Understanding how variation in vital rates interact to shape the trajectories of populations has long been understood to be a critical component of informed management and restoration efforts. However, an expanding body of work suggests that the expectations for population dynamics of ungulates may not be applicable to small, declining, or threatened populations. Populations of bighorn sheep (Ovis canadensis) suffered declines at the turn of the 20th century, and restoration efforts have been mixed such that many populations remain small and isolated. Here, we utilized survey data collected from 1983 to 2018 from 17 populations of bighorn sheep in Montana and Wyoming to estimate the parameters of a stage-specific population model that we used to (1) characterize the spatial and temporal variation in key vital rates including whether populations were stable, increasing, or declining; (2) estimate the contributions of vital rates to variation in population growth rates; and (3) evaluate potential sources of variation in lamb survival. We found substantial variation in all vital rates both among years and populations, strong evidence for an overall decline in nine of the 17 populations, and clear evidence for multiple combinations of vital rates that resulted in positive population trajectories. The contribution of ewe survival and lamb survival to the total variation in population growth rates varied among populations; however, declines in ewe survival dominated transitions of population trajectories from stable or increasing to declining, whereas reversals of declining population trajectories were dominated by improved lamb survival. We found strong evidence for a diverse set of associations between lamb survival and environmental covariates related to growing season and winter severity. The estimated relationships predict that environmental drivers can cause important changes in lamb survival and provide suggestive evidence that the presence of Mycoplasma ovipneumoniae is not sufficient to prevent population growth. Although our work demonstrates that the trajectories of these populations of bighorn sheep are driven by a variety of processes, the diversity of relationships between vital rates and population growth rates also suggests that there are multiple pathways to manage for population recovery.

中文翻译：

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大角羊种群动态变化的驱动因素

了解生命率的变化如何相互作用以塑造人口的轨迹长期以来一直被认为是知情管理和恢复工作的关键组成部分。然而，越来越多的工作表明，对有蹄类动物种群动态的预期可能不适用于小型、衰退或受威胁的种群。大角羊 ( Ovis canadensis) 的种群) 在 20 世纪之交遭受了衰退，恢复工作参差不齐，以至于许多人口仍然很小而且与世隔绝。在这里，我们利用从 1983 年到 2018 年从蒙大拿州和怀俄明州的 17 个大角羊种群收集的调查数据来估计特定阶段种群模型的参数，我们用来（1）表征关键生命率的空间和时间变化，包括人口是否稳定、增加或减少；(2) 估计生命率对人口增长率变化的贡献；(3) 评估羔羊存活率变异的潜在来源。我们发现年龄和人群的所有生命率都有很大差异，有力的证据表明 17 个人群中有 9 个总体下降，和明确的证据表明，生命率的多种组合导致了积极的人口轨迹。母羊存活率和羔羊存活率对种群增长率总变异的贡献因种群而异；然而，母羊存活率的下降主导了种群轨迹从稳定或增加到下降的转变，而种群轨迹下降的逆转主要是羔羊存活率的提高。我们发现了强有力的证据，证明羔羊存活率与与生长季节和冬季严重程度相关的环境协变量之间存在多种关联。估计的关系预测环境驱动因素会导致羔羊存活率发生重要变化，并提供暗示性证据表明存在 母羊存活率和羔羊存活率对种群增长率总变异的贡献因种群而异；然而，母羊存活率的下降主导了种群轨迹从稳定或增加到下降的转变，而种群轨迹下降的逆转主要是羔羊存活率的提高。我们发现了强有力的证据，证明羔羊存活率与与生长季节和冬季严重程度相关的环境协变量之间存在多种关联。估计的关系预测环境驱动因素会导致羔羊存活率发生重要变化，并提供暗示性证据表明存在 母羊存活率和羔羊存活率对种群增长率总变异的贡献因种群而异；然而，母羊存活率的下降主导了种群轨迹从稳定或增加到下降的转变，而种群轨迹下降的逆转主要是羔羊存活率的提高。我们发现了强有力的证据，证明羔羊存活率与与生长季节和冬季严重程度相关的环境协变量之间存在多种关联。估计的关系预测环境驱动因素会导致羔羊存活率发生重要变化，并提供暗示性证据表明存在 母羊存活率的下降主导了种群轨迹从稳定或增加到下降的转变，而种群轨迹下降的逆转主要是羔羊存活率的提高。我们发现了强有力的证据，证明羔羊存活率与与生长季节和冬季严重程度相关的环境协变量之间存在多种关联。估计的关系预测环境驱动因素会导致羔羊存活率发生重要变化，并提供暗示性证据表明存在 母羊存活率的下降主导了种群轨迹从稳定或增加到下降的转变，而种群轨迹下降的逆转主要是羔羊存活率的提高。我们发现了强有力的证据，证明羔羊存活率与与生长季节和冬季严重程度相关的环境协变量之间存在多种关联。估计的关系预测环境驱动因素会导致羔羊存活率发生重要变化，并提供暗示性证据表明存在卵型肺炎支原体不足以阻止人口增长。尽管我们的工作表明这些大角羊种群的轨迹是由多种过程驱动的，但生命率和种群增长率之间关系的多样性也表明，存在多种管理种群恢复的途径。