Indirect food web interactions mediated by rodent cycles: relative roles of lemmings and voles

Production cycles in birds are proposed as prime cases of indirect interactions in food webs. They are thought to be driven by predators switching from rodents to bird nests in the crash phase of rodent population cycles. Although rodent cycles are geographically widespread and found in different rodent taxa, bird production cycles appear to be most profound in the high Arctic where lemmings dominate. We hypothesized that this may be due to arctic lemmings inducing stronger predator responses than boreal voles. We tested this hypothesis by estimating predation rates in dummy bird nests during a rodent cycle in low arctic tundra. Here the rodent community consists of a spatially variable mix of one lemming (Lemmus lemmus) and two vole species (Myodes rufocanus and Microtus oeconomus) with similar abundances. In consistence with our hypothesis, lemming peak abundances predicted well crash-phase nest predation rates, whereas the vole abundances had no predictive ability. Corvids were found to be the most important nest predators. Lemmings appear to be accessible to the whole predator community which makes them particularly powerful drivers of food web dynamics.

鸟类繁殖周期被视为食物网（food web）间接相互作用的典型范例。现有研究认为，此类周期由捕食者在啮齿动物种群周期的崩溃阶段，从捕食啮齿类转向捕食鸟巢所驱动。尽管啮齿动物种群周期在地理上分布广泛，且涵盖不同啮齿类类群，但鸟类繁殖周期的显著效应仅集中在以旅鼠（lemming）为优势类群的高北极（high Arctic）区域。我们提出假说：这一现象或因北极旅鼠相较于北方田鼠，能够引发更强的捕食者响应。为验证该假说，我们在低北极苔原的啮齿动物周期中，通过人工鸟巢（dummy bird nests）估算了巢捕食率。该区域的啮齿动物群落由空间分布异质的1种旅鼠（灰旅鼠 *Lemmus lemmus*）与2种田鼠（棕背䶄 *Myodes rufocanus*、根田鼠 *Microtus oeconomus*）组成，三者丰度相近。研究结果与我们的假说一致：旅鼠的种群峰值能够较好预测崩溃期的鸟巢捕食率，而田鼠的丰度则无预测效力。此外，鸦科鸟类（corvids）被证实为最主要的鸟巢捕食者。旅鼠可被整个捕食者群落所利用，这使其成为驱动食物网动态的极强动力因子。