On each of these plots, they captured, marked and released the hares twice each year: in March, before the onset of the breeding season, and in October, at the onset of winter. The food supplements provided higher-quality nutrients than did plants growing in the forest. Of the two predator-exclusion blocks, one contained supplemental food to examine the influence of both resource availability and predation pressure. They treated the remaining two blocks with fertilizer to increase plant abundance. To test the effect of predation pressure, they enclosed two blocks with electric fences to exclude mammalian predators (hawks and owls retained access). They stocked two blocks with supplemental food for the duration of the experiment to test the effect of resource availability. The researchers used the remaining six to test the effects of resource availability, predation, and the interaction of both factors on snowshoe hare populations. They established nine 1 km 2 blocks in undisturbed forest. Krebs and colleagues have experimentally teased apart the influence of food abundance and predation on snowshoe hare ( Lepus americanus) populations in Canada. We cannot easily determine the extent to which each of these controls drives population cycles in the Swedish boreal forest, because this system is not amenable to caging experiments, but studies show that food and predation work together to regulate population sizes.įield experiments by Charles J. Thus both availability of resources and predation pressure affect the size of prey populations. Owl populations cycle in a similar manner, closely following the abundance of voles.Īs predator populations increase, they put greater strain on the prey populations and act as a top-down control, pushing them toward a state of decline. We would expect that the number of foxes in the population would increase as availability of their preferred food increases, and studies have demonstrated that this does, in fact, occur (Figure 1b).

Grouse and hare populations cycle in a manner comparable to those of voles, which suggests that food availability plays a role in regulating populations of these herbivores.įoxes prefer to consume voles and other small rodents, but will occasionally eat grouse and hares when voles are less abundant. When vole populations peak and competition for food is strongest, they turn to bark as a marginal food, and this shift in foraging behavior coincides with a population decline (Figure 1a). They grow more slowly, reproduce less, and populations decline. When preferred foods are scarce, individuals must turn to less desirable foods to prevent starvation. In years when their preferred food items are abundant, populations will grow. The availability of food acts as a bottom-up control that affects population size. Grouse, hares, and voles feed on vegetation, and the availability of their preferred foods will influence the population size of each.

Studies of these species have demonstrated linked population cycles in each of the prey species, with population peaks every 3-4 years (Figure 1). For example, red foxes ( Vulpes vulpes) in northern Sweden prey on voles, grouse, and hares. Quite often, these cycles co-occur with population cycles of other species in the same location. Some of the most notable examples of population changes occur in species that experience large, cyclic swings in population size. Additional factors, such as parasites and disease can further influence population dynamics. In reality, the interaction between these two forms of population control work together to drive changes in populations over time. More recently, scientists have discovered that predation can also influence the size of the prey population by acting as a top-down control. Such bottom-up control helped to regulate the population around carrying capacity. Researchers found that when resources (food, nesting sites, or refuges) were limited, populations would decline as individuals competed for access to the limiting resources. Ecologists have documented examples of such fluctuations in a wide variety of organisms, including algae, invertebrates, fish, frogs, birds, and mammals such as rodents, large herbivores, and carnivores.Įcologists have long wondered about the factors that regulate such fluctuations, and early research suggested that resource availability plays an important role. Populations of organisms do not remain constant the number of individuals within a population changes, sometimes dramatically, from one time period to the next.

Predator vs prey graph