Scale dependent niche conservatism found in fish communities of the largest freshwater lake in China

Investigations to niche evolution lead to controversial results in ecological studies: the competitive exclusion principle (CEP) predicts that closely-related species should be sufficiently divergent to coexist, whereas niche conservatism (NC) suggests that closely-related species should be more ecologically similar. Here, we employ stable isotope ratios (δ13C, δ15N) to estimate trophic niches and test niche evolution in fish communities of Poyang Lake, central China. At a broad scale involving 57 species, by examining the relationship between trophic niche differences along genetic distances and testing phylogenetic signals, we found trophic differences were positively associated with genetic distances, showing strong phylogenetic signals. We then focused on 7 Cultrinae species at local scale comparing trophic niche differences within and between sister species, closely-related species, and distantly-related species, and found that trophic differences between distantly-related species were significantly larger than those between closely-related species, supporting NC. However, trophic differences between sister species were larger than those between closely-related species, suggesting the importance of CEP but not NC. Therefore, our findings suggest that niche evolution operates in a scale-dependent manner: at a broad spatial and deep phylogenetic scale, NC may be present, whereas at a small local scale, CEP may be more applicable.