Data from: Demographic responses of a site-faithful and territorial predator to its fluctuating prey: long-tailed skuas and arctic lemmings

1. Environmental variability, through interannual variation in food availability or climatic variables, is usually detrimental to population growth. It can even select for constancy in key life-history traits, though some exceptions are known. Changes in the level of environmental variability are therefore important to predict population growth or life-history evolution. Recently, several cyclic vole and lemming populations have shown large dynamical changes, that might affect rodent predator demography or life histories. 2. Skuas constitute an important case study among rodent predators, because of their strongly saturating breeding productivity (they lay only two eggs) and high degree of site fidelity, in which they differ from nomadic predators raising large broods in good rodent years. This suggests that they cannot capitalize on lemming peaks to the same extent as nomadic predators, and might be more vulnerable to collapses of rodent cycles. 3. We develop a model for the population dynamics of long-tailed skuas feeding on lemmings to assess the demographic consequences of such variable and nonstationary prey dynamics, based on data collected in NE Greenland. The model shows that populations of long-tailed skua sustain well changes in lemming dynamics, including temporary collapses (e.g. 10 years). A high floater-to-breeder ratio emerges from rigid territorial behaviour and a long life expectancy, which buffers the impact of adult abundance's decrease on the population reproductive output. 4. The size of the floater compartment is affected by changes in both mean and coefficient of variation of lemming densities (but not cycle amplitude and periodicity per se). In Greenland, the average lemming density is below the threshold density required for successful breeding (including during normally cyclic periods). Due to Jensen's inequality, skuas therefore benefit from lemming variability; a positive effect of environmental variation. 5. Long-tailed skua populations are strongly adapted to fluctuating lemming populations, an instance of demographic lability in the reproduction rate. They are also little affected by poor lemming periods, if there are enough floaters, or juveniles disperse to neighbouring populations. The status of Greenland skua populations therefore strongly depends upon floater numbers and juvenile movements, which are not known. This reveals a need to intensify colour-ringing efforts on the long-tailed skua at a circumpolar scale.

1. 环境变异通过食物可获得性的年际波动或气候变量变化，通常会对种群增长产生不利影响。尽管已知存在少数例外情况，但这种变异甚至可能筛选出关键生活史性状保持恒定的个体。因此，环境变异水平的变化对于预测种群增长或生活史演化至关重要。近期，若干周期性田鼠与旅鼠种群出现了剧烈的动态变化，这可能会影响啮齿类捕食者的种群动态与生活史。
2. 贼鸥是啮齿类捕食者研究中的重要案例类群，这是因为它们的繁殖生产力具有极强的饱和特性（仅产2枚卵），且具有高度的繁殖位点保真度（site fidelity）——这一点与在啮齿类数量充足年份可抚育大量幼雏的游牧捕食者截然不同。这表明，贼鸥无法像游牧捕食者那样充分利用旅鼠种群的峰值期，因此可能更易受到啮齿类种群周期崩溃的影响。
3. 本研究基于格陵兰东北部收集的野外数据，构建了以旅鼠为食的长尾贼鸥种群动态模型，以评估这类可变且非平稳的猎物种群动态所带来的种群动态后果。模型结果显示，长尾贼鸥种群能够较好地适应旅鼠种群的动态变化，包括长达10年的暂时性种群崩溃。由于严格的领地行为与较长的寿命，种群中会形成较高的漂浮个体-繁殖个体比例，这一比例能够缓冲成体数量下降对种群繁殖产出的负面影响。
4. 漂浮个体库的规模同时受到旅鼠种群密度均值与变异系数（coefficient of variation）变化的影响（而非猎物种群周期的振幅与周期本身）。在格陵兰地区，旅鼠的平均密度低于成功繁殖所需的阈值密度（即便在正常的周期波动期亦是如此）。根据延森不等式（Jensen's inequality），贼鸥会从旅鼠种群的变异中获益，这体现了环境变异的正向效应。
5. 长尾贼鸥种群已高度适应旅鼠种群的波动，这是繁殖速率种群动态可塑性的一个典型案例。若存在足够的漂浮个体，或是幼鸟能够扩散至邻近种群，那么长尾贼鸥种群受旅鼠种群低迷期的影响极小。因此，格陵兰地区贼鸥种群的存续状况极大程度上取决于漂浮个体数量与幼鸟扩散行为，但目前这两项数据仍未被探明。这一结论凸显了在环极范围内加大长尾贼鸥环志标记工作力度的必要性。