To estimate a population, find the mean count in your sample, then scale up by area (quadrats) or use the proportion marked (capture-recapture). Then say how reliable the estimate is.
This lesson belongs to integrated biological reasoning. It follows the inheritance lesson. Confirm the exact ecology techniques for your exam year on the Cambridge subject page.
What are the two estimates?
Quadrat method: population = mean number per quadrat × (total area ÷ area of one quadrat).
Capture-recapture: population = (number marked first) × (size of second sample) ÷ (number marked in second sample).
Both are estimates, so they carry uncertainty. Random placement and enough repeats improve them.
Worked example 1 (invented data)
A field has an area of 400 m². A student places ten random quadrats, each 0.25 m², and counts daisies: 3, 5, 2, 4, 6, 3, 4, 5, 2, 6.
Step 1, total: 3 + 5 + 2 + 4 + 6 + 3 + 4 + 5 + 2 + 6 = 40.
Step 2, mean per quadrat: 40 ÷ 10 = 4.0.
Step 3, number of quadrat areas in the field: 400 ÷ 0.25 = 1600.
Step 4, estimate: 4.0 × 1600 = 6400 daisies.
Check by another route: 4.0 per 0.25 m² is 16 per m². Then 16 × 400 = 6400. The two routes agree.
Worked example 2 (invented data)
Beetles are caught and 30 are marked and released. Later 40 are caught, and 8 of them carry marks.
Estimate = 30 × 40 ÷ 8 = 1200 ÷ 8 = 150 beetles.
Check: 8 of 40 is one fifth marked. If one fifth of the population is the 30 marked beetles, the population is 30 × 5 = 150.
Energy in a food chain
If a producer holds 20 000 kJ and the primary consumer receives 2000 kJ, the transfer efficiency is 2000 ÷ 20 000 × 100 = 10%. The rest is lost as heat from respiration, in waste, or in parts not eaten.
The mistake to watch for
Mistaken answer: 4.0 × 400 = 1600 daisies.
This multiplies the mean per quadrat by the field area in m², ignoring that each quadrat is only 0.25 m². The count is per quadrat, not per m². Always ask: “per what area?”
Check yourself
1. Five 1 m² quadrats contain 2, 0, 1, 3 and 4 plants. The area is 50 m². Estimate the population.
Show answer
Total = 10. Mean = 10 ÷ 5 = 2.0 per m². 2.0 × 50 = 100 plants.
2. In a capture-recapture study, 20 animals are marked, then a sample of 50 contains 5 marked. Estimate the population.
Show answer
20 × 50 ÷ 5 = 1000 ÷ 5 = 200 animals.
3. A herbivore receives 1500 kJ from plants that held 15 000 kJ. Calculate the percentage transferred.
Show answer
1500 ÷ 15 000 × 100 = 10%.
Where this leads next
Move on to evaluating a biological conclusion with a measurement limitation. The practice set mixes all five lessons, and the scientific investigation critic is useful for sampling questions.
Scaling steps are quick to fix with a teacher watching your working in one-to-one Co-ordinated Sciences tuition.