Niche segregation on the landscape scale of two co-existing related congeners in the sympatric zone – modelling approach

Co-occurence of related congeners is one of the most complicated ecological phenomena. Although such species are characterised by niche conservatism, which should exclude their stable co-existence, a communities often consist of sister species. Thus, there must be some ecological mechanism that separates them spatially or temporally. This study advocates that the answer is niche divergence that results from interspecific competition. Despite numerous studies documenting this biotic interaction as an important ecological process that segregates species spatially, only few deal with niche divergence between allotopy and syntopy in a mosaic of a large sympatric zone. That is why I decided to analyse the co-existence of two sister species, the Whinchat Saxicola rubetra and the Stonechat Saxicola rubicola in a sympatric zone, where both species occur alongside in syntopic sites, and separately in allotopic sites. Besides, I analyse the Whinchat's potential niche divergence between sympatric and allopatric zones which can be designated on the territory of Poland. It is assumed that niche divergence resulting from interspecific competition can be observed on the basis of differences in habitat preferences of one or both species between syntopy and allotopy, and also for the Whinchat between the sympatric and the allopatric zone. The assumptions were tested in generalised mixed-effect modelling and contrast analyses based on a large dataset of bird density in Poland. Both species demonstrated shifted environmental preferences between syntopy and allotopy, while in allopatry both species had similar niche preferences. The Whinchat also indicated no differences between allopatry and allotopy. Thus, the adopted analytical system clearly proves that the co-existence of congeneric species is possible due to niche divergence, which most likely results from interspecific competition.