Density-dependent recruitment but not survival drives cyclic dynamics in a field vole population.

Arguably, the most fundamental question in population ecology is what drives patterns in the abundance of populations? Small rodents exhibiting regular multiannual cycles in abundance have long been a test bed for addressing this question. The prevailing orthodoxy, the predation hypothesis, contends that nonmigratory, specialist predators are necessary, and specialist and generalist predators, combined, are both necessary and sufficient, for causing population cycles. Thus, variations in survival, from predation, are the key drivers of the cycles. However, this, and other competing theories, have hitherto lacked supportive demographic evidence and hence a solid evidential foundation. Here, we provide such evidence, analyzing 10 y of monthly data from a cyclic field vole population. We find, contrary to the prevailing orthodoxy, that recruitment, not survival, varied substantially from phase to phase in the cycles, made the major contribution to variations in population growth rate, and had cycle-phase-specific negative delayed density dependence. These results, their consistency with what is known from other systems, and the weak demographic foundations of the predation hypothesis, together suggest recruitment, specifically breeding-season length, not predation, as the cycles' driving force. They therefore suggest that re-evaluation of the importance of the various determinants of population abundances, more generally, may be necessary.