Data from: Using experimentation to understand the 10-year snowshoe hare cycle in the boreal forest of North America

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.