Seasonal role of a specialist predator in rodent cycles: Ermine-lemming interactions in the High Arctic

The exact mechanisms behind population cycles remain elusive. An ongoing debate centers on whether predation by small mustelids is necessary and sufficient to generate rodent cycles, as stipulated by the specialist predator hypothesis (SPH). Specifically, the SPH predicts that the predator should respond numerically to the abundance of its prey with a delay of approximately one year, leading to delayed density-dependence in the dynamics of the prey population. Here, we analyze the numerical response of a small mustelid, the seasonality of its interaction with rodents, and its impact on population cycles using long-term seasonal data on ermines and cyclic lemmings in the High Arctic. Our results show that the numerical response of ermines to lemming fluctuations was delayed by one year and could mediate delayed density-dependence in lemming growth rate. The impact of ermines on the growth rate of lemmings was small but mostly circumscribed to winter, a critical period when shifts in cycle phases occur and direct density-dependence seems relaxed. Our simulations of lemming population with and without ermines suggest that these small mustelids are neither necessary, nor sufficient to generate cycles per se. However, the presence of small mustelids may be necessary to prolong the low-abundance phase and delay the recovery of lemming populations, promoting the presence of a multiannual low phase typical of lemming cycles. Our study corroborates the idea that population declines of cyclic populations are best explained by direct density-dependence; however, the delayed response of specialized predators induces the multiannual low phase and leads to longer periodicities, which are typically of 3-5 years in rodents.