Niche conservatism
The present study showed that, for the 57 species, in a broad scale, they exhibited niche conservatism. This is consistent with some previous studies. Eaton et al. (2006) plotted ecological differences with genetic distances for 3 clades of American blackbirds, resulting very similar patterns as in our present study, and concluded with a general pattern of niche conservatism. However, they did not do statistic test for these relationships. In our present study, we conducted statistic tests and found that, when genetic distances were small, there were significantly positive relationships between niche differences and genetic distances; while genetic distances were large, niche differences may reach saturation and showed a stable trend.
Based on these results, on the one hand, we proposed that there is a general pattern for niche conservatism, leading to the positive relationships between niche differences and genetic distances, especially when the genetic distance is within some limits. On the other hand, we proposed that, there is a limit restricting the divergence. In our present study, when genetic distances were above 0.24, the pairwise species differences reached saturation and showed a rather stable trend. Besides, distribution pattern of niche difference also showed that most species pairs were with limited divergence, only a few species were with large difference. We think that this is because that there are only limited trophic resources in the environment, leading to the limited niche divergence and trophic niche convergence.