Morphology of Orbulina universa and Globorotalia scitula across local extinctions during Sapropel S5 (Eastern Mediterranean Sea, c.126-121 ka)

Extinction is a remarkably difficult phenomenon to study under natural conditions. This is because the outcome of stress exposure and associated fitness reduction is not known until the extinction occurs and it remains unclear whether there is any phenotypic reaction of the exposed population that can be used to predict its fate. Here we take advantage of the fossil record, where the ecological outcome of stress exposure is known. Specifically, we analyze shell morphology of planktonic Foraminifera in sediment samples from the Mediterranean, during an interval preceding local extinctions. In two species representing different plankton habitats, we observe shifts in trait state and decrease in variance in association with non-terminal stress, indicating stabilizing selection. At terminal stress levels, immediately before extinction, we observe increased growth asymmetry and trait variance, indicating disruptive selection and bet-hedging. The pre-extinction populations of both species show a combination of trait states and trait variance distinct from all populations exposed to non-terminal levels of stress. This finding indicates that the phenotypic history of a population may allow the detection of threshold levels of stress, likely to lead to extinction. It is thus an alternative to population dynamics in studying and monitoring natural population ecology.

在自然环境中开展物种灭绝相关研究，堪称极具挑战性的课题。这是因为胁迫暴露及其伴随的适合度下降所产生的结局，直至种群灭绝发生前均无法获知；同时，暴露于胁迫的种群是否存在可用于预测其存续命运的表型反应，目前仍不明确。本研究借助化石记录展开分析——在此类记录中，胁迫暴露所带来的生态结局是已知的。具体而言，我们针对地中海海域沉积物样本中的浮游有孔虫（planktonic Foraminifera）壳体形态展开分析，研究时段覆盖局部灭绝发生前的沉积间隔。针对两种栖息于不同浮游生境的物种，我们观察到：在非终末胁迫条件下，种群出现性状状态偏移与性状方差降低的现象，这表明此时存在稳定选择（stabilizing selection）。而在灭绝发生前夕的终末胁迫阶段，我们观察到个体生长不对称性与性状方差均出现升高，这提示此时存在分裂选择（disruptive selection）与避险策略（bet-hedging）。两种物种的灭绝前种群，其性状状态组合与性状方差特征均与所有处于非终末胁迫水平的种群存在显著差异。这一发现表明，通过分析种群的表型历史，或可检测出可能引发灭绝的胁迫阈值水平。因此，该方法可作为种群动态研究的替代手段，用于自然种群生态学的研究与监测工作。