Data from: Predator foraging response to a resurgent dangerous prey

Prey switching occurs when a generalist predator kills disproportionately more of an abundant prey species and correspondingly spares a rarer species. Although this behaviour is a classic stabilizing mechanism in food web models, little is known about its operation in free-living systems which often include dangerous prey species that resist predation.
We used long-term (1995–2015) data from a large mammal system in northern Yellowstone National Park, USA, to understand how prey preference of a wild, generalist predator (Canis lupus) responds to a shift in prey species evenness involving rising numbers of dangerous prey (Bison bison) and dropping numbers of relatively safer prey (Cervus elaphus).
Contrary to the prey switching hypothesis, wolves attacked and killed disproportionately more of the rarer, but safer, species. Wolves maintained a strong preference against bison even when this species was more than twice as abundant as elk. There was also evidence that wolves were increasingly averse to hunting bison as relative bison abundance increased.
Wolves seldom hunted bison because capture success was limited to a narrow set of conditions: larger packs (>11 wolves) chasing smaller herds (10–20 bison) with calves. Wolves scavenged bison carrion instead and did so more frequently as bison abundance increased.
Our study demonstrates the overarching importance of prey vulnerability to understanding the prey preferences of generalist predators in ecological communities with dangerous prey. The formidable defences of such prey diminish the potential for switching and its stabilizing influence on population dynamics. In these communities, shifts from hunting to scavenging are perhaps more likely than shifts in prey preference. The assumption of switching may therefore overestimate the stability of multi-prey systems that include dangerous prey species.

猎物转换行为（prey switching）指广食性捕食者不成比例地捕食更为丰富的猎物物种，相应地更少取食稀有猎物物种的现象。尽管该行为是食物网模型中经典的稳定化机制，但人们对其在野生自然系统中的运作机制知之甚少——这类系统中往往存在具备反捕食能力的危险猎物物种。
本研究依托美国黄石国家公园北部大型哺乳动物群落1995–2015年的长期监测数据，旨在探究野生广食性捕食者灰狼（Canis lupus）的猎物偏好，如何响应猎物物种均匀度的变化：即危险猎物美洲野牛（Bison bison）种群数量上升，而相对安全的猎物马鹿（Cervus elaphus）种群数量下降。
研究结果与猎物转换假说相悖：灰狼不成比例地捕食了更为稀有但相对安全的马鹿种群。即便美洲野牛的种群丰度达到马鹿的两倍以上，灰狼仍强烈偏好回避捕食野牛。另有证据表明，随着美洲野牛相对丰度的提升，灰狼对捕食野牛的厌恶程度也随之升高。
灰狼极少主动捕食野牛，这是因为其捕猎成功率仅局限于极窄的条件范围：狼群规模大于11头，追逐带有幼崽的10–20头野牛组成的小群体。因此灰狼转而啃食美洲野牛的腐尸，且随着野牛种群丰度的提升，其腐食行为愈发频繁。
本研究表明，猎物脆弱性在理解包含危险猎物的生态群落中广食性捕食者的猎物偏好时，具有核心重要性。这类猎物的强大防御能力削弱了猎物转换行为的可能性，及其对种群动态的稳定化效应。在这类群落中，捕食行为向腐食行为的转变或许比猎物偏好的转变更为常见。因此，假设捕食者会发生猎物转换，可能会高估包含危险猎物的多猎物系统的种群稳定性。