Wyoming Cooperative Fish and Wildlife Research Unit, Department of Zoology and Physiology, University of Wyoming, Laramie, Wyoming 82071, USA.
Ecology. 2013 Jun;94(6):1245-56. doi: 10.1890/11-2298.1.
Migration is a striking behavioral strategy by which many animals enhance resource acquisition while reducing predation risk. Historically, the demographic benefits of such movements made migration common, but in many taxa the phenomenon is considered globally threatened. Here we describe a long-term decline in the productivity of elk (Cervus elaphus) that migrate through intact wilderness areas to protected summer ranges inside Yellowstone National Park, USA. We attribute this decline to a long-term reduction in the demographic benefits that ungulates typically gain from migration. Among migratory elk, we observed a 21-year, 70% reduction in recruitment and a 4-year, 19% depression in their pregnancy rate largely caused by infrequent reproduction of females that were young or lactating. In contrast, among resident elk, we have recently observed increasing recruitment and a high rate of pregnancy. Landscape-level changes in habitat quality and predation appear to be responsible for the declining productivity of Yellowstone migrants. From 1989 to 2009, migratory elk experienced an increasing rate and shorter duration of green-up coincident with warmer spring-summer temperatures and reduced spring precipitation, also consistent with observations of an unusually severe drought in the region. Migrants are also now exposed to four times as many grizzly bears (Ursus arctos) and wolves (Canis lupus) as resident elk. Both of these restored predators consume migratory elk calves at high rates in the Yellowstone wilderness but are maintained at low densities via lethal management and human disturbance in the year-round habitats of resident elk. Our findings suggest that large-carnivore recovery and drought, operating simultaneously along an elevation gradient, have disproportionately influenced the demography of migratory elk. Many migratory animals travel large geographic distances between their seasonal ranges. Changes in land use and climate that disparately influence such seasonal ranges may alter the ecological basis of migratory behavior, representing an important challenge for, and a powerful lens into, the ecology and conservation of migratory taxa.
迁移是许多动物增强资源获取能力同时降低被捕食风险的显著行为策略。历史上，这种运动的人口统计学优势使得迁移很普遍，但在许多类群中，这种现象被认为在全球范围内受到威胁。在这里，我们描述了通过完整的荒野地区迁移到美国黄石国家公园内受保护夏季牧场的麋鹿（Cervus elaphus）的生产力长期下降。我们将这种下降归因于有蹄类动物通常从迁移中获得的人口统计学优势的长期减少。在迁移的麋鹿中，我们观察到 21 年来繁殖力下降了 70%，怀孕率下降了 4 年，下降了 19%，这主要是由于年轻或哺乳期的雌性繁殖频率较低所致。相比之下，在常驻的麋鹿中，我们最近观察到繁殖力增加和怀孕率很高。栖息地质量和捕食的景观水平变化似乎是导致黄石公园迁移者生产力下降的原因。从 1989 年到 2009 年，迁移的麋鹿经历了一个越来越快的绿色化过程，持续时间也越来越短，这与春季-夏季气温升高和春季降水减少的情况一致，这也与该地区异常严重干旱的观察结果一致。现在，迁移的麋鹿还面临着四倍于常驻麋鹿的灰熊（Ursus arctos）和狼（Canis lupus）。这两种恢复的捕食者在黄石荒野中以高比例消耗迁移的麋鹿幼崽，但在常驻麋鹿的全年栖息地中，通过致命管理和人为干扰将其密度维持在低水平。我们的研究结果表明，大型食肉动物的恢复和干旱同时沿着海拔梯度发挥作用，对迁移麋鹿的种群动态产生了不成比例的影响。许多迁徙动物在其季节性范围内进行大地理距离的迁徙。这些季节性范围的土地利用和气候变化可能会改变迁徙行为的生态基础，这代表了对迁徙类群的生态学和保护的重要挑战和有力视角。
