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Quadrats · population estimates
Ecological sampling calculations
Estimate abundance from quadrats, calculate species frequency and use a simple mark–release–recapture estimate with its assumptions stated.
Method and worked examples ↓Learn the method
How to approach these questions
- Distinguish abundance (how many organisms) from frequency (how often the species occurs in sampled quadrats).
- Use the quadrat area in the same area unit as the habitat area.
- For a simple recapture estimate, N ≈ marked first × caught second ÷ marked in the second sample.
- The recapture estimate assumes a closed population, retained marks, mixing, and similar capture chances for marked and unmarked organisms.
Worked examples
Counts in five 0.25 m² quadrats are 2, 4, 3, 5 and 6. Estimate the population in a 50 m² habitat.
Mean = 20 ÷ 5 = 4 per quadrat. Density = 4 ÷ 0.25 = 16 m⁻². Population estimate = 16 × 50 = 800.
Thirty animals are marked. A second sample contains 40 animals, of which 6 are marked. Estimate the population.
N ≈ 30 × 40 ÷ 6 = 200. It is an estimate that depends on the sampling assumptions, not an exact census.
Keep practising
Topic map and study list · Practical data guides · Active recall decks
Course resources
AQA A-level Biology · OCR A A-level Biology · Textbook answers
Skill coverage follows the common A-level science subject content. Check your board’s current requirements: AQA 7402 and OCR A H420.