National Marine Fisheries Service, Northwest Fisheries Science Center, Montlake Blvd. East, Seattle, WA.
Department of Fisheries and Wildlife, Michigan State University, East Lansing, MI.
J Hered. 2018 Oct 31;109(7):709-723. doi: 10.1093/jhered/esy018.
The idealized concept of a population is integral to ecology, evolutionary biology, and natural resource management. To make analyses tractable, most models adopt simplifying assumptions, which almost inevitably are violated by real species in nature. Here, we focus on both demographic and genetic estimates of effective population size per generation (Ne), the effective number of breeders per year (Nb), and Wright's neighborhood size (NS) for black bears (Ursus americanus) that are continuously distributed in the northern lower peninsula of Michigan, United States. We illustrate practical application of recently developed methods to account for violations of 2 common, simplifying assumptions about populations: 1) reproduction occurs in discrete generations and 2) mating occurs randomly among all individuals. We use a 9-year harvest dataset of >3300 individuals, together with genetic determination of 221 parent-offspring pairs, to estimate male and female vital rates, including age-specific survival, age-specific fecundity, and age-specific variance in fecundity (for which empirical data are rare). We find strong evidence for overdispersed variance in reproductive success of same-age individuals in both sexes, and we show that constraints on litter size have a strong influence on results. We also estimate that another life-history trait that is often ignored (skip breeding by females) has a relatively modest influence, reducing Nb by 9% and increasing Ne by 3%. We conclude that isolation by distance depresses genetic estimates of Nb, which implicitly assume a randomly mating population. Estimated demographic NS (100, based on parent-offspring dispersal) was similar to genetic NS (85, based on regression of genetic distance and geographic distance), indicating that the >36000 km2 study area includes about 4-5 black-bear neighborhoods. Results from this expansive data set provide important insight into effects of violating assumptions when estimating evolutionary parameters for long-lived, free-ranging species. In conjunction with recently developed analytical methodology, the ready availability of nonlethal DNA sampling methods and the ability to rapidly and cheaply survey many thousands of molecular markers should facilitate eco-evolutionary studies like this for many more species in nature.
种群的理想化概念是生态学、进化生物学和自然资源管理的重要组成部分。为了使分析具有可操作性，大多数模型采用简化假设，但这些假设几乎不可避免地会被自然界中的真实物种所违背。在这里，我们关注的是美国密歇根州北部半岛连续分布的黑熊（Ursus americanus）的每代有效种群数量（Ne）、每年有效繁殖者数量（Nb）和 Wright 邻域大小（NS）的人口统计学和遗传估计。我们举例说明了最近开发的方法在解决 2 个常见的种群简化假设中的实际应用：1）繁殖发生在离散的世代中，2）交配在所有个体之间随机进行。我们使用了一个 9 年的超过 3300 只个体的捕获数据集，以及 221 对亲子对的遗传确定，来估计雄性和雌性的生命表参数，包括年龄特异性存活率、年龄特异性生育率和生育率的年龄特异性方差（这方面的经验数据很少）。我们发现，在两性中，同年龄段个体的繁殖成功率存在明显的过度分散，并且表明胎仔数量的限制对结果有很大的影响。我们还估计，另一个经常被忽略的生活史特征（雌性跳过繁殖）的影响相对较小，将 Nb 减少 9%，将 Ne 增加 3%。我们的结论是，由于距离隔离而导致的遗传估计的 Nb 降低，这是由于遗传估计隐含地假设了一个随机交配的种群。估计的人口统计学 NS（基于亲子扩散的 100）与遗传 NS（基于遗传距离和地理距离的回归的 85）相似，表明研究区 36000 多平方公里的范围内包含了大约 4-5 个黑熊栖息地。这个广泛数据集的结果提供了对违反假设时估计长寿命、自由放养物种进化参数的影响的重要见解。结合最近开发的分析方法，非致命性 DNA 采样方法的广泛应用以及快速、廉价地调查成千上万的分子标记的能力，应该会为更多的自然物种的生态进化研究提供便利。
