Data from: Spatiotemporal heterogeneity in prey abundance and vulnerability shapes the foraging tactics of an omnivore

1. Prey abundance and prey vulnerability vary across space and time, but we know little about how they mediate predator-prey interactions and predator foraging tactics. To evaluate the interplay between prey abundance, prey vulnerability, and predator space use, we examined patterns of black bear (Ursus americanus) predation of caribou (Rangifer tarandus) neonates in Newfoundland, Canada using data from 317 collared individuals (9 bears, 34 adult female caribou, 274 caribou calves). 2. During the caribou calving season, we predicted that landscape features would influence calf vulnerability to bear predation, and that bears would actively hunt calves by selecting areas associated with increased calf vulnerability. Further, we hypothesized that bears would dynamically adjust their foraging tactics in response to spatiotemporal changes in calf abundance and vulnerability (collectively, calf availability). Accordingly, we expected bears to actively hunt calves when they were most abundant and vulnerable, but switch to foraging on other resources as calf availability declined. 3. As predicted, landscape heterogeneity influenced risk of mortality, and bears displayed the strongest selection for areas where they were most likely to kill calves, which suggested they were actively hunting caribou. Initially, the per-capita rate at which bears killed calves followed a type-I functional response, but as the calving season progressed and calf vulnerability declined, kill rates dissociated from calf abundance. In support of our hypothesis, bears adjusted their foraging tactics when they were less efficient at catching calves, highlighting the influence that predation phenology may have on predator space use. Contrary to our expectations, however, bears appeared to continue to hunt caribou as calf availability declined, but switched from a tactic of selecting areas of increased calf vulnerability to a tactic that maximized encounter rates with calves. 4. Our results reveal that generalist predators can dynamically adjust their foraging tactics over short time scales in response to changing prey abundance and vulnerability. Further, they demonstrate the utility of integrating temporal dynamics of prey availability into investigations of predator-prey interactions, and move towards a mechanistic understanding of the dynamic foraging tactics of a large omnivore.1. 猎物丰度与猎物易被捕食性在空间与时间维度上均存在异质性，但目前学界对二者如何介导捕食者-猎物相互作用及捕食者觅食策略的机制仍不甚明晰。为评估猎物丰度、猎物易被捕食性与捕食者空间利用之间的相互关系，我们针对加拿大纽芬兰地区的美洲黑熊（Ursus americanus）捕食北美驯鹿（Rangifer tarandus）初生幼崽的行为模式展开研究，分析对象为317只佩戴追踪颈圈的个体（9只黑熊、34头成年雌性北美驯鹿、274头驯鹿幼崽）。 2. 在驯鹿产仔季，我们提出两项假设：其一，景观特征会影响幼崽遭遇黑熊捕食的风险；其二，黑熊会通过选择幼崽易被捕食风险更高的区域主动捕猎幼崽。进一步地，我们推测黑熊会根据幼崽丰度与易被捕食性的时空变化（二者统称为幼崽可获得性）动态调整其觅食策略。据此，我们预期当幼崽丰度最高且易被捕食性最强时，黑熊会主动捕猎幼崽；而当幼崽可获得性下降时，黑熊会转而捕食其他猎物资源。 3. 研究结果与预期一致：景观异质性会影响幼崽的死亡风险，且黑熊对幼崽被捕杀概率最高的区域表现出最强的生境选择偏好，这表明黑熊确实在主动捕猎驯鹿幼崽。研究初期，黑熊捕杀幼崽的个体捕杀率符合I型功能响应（type-I functional response）；但随着产仔季推进、幼崽易被捕食性下降，其捕杀率不再与幼崽丰度相关。结果支持了我们的假设：当黑熊捕猎幼崽的效率降低时，它们会调整觅食策略，这凸显了捕食物候对捕食者空间利用的潜在影响。但与我们的预期相悖的是，当幼崽可获得性下降时，黑熊并未停止捕猎驯鹿，而是从选择幼崽易被捕食风险更高的区域，转变为采用最大化幼崽遭遇概率的觅食策略。 4. 本研究结果表明，广食性捕食者可在短时间尺度内根据猎物丰度与易被捕食性的变化动态调整其觅食策略。此外，本研究证实了将猎物可获得性的时间动态纳入捕食者-猎物相互作用研究的价值，并为理解大型杂食动物的动态觅食策略提供了机制性视角。