Specialist and generalist natural enemies as an explanation for geographical gradients in population cycles of northern herbivores

Within Fennoscandia, two well-studied groups of herbivores exhibit clear geographical gradients in their population dynamics. Populations of a forest lepidopteran ( Epirrita autumnata , the autumnal moth) and voles of the genera Microtus and Clethrionomys show pronounced multi-annual cycles in the north but become more stable towards the south. Here we review empirical and theoretical studies on these species, mainly regarding the biological mechanisms that are assumed to generate the pattern of population dynamics in both systems. We conclude that the specialist/generalist predation hypothesis offers a common explanation for the population cycles and their geographical gradients irrespective of whether a herbivorous insect or small mammals are concerned. According to this hypothesis, originally developed for the Fennoscandian voles, but now applied also to E. autumnata , population cycles are generated by specialist natural enemies (predators for the voles and parasitoids for E. autumnata ). Furthermore, the dynamic shift from cycles to stability is assumed to be caused by an increase in the density and diversity of generalist natural enemies from north to south in Fennoscandia.