Stable isotope statistical analysis
δ 13C-δ 15N scatter diagram was used to reflect the trophic niche relationship among 57 species in GraphPad Prism version 5.0. For each species, the ellipse centroid is the mean δ 13C andδ 15N values among the samples. We then calculated Euclidean distances between ellipse centroids between species, which represents a measure of interspecific isotopic niche differences (Turner et al., 2010) in EXCEL 2019. Two centroids occupied different locations (i.e. different trophic niches) if the Euclidean distance between them is significantly greater than zero (Galetti et al., 2016). A box plot with Mann-Kendall Trend Test can be used to test whether Euclidean distances between ellipse centroids among species exhibit a significant tendency for genetic distance. This statistical analyse was conducted in the R package ‘trend’.
To identify the possible niche conservatism (NC) or competitive exclusion principle (CEP), all the data were pooled together, and trophic differences were compared between distantly related species, closely related species and sister species using Two-Related-Samples Nonparametric Test. This analyse was performed using STATISTICA 18.0. In this study, sister species were perceived as the closest relative of each other in the phylogenetic relationships of Cultrinae (e.g. C. erythropterus VS C. mongolicus ). Species in the same clade of Cultrinae were considered as closely related species (e.g. C. erythropterus VS C. alburnus ), and in the different clade of Cultrinae were regarded as distantly related species (e.g. C. erythropterus VS H. leucisculus ). Furthermore, the δ 13C and δ 15N bi-plots corresponding to trophic niche space were used to reflect the trophic niche relationship of 7 species. Considering the variation in sample size, the corrected standard ellipse areas (SEAc), which accounts for core trophic niche, were calculated based on the corrected isotope data and enclose 40% of the data (Jackson et al., 2011, 2012; Syväranta et al., 2013). The SEAc were used to evaluate the trophic niche width of each species and its distribution in trophic niche space (Yao et al., 2016). In addition, we calculated the overlapping SEAc of the 7 species in each sampled site. The analyses ofδ 13C and δ 15N bi-plots, SEAc and percentage of SEAc overlap were performed using the R package ‘SIBER’ and ‘SIAR’ (Jackson et al., 2011).