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3.7.2Additional lessons

Estimating Population Size — IB Biology SL Revision Notes & Topical Questions

5 min read 6 parts Lesson 83 of 142

📚 IB Biology SL Hub / Topical Revision
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Part 1 of 6Sampling Methods

3.7.2 Estimating Population Size
Why sample
  • It is rarely possible to count every organism, so a representative sample is used to estimate the whole population.
Random sampling
  • Sites are chosen using random numbers or a grid, which avoids bias and is suitable when the habitat is uniform.
Systematic sampling
  • Sites are placed at regular intervals along a line called a transect, which suits changes along an environmental gradient.
Sampling error
  • Samples never match the population exactly, so a larger sample reduces error and improves reliability.

Part 2 of 6Quadrats for Sessile Organisms

Frame quadrat
  • A square frame placed on the ground that is used to count plants or slow-moving animals in a known area.
Presence or absence
  • Records whether a species occurs in each quadrat.
Frequency
  • Records the percentage of quadrats in which the species is found.
Percentage cover
  • Estimates how much of the quadrat area is covered, which is useful when individuals are hard to count.
Mean and standard deviation
  • The mean gives the average, and a small standard deviation shows results are close to it.

Part 3 of 6Mark-Release-Recapture

This method is used for motile animals that cannot be counted using quadrats.

1
Capture as many individuals as possible and mark each one in a way that does not harm it or make it more likely to be eaten.
2
Release the marked animals so that they mix thoroughly with the rest of the population.
3
After a suitable time, capture a second sample and count the marked individuals in it.
4
Estimate the population size using the Lincoln index: N = (M × n) ÷ R.

Part 4 of 6Assumptions of the Method

Assumptions of the Method
Mixing
  • Marked individuals must spread evenly through the population before recapture.
No effect of marks
  • Marks must not affect survival, behaviour or chance of capture.
Closed population
  • There is little birth, death, immigration or emigration between the two samples.
Marks last
  • Marks must remain visible during the study.
SymbolMeaning
MNumber marked in first sample
nTotal number in second sample
RMarked individuals in second sample
NEstimated population size

Part 5 of 6Worked Example

Question

  • In a mark-release-recapture study, 120 beetles are marked and released.
  • A later sample of 90 beetles contains 18 marked individuals.
  • Estimate the population size.

  1. M = 120, n = 90, R = 18.
  2. N = (M × n) ÷ R = (120 × 90) ÷ 18.
  3. N = 10 800 ÷ 18 = 600.

Answer About 600 beetles

Part 6 of 6Exam Checklist & Tips

3.7.2 – Key Points for the Exam

  • Random sampling Choosing sample sites without bias.
  • Transect A line along which samples are taken systematically.
  • Quadrat A frame of known area used for sampling.
  • Mark-release-recapture A method for estimating population size of motile animals.
  • Percentage cover The percentage of ground covered by a species.

Common Mistakes

  • Using quadrats to count fast-moving animals.
  • Marking animals in a way that harms them or makes them easier for predators to see.
  • Swapping the values of n and R in the formula.
  • Treating a single small sample as accurate for the whole habitat.

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