Data from: Temperate grassland songbird species accumulate incrementally along a gradient of primary productivity

Global analyses of bird communities along elevation gradients suggest that bird diversity on arid mountains is primarily limited by water availability, not temperature or altitude. However, the mechanism by which water availability, and subsequently primary productivity, increases bird diversity is still unclear. Here we evaluate two possible mechanisms from species-energy theory. The more individuals hypothesis proposes that a higher availability of resources increases the total number of individuals that can be supported, and therefore the greater number of species that will be sampled. By contrast, the more specialization hypothesis proposes that increasing resource availability will permit specialists to exploit otherwise rare resources, thus increasing total diversity. We used 5 years of surveys of grassland songbird communities along an elevational gradient in British Columbia, Canada, to distinguish between these hypotheses. Vegetation changed markedly in composition along the gradient and contrary to the expectations of the more specialization hypothesis, bird community composition was remarkably constant. However, both total abundance and species richness of birds increased with increasing water availability to plants. When we used rarefaction to correct species richness for differences in total abundance, much of the increase in bird diversity was lost, consistent with the expectations of the more individuals hypothesis. Furthermore, high species richness was associated with reductions in territory size of common bird species, rather than the fine-scale spatial partitioning of the landscape. This suggests that bird diversity increases when greater resource availability allows higher densities rather than greater habitat specialization. These results help explain a pervasive global pattern in bird diversity on arid mountains, and suggest that in such landscapes conservation of grassland birds is strongly linked to climate and hydrology.

沿海拔梯度开展的鸟类群落全球分析表明，干旱山地的鸟类多样性主要受水资源可利用量限制，而非温度或海拔。然而，水资源可利用量及随之而来的初级生产力提升鸟类多样性的具体机制仍不明晰。本研究从物种能量理论（species-energy theory）出发，评估了两种可能的作用机制。个体数量更多假说（more individuals hypothesis）认为，资源可利用量提升会增加生态系统所能承载的个体总数量，进而使得被采样到的物种数量更多。与之相对的特化程度更高假说（more specialization hypothesis）则提出，资源可利用量提升可允许特化物种利用原本稀缺的资源，从而提升总物种多样性。我们依托加拿大不列颠哥伦比亚省一处海拔梯度上的5年草原鸣禽群落调查数据，对这两种假说进行了验证区分。沿该海拔梯度，植被组成发生了显著变化；但与特化程度更高假说的预期相悖，鸟类群落组成却保持了极高的稳定性。不过，鸟类总个体数与物种丰富度均随植物可利用水资源量的提升而增加。当我们采用稀疏化分析法（rarefaction）针对总个体数差异对物种丰富度进行校正后，鸟类多样性的提升幅度大幅降低，这与个体数量更多假说的预期相符。此外，高物种丰富度与常见鸟类的领域面积缩小相关，而非与景观的精细空间分区有关。这表明，当更高的资源可利用量支持了更高的种群密度，而非更高的生境特化程度时，鸟类多样性会得以提升。本研究结果有助于解释干旱山地鸟类多样性的全球性普遍格局，并表明在这类景观中，草原鸣禽的保护与气候和水文状况密切相关。