Large herbivores in a partially migratory population search for the ideal free home.
Large herbivores in a partially migratory population search for the ideal free home.
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迁移是跨分类群在时间异质性景观中获取资源的一种策略。迁徙的种群可以获得更高的丰度，因为这样的迁徙可以获得更高质量的资源，或者减少被捕食的风险，从而提高适应性。然而，大多数迁徙物种发生在部分迁徙种群中，即迁徙和非迁徙个体的混合种群。人们认为，在部分迁移人口中，迁移人口的比例可以通过(1)种群水平的进化稳定状态来维持，在这种状态下，与密度相关的生命率对迁移者和居民起作用，以平衡适合度；或者(2)有条件迁移，在这种状态下，迁移倾向受个体状态的影响。然而，在许多方面，迁徙也是栖息地选择的一种形式，迁徙者和居民的比例可能是密度依赖的栖息地选择的结果。在此，我们检验了理想自由分布理论（IFD）是否可以解释部分迁移种群中不同迁移策略的共存。IFD预测个体表现出密度依赖的生命率，并选择不同的迁移策略来最大化个体适应度，从而导致策略之间的适应度相等（λ）。我们在一个部分迁徙的麋鹿种群中测试了IFD的预测，该种群在19年的人口统计数据和迁徙策略转换率中下降了70%。我们发现了居民怀孕和成年女性生存的密度依赖证据，这为改变策略提供了健康激励。尽管不同的迁徙策略在生命率上存在差异，但平均λ（适应度）是相等的。然而，正如IFD所预测的那样，个体会转向更适合自己的策略。我们的分析表明，部分迁移可能是由遵循理想自由分布的策略选择驱动的。这些发现强化了大型食草动物跨分类群的迁移可能是一种多态行为，其中迁移策略选择是由不同的成本和收益决定的，由密度依赖介导。
Migration is a tactic used across taxa to access resources in temporally heterogenous landscapes. Populations that migrate can attain higher abundances because such movements allow access to higher quality resources, or reduction in predation risk resulting in increased fitness. However, most migratory species occur in partially migratory populations, a mix of migratory and non‐migratory individuals. It is thought that the portion of migrants in a partial migration population is maintained either through (1) a population‐level evolutionary stable state where counteracting density‐dependent vital rates act on migrants and residents to balance fitness or (2) conditional migration, where the propensity to migrate is influenced by the individual's state. However, in many respects, migration is also a form of habitat selection and the proportion of migrants and residents may be the result of density‐dependent habitat selection. Here, we test whether the theory of Ideal Free Distribution (IFD) can explain the coexistence of different migratory tactics in a partially migratory population. IFD predicts individuals exhibit density‐dependent vital rates and select different migratory tactics to maximize individual fitness resulting in equal fitness (λ) between tactics. We tested the predictions of IFD in a partially migratory elk population that declined by 70% with 19 years of demographic data and migratory tactic switching rates from >300 individuals. We found evidence of density dependence for resident pregnancy and adult female survival providing a fitness incentive to switch tactics. Despite differences in vital rates between migratory tactics, mean λ (fitness) was equal. However, as predicted by the IFD, individuals switched tactics toward those of higher fitness. Our analysis reveals that partial migration may be driven by tactic selection that follows the ideal free distribution. These findings reinforce that migration across taxa may be a polymorphic behavior in large herbivores where migratory tactic selection is determined by differential costs and benefits, mediated by density dependence.
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