Abstract
BackgroundInterspecific competition is known to be strongest between those species that are both closely related and sympatric. Egrets are colonially nesting wetland birds that often overlap and can therefore be expected to compete in roosting and nesting habitat as well as in diet. According to the niche partitioning hypothesis, it is to be expected that these similar species would show differentiation in at least one of the main niche dimensions to reduce competition. We tested niche partitioning between the colonially nesting Little Egret (Egretta garzetta) and Cattle Egret (Bubulcus ibis) in temporal, spatial and trophic dimensions.MethodsField study was conducted in three mixed egret colonies in Yangxian County, southwest Shaanxi Province, central China. For each nest colony we recorded its spatial location, the height of nesting trees and of nests, the height of roosting trees and of roosting individuals within the trees. We determined the first egg-laying and first hatching dates of the two species. Craw dissection of storm-killed egret nestlings was used to measure the diet. Six transects were surveyed to study foraging habitat selection.ResultsWe found that hatching time of Little Egrets peaked earlier (by about 1 month) than that of Cattle Egrets. Cattle Egrets nested and roosted higher than Little Egrets. The foraging habitats used by Little Egrets were dominated by river banks (73.49%), followed by paddy fields (13.25%) and reservoirs (10.84%), whereas Cattle Egret foraging sites were characterized by grasslands (44.44%), paddy fields (33.33%) and river banks (22.22%). Little Egrets consumed more fishes (65.66%) and Odonata larvae (13.69%) than Cattle Egrets, while Cattle Egrets were found feeding mainly on Coleoptera (29.69%) and Orthoptera (23.29%). Little Egrets preyed on larger mean biomasses of food items than Cattle Egrets.ConclusionsOur results confirm the niche partitioning hypothesis as a mechanism for coexistence among ecologically similar species. In two coexisting egret species, niche partitioning is multidimensional, such that the two coexistent species occupy differing ecological space based on all three temporal, spatial and trophic niche dimensions.
Highlights
Interspecific competition is known to be strongest between those species that are both closely related and sympatric
Niche theory predicts that similar species when in coexistence should show differentiation in at least one of the main niche dimensions in order to escape the negative impacts of interspecific competition (MacArthur and Levins 1967; Schoener 1974; Hutchinson 1991; Julliard et al 2006; Cameron et al 2007)
Reproductive time Little Egrets started breeding on 7 April and ended by 4 July, and Cattle Egrets started breeding on 1 May and ended by 2 August
Summary
Interspecific competition is known to be strongest between those species that are both closely related and sympatric. There were a few studies on nest site selection (Jenni 1969; McCrimmon 1978; Burger 1979; Parejo et al 1999; Metallaoui et al 2020) and temporal separation between species within heron colonies (Weber 1975; Burger 1978; Ashkenazi and Yom-Tov 1997). Despite those separate studies of diet, nest site selection and nesting phenology, we still do not have a comprehensive understanding of niche partitioning that integrates the temporal, spatial and trophic dimensions of sympatric ardeid species
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