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.

密度依赖型生境选择已被用于预测和解释物种在不同生境间的丰度格局。生境适宜性的非密度依赖组分——热适宜性，通常是占动物物种绝大多数的变温动物（ectotherms）生境选择的最为核心的影响因子。变温动物种群密度可升至较高水平，此时有限资源的竞争加剧会导致单一个体的适合度在某一生境内下降。因此，在判断哪个生境能带来最高适合度收益时，密度与热适宜性可能会给出相互矛盾的信息，变温动物的生境选择也可能表现为非密度依赖型。
本研究以分布于10个样地的饰纹树蜥（Urosaurus ornatus）为研究对象，每个样地均跨越两种相邻生境——冲沟生境与高地生境，以此检验“即便生境间热适宜性存在差异，生境选择仍为密度依赖型”这一假说。我们首先验证了各生境内的适合度替代指标随种群密度升高而下降，这表明密度对生境适宜性存在依赖效应；同时我们证实了两种生境的适宜性（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 analysis）检验了两个预测：其一，生境选择依赖于种群密度；其二，同一样地内两种生境中的适合度替代指标相等。
研究结果显示：月存活率随种群密度升高而下降，且冲沟生境的猎物丰度与热适宜性均高于高地生境；饰纹树蜥更偏好食物更丰富、热适宜性更高的生境；两种生境中的蜥蜴密度呈正相关；且不同生境间蜥蜴的适合度替代指标无显著差异。尽管生境间热适宜性存在差异，但这些观测结果与密度依赖型生境选择假说相符。
我们推测，当种群密度较高且环境温度接近其最适性能范围时，密度依赖型生境选择在陆生变温动物中普遍存在。但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减弱。