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

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Two major theoretical concepts of niche evolution lead to conflicting predictions 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 test this conundrum by employing stable isotope ratios (δ13C, δ15N) to estimate trophic niches and test niche evolution in fish communities of Poyang Lake, central China. At a broad phylogenetic scale involving 57 species, we examined the relationships between trophic niche differences along genetic distances and tested phylogenetic signals. We found that trophic differences were positively associated with genetic distances when genetic distances were less than 0.24, showing strong phylogenetic signal, but not when larger than 0.24. We then focused on seven Cultrinae species coexisting at a local scale and compared trophic niche differences within and between sister species, closely-related species, and distantly-related species. We found that trophic differences between distantly-related species were significantly larger than those between closely-related species at a broad spatial scale, supporting NC. However, trophic differences between sister species were larger than those between closely-related species at a small local scale (individual sampling sites), suggesting the importance of CEP not NC. Hence, our findings suggest that niche evolution operates in a scale-dependent manner: in a phylogenetic scale (time scale), NC predictions were met well below a certain range, not above that range; at a spatial scale, CEP predictions were met for coexisting sister species, however the other species followed the NC predictions.