The parasite regulation hypothesis revisited: helminth and coccidian evidence from voles and lemmings

Population cycles in voles and lemmings have long stimulated interest in parasites as potential density-dependent regulators. Nevertheless, the evidence remains fragmented. This review was motivated by our initial finding that several published claims about the effects of parasites on cyclic rodents were based on sparse data, aggregation, prevalence-based inference, or other methodological limitations, which calls for a reassessment of the available evidence. We surveyed observational, experimental, modeling, and methodological studies addressing the effects of helminth and coccidian parasites on cyclic rodent populations. We focused particularly on re-evaluating published quantitative evidence; reconstructing effect sizes; assessing statistical limitations; and comparing reproductive, condition-related, and demographic outcomes across host–parasite systems. The most consistent findings were parasite aggregation, marked seasonality, and close associations between infection patterns and host demography. The reproductive effects were generally small. Trichuris arvicolae provided the clearest evidence for direct reproductive costs among the helminths reviewed. However, the effect was modest, and the pup-mass analysis was potentially affected by offspring-level pseudoreplication. Evidence for Syphacia -associated reproductive effects was weaker than often assumed because infected reproductive subgroups were small and burden distributions were aggregated. Evidence for Eimeria -related condition or survival effects was suggestive but mixed, with positive findings in some systems but no consistent evidence for delayed density-dependent regulation. Previously reported apparent combined effects were often sensitive to the inclusion of sparse or methodologically problematic datasets. The reviewed evidence does not support a universal parasite-driver model of vole or lemming cycles. Parasites may modify condition, reproduction, survival, or recruitment under specific ecological circumstances, but the available evidence does not establish them as consistent primary regulators across systems. Future progress will require longitudinal burden-based studies, experimental manipulations, studies investigating multiple parasite groups in parallel, and the disclosure of raw demographic and parasitological data.