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).