Data from: Competition between apex predators? Brown bears decrease wolf kill rate on two continents

Trophic interactions are a fundamental topic in ecology, but we know little about how competition between apex predators affects predation, the mechanism driving top-down forcing in ecosystems. We used long-term datasets from Scandinavia (Europe) and Yellowstone National Park (North America) to evaluate how grey wolf (Canis lupus) kill rate was affected by a sympatric apex predator, the brown bear (Ursus arctos). We used kill interval (i.e. the number of days between consecutive ungulate kills) as a proxy of kill rate. Although brown bears can monopolize wolf kills, we found no support in either study system for the common assumption that they cause wolves to kill more often. On the contrary, our results showed the opposite effect. In Scandinavia, wolf packs sympatric with brown bears killed less often than allopatric packs during both spring (after bear den emergence) and summer. Similarly, the presence of bears at wolf-killed ungulates was associated with wolves killing less often during summer in Yellowstone. The consistency in results between the two systems suggests that brown bear presence actually reduces wolf kill rate. Our results suggest that the influence of predation on lower trophic levels may depend on the composition of predator communities.

营养级互作（trophic interactions）是生态学的核心研究议题之一，但学界对顶级捕食者（apex predators）间的竞争如何影响捕食行为——这一驱动生态系统下行调控（top-down forcing）的核心机制——的认知仍较为匮乏。本研究依托欧洲斯堪的纳维亚半岛与北美黄石国家公园的长期数据集，评估了同域分布的顶级捕食者棕熊（Ursus arctos）对灰狼（Canis lupus）猎杀率的影响。我们以猎杀间隔（kill interval，即连续两次有蹄类猎物猎杀之间的天数）作为猎杀率的替代指标。尽管棕熊可独占灰狼的猎杀成果，但在两个研究系统中，学界普遍认为棕熊会迫使灰狼更频繁开展猎杀的假设均未得到支持。与之相反，本研究结果呈现出截然相反的效应：在斯堪的纳维亚半岛，与棕熊同域分布的灰狼种群在春季（棕熊出眠后）与夏季的猎杀频率均低于异域分布的灰狼种群。同样，在黄石国家公园，夏季时灰狼猎杀的有蹄类猎物旁存在棕熊的情况下，灰狼的猎杀频率同样更低。两个研究系统的结果具有高度一致性，表明棕熊的存在实则会降低灰狼的猎杀率。本研究结果提示，捕食作用对低营养级生物的影响，可能取决于捕食者群落的组成结构。