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.