Phase dependence and population cycles in a large-mammal predator-prey system

Specialized enemies, such as predators and parasitoids, play an important role in the population cycles of small animals by generating delayed density dependence. We investigated the role of predation in population cycles in an undisturbed large-mammal system using long-term data on the sustained density fluctuations of wolves and moose on Isle Royale (Michigan, USA). Nonlinear time-series analysis revealed that wolves display phase-dependent dynamics with stronger density dependence during the decline phase than during the increase phase. This phase dependence was also reflected in predation rates: the number of moose killed daily by wolves was greater during the wolf increase phase than during the wolf decline phase. Accordingly, moose displayed multi-annual cycles generated by an interaction between weak self-regulation and strong delayed density dependence during periods of wolf increase, and strong self-regulation with negligible delayed density dependence during periods of wolf decline. This constitutes, to our knowledge, the first formal documentation of population cycles in large mammals. By making use of long-term data at both trophic levels, as well as data on predator behavior, this analysis may shed light on the mechanisms through which predators contribute to population cycles of prey in other taxa.