Data from: The geography of demography: long-term demographic studies and species distribution models reveal a species border limited by adaptation

Potential causes of species’ geographic distribution limits fall into two broad classes: (1) limited adaptation across spatially variable environments and (2) limited opportunities to colonize unoccupied areas. Combining demographic studies, analyses of demographic responses to environmental variation, and species distribution models, we investigated the causes of range limits in a model system, the eastern border of the California annual plant Clarkia xantiana ssp. xantiana. Vital rates of 20 populations varied with growing season temperature and precipitation: fruit number and overwinter survival of 1-year-old seeds declined steeply, while current-year seed germination increased modestly along west-to-east gradients in decreasing temperature, decreasing mean precipitation, and increasing variation in precipitation. Long-term stochastic finite rate of increase, ls, exhibited a fourfold range and varied among geologic surface materials as well as with temperature and precipitation. Growth rate declined significantly toward the eastern border, falling below 1 in three of the five easternmost populations. Distribution models employing demographically important environmental variables predicted low habitat favorability beyond the eastern border. Models that filtered or weighted population presences by ls predicted steeper eastward declines in favorability and assigned greater roles in setting the distribution to among-year variation in precipitation and to geologic surface material. These analyses reveal a species border likely set by limited adaptation to declining environmental quality.

物种地理分布边界的潜在成因可分为两大类别：(1) 空间异质环境下的适应能力受限；(2) 拓殖未占据区域的机会不足。本研究结合种群统计研究（demographic studies）、环境响应的种群动态分析以及物种分布模型（Species Distribution Models），以加州一年生植物黄茎克拉花（Clarkia xantiana ssp. xantiana）的东部边界为模式系统，探究了其分布边界的成因。20个种群的生命率（vital rates）随生长季温度与降水发生变化：沿西至东的环境梯度，随着温度降低、平均降水量减少以及降水变异程度升高，果实数量与1年生种子的越冬存活率急剧下降，而当年种子萌发率则小幅上升。长期随机有限增长速率（ls）的变化幅度达四倍之多，且在不同地质地表材质间存在差异，同时亦随温度与降水条件变化而改变。种群增长速率朝向东边界显著下降，在最东部的5个种群中有3个降至1以下。采用具有重要种群动态意义的环境变量构建的物种分布模型，预测了东边界以外区域的生境适宜度较低。而通过ls对种群出现点进行筛选或加权的物种分布模型，则预测出适宜度沿东向下降得更为显著，且将降水的年际变异与地质地表材质在塑造物种分布中的作用赋予了更高权重。本研究表明，该物种的分布边界极有可能由适应不断下降的环境质量的能力受限所决定。