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Predator-prey relationshipsIB MYP Biology: Revision notes

Section 1

Predators and prey

A predator is an animal that hunts, kills and eats other animals. The animal it eats is the prey. Predators and prey affect each other's numbers, because each depends on the other.

For example, lynx are predators of snowshoe hares, and cheetahs are predators of gazelles.

Key termspredatorpreypopulation

Section 2

Predator-prey cycles and graphs

When the populations of a predator and its prey are plotted on a graph over time, both rise and fall in regular cycles.

  1. Prey numbers are high, so there is plenty of food for predators.
  2. Predator numbers increase.
  3. More predators eat more prey, so prey numbers fall.
  4. With less food, predator numbers fall.
  5. With fewer predators, prey numbers recover, and the cycle repeats.

The predator peak comes after the prey peak. This delay is a time lag, because predators need time to breed and grow. Predator numbers are limited by their food supply, and prey numbers are limited by predation and by their own food.

Key termscycletime lagpeak
Exam tip

Reading a graph: find the prey peak first, then the predator peak that follows it, and quote the number of years between them.

Section 3

Interpreting population data

When you describe a predator-prey graph or data, use numbers.

  • Describe the pattern: both populations rise and fall, and the predator peaks later than the prey peak.
  • Give a figure: for example, hares peak at 140 000 in year 4 and again in year 14, so the cycle is 10 years long.
  • Explain it: link each change to food supply or predation.

If prey numbers fall for another reason (for example disease), predators lose food and also fall.

Key termsinterpretcycle length
Common mistake

Do not say that predator numbers fall 'because they are eaten by the prey'. They fall because there is less food.

Section 4

Adaptations of predators

An adaptation is a feature that helps an organism survive and reproduce in its habitat. Predators have adaptations to catch prey.

  • Eyes at the front of the head give good judgement of distance (cheetah, lynx).
  • Streamlined body and powerful muscles for fast running.
  • Sharp teeth and claws to catch and kill.
  • Camouflage to approach prey unseen.
Key termsadaptationcamouflage

Section 5

Adaptations of prey

Prey have adaptations to avoid being caught.

  • Eyes at the sides of the head give a wide field of view, to spot predators approaching.
  • Camouflage: a brown gazelle is hard to see against dry grass.
  • Speed and agility: long legs for fast running.
  • Warning colours, spines or poison to put predators off.

Prey that survive reproduce and pass on their adaptations.

Key termswarning coloursfield of view

Must Know

  • Predator-prey populations rise and fall in cycles
  • The predator peak comes after the prey peak: a time lag
  • Predator numbers depend on prey, so a fall in prey makes predators fall
  • Predators: eyes at front, speed, sharp teeth. Prey: eyes at sides, camouflage, speed

That's the notes covered.

Carry on to the next subtopic.

Exam questions on Predator-prey relationships

  1. In a nature reserve in Canada, the populations of snowshoe hares (prey) and lynx (predators) were counted every year for 20 years. The hare population peaked at about 140 000 in year 4 and again in year 14. The lynx population peaked at about 60 000 in year 6 and again in year 16.
    Explain why each lynx peak comes after the hare peak.2 marks
  2. A cheetah hunts gazelles on an African grassland. The cheetah has a streamlined body, powerful leg muscles and eyes at the front of its head. The gazelle has eyes at the sides of its head, long legs for fast running and a brown coat.
    Explain how the gazelle's brown coat helps it to survive.2 marks
  3. A class investigates camouflage using paper 'insects' on a green lawn. In each trial the teacher scatters 20 green, 20 yellow and 20 red paper insects, all the same size, over the same area of the lawn. Ten students act as predators and each has 20 seconds to collect as many insects as they can. The trial is repeated three times with fresh insects. In total, the students collected 9 green, 24 yellow and 41 red insects.
    Identify the independent variable, the dependent variable and one control variable in this investigation.3 marks
See the full worksheet

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).