Data from: Landscape-scale eco-evolutionary dynamics: selection by seed predators and fire determine a major reproductive strategy

Recent work in model systems has demonstrated significant effects of rapid evolutionary change on ecological processes (eco-evolutionary dynamics). Fewer studies have addressed whether eco-evolutionary dynamics structure natural ecosystems. We investigated variation in the frequency of serotiny in lodgepole pine (Pinus contorta), a widespread species in which postfire seedling density and ecosystem structure are largely determined by serotiny. Serotiny, the retention of mature seeds in cones in a canopy seed bank, is thought to be an adaptation for stand-replacing fire, but less attention has been paid to the potential selective effects of seed predation on serotiny. We hypothesized that spatial variation in percentage serotiny in lodgepole pine forests results from an eco-evolutionary dynamic where the local level of serotiny depends on the relative strengths of conflicting directional selection from fire (favoring serotiny) and seed predation (favoring cones that open at maturity). We measured percentage serotiny, the abundance of American red squirrels (Tamiasciurus hudsonicus; the primary pre-dispersal seed predator of lodgepole pine), and several measures of forest structure in Yellowstone National Park, USA. Fire frequency strongly predicted the frequency of serotiny, a pattern that is well-supported in the literature. At sites with high fire frequency (return intervals of 135–185 years) where fire favors increased serotiny, squirrel abundance was negatively associated with serotiny, suggesting that selection from predation can overwhelm selection from fire when squirrels are abundant. At sites with low fire frequency (return intervals of 280–310 years), serotiny was nearly universally uncommon (<10%). Finally, forest structure strongly predicted squirrel density independently of serotiny, and serotiny provided no additional explanatory power, suggesting that the correlation is caused by selection against serotiny exerted by squirrels, rather than squirrels responding to variation in percentage serotiny.

近年来模型系统相关研究已证实，快速进化过程对生态过程具有显著影响（生态进化动力学（eco-evolutionary dynamics））。针对生态进化动力学能否塑造自然生态系统的相关研究仍较为匮乏。本研究聚焦于扭叶松（Pinus contorta）——一种分布广泛的树种，其火烧后幼苗密度与生态系统结构在很大程度上由种子宿存（serotiny）特性决定——的种子宿存频率变异。种子宿存指成熟种子留存于冠层球果种子库的特性，通常被认为是应对林分毁灭性火灾的适应策略，但针对种子捕食对种子宿存的潜在选择作用的研究仍较少。我们提出假说：扭叶松林分中种子宿存率的空间变异源于一种生态进化动力学过程，即局域种群的种子宿存水平取决于火灾（倾向于促进种子宿存）与种子捕食（倾向于促进成熟时开裂的球果）这两种相反方向定向选择压力的相对强度。我们在美国黄石国家公园开展了相关测量，包括种子宿存率、美洲红松鼠（Tamiasciurus hudsonicus，扭叶松扩散前种子的主要捕食者）的丰度，以及多项森林结构指标。研究结果显示，火灾频率可显著预测种子宿存频率，这一结论已得到现有文献的充分支持。在火灾频率较高（火灾回归间隔为135–185年）、火灾倾向于促进种子宿存的样地中，松鼠丰度与种子宿存率呈负相关关系，表明当松鼠种群数量较高时，捕食带来的选择压力可抵消火灾带来的选择压力。在火灾频率较低（火灾回归间隔为280–310年）的样地中，种子宿存率普遍极低（<10%）。此外，森林结构可独立于种子宿存率显著预测松鼠密度，而种子宿存率未提供额外的解释能力，这表明二者的相关性是由松鼠对种子宿存的选择淘汰作用所致，而非松鼠响应种子宿存率的变异。