Analysis of niche conservatism among the 57 species
In the present study, each of the 57 species showed differences in their niches indicated by the δ 13C andδ 15N values. In overall, the stable isotope values ranged from -36.707 to -18.129 for δ 13C, represented by Cirrhinus mrigala and Misgurnus anguillicaudatus ; and from 5.184 to 21.525 forδ 15N, represented by Paramisgurnus dabryanus and Neosalanx brevirostris (Fig. 2). Pairwise species niche differences (EDBEC, Euclidean distances between ellipse centroid) ranged from 0.063 to 15.256, with the shortest distance occurring between two bitterling species (Acheilognathus rhombeus and Rhodeus sinensis ), and the most distant between a minnow and a loach, Pseudolaubuca engraulis and Misgurnus anguillicaudatus (Fig. 3).
The plot of pairwise species trophic niche differences against genetic distances showed a rough positive pattern, indicating possible niche conservatism. Further analysis showed that, the positive relationships were significant while genetic distances less than 0.24, but niche differences may reach saturation and showed a stable trend while genetic distances larger than 0.24. Besides, at the very far genetic distance (Fig. 3, Fig. 4), some species pairs showed very little niche differences, indicating niche convergence. For instance, trophic differences (EDBEC) between Silurus asotus andSiniperca scherzeri was only 0.35.
In addition, we also found that species niche differences were distributed in skew, with most species pairs (87.22%) bearing not very much differences (EDBEC less than 6), while only a few species pairs (12.78%) had considerable differences in their trophic niches (EDBEC more than 6) (Fig. 5).