INTRODUCTION

Predation is a trophic interaction in which an individual of one species (a predator) kills and/or consumes individuals of another species (its prey). Predator and prey numbers sometimes follow oscillating cycles that are closely linked—that is, as prey numbers rise and fall, predator cycles show a similar, but lagging, rise and fall. For example, when prey are abundant, predators consume more prey and also become abundant. As predators become more abundant, they cause prey populations to fall, and this leads to a decline in predators as well. In the past, ecologists thought that the tight link between prey and predators controlled the cycles. But the control may be more complex, or come from outside the predator–prey interaction.

The community in this simulation consists of a meadow that provides food and habitat for voles and owls. The number of voles (prey) in the community is dependent on grass as food (positive effect) and owls as predators (negative effect). The owl (predator) population is dependent on voles for food. The simulation is based on a mathematical model.

For this simulation, the assumptions are that in a given season, the abundance of meadow grass (A) is determined by rainfall and stays constant throughout the season. The amount of grass available determines the number of voles that can be produced during the season. Increases in the vole (prey) population (B) follows a simple exponential growth trajectory. However, although voles use the grass as a food source, they have a negligible effect on the abundance of the grass.

The vole population growth rate is also affected by the number of owls (predators) in the system. Therefore, the model subtracts a parameter that is a function of both the vole population (B) and owl population (C). This parameter measures the efficiency of predation—that is, the percentage of prey that predators consume. If the percentage is small, the predator population has little effect on the growth rate of the prey population. In contrast, if it is large, the predator population has a negative effect on the prey population and it declines.

In the simulation, all the model parameters are fixed except the growth rate of the voles and owls. You can change this growth rate by selecting the seasonal rainfall (drought, average rainfall, and above average rainfall), which will determine the amount of grass available for voles as food.

Go to the simulation to explore the population dynamics in the simulated meadow community. Note the relationships between the populations over time.

CONCLUSION

As you ran the simulation, you likely noticed the cycling of the predator (owl) and the prey (vole) populations. When the prey increase in abundance, the predators do as well, causing the prey numbers to decline. When the prey decline, so do the predators. At some point, the prey population starts to increase again due to the lack of predation, which then allows the predator population to increase as well.

Thus, the cycling of the predator follows, or lags behind, that of the prey. Predator numbers only decrease after the number of prey declines, and they only increase after the prey numbers increase. The cycling is most obvious when a predator is entirely, or almost entirely, dependent on that prey species. If the owls also eat mice, rats, and rabbits, their dependence on voles as prey will be less, and the cycle will be less linked and less obvious.

The model is designed to illustrate the effect of primary producers (grass) on these coupled cycles of predator and prey in the meadow ecosystem. Under average rainfall, the vole population shows a continuing cycle, and owls show a lower-amplitude cycle that lags behind the vole cycle. Under drought conditions, population sizes and fluctuations of both voles and owls decrease. With lower grass production, voles have less food and produce fewer young. Their lower population in turn decreases food available for owls, whose population decreases as a result. The cycles of both are longer and have a lower amplitude than under average rainfall. When rainfall is above average, more grass is produced, so the peak population (amplitude) of voles is much higher and closer together. The owl population still lags behind (responding to the vole population), but their peaks are also higher.

Thus, the community exerts both top-down control, with growing predator populations controlling prey numbers, and bottom-up control, with an increase or decrease in food (grass) availability directly controlling the prey and indirectly controlling the predator. In either case, cycling will be most pronounced if the predator is strongly dependent on that prey for food.

The coupled predator–prey model used here forms the basis of many models used today in the analysis of population dynamics. This type of cycling is observed in nature, but it is not overwhelmingly common. By itself, it is not sufficient to model many predator–prey systems found in nature. As shown here, the food available to prey also exerts considerable control over both prey and predator populations.