1. Population cycles have long fascinated ecologists from the time of Charles Elton in the 1920s. The discovery of large population fluctuations in undisturbed ecosystems challenged the idea that pristine nature was in a state of balance. The 10-year cycle of snowshoe hares (Lepus americanus Erxleben) across the boreal forests of Canada and Alaska is a classic cycle, recognized by fur traders for more than 300 years. 2. Since the 1930s ecologists have investigated the mechanisms that might cause these cycles. Proposed causal mechanisms have varied from sunspots to food supplies, parasites, diseases, predation, and social behaviour. Both the birth rate and the death rate change dramatically over the cycle. Social behaviour was eliminated as a possible cause because snowshoe hares are not territorial and do not commit infanticide. 3. Since the 1960s large-scale manipulative experiments have been used to discover the major limiting factors. Food supply and predation quickly became recognized as potential key factors causing the cycle. Experiments adding food and restricting predator access to field populations have been decisive in pinpointing predation as the key mechanism causing these fluctuations. 4. The immediate cause of death of most snowshoe hares is predation by a variety of predators, including the Canada lynx (Lynx canadensis Kerr). The collapse in the reproductive rate is not due to food shortage as was originally thought, but is a result of chronic stress from predator chases. 5. Five major issues remain unresolved. First, what is the nature of the predator-induced memory that results in the prolonged low phase of the cycle? Second, why do hare cycles form a travelling wave, starting in the centre of the boreal forest in Saskatchewan and travelling across western Canada and Alaska? Third, why does the amplitude of the cycle vary greatly from one cycle to the next in the same area? Fourth, do the same mechanisms of population limitation apply to snowshoe hares in eastern North American or in similar ecosystems across Siberia? Finally, what effect will climatic warming have on all the above issues? The answers to these questions remain for future generations of biologists to determine.
Usage Notes:
1_Metadata for Kluane Hare Grids
Descriptive data for the data given in the following 5 files
2_Phases of Hare Cycle
Describes the phases of the ten-year cycle of snowshoe hares for the years of study in the Yukon
3_Monitoring Data for Small Mammals
The detailed data for the 3 main species discussed in this paper for the years studied, population size and confidence limits.
4_Controls Hare Live trap data Kluane
The detailed demographic data for snowshoe hares on the control grids by capture date over all the years of study and all the grids that were controls.
5_Feeding Experiment Data Figure 1
The data used in Figure 1 for the feeding experiment on hares.
6_Fence+Food Hare Capture Data
The detailed demographic data for the 1986-96 fence and food individual snowshoe hares by capture time, for this experiment that has been critical for our understanding.