Investigations to niche evolution have received controversial results in recent times, leading to some confusions in ecological studies: competitive exclusion principle (CEP) predicts that closely related species should be adequately divergent; while niche conservatism (NC) predicts that closely related species should be ecologically most similar. In the present study, by using stable isotope to signify trophic niche, we analyzed niche divergences of 57 fishes from a lake (Poyang Lake, central China) at the phylogenetic scale, as well as niche divergences and overlaps of 7 fishes (same subfamliy) at 5 sites of this lake, and comprehensively investigated the concepts of NC and competitive exclusion principle (CEP). The results showed that, NC exists on a large scale (trophic differences showing positive relationships with genetic distances), but not at the local scale (sister species showing larger differences than closely related species, and almost no trophic overlap), where the CEP works better. Besides, trophic convergence was also found between some very distantly related species, leading to the possible limited divergence. We believe these findings will contribute to future theoretical and empirical niche explorations.

The spatial-temporal patterns of fish assemblages in lotic systems can provide useful information in developing effective conservation measures. This study aimed to explore the spatiotemporal changes in fish assemblage and their association with environmental factors in mountain streams of Ren River, southwest China. Filed investigations were conducted at 18 sites during rainy and dry season in 2017. A total of 21 species, belonged to 3 orders, 8 families and 19 genera, were collected. Analysis of similarities (ANOSIM) showed fish assemblages structure varied significantly at the spatial scale, but not at the temporal scale. In low order sites, fish assemblages were mainly dominated by cold water and rheophilic species (e.g. Rhynchocypris oxycephalus, Scaphesthes macrolepis, Metahomaloptera omeiensis and Gnathopogon herzensteini), while those in high order sites were predominated by warm water and eurytopicity or stagnophilic species (e.g. Squalidus argentatus, Hemiculter leucisculus and Zacco platypus). Canonical correspondence analysis (CCA) showed fish assemblages were structured by a combination of large-scale landscape factors (e.g. altitude and C-link) and small-scale habitat features (e.g. channel width, water temperature and depth). Among these factors, landscape factors had the greatest influence on fish assemblage, while local habitat measures played less important roles or just acted in certain season.