Predation is an ecological interaction where one organism, the predator, hunts and consumes another organism, the prey. This relationship is a fundamental driver of energy transfer within an ecosystem. Population cycles describe the regular, alternating periods of growth and decline observed in the numbers of certain animal species over time. The dynamic link between predators and their prey is a primary mechanism behind these predictable fluctuations in population size, though other ecological factors modify this classic biological phenomenon.
Understanding Population Cycles
A true population cycle is distinguished from random fluctuations by its defined periodicity and amplitude. These cycles are often observed in species living in simpler ecosystems, such as the North American boreal forest. A widely documented example involves the snowshoe hare (Lepus americanus) and its specialist predator, the Canada lynx (Lynx canadensis).
Historical fur-trapping records spanning over a century demonstrate a recurring pattern in the populations of both species. The snowshoe hare population regularly experiences a dramatic increase and subsequent crash over a period that typically ranges between 8 and 11 years. The lynx population tracks this pattern closely, but its peaks and troughs are consistently delayed.
The Mechanism of Predator-Prey Oscillation
The cyclical pattern is a result of a continuous, density-dependent feedback loop between the two populations, where the size of one directly affects the growth rate of the other. The process begins when the prey population is at a low point and predation pressure is minimal.
With few predators present, the prey species, such as the snowshoe hare, can reproduce successfully, leading to a rapid and large increase in their numbers. This abundance of food then provides the necessary resources for the predator population to thrive, as better nutrition allows for greater reproductive success and higher survival rates for their young. The predator population then begins to climb, but with a noticeable delay, as it takes time for offspring to mature and for the population to reach a size capable of impacting the now-large prey base.
As the number of predators reaches its peak, the intense hunting pressure begins to overwhelm the prey population, causing a sharp decline in prey numbers. This high density of predators actively hunting is the primary force that drives the prey population down from its maximum size. Immediately following the prey’s decline, the predator population soon faces a severe food shortage. With their primary food source scarce, predators experience increased mortality and reduced reproduction, leading to a subsequent crash in their own numbers. Once the predator population falls to a low level, the pressure on the prey is released, allowing the prey population to begin its recovery, which restarts the entire cycle.
The Role of Time Lag in Predictive Modeling
The sustained oscillation between predator and prey populations depends on the concept of a time lag. This lag represents the delay between the point when the prey population reaches its maximum size and the subsequent point when the predator population reaches its maximum. Without this delay, the populations would reach a stable equilibrium where both numbers remain constant, rather than exhibiting the observed alternating booms and busts.
The time lag occurs because predators cannot instantly increase their numbers in response to abundant prey. It takes a period for predators to detect the increase in prey, reproduce, and for the new generation of predators to grow large enough to significantly impact the prey population. For the lynx and hare, this delay is approximately one to two years.
Scientists use mathematical frameworks to model these dynamics, which predict the characteristic wave-like curves of the populations. The models utilize variables that account for the growth rate of the prey and the efficiency of the predator. The incorporation of a time delay is what shifts the outcome from a steady state to a continuous, self-generating oscillation.
Non-Predation Factors That Alter Cycles
While predation is the major force driving the cycle, external factors introduce complexity. One major factor is food scarcity for the prey species itself, which introduces a “bottom-up” control to the “top-down” influence of predation. As the snowshoe hare population peaks, they can over-graze their local vegetation, leading to a decline in their nutritional resources and contributing to their population crash alongside increased predation.
Disease outbreaks can also accelerate the decline phase of a cycle, particularly in dense or stressed populations. When a prey population is at its maximum, the close proximity of individuals makes the transmission of pathogens faster and more widespread. Habitat fragmentation, caused by human development, further alters these dynamics by dividing continuous habitats into smaller, isolated patches.
Fragmentation can decrease the amplitude of the population cycle by limiting the maximum size a population can reach. It can also shift the balance of the relationship by increasing predation risk near habitat edges, or by causing a “mesopredator release” where the disappearance of large predators allows medium-sized predators to increase unchecked, changing the overall pressure on the prey.