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C4.1.4—Capture-mark-release-recapture

Capture-mark-release-recapture explains how ecological evidence links organisms, resources and interactions to population size, distribution or community structure in a habitat.

Syllabus
First assessment 2025
Objective
C4.1.4
Level
SL

Exam analysis

Chance of appearing1%of analysed past papers
Latest appearanceMay 2025
Most common paperPaper1A
Typical marks1–3

Common command terms

  • Describe
  • Outline
  • Explain
  • Identify

Scoring notes

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

Recent exam appearances

May 2025Paper1B ["SL"] · TZ33(a)[ 3 ]C4.1.4—Capture-mark-release-recapture
May 2025Paper1A ["SL"] · TZ126[ 1 ]C4.1.4—Capture-mark-release-recapture
Practice this objective

Coverage 2025–2025 · Updated 16 Jul 2026

Capture–mark–recapture estimates mobile populations

Capture–mark–recapture estimates mobile populations.

A first sample is captured, marked harmlessly and released. After mixing, a second sample is taken; the marked fraction helps estimate the total population.

N≈(first caught × second caught)/marked recaptured; check mixing, mark retention and equal catchability.

If 40 fish are marked, 50 are caught later and 10 are marked, the estimate is about 200 fish.

The calculation fails if marks are lost, animals learn the trap or the population changes between samples.

Capture-mark-release-recapture exam focus

Assessment in practice

2–6 marks
How it is assessed

This objective is assessed through structured response, commonly using Describe / Outline / Explain.

Command terms

Describe / Outline / Explain / Identify

What earns marks

Key ideas include Capture Mark Release Recapture, Estimates, Motile, Animal, each tied to evidence about abundance, distribution, survival or species effects.

Watch for

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

Representative question

Question 1

[Maximum number: 6]

Explain the technique used to estimate the population size of a named species of organism that is able to move.

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

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