Many large herbivore populations are partially migratory, in which the population is comprised of both non-migratory (resident) and migratory individuals. Density-dependence contributes to regulating the dynamics of partially migratory populations by altering habitat selection, vital rates, or rates of behavioral switching between migratory tactics. Studies of mechanisms leading to these shifts have focused mainly on their behavior on summer range, overlooking the potential for density-dependent effects during winter that may influence decisions to migrate. We hypothesized that competition for food and safety from wolf predation risk on winter ranges would differentially affect habitat selection, movements, and grouping behavior of migrant and resident female North American elk (Cervus canadensis) on their sympatric winter range. We used GPS locations from 92 adult female elk in 155 elk-winters at Ya Ha Tinda, Alberta, Canada, over a 14-year period when the elk population declined by ∼70% to test our hypotheses. Elk showed consistently strong selection for areas of high forage biomass that corresponded to longer residence times and shorter return times to areas of high forage biomass. The strength of the selection diminished at high elk population size as did the extent to which elk traded off forage for safety from wolf predation risk. Elk increased movement rates and extended return times only to the riskiest areas. Median group size and mean sociality among elk increased at low population size, with resident elk groups being larger and more cohesive than migrant groups. Similar density-dependent responses by migrant and resident female elk on sympatric winter range indicate resident elk do not alter foraging behaviors to compensate for exposure to low nutritional resources in summer, implicating seasonal differences in nutrition are not mediated by winter densities in this system. We discuss the implications of competition on winter ranges for the maintenance of partial migration in ungulates in montane systems.

中文翻译：

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部分洄游麋鹿种群在同域冬季范围内的密度依赖性觅食行为

许多大型食草动物种群是部分迁徙的，其中种群由非迁徙（居民）和迁徙个体组成。密度依赖性有助于通过改变栖息地选择、生命率或迁徙策略之间的行为转换率来调节部分迁徙种群的动态。导致这些转变的机制的研究主要集中在它们在夏季范围内的行为上，而忽略了可能影响迁移决定的冬季密度依赖效应的可能性。我们假设冬季范围内狼捕食风险对食物和安全的竞争将不同地影响迁徙和常驻雌性北美麋鹿 (Cervus canadensis) 在其同域冬季范围内的栖息地选择、运动和分组行为。在 14 年的时间里，当麋鹿数量下降了约 70% 时，我们使用了加拿大艾伯塔省 Ya Ha Tinda 155 个麋鹿冬季的 92 头成年雌性麋鹿的 GPS 位置来检验我们的假设。麋鹿对高草料生物量区域表现出一贯的强烈选择，这对应于较长的停留时间和较短的返回高草料生物量区域的时间。选择的强度随着麋鹿种群规模的增加而减弱，麋鹿为了避免狼捕食风险而牺牲草料的程度也是如此。麋鹿只在风险最高的地区增加了移动率并延长了返回时间。麋鹿的中位数群体规模和平均社会性在低种群规模时增加，居民麋鹿群体比迁徙群体更大，更具凝聚力。迁徙和常驻雌性麋鹿在同域冬季范围内的类似密度依赖性反应表明，常驻麋鹿不会改变觅食行为来补偿夏季暴露于低营养资源，这意味着营养的季节性差异不是由该系统中的冬季密度介导的。我们讨论了竞争对维持山地系统有蹄类动物部分迁徙的冬季活动的影响。