Does Litter Size Variation Affect Models of Terrestrial Carnivore Extinction Risk and Management?

Background
Individual variation in both survival and reproduction has the potential to influence extinction risk. Especially for rare or threatened species, reliable population models should adequately incorporate demographic uncertainty. Here, we focus on an important form of demographic stochasticity: variation in litter sizes. We use terrestrial carnivores as an example taxon, as they are frequently threatened or of economic importance. Since data on intraspecific litter size variation are often sparse, it is unclear what probability distribution should be used to describe the pattern of litter size variation for multiparous carnivores.
Methodology/Principal Findings
We used litter size data on 32 terrestrial carnivore species to test the fit of 12 probability distributions. The influence of these distributions on quasi-extinction probabilities and the probability of successful disease control was then examined for three canid species – the island fox Urocyon littoralis, the red fox Vulpes vulpes, and the African wild dog Lycaon pictus. Best fitting probability distributions differed among the carnivores examined. However, the discretised normal distribution provided the best fit for the majority of species, because variation among litter-sizes was often small. Importantly, however, the outcomes of demographic models were generally robust to the distribution used.
Conclusion/Significance
These results provide reassurance for those using demographic modelling for the management of less studied carnivores in which litter size variation is estimated using data from species with similar reproductive attributes.

**背景**
个体在存活与繁殖方面的异质性均可能对物种灭绝风险产生影响。对于珍稀或受威胁物种而言，可靠的种群模型需充分纳入种群统计不确定性（demographic uncertainty）。本研究聚焦于种群统计随机性（demographic stochasticity）的一类重要形式：窝仔数（litter size）变异。我们以陆生食肉目动物（terrestrial carnivores）作为研究类群，因其多处于受威胁状态或具备重要经济价值。由于种内（intraspecific）窝仔数变异的相关数据通常较为匮乏，目前尚无法明确应采用何种概率分布（probability distribution）来刻画多胎（multiparous）食肉动物的窝仔数变异模式。

**方法与主要结果**
我们收集了32种陆生食肉目动物的窝仔数数据，以此检验12种概率分布的拟合效果。随后针对3种犬科（canid）动物——岛屿灰狐（Urocyon littoralis）、赤狐（Vulpes vulpes）以及非洲野犬（Lycaon pictus），分析了不同概率分布对准灭绝概率（quasi-extinction probability）与疾病防控成功概率的影响。受试食肉动物的最优拟合概率分布存在差异，但由于多数物种的窝仔数变异幅度较小，离散化正态分布（discretised normal distribution）为绝大多数物种提供了最优拟合效果。值得注意的是，种群统计模型的输出结果总体上对所选用的概率分布具有稳健性。

**结论与意义**
本研究结果为那些基于具有相似繁殖特征的物种数据估算窝仔数变异、并利用种群统计模型开展未充分研究的食肉动物管理工作的研究者提供了信心与依据。