This deep dive covers Predator-Prey Cycles within Ecosystems Communities for GCSE Biology. Topic 1: Ecosystems Communities It is section 9 of 15 in this topic. Use this deep dive to connect the idea to the wider topic before moving on to questions and flashcards.
🔄 Predator-Prey Cycles
In a stable community, the populations of predators and prey rise and fall in repeating cycles:
- Prey population increases — plenty of food, few predators
- Predator population increases — more prey available to eat
- Prey population decreases — more predators hunting them
- Predator population decreases — less food available (prey numbers are low)
- Cycle repeats — with fewer predators, prey numbers recover, and the cycle starts again
The Time Lag
The predator population always peaks AFTER the prey population — there is a time lag because it takes time for predators to breed when food is plentiful, and time for them to die off when food becomes scarce.
Reading the Graph
On a predator-prey graph, the two lines oscillate (go up and down) together, but the predator line is always slightly behind the prey line. The prey line peaks first, then the predator line peaks shortly after.
Exam skill: When describing a predator-prey graph, always mention: (1) both populations oscillate, (2) predator peaks AFTER prey, (3) explain WHY using the food availability link.
Test yourself: predator-prey cycles
Apply predator-prey cycles to scenarios you have not seen before.
Explain why there is a time lag between the peak in the prey population and the peak in the predator population.2 marks
When prey is abundant, predators have more food so more survive and reproduce, but it takes time for young predators to be born and grow before the extra food shows up as a rise in predator numbers.
Examiners need the causal link between food supply and reproduction TIME. 'Predators eat more prey so predator numbers rise' with no mention of the delay only scores 1 of 2.
A gamekeeper starts shooting foxes to protect pheasants. Foxes in the same habitat also prey on rabbits. Predict and explain what would happen to the rabbit population.2 marks
The rabbit population would increase, because with fewer foxes there is less predation on rabbits, so a greater proportion survive to reproduce.
Full marks needs the chain: fewer predators, less predation, more survival, population rise. 'Rabbits increase because there are fewer foxes' is only 1 of 2 — it skips the mechanism.
One lake shows a regular repeating predator-prey cycle. In a second lake the prey population crashes to almost zero and never recovers. Suggest one reason why the second lake does not show a stable cycle.2 marks
The prey may have no refuge or alternative food source to fall back on, so predators can keep finding and eating the last remaining individuals until the population cannot recover.
Any ecologically sound reason is accepted — no refuge, no alternative prey, habitat loss — but it must be explained, not just named.
Practice questions for Ecosystems Communities
What is a community in ecology?
Explain what is meant by interdependence in an ecosystem.