3.8.2Additional lessons
Estimating Population Size — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 11 of 101
Section 1 of 5Why We Estimate
Counting is impractical
- In most habitats it is not possible to count every individual, so ecologists sample and estimate.
Sampling
- Small areas or groups are measured and the results are scaled up to the whole habitat.
Reliability
- Larger and randomised samples give estimates that are closer to the true population size.
Choice of method
- Sessile organisms such as plants are sampled with quadrats, while mobile animals need capture-recapture.
Section 2 of 5Quadrat Sampling
Quadrats suit plants and slow-moving animals that stay in place while being counted.
1
Decide the area of the habitat and choose a quadrat of known size, such as 0.25 m².
2
Use random numbers to generate coordinates on a grid laid over the area so that the placement of each quadrat is unbiased.
3
Place a quadrat at each coordinate and count the individuals of the species inside, or estimate percentage cover.
4
Calculate the mean number per quadrat and divide by the quadrat area to obtain density per square metre.
5
Multiply the density by the total area of the habitat to estimate the population size.
Section 3 of 5Capture-Mark-Release-Recapture
Capture-Mark-Release-Recapture
First sample
- Capture as many animals as possible, mark each harmlessly and release them back into the habitat.
Mixing time
- Allow time for the marked animals to mix randomly with the unmarked population.
Second sample
- Capture another sample and count the total caught and how many of them carry marks.
Lincoln index
- Population size is estimated as the first sample multiplied by the second sample, divided by the number of marked animals recaptured.
Assumptions
- No births, deaths or migration occur, marking does not harm or change behaviour, and marks stay visible.
| Method | Used for | Main limitation |
|---|---|---|
| Random quadrats | Plants, sessile animals | Sample may not represent the area |
| Mark-recapture | Mobile animals | Relies on several assumptions |
Section 4 of 5Worked Example
Question
- A beetle population is sampled.
- In the first sample 40 beetles are caught, marked and released.
- A second sample of 50 beetles contains 10 marked beetles.
- Estimate the population size.
- Write the Lincoln indexN = (n1 × n2) ÷ m,What the symbols mean: N = estimated population size · n1 = number caught and marked first · n2 = number caught the second time · m = number of marked ones in the second catch
- where n1 is the number marked in the first sample, n2 the size of the second sample and m the number of marked beetles recaptured.
- Substitute the valuesN = (40 × 50) ÷ 10.
- Calculate2000 ÷ 10 = 200.
Answer The estimated population size is 200 beetles.
Section 5 of 5Exam Checklist & Tips
3.8.2 – Key Points for the Exam
- Quadrat A square frame of known area used for sampling.
- Random sampling Choosing sample sites so every position has the same chance of selection.
- Density Number of individuals per unit area.
- Lincoln index An equation for population size from mark and recapture data.
- Sessile Fixed in position and not able to move.
Common Mistakes
- Placing quadrats where the organisms look most plentiful, which biases the result.
- Using too few quadrats, so the estimate is unreliable.
- Writing the Lincoln index upside down or using the recaptured number in the wrong place.
- Marking animals in a way that makes them easier for predators to catch.
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