Data from: Dietary niche variation and its relationship to lizard population density

(1) Insular species are predicted to broaden their niches, in response to having fewer competitors. They can thus exploit a greater proportion of the resource spectrum. In turn, broader niches are hypothesized to facilitate (or be a consequence of) increased population densities. (2) We tested whether insular lizards have broader dietary niches than mainland species, how it relates to competitor and predator richness, and the nature of the relationship between population density and dietary niche breadth. (3) We collected population density and dietary niche breadth data for 36 insular and 59 mainland lizard species, and estimated competitor and predator richness at the localities where diet data were collected. We estimated dietary niche shift by comparing island species to their mainland relatives. We controlled for phylogenetic relatedness, body mass, and the size of the plots over which densities were estimated. (4) We found that island and mainland species had similar niche breadths. Dietary niche breadth was unrelated to competitor and predator richness, on both islands and the mainland. Population density was unrelated to dietary niche breadth across island and mainland populations. (5) Our results indicate that dietary generalism is not an effective way of increasing population density nor is it result of lower competitive pressure. A lower variety of resources on islands may prevent insular animals from increasing their niche breadths even in the face of few competitors.

(1) 岛屿物种（insular species）因竞争者数量更少，被预测会拓宽其生态位（niche）。借此它们可利用更广泛的资源谱。反之，较宽的生态位被假设会促进种群密度提升（或为种群密度提升的结果）。
(2) 本研究旨在检验岛屿蜥蜴的饮食生态位（dietary niche）是否较大陆物种更宽、饮食生态位宽度与竞争者、捕食者丰富度的关联，以及种群密度与饮食生态位宽度之间的关系本质。
(3) 我们收集了36种岛屿蜥蜴与59种大陆蜥蜴的种群密度及饮食生态位宽度数据，并在饮食数据采集的样地处估算了竞争者与捕食者的丰富度。我们通过将岛屿物种与其大陆近缘类群进行对比，估算了饮食生态位的偏移情况。同时，我们控制了系统发育亲缘关系、体质量以及种群密度估算所用样地的面积这三个变量。
(4) 研究结果显示，岛屿与大陆蜥蜴物种的生态位宽度并无显著差异。无论是在岛屿还是大陆环境中，饮食生态位宽度均与竞争者、捕食者丰富度无关联。在岛屿与大陆种群中，种群密度与饮食生态位宽度均无显著相关关系。
(5) 本研究结果表明，饮食泛化既非提升种群密度的有效途径，也并非竞争压力更低的结果。岛屿上的资源种类更少，即便竞争者稀少，也可能阻碍岛屿动物拓宽其生态位宽度。