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Partitioning the effects of deterministic and stochastic processes on species extinction risk
Ecological Complexity ( IF 3.5 ) Pub Date : 2019-04-01 , DOI: 10.1016/j.ecocom.2019.03.004
Tak Fung , James P. O'Dwyer , Ryan A. Chisholm

Abstract Species populations are subjected to deterministic and stochastic processes, both of which contribute to their risk of extinction. However, current understanding of the relative contributions of these processes to species extinction risk is far from complete. Here, we address this knowledge gap by analyzing a suite of models representing species populations with negative intrinsic growth rates, to partition extinction risk according to deterministic processes and two broad classes of stochastic processes – demographic and environmental variance. Demographic variance refers to random variations in population abundance arising from random sampling of events given a particular set of intrinsic demographic rates, whereas environmental variance refers to random abundance variations arising from random changes in intrinsic demographic rates over time. When the intrinsic growth rate was not close to zero, we found that deterministic growth was the main driver of mean time to extinction, even when population size was small. This contradicts the intuition that demographic variance is always an important determinant of extinction risk for small populations. In contrast, when the intrinsic growth rate was close to zero, stochastic processes exerted substantial negative effects on the mean time to extinction. Demographic variance had a greater effect than environmental variance at low abundances, with the reverse occurring at higher abundances. In addition, we found that the combined effects of demographic and environmental variance were often substantially lower than the sum of their effects in isolation from each other. This sub-additivity indicates redundancy in the way the two stochastic processes increase extinction risk, and probably arises because both processes ultimately increase extinction risk by boosting variation in abundance over time.

中文翻译:

划分确定性和随机过程对物种灭绝风险的影响

摘要 物种种群受到确定性和随机性过程的影响,这两者都会导致它们灭绝的风险。然而,目前对这些过程对物种灭绝风险的相对贡献的理解还远未完成。在这里,我们通过分析一组代表具有负内在增长率的物种种群的模型来解决这一知识差距,根据确定性过程和两大类随机过程——人口和环境变化来划分灭绝风险。人口统计学方差是指在给定一组特定的固有人口统计率的情况下,随机抽样事件引起的人口丰度的随机变化,而环境方差是指内在人口率随时间的随机变化引起的随机丰度变化。当内在增长率不接近于零时,我们发现确定性增长是平均灭绝时间的主要驱动因素,即使人口规模很小。这与人口差异始终是小种群灭绝风险的重要决定因素的直觉相矛盾。相比之下,当内在增长率接近于零时,随机过程对平均灭绝时间产生了实质性的负面影响。人口统计学差异在低丰度时比环境差异具有更大的影响,而在高丰度时则相反。此外,我们发现,人口和环境差异的综合影响通常远低于它们彼此孤立的影响之和。这种次可加性表明两个随机过程增加灭绝风险的方式存在冗余,并且可能是因为这两个过程最终通过增加丰度随时间的变化来增加灭绝风险。
更新日期:2019-04-01
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