Population cycles: generalities, exceptions and remaining mysteries

Abstract

Population cycles are one of nature's great mysteries. For almost a hundred years, innumerable studies have probed the causes of cyclic dynamics in snowshoe hares, voles and lemmings, forest Lepidoptera and grouse. Even though cyclic species have very different life histories, similarities in mechanisms related to their dynamics are apparent. In addition to high reproductive rates and density-related mortality from predators, pathogens or parasitoids, other characteristics include transgenerational reduced reproduction and dispersal with increasing-peak densities, and genetic similarity among populations. Experiments to stop cyclic dynamics and comparisons of cyclic and noncyclic populations provide some understanding but both reproduction and mortality must be considered. What determines variation in amplitude and periodicity of population outbreaks remains a mystery.

Keywords: forest Lepidoptera; lemmings; maternal effects; red grouse; snowshoe hares; voles.

Figure 1.

Population trend (bars) for snowshoe hares and lynx track index (line) in the Kluane Lake region of the Yukon Territory. Data from C.J. Krebs http://www.zoology.ubc.ca/~krebs/kluane.html. (Online version in colour.)

Figure 2.

Red grouse bag time-series showing variation in temporal patterns among two different moors in the United Kingdom. Data from Darren Shaw and Dan Hayden, University of Glasgow. (Online version in colour.)

Figure 3.

An example of cycles of a vole. Numbers of Myodes rufocanus snap trapped in the autumn in northern Finland. Data collected by Heikki Henttonen and provided by Charles Krebs. (Online version in colour.)

Figure 4.

(a) Population and fecundity trends for a population of Western tent caterpillars on Galiano Island. (b) Population and trends in infection by nucleopolyhedrovirus for the same population (see [34] for methods). (Online version in colour.)