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.1. 种间竞争（interspecific competition）在塑造物种分布格局中的作用一直存在广泛争议，而同时兼顾大、小空间尺度过程的相关研究仍近乎空白。理论上，生态位（niche）重叠的物种种间竞争应促使二者在同域分布区（sympatric zones）发生生态位分化，以降低竞争成本。诸多物种在同域分布区内呈现镶嵌分布格局：同栖位点（syntopic sites）为两物种共同占据，而异栖位点（allotopic sites）则仅存单一物种。目前尚不清楚此类镶嵌分布究竟是竞争驱动的生态位分化所致，还是由环境梯度介导的物种丰度向分布区边缘衰减所引发。 2. 若种间竞争确实发挥作用，则应观测到以下现象：(1) 两物种（或其中之一）的生境偏好于同栖与异栖区间存在差异；(2) 两物种的生境偏好于异域分布区（allopatry）与异栖区间存在差异。此外，(3) 两物种在异栖区的生境偏好分化程度应高于异域分布区；(4) 异域分布区内两物种的生境偏好或无显著差异。最后，(5) 竞争从属物种的生境偏好偏移程度总体上应高于竞争优势物种。 3. 本研究采用了一套独特的数据集，涵盖两近缘雀形目（passerine）物种——普通夜莺（Luscinia megarhynchos）与鸫夜莺（Luscinia luscinia）——在其同栖区、异栖区及异域分布区的丰度数据。研究采用广义混合效应模型（generalized mixed-effects modelling）框架对上述预测进行了检验。 4. 在校正与物种接触带垂直的环境梯度效应后，本研究除一条预测未获支持外，其余全部得到了强有力的验证。两物种的生境偏好于同栖与异栖区间，以及异域分布区与异栖区间均发生了显著偏移。异域分布区内两物种偏好相同的生境，但在异栖区二者的生境偏好却呈现出显著分化：普通夜莺的丰度随干燥温暖且草地覆盖度较低的生境梯度升高而增加，而鸫夜莺的丰度变化趋势则恰好相反。第五条预测未获支持。 5. 本研究结果表明，近缘物种种间竞争可导致其在整个地理分布范围内的生境利用发生显著改变，并伴随同域分布区的生境偏好分化。在此场景下，物种会“逃离”竞争压力，迁至被竞争者规避的生境所覆盖的异栖区。因此，本研究认为种间竞争是调控大、小空间尺度下物种分布格局的重要驱动因子。