Data from: Functional traits reveal the expansion and packing of ecological niche space underlying an elevational diversity gradient in passerine birds

Variation in species richness across environmental gradients may be associated with an expanded volume or increased packing of ecological niche space. However, the relative importance of these alternative scenarios remains unknown, largely because standardised information on functional traits and their ecological relevance is lacking for major diversity gradients. Here we combine data on morphological and ecological traits for 523 species of passerine birds distributed across an Andes-to-Amazon elevation gradient. We show that morphological traits capture substantial variation in species dietary (75%) and foraging niches (60%) when multiple independent trait dimensions are considered. Having established these relationships, we show that the 14-fold increase in species richness towards the lowlands is associated with both an increased volume and density of functional trait space. However, we find that increases in volume contribute little to changes in richness, with most (78%) lowland species occurring within the range of trait space occupied at high elevations. Taken together, our results suggest that high species richness is mainly associated with a denser occupation of functional trait space, implying an increased specialisation or overlap of ecological niches, and supporting the view that niche packing is the dominant trend underlying gradients of increasing biodiversity towards the lowland tropics.

物种丰富度（species richness）沿环境梯度的变异，可能与生态位空间的体积扩张或填充度提升相关。然而，这两种情形的相对重要性仍未明确，这主要是因为针对主要多样性梯度，尚缺乏标准化的功能性状（functional trait）及其生态相关性数据。本研究整合了沿安第斯山脉至亚马逊海拔梯度分布的523种雀形目鸟类的形态与生态性状数据。研究结果显示，当考虑多个独立性状维度时，形态性状能够解释物种饮食生态位75%、觅食生态位60%的显著变异。在明确上述关联后，我们发现向低地方向物种丰富度提升14倍的现象，同时伴随着功能性状空间的体积扩张与密度提升。但进一步分析表明，性状空间体积的增加对物种丰富度变化的贡献极小：78%的低地物种均处于高海拔地区已占据的性状空间范围内。综合来看，本研究结果表明，高物种丰富度主要与功能性状空间的更致密填充相关，这意味着生态位的特化程度提升或重叠程度增加，同时支持了"生态位填充是驱动生物多样性向低地热带地区递增梯度的主导趋势"这一学术观点。