Introduction
The concept of ecological niche is fundamental in ecology, explorations to which have greatly prompted the advances of ecological science. Traditionally, studies on this concept have been focused on the ecological aspect, with the core parts being the understanding of species coexistence mechanism, or the famous competitive exclusion principle (CEP). It states that “two species with identical niches (and compete for a single resource) cannot coexist together indefinitely” (Gause, 1934; Hardin, 1960; De León et al. , 2014). According to this, closely related species should be adequately divergent (Svensson, 2012; Wang et al. , 2015; Liu et al. , 2018; Silva et al., 2020). The CEP has so much influential on the whole ecological studies that almost all theoretical investigations and empirical data explanations are based on it (MacArthur, 1958; Holt, 2013).
However, in some recent times, niche evolution or niche divergence has drawn great attention resulting in a new principle of niche conservatism (NC) (Webb et al., 2002; Ackerly, 2003; Ackerly et al., 2006; Kraft et al. , 2007; Swenson & Enquist, 2009). According to it, a species tends to retain aspects of its fundamental niche over space and time, therefore, closely related species should be ecologically most similar (Harvey & Pagel, 1991; Losos et al., 2003; Wiens & Graham, 2005; Diniz-Filho & Bini, 2007). Apparently, these two principles (CEP and NC) have some contradictions on their predictions to niche relationships of closely related species.
At present, due to the longer research history, the CEP has been investigated extensively and accepted as a general rule (Davydova et al., 2003; López-Gómez & Molina-Meyer, 2006; Hening & Nguyen, 2020). However, studies on the NC over macroevolutionary time have received different results. Supports for NC have been found in amphibians (Hu et al., 2016), freshwater arthropods (Morinièreet et al., 2016) and plants (Ricklefs & Latham, 1992; Prinzing et al., 2001; Crisp et al., 2009; Liu et al., 2012, 2015), while contradictions found in studies such as microhabitat preferences in Anolis lizards (Losos et al., 2003), Eltonian niche of 32 freshwater fishes (Comte et al., 2017), climatic, habitat and trophic niches of a European avian assemblage (Pearman et al., 2014). Therefore, given all the contradictions, it has not been explored whether CEP and NC can be synthesized together.
In the present study, by using stable isotope analysis to signify trophic niche and represent species ecological niche of 57 fish species in 13 families and 10 orders collected from the Poyang Lake, central China, we examined niche evolution or conservatism at a phylogenetic scale and test NC at phylogenetic scale. Besides, we analyzed niche divergence among 7 species from a subfamily (Cultrinae, Cyprinidae) to test the CEP by comparing the niche differences and overlaps in their coexisting local sites. We found that the two principles work at different scale: CEP present at local species coexistence scale, while NC present at relatively larger evolutionary scale. Therefore, both two principles can be synthesized in this framework. And this further understanding to niche evolution will be beneficial for theoretical study to species coexistence mechanism and give hints for future conservation.