Morphologically cryptic amphipod species are “ecological clones” at regional but not at local scale: a case study of four niphargus species

Recent studies indicate that morphologically cryptic species may be ecologically more different than would be predicted from their morphological similarity and phylogenetic relatedness. However, in biodiversity research it often remains unclear whether cryptic species should be treated as ecologically equivalent, or whether detected differences have ecological significance. In this study, we assessed the ecological equivalence of four morphologically cryptic species of the amphipod genus Niphargus. All species live in a small, isolated area on the Istrian Peninsula in the NW Balkans. The distributional ranges of the species are partially overlapping and all species are living in springs. We reconstructed their ecological niches using morphological traits related to feeding, bioclimatic niche envelope and species’ preference for epihypogean habitats. The ecological meaning of differences in niches was evaluated using distributional data and co-occurrence frequencies. We show that the species comprise two pairs of sister species. All species differ from each other and the degree of differentiation is not related to phylogenetic relatedness. Moreover, low co-occurrence frequencies in sympatric zones imply present or past interspecific competition. This pattern suggests that species are not differentiated enough to reduce interspecific competition, nor ecologically equivalent to co-exist via neutral dynamics. We tentatively conclude that the question of ecological equivalence relates to the scale of the study: at a fine scale, species’ differences may influence dynamics in a local community, whereas at the regional level these species likely play roughly similar ecological roles

Epi-hypogean distribution of the studied species, corrected for geological basement.

<p>Epi-hypogean distribution of the studied species, corrected for geological basement.</p

Summary of all results.

<p>¹ Numbers rang species pairs according to the similarity of the species ecological niche: 1- little difference, 6—maximum differences.</p><p>² All species exhibit a clumped distribution (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0134384#pone.0134384.t004" target="_blank">Table 4</a>).</p><p>Summary of all results.</p

Distribution of focal species and the studied area (the Istrian Peninsula).

<p>The inset map indicates the geographic position of the study area within Europe. Species presence records were superimposed on a SRTM Shaded Relief (Central North) layer available from ESRI.</p

Species`differences in morphological traits related to feeding biology.

<p>*diagnostic character between <i>N</i>. <i>krameri</i> A and <i>N</i>. <i>krameri</i> B.</p><p>Species`differences in morphological traits related to feeding biology.</p

BioClim variables used in modelling the bioclimatic niches.

<p>0.7, 0.8, 0.9 refer to threshold of the maximum correlation (Spearman’s rank) between BioClim variables value beyond which the data were considered as independent.</p><p>*Isothermality = [Mean Diurnal Range (Mean of monthly (max temp–min temp)) / Temperature Annual Range]</p><p>BioClim variables used in modelling the bioclimatic niches.</p

Evidence for clumped distribution of focal species.

<p>* NN Ratio = observed mean distance/expected mean distance, if NNR<1 then distribution is clumped, if NNR>1 then distribution is equally dispersed.</p><p>Evidence for clumped distribution of focal species.</p

Ecological similarity between species pairs along individual niche axes and the joint niche.

<p>¹Overall morphological similarity calculated from <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0134384#pone.0134384.t002" target="_blank">Table 2</a>, normalized values.</p><p>²Difference in preference for surface habitats, normalized values.</p><p>^{0.7, 0.8, 0.9} The values denote maximum Spearman’s rank correlation between BioClim variables allowed in calculation of Schoener’s D index. For calculations of the joint niche values of D index were standardized.</p><p>*A statistically significant difference in values of selected BioClim variables for the species pair.</p><p>Ecological similarity between species pairs along individual niche axes and the joint niche.</p

Bayesian phylogenetic trees of the focal species complexes using concatenated alignments for 28S and COI gene fragment.

<p>The numbers on nodes indicate posterior probabilities for Bayesian trees and bootstrap support values for maximum likelihood trees. Patristic, K2P and uncorrected p-distances between A and B species within <i>N</i>. <i>krameri</i> and <i>N</i>. <i>spinulifemur</i> are 0.34, 0.14, 0.12 and 0.36, 0.19, 0.16, respectively. Coloured dots at some terminals indicate localities of co-occurrences with the respective species. The species are: <i>Niphargus krameri</i> A (NKA, red), <i>Niphargus krameri</i> B (NKB, blue), <i>Niphargus spinulifemur</i> A (NSA, green), and <i>Niphargus spinulifemur</i> B (NSB, yellow).</p