1. Department of Ecology and Evolutionary Biology, University of Connecticut, Storrs, Connecticut 06269, USA
2. Department of Ecological Botany, Umeå University, S-901 87 Umeå, Sweden
3. Department of Animal Ecology, Umeå University, S-901 87 Umeå, Sweden
4. Finnish Forest Research Institute, Vantaa Research Center, PO Box 18, FIN-01301 Vantaa, Finland

Correspondence to: P. Turchin¹ Correspondence and requests for materials should be addressed to P.T. (e-mail: Email: Peter.Turchin@UConn.edu).

Abstract

Large oscillations in the populations of Norwegian lemmings have mystified both professional ecologists and lay public^{1, 2, 3}. Ecologists suspect that these oscillations are driven by a trophic mechanism^{4, 5}: either an interaction between lemmings and their food supply, or an interaction between lemmings and their predators. If lemming cycles are indeed driven by a trophic interaction, can we tell whether lemmings act as the resource ('prey') or the consumer ('predator')? In trophic interaction models, peaks of resource density generally have a blunt, rounded shape, whereas peaks of consumer density are sharp and angular. Here we have applied several statistical tests to three lemming datasets and contrasted them with comparable data for cyclic voles. We find that vole peaks are blunt, consistent with their cycles being driven by the interaction with predators. In contrast, the shape of lemming 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' explanation of all cyclic rodent dynamics.