Department of Forest Sciences, University of British Columbia, 2424 Main Mall, Vancouver, British Columbia V6T 1Z4, Canada.
BMC Ecol. 2012 Jun 29;12:9. doi: 10.1186/1472-6785-12-9.
The life history strategy of a species can influence how populations of that species respond to environmental variation. In this study, we used a matrix modeling approach to examine how life history differences among sympatric rock and white-tailed ptarmigan affect the influence of demographic rates on population growth (λ) and the potential response to a changing climate. Rock ptarmigan have a slower life history strategy than white-tailed ptarmigan in the study region with lower annual reproductive effort but higher adult survival.
Based on data from a 5-year field study, deterministic estimates of λ indicated that populations were stable for rock ptarmigan (λ = 1.01), but declining for white-tailed ptarmigan (λ = 0.96). The demographic rates with the highest elasticity for rock ptarmigan were the survival of after-second year females, followed by juvenile survival and success of the first nest. For white-tailed ptarmigan, juvenile survival had the highest elasticity followed by success of the first nest and survival of second-year females. Incorporating stochasticity into the demographic rates led to a 2 and 4% drop in λ for rock and white-tailed ptarmigan respectively. Using data from the first three years we also found that population growth rates of both species were depressed following an increased frequency of severe years, but less so for rock ptarmigan which showed greater resilience under these conditions.
Our results provide evidence that populations of closely related species can vary in their response to environmental change as a consequence of life history differences. Rock ptarmigan, with a slower life history, are more responsive to demographic rates that influence survival and older life stages but this response is tempered by the extent of variability in each of the rates. Thus, predictions need to consider both aspects in modeling population response to a varying climate. Juvenile survival was a highly influential rate for both species, but the period from independence to first breeding is a poorly understood stage for many bird species. Additional study on juvenile survival, the influence of density dependence and the effects of predators as the mechanism driving survival-reproduction tradeoffs are all areas requiring further study.
物种的生活史策略会影响该物种的种群对环境变化的反应。在这项研究中，我们使用矩阵建模方法来研究同域的岩雷鸟和白尾雷鸟的生活史差异如何影响种群增长率（λ）以及对气候变化的潜在反应。在研究区域，岩雷鸟的生活史策略比白尾雷鸟慢，繁殖年的繁殖努力较低，但成年存活率较高。
根据 5 年野外研究的数据，λ 的确定性估计表明，岩雷鸟种群稳定（λ=1.01），而白尾雷鸟种群下降（λ=0.96）。对岩雷鸟具有最高弹性的人口动态率是第二年雌性的存活率，其次是幼鸟的存活率和第一个巢的成功率。对白尾雷鸟来说，幼鸟的存活率最高，其次是第一个巢的成功率和第二年雌性的存活率。将随机性纳入人口动态率导致岩雷鸟和白尾雷鸟的λ分别下降了 2%和 4%。利用前三年的数据，我们还发现，在恶劣年份频率增加的情况下，两种物种的种群增长率都有所下降，但岩雷鸟的下降幅度较小，在这些条件下表现出更大的弹性。
我们的研究结果提供了证据，表明密切相关的物种的种群对环境变化的反应可能因生活史差异而有所不同。岩雷鸟的生活史较慢，对影响存活率和老年生命阶段的人口动态率的反应更为敏感，但这种反应受到每个率的变异性程度的影响。因此，在对不断变化的气候进行建模时，预测需要考虑这两个方面。幼鸟的存活率对两种物种都有很大的影响，但从独立到第一次繁殖的阶段对许多鸟类物种来说都是一个理解不足的阶段。进一步研究幼鸟的存活率、密度依赖的影响以及作为生存-繁殖权衡的驱动机制的捕食者的影响，都是需要进一步研究的领域。
