Data from: Patterns and drivers of intraspecific variation in avian life history along elevational gradients: a meta-analysis

Elevational gradients provide powerful natural systems for testing hypotheses regarding the role of environmental variation in the evolution of life-history strategies. Case studies have revealed shifts towards slower life histories in organisms living at high elevations yet no synthetic analyses exist of elevational variation in life-history traits for major vertebrate clades. We examined (i) how life-history traits change with elevation in paired populations of bird species worldwide, and (ii) which biotic and abiotic factors drive elevational shifts in life history. Using three analytical methods, we found that fecundity declined at higher elevations due to smaller clutches and fewer reproductive attempts per year. By contrast, elevational differences in traits associated with parental investment or survival varied among studies. High-elevation populations had shorter and later breeding seasons, but longer developmental periods implying that temporal constraints contribute to reduced fecundity. Analyses of clutch size data, the trait for which we had the largest number of population comparisons, indicated no evidence that phylogenetic history constrained species-level plasticity in trait variation associated with elevational gradients. The magnitude of elevational shifts in life-history traits were largely unrelated to geographic (altitude, latitude), intrinsic (body mass, migratory status), or habitat covariates. Meta-population structure, methodological issues associated with estimating survival, or processes shaping range boundaries could potentially explain the nature of elevational shifts in life-history traits evident in this data set. We identify a new risk factor for montane populations in changing climates: low fecundity will result in lower reproductive potential to recover from perturbations, especially as fewer than half of the species experienced higher survival at higher elevations.

海拔梯度为检验环境变异在生活史策略（life-history strategies）演化中所扮演角色的假说，提供了极具说服力的天然实验系统。已有案例研究显示，栖息于高海拔区域的生物，其生活史会向慢节奏方向演化，但目前尚无针对主要脊椎动物支系（vertebrate clades）生活史性状沿海拔梯度变异的综合分析。本研究聚焦两大科学问题：其一，全球范围内鸟类物种的配对种群中，生活史性状随海拔梯度的变化规律；其二，驱动生活史沿海拔梯度发生偏移的生物与非生物因子。通过三种分析方法，我们发现：高海拔区域的繁殖力显著下降，这源于更小的窝卵数与每年更少的繁殖尝试次数。与之相对，与亲本投入（parental investment）或存活相关的性状在海拔间的差异，在不同研究中表现并不统一。高海拔种群的繁殖季更短且启动更晚，但发育周期更长，这表明时间约束是导致繁殖力下降的重要诱因之一。针对窝卵数这一拥有最多种群配对样本的性状开展的分析显示：并无证据表明系统发育历史（phylogenetic history）会限制物种在海拔梯度相关性状变异中的表型可塑性（phenotypic plasticity）。生活史性状沿海拔梯度偏移的幅度，在很大程度上与地理因子（海拔、纬度）、内在因子（体重、迁徙状态）或生境协变量并无关联。集合种群（meta-population）结构、存活估计相关的方法学问题，或是塑造物种分布区边界的生态过程，或可解释本数据集所呈现的生活史性状沿海拔梯度偏移的特征。本研究还为气候变化背景下的山地种群识别出一项新的风险因子：低繁殖力会导致种群从扰动中恢复的繁殖潜力大幅下降，尤其是在仅不足半数的物种在高海拔区域拥有更高存活率的情况下。