3.7.2Additional lessons
Estimating Population Size — IB Biology SL Revision Notes & Topical Questions
5 min read 6 parts Lesson 83 of 142
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 6Sampling Methods
Why sample
- Why sample It is rarely possible to count every organism, so a representative sample is used to estimate the whole population.
Random sampling
- Random sampling Sites are chosen using random numbers or a grid, which avoids bias and is suitable when the habitat is uniform.
Systematic sampling
- Systematic sampling Sites are placed at regular intervals along a line called a transect, which suits changes along an environmental gradient.
Sampling error
- 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
- 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
- Presence or absence Records whether a species occurs in each quadrat.
Frequency
- Frequency Records the percentage of quadrats in which the species is found.
Percentage cover
- Percentage cover Estimates how much of the quadrat area is covered, which is useful when individuals are hard to count.
Mean and standard deviation
- 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
- Mixing Marked individuals must spread evenly through the population before recapture.
No effect of marks
- No effect of marks Marks must not affect survival, behaviour or chance of capture.
Closed population
- Closed population There is little birth, death, immigration or emigration between the two samples.
Marks last
- Marks last Marks must remain visible during the study.
| Symbol | Meaning |
|---|---|
| M | Number marked in first sample |
| n | Total number in second sample |
| R | Marked individuals in second sample |
| N | Estimated 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.
- M = 120, n = 90, R = 18.
- N = (M × n) ÷ R = (120 × 90) ÷ 18.
- 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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