Predator-prey cycles: rabbits and foxes sim
Set how much grass the rabbits have and how many foxes go in, step a model meadow one year at a time, and watch the rabbit and fox numbers rise and fall in cycles, the fox peaks a few years behind the rabbit peaks.
Ecology: predator-prey cycles in a stable community, with the predator peak lagging behind the prey peak, far fewer predators than prey, and the effect of the prey's food supply · covers the GCSE topic
What the sim shows
Predator-prey cycles: rabbits (the prey) eat grass, and foxes (the predators) eat rabbits. In a stable community the numbers of predators and prey rise and fall in cycles. When rabbits are plentiful, the foxes have plenty of food, so more fox cubs survive and the foxes increase. More foxes eat more rabbits, so the rabbits fall. With fewer rabbits, the foxes go short of food and fall too, which lets the rabbits rise again. The fox peak always comes a little after the rabbit peak (a lag), because the foxes need time to breed, and there are always fewer foxes than rabbits (usually far fewer), because each fox must eat many rabbits. The grass sets how many rabbits the meadow can feed. The real Canadian lynx and snowshoe hare, known from fur trappers' records of pelts traded over about a hundred years, rose and fell like this about every 10 years, the lynx peaks a little after the hare peaks (in this model the lag is about 3 years; the real lynx lag is shorter). Simplified on purpose: the numbers are illustrative (≈), from a textbook model (the Rosenzweig-MacArthur model), not from a real meadow; only three things are modelled: grass, rabbits and foxes; real foxes also eat voles, birds and insects, and real rabbits face disease (such as myxomatosis), weather and other predators; the grass is the same every year; real grass grows back and is eaten down; the populations change smoothly through each year; real rabbits breed in seasons and real foxes have one litter a year; no animals move into or out of the meadow; the model tells you whole animals each year, but inside it the numbers change smoothly; a population below half an animal has died out and stays out; with 6 turf tiles or fewer the swings fade in this model (very slowly with exactly 6), and with more they keep going; real cycles are kept going by many more things; the counters on the bench stand for 50 rabbits or 2 foxes each, and the grey and orange counters are a key, not real colours.
- Lit: a fox counter to drag from the dish onto the fox column
- Counter columns: grey = rabbits (1 counter = 50), orange = foxes (1 counter = 2)
- Green tile = grass the rabbits eat; brown = bare soil
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