Numerical response of a mammalian specialist predator to multiple prey dynamics in Mediterranean farmlands

The study of rodent population cycles has greatly contributed, both theoretically and empirically, to our understanding of the circumstances under which predator-prey interactions destabilize populations. According to the specialist predator hypothesis, reciprocal interactions between voles and small predators that specialize on voles, such as weasels, can cause multi-annual cycles. A fundamental feature of classical weasel-vole models is a long time-lag in the numerical response of the predator to variations in prey abundance: weasel abundance increases with that of voles and peaks approximately one year later. We investigated the numerical response of the common weasel (Mustela nivalis) to fluctuating abundances of common voles (Microtus arvalis) in recently colonized agro-steppes of Castilla-y-Léon, north-western Spain, at the southern limit of the species' range. Populations of both weasels and voles exhibited multi-annual cycles with a 3 year-period. Weasels responded quickly and numerically to changes in common vole abundance, with a time-lag between prey and weasel abundance that did not exceed 4 months and occured during the breeding season, reflecting the quick conversion of prey into predator offspring and/or immigration to sites with high vole populations. We found no evidence of a sustained, high weasel abundance following vole abundance peaks. Weasel population growth rates showed spatial synchrony across study sites c.60 km apart. Weasel dynamics were more synchronized with that of common voles than with other prey species (mice or shrews). However, asynchrony within, as well as among sites, in the abundance of voles and alternative prey suggests that weasel mobility could allow them to avoid starvation during low vole phases, precluding the emergence of prolonged time-lag in the numerical response to voles. Our observations are inconsistent with the specialist predator hypothesis as currently formulated and suggest that weasels might follow rather than cause the vole cycles in north-western Spain. The reliance of a specialized predator on a functional group of prey such as small rodents does not necessarily lead to a long delay in the numerical response by the predator, depending on the spatial and inter-specific synchrony in prey dynamics. This article is protected by copyright. All rights reserved.