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C4.1.16—Predator-prey relationships

Predator-prey relationships explains how ecological evidence links organisms, resources and interactions to population size, distribution or community structure in a habitat.

Syllabus
First assessment 2025
Objective
C4.1.16
Level
HL

Exam analysis

Chance of appearing3%of analysed past papers
Latest appearanceMay 2019
Most common paperPaper3
Typical marks1–2

Common command terms

  • Identify
  • Compare
  • Describe
  • Predict
  • Explain
  • Contrast
  • Suggest
  • Determine

Scoring notes

Common mistake
Confusing a population with a community or with all organisms in an ecosystem.

Recent exam appearances

May 2019Paper3 ["HL"] · TZ26(a)(ii)[ 2 ]C4.1.16—Predator-prey relationships
May 2019Paper3 ["HL"] · TZ26(a)(i)[ 1 ]C4.1.16—Predator-prey relationships
May 2015Paper3 ["HL"] · TZ110(d)[ 2 ]C4.1.16—Predator-prey relationships
November 2014Paper3 ["HL"] · TZ01(d)[ 2 ]C4.1.16—Predator-prey relationships
Practice this objective

Coverage 2014–2019 · Updated 16 Jul 2026

Predator and prey numbers can oscillate

Predator–prey interactions can regulate both animal populations through density-dependent feedback, often producing time-lagged cycles.

More prey support predator survival and reproduction, so predator numbers rise later. Increased predation then lowers prey numbers; food shortage subsequently lowers predator numbers, allowing prey recovery.

Long-term snowshoe hare and Canada lynx records show repeated abundance cycles in which lynx peaks generally follow hare peaks, consistent with the delayed predator response.

Trace hare increase → more food for lynx → delayed lynx increase → higher hare mortality → hare decline → lynx decline from reduced food.

A lagged correlation supports but does not alone prove a predator-driven cycle. Food supply, disease, climate and sampling methods can also influence both populations.

Predator-prey relationships

Assessment in practice

2 marks
How it is assessed

This objective is assessed through structured response, commonly using Identify / Compare / Describe.

Command terms

Identify / Compare / Describe / Predict / Explain / Contrast / Suggest / Determine

What earns marks

Build the answer around this relationship: Predator-prey relationships must be linked to the correct ecological unit, method or species interaction.

Watch for

Confusing a population with a community or with all organisms in an ecosystem.

Representative question

Question 1

[Maximum number: 3]

Describe the effect of removing trout on frog density in Upper and Lower LeConte Lakes.

Populations and Communities

  • A population is one species in an area; a community is all interacting populations there.
  • Estimate abundance with unbiased sampling: quadrats for sessile organisms and capture–mark–release–recapture for mobile animals, checking each method’s assumptions.
  • Density-dependent competition, predation, disease and waste create negative feedback around carrying capacity; exponential growth slows into a sigmoid curve as limits strengthen.
  • Classify interspecific relationships by costs and benefits: predation, herbivory, competition, mutualism, parasitism and pathogenicity.
  • Invasive species may escape controls and displace endemic species. Removal experiments can reveal competition and fundamental versus realized niches.
  • Chi-squared tests assess species association from observed and expected quadrat counts.
  • Predator peaks usually lag prey peaks; top-down control begins with consumers, while bottom-up control begins with resources or producers.

Concept essentials

  • Predator-prey relationships must be linked to the correct ecological unit, method or species interaction.
  • The evidence for predator-prey relationships depends on measurable abundance, distribution, survival or resource patterns.
  • Predator-prey relationships is clearest when cause, ecological process and population outcome are kept distinct.
  • Field or graph evidence for predator-prey relationships needs biological interpretation, not values alone.