Numerical response of mammalian carnivores to rodents affects bird reproduction in temperate forests: A case of apparent competition?

Resource pulses such as mast seeding in temperate forests may affect interspecific interactions over multiple trophic levels and link different seed and nonseed consumers directly via predation or indirectly via shared predators. However, the nature and strength of interactions among species remain unknown for most resource pulse-driven ecosystems. We considered five hypotheses concerning the influence of resource pulses on the interactions between rodents, predators, and bird reproduction with data from northern Switzerland collected between 2010 and 2015. In high-rodent-abundance-years (HRAYs), wood warbler (Phylloscopus sibilatrix) nest survival was lower than in low-rodent-abundance-years, but rodents were not important nest predators, in contrast to rodent-hunting predators. The higher proportion of nests predated by rodent-hunting predators and their increased occurrence in HRAYs suggests a rodent-mediated aggregative numerical response of rodent-hunting predators, which incidentally prey on the wood warbler's ground nests. There was no evidence that rodent-hunting predators responded behaviorally by switching prey. Lastly, nest losses caused by nonrodent-hunting predators were not related to rodent abundance. We show that wood warblers and rodents are linked via shared predators in a manner consistent with apparent competition, where an increase of one species coincides with the decrease of another species mediated by shared predators. Mast seeding frequency and annual seed production appear to have increased over the past century, which may result in more frequent HRAYs and generally higher peaking rodent populations. The associated increase in the magnitude of apparent competition may thus at least to some extent explain the wood warbler's decline in much of Western Europe.