size at higher latitudes. Species richness predictors did not vary between morphology and MMs but the latter increased the proportion of variance in range size explained by historical climates. These findings demonstrate that our knowledge of groundwater biodiversity determinants is robust to overlooked cryptic species because the latter are homogeneously distributed along environmental gradients. Yet, our findings call for incorporating multiple species delimitation methods into the analysis of large‐scale biodiversity patterns across a range of taxa and ecosystems.

Niches within a niche: ecological differentiation of subterranean amphipods across Europe's interstitial waters

“…While we had genetic data for the majority of species, we are missing these data at the population level, and would expect that at least some of the nominal species contain multiple cryptic species. Cryptic species are a common phenomenon in Niphargus (Lefébure et al , Trontelj et al , Meleg et al , Delić et al , b, Eme et al ) and may be broadly sympatric or even co‐occurring (Fišer et al 2015b, Delić et al ). Cryptic species may increase the complexity of local communities and underestimate taxonomic structure, possibly affecting the conclusions (Fišer et al ).…”

Cryptic species as a window into the paradigm shift of the species concept

“…Use of different molecular markers and species delineation methods revealed conflicting results about the number of species. Alternative species delimitations suggested by different methods were not in conflict with each other, but differed in the degree of splitting; distance methods typically turned to be more conservative (Copilaş‐Ciocianu & Petrusek, ; Delić, Trontelj, Rendoš, & Fišer, ; Delić, Švara, et al., ; Eme et al., ; Katouzian et al., ; Weiss et al., ). In such cases, authors either provided a rough estimate of species number (Copilaş‐Ciocianu & Petrusek, ; Weiss et al., ), opted for a particular species hypothesis by ranking the efficiency of different methods (e.g., conservative ABGD was preferred over PTP and bGMYC; Katouzian et al., ), reconsidered the results within spatial context (presence of syntopy (Delić, Trontelj, et al., )) or drew on other studies to justify the choice of a particular threshold value.…”

“…The authors found that cryptic species were homogeneously distributed among taxa and regions and concluded “that cryptic metazoan diversity can be treated as random error in biodiversity assessments.” Their meta‐analysis was later criticized for several methodological problems (Trontelj & Fišer, ), and additional analyses almost a decade later suggested that cryptic species actually may be heterogeneously distributed across animal phyla (Pérez‐Ponce de León & Poulin, ). The evidence for homogenous distribution of cryptic species across geographic regions is conflicting (Eme et al., ; Gill et al., ; Voda, Dapporto, Dinca, & Vila, ); perhaps an heterogeneous distribution can be detected only at global geographic scales.…”

“…Whether symmetrical and asymmetrical fragmentations of morphospecies ranges are homogeneously distributed across geographic regions has yet to be determined. One study indicates, however, that consideration of cryptic species does not affect continental scale patterns of range size, the so‐called Rapoport effect (i.e., the pattern of increasing range size of morphological species at higher latitudes in the Palearctic region; Stevens, ) (Eme et al., ).…”

Finding answers in the dark: caves as models in ecology fifty years after Poulson and White

“…ants: Pape 2016; diatoms: Falasco et al 2014;fungi and yeasts: Vanderwolf et al 2013;nematodes: Du Preez et al 2017;spiders: Mammola andIsaia 2017b, Mammola et al 2018a). Nonetheless, it has been recently shown that the existence of cryptic diversity does not always represent a major problem when investigating the ecological factors driving patterns of subterranean species richness (Eme et al 2018). Nonetheless, it has been recently shown that the existence of cryptic diversity does not always represent a major problem when investigating the ecological factors driving patterns of subterranean species richness (Eme et al 2018).…”