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（elevational gradients）为检验关于环境变异在生活史策略（life-history strategies）演化中作用的假说提供了强有力的天然实验系统。已有案例研究揭示，栖息于高海拔地区的生物其生活史趋于缓慢，但目前尚无针对主要脊椎动物支系生活史性状（life-history traits）海拔变异的综合分析。本研究开展了两项核心分析：（i）剖析全球鸟类物种种群配对中，生活史性状随海拔的变化规律；（ii）明确驱动生活史性状海拔偏移的生物与非生物因子。本研究采用三种分析方法，结果发现，随着海拔升高，繁殖力因窝卵数（clutch size）更小、每年繁殖尝试次数更少而降低。与之相对，与亲本投入或存活相关的性状的海拔差异则因研究而异。高海拔种群的繁殖季更短且启动更晚，但发育周期更长，这表明时间限制会导致繁殖力下降。针对窝卵数——这一我们拥有最多种群比较数据的性状——的分析显示，并无证据表明系统发育历史会限制物种水平上与海拔梯度相关的性状变异的可塑性。生活史性状的海拔偏移幅度，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）或生境协变量无关。元种群结构（meta-population structure）、存活估算相关的方法学问题，或是塑造物种分布边界的过程，或可解释本数据集所呈现的生活史性状海拔偏移的本质。本研究还识别了气候变化背景下山地种群面临的一项新风险因子：低繁殖力将导致种群从扰动中恢复的繁殖潜力更低，尤其是仅有不到一半的物种在高海拔地区拥有更高的存活率。