Range-Wide Latitudinal and Elevational Temperature Gradients for the World's Terrestrial Birds: Implications under Global Climate Change

Species' geographical distributions are tracking latitudinal and elevational surface temperature gradients under global climate change. To evaluate the opportunities to track these gradients across space, we provide a first baseline assessment of the steepness of these gradients for the world's terrestrial birds. Within the breeding ranges of 9,014 bird species, we characterized the spatial gradients in temperature along latitude and elevation for all and a subset of bird species, respectively. We summarized these temperature gradients globally for threatened and non-threatened species and determined how their steepness varied based on species' geography (range size, shape, and orientation) and projected changes in temperature under climate change. Elevational temperature gradients were steepest for species in Africa, western North and South America, and central Asia and shallowest in Australasia, insular IndoMalaya, and the Neotropical lowlands. Latitudinal temperature gradients were steepest for extratropical species, especially in the Northern Hemisphere. Threatened species had shallower elevational gradients whereas latitudinal gradients differed little between threatened and non-threatened species. The strength of elevational gradients was positively correlated with projected changes in temperature. For latitudinal gradients, this relationship only held for extratropical species. The strength of latitudinal gradients was better predicted by species' geography, but primarily for extratropical species. Our findings suggest threatened species are associated with shallower elevational temperature gradients, whereas steep latitudinal gradients are most prevalent outside the tropics where fewer bird species occur year-round. Future modeling and mitigation efforts would benefit from the development of finer grain distributional data to ascertain how these gradients are structured within species' ranges, how and why these gradients vary among species, and the capacity of species to utilize these gradients under climate change.

在全球气候变化背景下，物种的地理分布正追踪纬度与海拔方向的地表温度梯度。为评估物种在空间尺度上追踪此类温度梯度的可行性，我们针对全球陆生鸟类开展了此类温度梯度陡度的首次基线评估。在9014种鸟类的繁殖分布区内，我们分别针对全部鸟类及其中一个鸟类亚群，刻画了沿纬度与海拔方向的温度空间梯度特征。我们针对受威胁物种与非受威胁物种，在全球尺度上总结了此类温度梯度，并基于物种的地理特征（分布区大小、形状与朝向）以及气候变化情景下的温度预估变化，分析了温度梯度陡度的变化规律。非洲、北美西部、南美西部以及中亚地区的物种，其海拔温度梯度最为陡峭；而澳大拉西亚、岛屿印度-马来亚区与新热带低地区的物种，其海拔温度梯度则最为平缓。非热带物种（尤其是北半球的非热带物种）的纬度温度梯度最为陡峭。受威胁物种的海拔温度梯度更为平缓，而受威胁与非受威胁物种间的纬度温度梯度差异极小。海拔温度梯度的强度与温度预估变化呈正相关。对于纬度温度梯度而言，这一相关关系仅存在于非热带物种中。纬度温度梯度的强度可通过物种地理特征更好地预测，但这一规律主要适用于非热带物种。本研究结果表明，受威胁物种与更为平缓的海拔温度梯度相关联；而陡峭的纬度温度梯度则多存在于热带以外区域，这些区域的常年栖息鸟类物种相对较少。未来的建模与气候减缓工作，可通过开发更细粒度的物种分布数据获益，以此明确温度梯度在物种分布区内的结构特征、物种间温度梯度差异的模式与成因，以及气候变化背景下物种利用此类温度梯度的能力。