Data from: Demographic compensation does not rescue populations at a trailing range edge

Species' geographic ranges and climatic niches are likely to be increasingly mismatched due to rapid climate change. If a species' range and niche are out of equilibrium, then population performance should decrease from high-latitude "leading" range edges, where populations are expanding into recently ameliorated habitats, to low-latitude "trailing" range edges, where populations are contracting from newly unsuitable areas. Demographic compensation is a phenomenon whereby declines in some vital rates are offset by increases in others across time or space. In theory, demographic compensation could increase the range of environments over which populations can succeed and forestall range contraction at trailing edges. An outstanding question is whether range limits and range contractions reflect inadequate demographic compensation across environmental gradients, causing population declines at range edges. We collected demographic data from 32 populations of the scarlet monkeyflower (Erythranthe cardinalis) spanning 11˚ latitude in western North America and used integral projection models to evaluate population dynamics and assess demographic compensation across the species' range. During the 5-year study period, which included multiple years of severe drought and warming, population growth rates decreased from north to south, consistent with leading-trailing dynamics. Southern populations at the trailing range edge declined due to reduced survival, growth, and recruitment, despite compensatory increases in reproduction and faster life history characteristics. These results suggest that demographic compensation may only delay population collapse without the return of more favorable conditions or the contribution of other buffering mechanisms such as evolutionary rescue.

受快速气候变化影响，物种的地理分布范围与气候生态位（climatic niche）的匹配度或将持续降低。若某物种的分布范围与气候生态位失衡，则种群表现将从高纬度的“前沿”分布边界——此处种群正扩张至新近改善的生境——向低纬度的“后缘”分布边界逐渐衰减，而在后缘边界处，种群正从新近不再适宜的栖息区域收缩退场。种群统计补偿（demographic compensation）是指在时间或空间维度上，部分关键生命率（vital rates）的下降可通过其他生命率的上升实现抵消的现象。从理论层面而言，种群统计补偿可拓宽种群成功存续的环境范围，并延缓后缘分布边界处的分布范围收缩。当前尚未解决的核心问题是，分布范围边界与范围收缩现象，是否源于跨环境梯度的种群统计补偿不足，进而导致分布边界处的种群数量下降。本研究采集了北美西部11个纬度跨度内的32个猩红色沟酸浆（Erythranthe cardinalis）种群的种群统计数据，并采用积分投影模型（integral projection models）评估了该物种种群动态及其分布范围内的种群统计补偿情况。在涵盖多年严重干旱与气候变暖的5年研究周期内，种群增长率从北向南逐渐降低，这与前沿-后缘动态理论的预期一致。尽管南部后缘分布边界的种群在繁殖能力与更快的生活史特征方面出现了补偿性提升，但由于存活、生长与种群补充量的下降，此类种群仍出现了衰退。上述结果表明，若无法回归更适宜的环境条件，或缺乏进化救援（evolutionary rescue）等其他缓冲机制的助力，种群统计补偿或许仅能延缓种群崩溃的进程。