Data from: Density-dependent habitat selection predicts fitness and abundance in a small lizard

Density-dependent habitat selection has been used to predict and explain patterns of abundance of species between habitats. Thermal quality, a density-independent component of habitat suitability, is often the most important factor for habitat selection in ectotherms which comprise the vast majority of animal species. Ectotherms may reach high densities such that individual fitness is reduced in a habitat due to increased competition for finite resources. Therefore, density and thermal quality may present conflicting information about which habitat will provide the highest fitness reward and ectotherm habitat selection may be density-independent. Using ornate tree lizards Urosaurus ornatus at 10 sites each straddling two adjacent habitats (wash and upland), we tested the hypothesis that habitat selection is density-dependent even when thermal quality differs between habitats. We first tested that fitness proxies decline with density in each habitat, indicating density-dependent effects on habitat suitability. We also confirmed that the two habitats vary in suitability (quantified by food abundance and thermal quality). Next, we tested the predictions that habitat selection depends on density with isodar analyses and that fitness proxies are equal in the two habitats within a site. We found that monthly survival rates decreased with density, and that the wash habitat had more prey and higher thermal quality than the upland habitat. Lizards preferred the habitat with more food and higher thermal quality, lizard densities in the two habitats were positively correlated, and fitness proxies of lizards did not differ between habitats. These patterns are consistent with density-dependent habitat selection, despite differences in thermal quality between habitats. We expect that density-dependent habitat selection is widespread in terrestrial ectotherms when densities are high and temperatures are close to their optimal performance range. In areas where thermal quality is low, however, we expect that depletable resources, such as food, become less limiting because assimilating resources is more difficult.密度依赖型栖息地选择（density-dependent habitat selection）常被用于预测和解释物种在不同栖息地间的丰度分布模式。热适宜性（thermal quality）作为栖息地适宜性的密度非依赖组分，通常是占动物物种绝大多数的变温动物（ectotherm）栖息地选择的最关键影响因素。变温动物可能达到较高种群密度，此时栖息地内有限资源的竞争加剧，会导致个体适合度（fitness）下降。因此，密度与热适宜性可能就“哪个栖息地能提供最高适合度收益”这一问题给出相悖的结论，此时变温动物的栖息地选择可能呈现密度非依赖特性。 本研究以分布于10个样地的饰树蜥（Urosaurus ornatus）为研究对象，每个样地均横跨两种相邻栖息地：冲沟生境（wash）与高地生境（upland），以此检验“即便栖息地间热适宜性存在差异，栖息地选择仍为密度依赖型”这一假说。我们首先验证了各栖息地内适合度替代指标（fitness proxy）随种群密度升高而下降，这表明密度对栖息地适宜性存在依赖效应。同时我们证实，两种栖息地的适宜性（通过食物丰度与热适宜性量化）存在差异。随后，我们通过等密度线分析（isodar analyses）检验了两项预测：一是栖息地选择依赖于种群密度，二是同一样地内两种栖息地的适合度替代指标无显著差异。 研究结果显示：月存活率随种群密度升高而下降；冲沟生境的猎物资源更丰富，且热适宜性优于高地生境。饰树蜥更偏好食物更丰富、热适宜性更佳的栖息地；两种生境中的蜥蜴种群密度呈正相关；且两种栖息地内蜥蜴的适合度替代指标并无差异。尽管栖息地间热适宜性存在差异，但上述观测结果与密度依赖型栖息地选择假说相符。 我们推测，当种群密度较高且环境温度接近其最佳生理性能阈值时，密度依赖型栖息地选择在陆生变温动物中广泛存在。然而，在热适宜性较低的区域，由于生物体对资源的同化过程更为困难，诸如食物这类可耗尽资源的限制作用会有所减弱。