Functional responses in animal movement explain spatial heterogeneity in animal-habitat relationships

1. Understanding why heterogeneity exists in animal-habitat spatial relationships is critical for identifying the drivers of animal distributions. Functional responses in habitat selection – whereby animals adjust their habitat selection depending on habitat availability – are useful for describing animal-habitat spatial heterogeneity. However, they could be yielded by different movement tactics, involving contrasting inter-specific interactions. 2. Identifying functional responses in animal movement, rather than in emergent spatial patterns like habitat selection, could disentangle the effects of different movement behaviours on spatial heterogeneity in animal-habitat relationships. This would clarify how functional responses in habitat selection emerge and provide a general tool for understanding the mechanistic drivers of animal distributions. 3. We tested this approach using data from GPS-collared woodland caribou (Rangifer tarandus), a prey species under top-down control. We tested how caribou selected and moved with respect to a key resource (lichen-conifer stands) as a function of the availability of surrounding refuge land-cover (closed-conifer stands), using step selection functions. 4. Caribou selected resource patches more strongly in areas richer in refuge land-cover – a functional response in habitat selection. However, adjustments in multiple movement behaviours could have generated this pattern: stronger directed movement towards resources patches and/or longer residency within resource patches, in areas richer in refuges. Different contributions of these behaviours would produce contrasting forager spatial dynamics. 5. We identified functional responses in both movement behaviours: caribou were more likely to move towards resource patches in areas richer in refuge land-cover, and to remain in these patches during movement steps. This tactic enables caribou to spend longer foraging in safer areas where they could rapidly seek refuge in dense cover when predators are detected. 6. Our study shows that functional responses in movement can expose the context-dependent movement decisions that generate heterogeneity in animal-habitat spatial relationships. We used these functional responses to characterise anti-predator movement tactics employed by a large herbivore, but they could be applied in many different scenarios. The movement rules from functional responses in movement are well-suited to integration in spatial explicit individual-based models for forecasting animal distributions in landscapes undergoing environmental change.

1. 探究动物与生境的空间关系中异质性的成因，对明确动物分布的驱动因子至关重要。生境选择功能响应（habitat selection functional responses）——即动物依据生境可获得性调整其生境选择的过程——可有效刻画动物与生境间的空间异质性（spatial heterogeneity）。然而，这类响应可由不同的移动策略所产生，且涉及截然不同的种间相互作用。
2. 相较于从生境选择等涌现空间模式中识别功能响应，直接从动物移动行为中识别功能响应，能够厘清不同移动行为对动物与生境关系中空间异质性的影响。这将阐明生境选择功能响应的形成机制，并为解析动物分布的机制性驱动因子提供通用研究工具。
3. 本研究利用佩戴GPS项圈的林地驯鹿（Rangifer tarandus）——一种受顶下行控制（top-down control）的猎物种群——的监测数据验证了该研究方法。我们采用步选择函数（step selection functions），探究了驯鹿对关键资源（地衣-针叶林林分）的选择与移动模式如何随周边避难所土地覆盖类型（封闭针叶林林分）的可获得性发生变化。
4. 在避难所土地覆盖更丰富的区域，驯鹿对资源斑块的选择强度更高——这属于生境选择功能响应。然而，多种移动行为的调整均可产生该模式：在避难所更丰富的区域，驯鹿会更精准地定向移动至资源斑块，或在资源斑块内停留更久。这些行为的不同贡献方式，会产生截然不同的觅食者空间动态。
5. 我们在两种移动行为中均识别出了功能响应：在避难所土地覆盖更丰富的区域，驯鹿更倾向于向资源斑块移动，且在移动步程中会停留在这些斑块内。该策略使驯鹿能够在更安全的区域花费更长时间觅食，一旦侦测到捕食者，便可快速退入茂密植被中寻求庇护。
6. 本研究表明，移动行为中的功能响应能够揭示驱动动物与生境空间关系异质性的、依赖于环境背景的移动决策。我们利用这类功能响应刻画了大型植食动物所采用的反捕食移动策略，但其可推广至诸多不同研究场景。基于移动功能响应得到的移动规则，非常适合整合入空间显式个体基模型，以预测环境变化景观中的动物分布情况。