Rotella Jay J, Link William A, Nichols James D, Hadley Gillian L, Garrott Robert A, Proffitt Kelly M
Ecology Department, Montana State University, Bozeman, Montana 59717, USA.
Ecology. 2009 Apr;90(4):975-84. doi: 10.1890/08-0971.1.
Much of the existing literature that evaluates the roles of density-dependent and density-independent factors on population dynamics has been called into question in recent years because measurement errors were not properly dealt with in analyses. Using state-space models to account for measurement errors, we evaluated a set of competing models for a 22-year time series of mark-resight estimates of abundance for a breeding population of female Weddell seals (Leptonychotes weddellii) studied in Erebus Bay, Antarctica. We tested for evidence of direct density dependence in growth rates and evaluated whether equilibrium population size was related to seasonal sea-ice extent and the Southern Oscillation Index (SOI). We found strong evidence of negative density dependence in annual growth rates for a population whose estimated size ranged from 438 to 623 females during the study. Based on Bayes factors, a density-dependence-only model was favored over models that also included environmental covariates. According to the favored model, the population had a stationary distribution with a mean of 497 females (SD = 60.5), an expected growth rate of 1.10 (95% credible interval = 1.08-1.15) when population size was 441 females, and a rate of 0.90 (95% credible interval = 0.87-.93) for a population of 553 females. A model including effects of SOI did receive some support and indicated a positive relationship between SOI and population size. However, effects of SOI were not large, and including the effect did not greatly reduce our estimate of process variation. We speculate that direct density dependence occurred because rates of adult survival, breeding, and temporary emigration were affected by limitations on per capita food resources and space for parturition and pup-rearing. To improve understanding of the relative roles of various demographic components and their associated vital rates to population growth rate, mark-recapture methods can be applied that incorporate both environmental covariates and the seal abundance estimates that were developed here. An improved understanding of why vital rates change with changing population abundance will only come as we develop a better understanding of the processes affecting marine food resources in the Southern Ocean.
近年来，许多评估密度依赖和密度独立因素对种群动态影响的现有文献受到了质疑，因为在分析中没有妥善处理测量误差。我们使用状态空间模型来处理测量误差，对一组竞争模型进行了评估，该模型基于在南极埃雷布斯湾研究的雌性威德尔海豹（Leptonychotes weddellii）繁殖种群22年的重捕估计丰度时间序列。我们测试了增长率中直接密度依赖的证据，并评估了平衡种群规模是否与季节性海冰范围和南方涛动指数（SOI）有关。我们发现，对于一个在研究期间估计规模从438只到623只雌性的种群，其年增长率存在强烈的负密度依赖证据。基于贝叶斯因子，仅密度依赖模型比同时包含环境协变量的模型更受青睐。根据受青睐的模型，该种群具有一个平稳分布，均值为497只雌性（标准差 = 60.5），当种群规模为441只雌性时，预期增长率为1.10（95%可信区间 = 1.08 - 1.15），对于553只雌性的种群，增长率为0.90（95%可信区间 = 0.87 - 0.93）。一个包含SOI影响的模型确实得到了一些支持，并表明SOI与种群规模之间存在正相关关系。然而，SOI的影响不大，并且包含该影响并没有大幅降低我们对过程变化的估计。我们推测直接密度依赖的发生是因为成年个体的生存、繁殖和临时迁出率受到人均食物资源以及分娩和育幼空间限制的影响。为了更好地理解各种人口统计学组成部分及其相关生命率对种群增长率的相对作用，可以应用标记重捕方法，该方法结合环境协变量和在此开发的海豹丰度估计。只有当我们更好地理解影响南大洋海洋食物资源的过程时，才能更深入地理解为什么生命率会随着种群丰度的变化而变化。
