Data from: Caribou avoiding wolves face increased predation by bears – caught between Scylla and Charybdis

1. Prey may trade off resource acquisition with mortality risk by using various habitat-selection strategies. Empirical assessments have shown that the functional and numerical responses of predators to human disturbances are variable, yet spatial changes in predation risk by two predators have seldom been studied for prey occurring in human-modified landscapes. Using the boreal caribou (Rangifer tarandus caribou) – grey wolf (Canis lupus) – black bear (Ursus americanus) system in eastern Canada, we investigated whether responses of prey towards one predator could concomitantly increase risk of predation from another predator exhibiting a different foraging tactic. 2. We investigated trade-offs made by solitary caribou females and mothers accompanied by their calf during the period of highest calf vulnerability, and compared the behaviour of mothers that would eventually lose their calf to predation to that of mothers whose calf survived until the following year. We modelled habitat selection using different metrics of forage based on field measurements and digital maps, and developed empirical models of predation risk and prey behaviour using GPS data collected on both predators and prey. 3. Mothers accompanied by their calf seemed to compromise foraging opportunities for safety, as opposed to solitary females who showed no particular avoidance of areas used by predators. Although caribou mothers adopted selection strategies that could have protected their offspring from wolves, females that eventually lost their calf to predation selected for vegetative associations that were favourable to bears. 4. Synthesis and applications. We determined that mothers that most strongly avoided suitable wolf habitat were also those that most strongly selected suitable bear habitat, suggesting that by using anti-predator strategies aimed at reducing predation risk from wolves, caribou exposed their offspring to increased predation risk from bears. This result is of paramount conservation value as bears were responsible for 94% of caribou calf kills in this system. In the short term, conservation efforts for boreal caribou may benefit from the management of bear populations by means of liberal hunting regulations or predator control. In the long term, however, these actions should be used in conjunction with the protection of potential calving areas away from cutblocks and roads.

1. 猎物可通过多种栖息地选择策略，在资源获取与死亡风险之间达成权衡。已有实证评估显示，捕食者对人类干扰的功能响应与数量响应存在变异，但针对人为改造景观中的猎物，两种捕食者的捕食风险空间变化却鲜有研究。本研究以加拿大东部的北方驯鹿（boreal caribou, Rangifer tarandus caribou）-灰狼（grey wolf, Canis lupus）-美洲黑熊（black bear, Ursus americanus）系统为研究对象，探讨猎物对一种捕食者的响应是否会同时提升其暴露于另一种觅食策略不同的捕食者的捕食风险。
2. 本研究聚焦幼崽最易受攻击时期，分别对独居雌性驯鹿与携带幼崽的母驯鹿开展权衡分析，并将最终因捕食失去幼崽的母驯鹿行为，与幼崽存活至次年的母驯鹿行为进行对比。研究基于野外实测数据与数字地图，采用多种饲草度量指标构建栖息地选择模型，并利用捕食者与猎物的GPS定位数据，构建捕食风险与猎物行为的实证模型。
3. 与独居雌性驯鹿未刻意避开捕食者活动区域不同，携带幼崽的母驯鹿似乎为保障安全而牺牲了部分觅食机会。尽管母驯鹿采取的栖息地选择策略本可帮助后代规避灰狼的捕食，但最终因捕食失去幼崽的母驯鹿，却选择了对美洲黑熊有利的植被群落生境。
4. 综合与应用：本研究证实，那些最大程度规避适宜灰狼栖息生境的母驯鹿，同时也最大程度选择了适宜美洲黑熊的栖息生境，这表明母驯鹿为采取针对灰狼的反捕食策略以降低其捕食风险，反而让后代暴露于更高的美洲黑熊捕食风险中。该结果具备至关重要的保护学价值——本系统中94%的驯鹿幼崽死亡均由美洲黑熊造成。短期来看，通过放宽狩猎法规或开展捕食者管控来管理黑熊种群，可助力北方驯鹿的保护工作；但从长期视角出发，此类措施需与保护远离采伐迹地与道路的潜在产崽区域相结合，方能发挥最大效用。