Supplemental material for: The dynamics of bird diversity in the new world

Three prominent explanations have been proposed to explain the dramatic differences in species richness across regions and elevations, (1) time for speciation, (2) diversification rates, and (3) ecological limits. But the relative importance of these explanations and, especially, their interplay and possible synthesis remain largely elusive. Integrating diversification analyses, null models, and GIS, I study avian richness across regions and elevations of the New World. My results reveal that even though the three explanations are differentially important (with ecological limits playing the dominant role), each contributes uniquely to the formation of richness gradients. Further, my results reveal the likely interplay between the explanations. They indicate that ecological limits hinder the diversification process, such that the accumulation of species within a region gradually slows down over time. Yet, it does not seem to converge toward a hard ceiling on regional richness. Instead, species-rich regions show suppressed, but continued, diversification, coupled with signatures of possible competition (esp. Neotropical lowlands). Conversely, species-poor, newly-colonized regions show fast diversification and weak to no signs of competition (esp. Nearctic highlands). These results held across five families of birds, across grid cells, biomes, and elevations. Together, my findings begin to illuminate the rich, yet highly consistent, interplay of the mechanisms that together shape richness gradients in the New World, including the most species-rich biodiversity hotspots on the planet, the Andes and the Amazon.

目前已有三种主流解释被提出，用以阐释不同区域与海拔梯度间物种丰富度的显著差异：(1) 物种形成时间（time for speciation），(2) 分化速率（diversification rates），以及(3) 生态限制（ecological limits）。但这些解释的相对重要性，尤其是它们之间的相互作用与潜在整合机制，在很大程度上仍不甚明晰。本研究整合分化分析、零模型（null models）与地理信息系统（GIS）技术，探究新大陆不同区域与海拔梯度的鸟类物种丰富度格局。研究结果显示，尽管三种解释的重要性各不相同（其中生态限制占据主导地位），但每一种都对物种丰富度梯度的形成有着独特贡献。进一步而言，本研究结果还揭示了这些解释背后潜在的相互作用机制：生态限制会阻碍物种分化过程，使得区域内的物种积累速率随时间推移逐渐放缓。然而，这一过程似乎并未收敛至区域物种丰富度的绝对上限。与之相反，物种丰富的区域虽存在分化速率受抑的情况，但分化仍在持续，同时伴随潜在的种间竞争信号（尤以新热带低地为甚）。反之，物种匮乏且新近拓殖的区域则呈现出快速的物种分化，几乎无明显种间竞争迹象（尤以新北区高地为甚）。上述研究结果在5个鸟类类群、不同网格单元、生物群系以及海拔梯度中均保持一致。综上，本研究的发现初步阐明了塑造新大陆物种丰富度梯度的多种机制之间复杂且高度一致的相互作用关系，其中包括全球物种最丰富的生物多样性热点地区——安第斯山脉与亚马孙流域。