Predators and Prey: A Food Chain
Watch rabbits and foxes share a field: rabbits breed on the grass, foxes breed on the rabbits. Because the foxes only multiply after the rabbits have, each peak in the foxes comes after the peak in the rabbits, so the two numbers rise and fall in repeating cycles. Change the food, the foxes' hunting success and how fast they die, or add a drought, disease or the removal of the foxes, to find when the animals live side by side, cycle, or die out.
Preview — sign up free to change the numbers yourself
What you can adjust
- Starting rabbits
- 10 – 800
- Starting foxes
- 1 – 60
- Rabbit birth rate
- 0.5 – 4 /year
- Food for rabbits
- 200 – 2000 rabbits
- Fox hunting success
- 0.02 – 0.3
- Fox death rate
- 0.2 – 2 /year
- Years to watch
- 5 – 60 years
- Event at the halfway point
- 0 – 3
How many rabbits live in the field at the start.
How many foxes live in the field at the start.
How fast the rabbits would multiply with plenty of food and no foxes (births minus natural deaths). A higher rate lets the rabbits bounce back faster after a crash.
How many rabbits the grass can feed. More food does not make the field steadier: with a lot of food the rabbits and foxes swing further up and down, and a swing can end with one of them dying out.
How easily a fox finds a rabbit: the number of rabbits one fox catches in a year for every rabbit in the field, when rabbits are scarce. A fox can only eat so many rabbits, so catches level off when rabbits are plentiful.
The share of foxes that die of natural causes each year. If it is too high for the rabbits to feed enough foxes, the foxes die out.
How many years of the food chain are shown. Each year takes about 0.4 seconds to play.
Something that happens exactly halfway through the run: a drought that halves the grass, all the foxes removed (and none return), or a disease that kills half the rabbits.
About this simulation
What it shows
A field with grass, rabbits and foxes, year by year. A graph draws the number of rabbits and foxes against the years, and a small loop plots the foxes against the rabbits so you can see how the two numbers chase each other.
The key idea
More rabbits means more food for the foxes, so the foxes increase a little later. More foxes then catch more rabbits, so the rabbits fall, and then the foxes go hungry and fall too. That delay is why each fox peak comes after a rabbit peak, and why the numbers cycle instead of staying still.
Try this
Lower Food for rabbits from 1000 to 250: the swings fade away and the rabbits and foxes settle at steady numbers. Raise it to 2000 and the swings grow so deep that the rabbits die out.
A simple model: no seasons and no other animals. Each fox can only eat about one rabbit a week, and a population that drops below one animal is counted as extinct for good. The rabbits and foxes are just an illustration: the same pattern is seen in other food chains.