Melbourne Brett A, Hastings Alan
Department of Ecology and Evolutionary Biology, University of Colorado, Boulder, Colorado 80309, USA.
Nature. 2008 Jul 3;454(7200):100-3. doi: 10.1038/nature06922.
Extinction risk in natural populations depends on stochastic factors that affect individuals, and is estimated by incorporating such factors into stochastic models. Stochasticity can be divided into four categories, which include the probabilistic nature of birth and death at the level of individuals (demographic stochasticity), variation in population-level birth and death rates among times or locations (environmental stochasticity), the sex of individuals and variation in vital rates among individuals within a population (demographic heterogeneity). Mechanistic stochastic models that include all of these factors have not previously been developed to examine their combined effects on extinction risk. Here we derive a family of stochastic Ricker models using different combinations of all these stochastic factors, and show that extinction risk depends strongly on the combination of factors that contribute to stochasticity. Furthermore, we show that only with the full stochastic model can the relative importance of environmental and demographic variability, and therefore extinction risk, be correctly determined. Using the full model, we find that demographic sources of stochasticity are the prominent cause of variability in a laboratory population of Tribolium castaneum (red flour beetle), whereas using only the standard simpler models would lead to the erroneous conclusion that environmental variability dominates. Our results demonstrate that current estimates of extinction risk for natural populations could be greatly underestimated because variability has been mistakenly attributed to the environment rather than the demographic factors described here that entail much higher extinction risk for the same variability level.
自然种群的灭绝风险取决于影响个体的随机因素，可通过将这些因素纳入随机模型来进行估计。随机性可分为四类，包括个体层面出生和死亡的概率性质（种群统计学随机性）、不同时间或地点种群水平出生和死亡率的变化（环境随机性）、个体的性别以及种群内个体间生命率的变化（种群统计学异质性）。此前尚未开发出包含所有这些因素的机制随机模型来研究它们对灭绝风险的综合影响。在此，我们使用所有这些随机因素的不同组合推导出一族随机里克模型，并表明灭绝风险强烈依赖于导致随机性的因素组合。此外，我们表明只有使用完整的随机模型才能正确确定环境和种群统计学变异性的相对重要性，进而确定灭绝风险。使用完整模型，我们发现种群统计学随机性来源是赤拟谷盗（红粉甲虫）实验室种群变异性的主要原因，而仅使用标准的更简单模型会得出环境变异性占主导的错误结论。我们的结果表明，目前对自然种群灭绝风险的估计可能被大大低估，因为变异性被错误地归因于环境，而不是这里描述的种群统计学因素，对于相同变异性水平，后者带来的灭绝风险要高得多。
