The relationship between the Canada Lynx (Lynx canadensis) and the Snowshoe Hare (Lepus americanus) is a classic example of population dynamics in the North American boreal forests. The hare serves as the primary food source for the lynx, resulting in predictable fluctuations in their numbers over time. This predator-prey interaction produces a pattern of population booms and busts that has fascinated ecologists for centuries.
The Classic Predator-Prey Cycle
The population numbers of the lynx and the hare follow a cyclical pattern, peaking and crashing regularly over approximately 8 to 11 years. This fluctuation is driven by a straightforward ecological mechanism: the availability of the hare population directly influences the reproductive success and survival of the lynx.
When the hare population increases rapidly, it provides an abundant food supply for the lynx. This surplus allows the lynx to reproduce more successfully, resulting in a delayed increase in the predator’s population size. The lynx population peak typically lags behind the hare peak by about one to two years.
As the number of lynx reaches its maximum, increased predation pressure quickly drives the hare population down. With their primary food source disappearing, the large lynx population faces starvation and a sharp decline in reproductive output. This scarcity causes the lynx population to crash. Once lynx numbers are low, predation pressure is significantly reduced, allowing the hare population to begin its recovery and start the cycle anew.
Historical Evidence of the Population Swing
The cyclical nature of the lynx and hare populations is supported by historical data. For over 200 years, the Hudson’s Bay Company maintained detailed records of the pelts acquired from trappers across Canada. These comprehensive records, dating back to the late 1700s, provide a long-term proxy for the population sizes of both species.
The consistent correlation in the trade data—with lynx pelt numbers peaking shortly after hare pelt numbers—provided the initial proof of the predator-prey cycle. While the number of pelts collected does not represent the exact total population, the data reliably track the relative abundance of the animals over time. This continuous record established the lynx-hare relationship as the definitive example of a predator-prey oscillation in ecology, confirming the approximately 10-year cycle over many generations.
Beyond Predation: Factors Influencing Hare Population Swings
While lynx predation is a primary factor, modern ecological research shows that the hare population crash is a complex event influenced by multiple stressors. The decline is often initiated or worsened by resource depletion and physiological stress, which act alongside increased predation.
Resource Depletion and Malnutrition
During the peak phase of the cycle, high hare density leads to intense browsing on their main winter food source, including woody shrubs and willow. This heavy consumption causes food limitation and malnutrition for the hares. Plants may also produce secondary compounds as a defense mechanism after heavy browsing, making the remaining food less nutritious and stressing the hares further.
Physiological Stress
Another factor is the physiological toll that persistent predator threat takes on the hares. High densities of predators, including the lynx, coyotes, and owls, cause chronic stress in the hare population. This chronic stress elevates stress hormones, which reduce the hares’ reproductive success and weaken their immune systems.
Stressed female hares may produce fewer offspring, and the young often have higher stress hormones themselves, reducing their ability to survive and reproduce. This intergenerational effect contributes to the sharp decline in the hare population and can lengthen the subsequent low phase of the cycle. The hare population crash results from a combination of increased mortality from predation, reduced birth rates from stress, and poor body condition from food limitation.