BACKGROUND Determining the relative contribution of intrinsic and extrinsic factors to fluctuations in population size, trends and demographic composition is analytically complex. It is often only possible to examine the combined effects of these factors through measurements made over long periods, spanning an array of population densities or levels of food availability. Using age-structured mark-recapture models and datasets spanning five decades (1950-1999), and two periods of differing relative population density, we estimated age-specific probabilities of survival and examined the combined effects of population density and environmental conditions on juvenile survival of southern elephant seals at Macquarie Island. RESULTS First-year survival decreased with density during the period of highest population size, and survival increased during years when the Southern Oscillation Index (SOI) anomaly (deviation from a 50-year mean) during the mother's previous foraging trip to sea was positive (i.e., El Niño). However, when environmental stochasticity and density were considered together, the effect of density on first-year survival effectively disappeared. Ignoring density effects also leads to models placing too much emphasis on the environmental conditions prevailing during the naïve pup's first year at sea. CONCLUSION Our analyses revealed that both the state of the environment and population density combine to modify juvenile survival, but that the degree to which these processes contributed to the variation observed was interactive and complex. This underlines the importance of evaluating the relative contribution of both the intrinsic and extrinsic factors that regulate animal populations because false conclusions regarding the importance of population regulation may be reached if they are examined in isolation.

中文翻译：

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南部象海豹长期生存趋势的内在和外在驱动因素之间复杂的相互作用。

背景技术确定内在因素和外在因素对人口规模，趋势和人口组成波动的相对贡献在分析上是复杂的。通常，只有通过长期测量，跨越一系列人口密度或食物供应水平，才能检查这些因素的综合影响。我们使用年龄结构化的标记夺回模型和数据集（跨越五个十年（1950-1999年）以及两个相对人口密度不同的时期），估算了特定年龄段的生存概率，并研究了人口密度和环境条件对青少年生存的综合影响麦格理岛的南部象海豹。结果在人口规模最大的时期，第一年生存率随密度的降低而降低，在母亲之前的海上觅食之旅中，当南方涛动指数（SOI）异常（偏离50年平均值）为正数（即厄尔尼诺现象）时，存活率增加了。但是，当同时考虑环境随机性和密度时，密度对第一年生存的影响实际上消失了。忽略密度效应还导致模型过分强调幼稚海第一年的主要环境条件。结论我们的分析表明，环境状况和人口密度共同影响了青少年的生存，但是这些过程对所观察到的变化的贡献程度是互动的和复杂的。