Migration in geographic and ecological space by a large herbivore

Partial migration, when only part of the population migrates seasonally while the other part remains resident on the shared range, is the most common form of migration in ungulates. Migration is often defined by spatial separation of seasonal ranges and consequently, classification of individuals as migrants or residents is usually only based on geographic criteria. However, the underlying mechanism for migration is hypothesized to be movement in response to spatiotemporal resource variability and thus, migrants are assumed to travel an ‘ecological distance’ or shift their realized ecological niches. While ecological and geographic distances should be related, their relationship may depend on landscape heterogeneity. Here, we tested the utility of ecological niche theory to both classify migratory individuals and to understand the underlying ecological factors for migratory behavior. We developed an integrative approach combining measures in geographic and ecological niche space and used this to classify and explain migratory behavior of 71 annual roe deer (Capreolus capreolus) movement trajectories in five European study areas. Firstly, to assess the utility of the ecological distance concept for classifying migratory behavior, we tested whether roe deer sought the same ecological conditions year-round or moved to different ecological conditions by measuring the annual ecological distance travelled and the seasonal niche overlap using multivariate statistics. Comparing methods to classify migrants and residents based on geographic and ecological niche space, we found that migratory roe deer switched between seasons both in geographic and in ecological dimensions. Secondly, we tested which seasonal ecological factors separated resident from migrant niches using discriminant analysis and which broad-scale determinants (e.g., spatiotemporal forage variation and population density) predicted migration probability using generalized linear models. Our results indicated that factors describing forage and topographic variability discriminated seasonal migrant from resident niches. Determinants for predicting migration probability included the temporal variation (seasonality) and also the spatial variability of forage patches. Lastly, we also found suggestive evidence for a positive relationship between population density and migration probability. By applying the ecological niche concept to the study of partial migration in ungulates, our work underlines that partial migration is a form of behavioral plasticity.

部分迁徙（Partial migration）是有蹄类动物中最为常见的迁徙形式，指种群内仅有部分个体进行季节性迁徙，其余个体则留居于共享栖息范围内。迁徙通常以季节性栖息范围的空间分隔为定义依据，因此对个体的迁徙者与留居者分类通常仅采用地理标准。然而，学界普遍假设迁徙的底层机制是个体为响应时空资源变动而产生的移动行为，因此迁徙者被认为是行进了一段‘生态距离’，或是改变了自身的实际生态位（realized ecological niche）。尽管生态距离与地理距离理应存在关联，但二者的关联关系可能取决于景观异质性。本研究旨在检验生态位理论在迁徙个体分类以及解析迁徙行为底层生态驱动因素两方面的应用价值。我们开发了一种整合地理与生态位空间度量的综合分析方法，并利用该方法对欧洲5个研究区域内71条年度狍（Capreolus capreolus）的移动轨迹的迁徙行为进行分类与解析。首先，为评估生态距离概念在迁徙行为分类中的应用价值，我们通过多元统计方法测量年度行进生态距离与季节生态位重叠度，以此检验狍是否全年维持相同的生态条件，或是会转移至不同的生态条件中。对比基于地理空间与生态位空间的迁徙者-留居者分类方法后，我们发现迁徙狍在季节更替时，无论是地理维度还是生态维度均发生了栖息环境的切换。其次，我们通过判别分析（discriminant analysis）解析了区分留居者与迁徙者生态位的季节性生态因子，并利用广义线性模型（generalized linear model）探究了哪些宏观尺度驱动因子（如时空饲草资源变动与种群密度）能够预测迁徙概率。研究结果显示，表征饲草资源与地形异质性的因子能够有效区分季节性迁徙者与留居者的生态位。能够预测迁徙概率的驱动因子包括时间维度变动（即季节性）以及饲草斑块的空间异质性。最后，我们还发现了种群密度与迁徙概率呈正相关的佐证性证据。本研究将生态位概念应用于有蹄类部分迁徙的研究，证实部分迁徙是一种行为可塑性的表现形式。