Are lemmings prey or predators?

Large oscillations in the populations of Norwegian lemmings have mystified both professional ecologists and lay public(1-3). Ecologists suspect that t hese oscillations are driven by a trophic mechanism(4,5): either an interac tion between lemmings and their food supply, or an interaction between lemm ings and their predators. If lemming cycles are indeed driven by a trophic interaction, can we tell whether lemmings act as the resource ('prey') or t he consumer ('predator')? In trophic interaction models, peaks of resource density generally have a blunt, rounded shape, whereas peaks of consumer de nsity are sharp and angular. Here we have applied several statistical tests to three lemming datasets and contrasted them with comparable data for cyc lic voles, We find that vole peaks are blunt, consistent with their cycles being driven by the interaction with predators. In contrast, the shape of l emming peaks is consistent with the hypothesis that lemmings are functional predators, that is, their cycles are driven by their interaction with food plants. Our findings suggest that a single mechanism, such as interaction between rodents and predators, is unlikely to provide the 'universal' expla nation of all cyclic rodent dynamics.