MrBadmusAI
  1. KS3
  2. Biology
  3. Ecosystems and interdependence
  4. Predator and prey

Ecosystems and interdependence · Model

Predator and prey

A fox eating rabbits is bad news for the rabbits. A fox population eating a rabbit population is something stranger: two numbers chasing each other round in a circle, neither able to win.

Start here

Foxes have eaten rabbits for thousands of years. There are still rabbits.

Every predator is better at hunting than its prey is at escaping, at least sometimes, or it would starve. Run that forward for a few thousand generations and you might expect the prey to be gone and the predator with it. Instead both are still here.

Why do predators not wipe out their prey?

At the bench · one field, two populations

Run the years and watch the lag

year 0

Two rules, and nothing else. Rabbits breed and are eaten; foxes eat and die. Look at when each peak happens — not how high, when.

Rabbits 800

Foxes 120

amber = rabbits · green = foxes · each pair is one year, oldest on the left · the two are scaled separately

Nothing has happened yet. Look at the shape before you read anything else.

One turn of the rabbit and fox cycle, with the gap between the two peaks measuredA line graph of one complete turn of the cycle. Along the bottom is a year axis, ticked every year and numbered every second year. Up the left-hand side is a scale for the rabbits and up the right-hand side a separate scale for the foxes, because the two are counted on their own scales. Two lines cross the plot. The rabbit line is solid and thicker; the fox line is dashed. Each carries a dot at every year, which is where the numbers actually are, and each is named at its right-hand end rather than in a key. The rabbit line rises to a peak and then falls away. The fox line is still rising while the rabbit line falls, reaches its own peak later, and only then turns down, after which the rabbit line begins to climb again. The rabbit peak is marked with a filled circle and the fox peak with an open one, each labelled with the year it happens in, and a dashed line drops from each down to the year axis. Between those two dropped lines, below the axis, a measured line with a tick at each end spans the gap and is labelled with the number of years in it. A line at the foot of the figure repeats that the two are counted on their own scales and that what to look at is when each peak happens, not how high it is.ONE TURN OF THE CYCLE, FROM THE FIELD ON THIS PAGE025050075010000100200300400Rabbits countedFoxes counted131517192123252729yearRabbitsFoxesRabbits peakyear 20Foxes peakyear 255 yearsRabbits — solid, filled peakFoxes — dashed, open peakThe two are counted on their own scales. Look at WHEN each peak happens — not how high, when.
The fox line is still going up while the rabbit line is already coming down, and it does not turn over until years later. That gap is the whole idea: foxes cannot become more foxes the moment there is more to eat — breeding takes time — so the fox peak always follows the rabbit peak rather than arriving with it. Look at when each peak happens, not how high it is: the two are counted on their own scales.

The cycle, in four steps

It has no beginning, so start anywhere.

  • 1

    Plenty of prey

    Rabbits are numerous, so foxes are well fed. More cubs survive their first winter, and the fox population starts to climb.

  • 2

    Predators rise

    More foxes hunting means more rabbits eaten than are born. The rabbit population turns and begins to fall — while the fox population is still rising.

  • 3

    Prey becomes scarce

    Now there are many foxes and few rabbits. Foxes go hungry, fewer cubs survive, and the fox population falls in its turn.

  • 4

    Predators rise no longer

    With few foxes left, the surviving rabbits breed almost unchecked and the population recovers. Which returns you to step 1, some years later.

Key fact

Predator and prey numbers rise and fall together, and the predator peak always comes after the prey peak. Each population limits the other, so neither wipes the other out and neither grows without limit.

Think again

“The two peaks happen at the same time — more rabbits, more foxes.”

More rabbits does eventually mean more foxes, and eventually is the entire content of this lesson. A fox that finds plenty of food this spring cannot turn that into extra foxes until its cubs are born and survive, so the fox numbers respond to last year's rabbits, not this year's. Look at the chart on the bench: the green peak sits to the right of the amber one every single time. That delay is what makes the pattern a cycle rather than two lines moving together. Without a lag the populations would simply settle at a steady value and stay there; with one, each overshoots the level it could sustain, corrects, overshoots the other way, and swings again — a little less far each time. Delayed feedback producing oscillation is not just biology — it is why a shower runs hot then cold when the tap is slow to respond.

“Remove the predators and the prey will do brilliantly.”

Press remove every fox on the bench and watch what actually happens. The rabbits climb steeply, exactly as expected, and then they stop — not because anything is eating them, but because there is only so much grass. Every environment has a ceiling set by food, water, space and disease, and a population that reaches it stays there in a state that is nothing like doing brilliantly: crowded, hungry and vulnerable to any disease that gets in. Predators are one limit among several, and often not the tightest one. This is why removing a predator to help a prey species so rarely works as intended, and it is the beginning of Disturbing a food web, where the consequences spread outwards to species nobody was thinking about.

Mastery ladder

Not started yet.

Rungs 3 and 4 you mark yourself.

Rung 1 · Read the cycle

Rabbit numbers in a field have been falling for two years. What is the fox population most likely to do next?

Rung 2 · The one that catches people

On a predator–prey graph, how do the two peaks line up?

Rung 3 · Explain one full cycle

Describe one complete predator–prey cycle in a field of rabbits and foxes, starting from a year when rabbits are plentiful. Explain what causes each change rather than just describing it.

Rung 4 · Take it somewhere new

A farmer plans to remove every fox from her land to protect her chickens. Using the model, predict what will happen to the rabbit population over the next few years, what else on the farm might be affected, and name one thing the model does not account for.

Key note

Predator and prey populations cycle. Plenty of prey lets predator numbers rise; rising predator numbers push prey numbers down; scarce prey starves the predators; falling predator numbers let the prey recover. The predator peak always follows the prey peak, because breeding takes time. Prey numbers are also limited by food and space, so removing the predators does not let them rise for ever.

Going further

The best-known data on this were not collected by scientists. The Hudson's Bay Company bought furs in Canada for two centuries and kept careful records of how many lynx and snowshoe hare pelts came in each year, and when ecologists later plotted those ledgers they found two wavy lines rising and falling on a cycle of roughly ten years, with the lynx peak trailing the hare peak. A linked cycle in two species, recovered from a company's account books: that is what the data show, and it is the thing worth learning from them. What they do not show is one animal governing the other. Hare numbers rise and fall for reasons that reach well past lynx — the shoots and twigs they feed on are stripped after a peak and take years to grow back, and hares living under heavy pressure raise fewer young. Lynx move within that cycle as much as they drive it. The record itself also needs reading with care: those ledgers count pelts brought in by trappers, not hares alive in the forest, so the price of fur and the number of people out trapping are in the line too. A ten-year rhythm that survives all of that is a real rhythm. It is still not a census.

Before this lesson

Connects to

At GCSE this becomes

  • Population dynamics, carrying capacity, and interpreting real predator–prey data with its irregularities.

Where to next

Ask Mr Badmus AI

Want to predict what your own change to the field would do?

The bench runs a two-species model with a fixed grass supply and no weather, disease, migration or alternative prey. Real populations are affected by all of those, cycle less tidily, and sometimes do not cycle at all. The model is here to show why a lag produces a cycle, not to predict a real field.

Lesson content © MrBadmusAI.

Mr. Badmus AI

KS3 Science Tutor

preview