Data from: Competition-driven niche segregation on a landscape scale: evidence for escaping from syntopy toward allotopy in two coexisting sibling passerine species

1. The role of interspecific competition for generating patterns in
species’ distribution is hotly debated and studies taking into account
processes occurring at both large and small spatial scales are almost
missing. Theoretically, competition between species with overlapping
niches should result in divergence of their niches in sympatry to reduce
the costs of competition. Many species show a mosaic distribution within
sympatric zones, with the syntopic sites occupied by both species, and
allotopic sites where only one species occurs. It is unclear whether such
mosaics arise as a consequence of competition-driven niche segregation or
due to the decline of their abundances towards range edges driven by
environmental gradients. 2. If the interspecific competition matters, we
should observe (i) a shift in habitat preferences of one or both species
between syntopy and allotopy, and (ii) between allopatry and allotopy.
Moreover, (iii) species should show greater divergence in their habitat
preferences in allotopy than in allopatry where (iv) no differences in
habitat preferences may occur. Finally, (v) shifts should be generally
greater in the competitively subordinate species than in the dominant
species. 3. We used a unique dataset on abundance of two closely related
passerine species, the Common Nightingale (Luscinia megarhynchos) and the
Thrush Nightingale (Luscinia luscinia), collected across their syntopy,
allotopy and allopatry. The predictions were tested within a generalized
mixed-effects modelling framework. 4. After accounting for environmental
gradients perpendicular to the species’ contact zone, we found a strong
support for all but one prediction. Habitat preferences of both species
shifted markedly between syntopy and allotopy, as well as between
allopatry and allotopy. Whereas the species preferred the same habitats in
allopatry, their preferences became strikingly different in allotopy where
the abundance of the Common Nightingale increased towards dry and warm
sites with low coverage of pastures, while the abundance of the Thrush
Nightingale showed exactly opposite trends. Fifth prediction was not
supported. 5. Our results indicate that the competition between closely
related species can result in considerable changes in habitat use across
their geographic ranges accompanied with divergence in their habitat
preferences in sympatry. Here the species “escape” from competition to
allotopic sites covered by habitats avoided by the competitor. Therefore,
we argue that the interspecific competition is an important driver of
species’ distribution at both large and small spatial scales.