Choose field methods and use the Lincoln index
Use quadrats, transects and mark-release-recapture to estimate distribution or abundance.
Section 18.2 · Biodiversity
A Level; Cambridge International Biology 9700, 2028–2030. Written responses are self-checked; this does not certify supervised fieldwork or practical competence.
Before you begin
- Classification compares shared characteristics; populations occupy environments and contain variation.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
In the Lincoln index, what does m₂ represent?
What you will learn
- Select frame quadrats, line or belt transects for suitable survey questions
- Use the supplied Lincoln index and state its assumptions
Build the explanation
Frame quadrats estimate abundance or frequency for organisms that can be counted in a defined area. A line transect records changes along a line; a belt transect samples a strip and can quantify distribution across an environmental gradient. Mark-release-recapture estimates mobile-animal population size using N≈n₁n₂/m₂, where n₁ are marked in the first sample, n₂ is the second sample, and m₂ are marked recaptures.
Use evidence within scope
Lincoln estimates assume a closed population between samples, marks are retained and do not change survival or capture, and marked animals mix back into the population. Very small recapture counts make estimates unstable; the formula is provided in the examination.
Pause and explain
A second sample contains 50 animals, 10 of which were marked; the first sample marked 25. Estimate N and state one assumption.
Worked example
First catch and mark 30 animals. Later catch 40, including 6 marked animals. Estimate population size.
Show the worked solution
- N≈n₁n₂/m₂.
- Substitute 30×40÷6.
Answer N≈200 animals, assuming marking, mixing and capture conditions are suitable.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Which method suits recording plant abundance in fixed plots?
Give me a hint
Identify the required evidence and sequence the reasoning.
Compare my reasoning
- Organisms are counted in a defined area.
- Use frame quadrats.
Frame quadrats are suitable for estimating plant abundance in defined plots.
Look for these in your work
- Relevant evidence selected
- Reasoning linked to conclusion
A belt transect crosses a wet-to-dry gradient. What pattern can it reveal?
Give me a hint
Use the supplied example or data to justify a conclusion.
Compare my reasoning
- Record organisms across positions in the strip.
- Compare distribution along the environmental change.
A belt transect can show how abundance or distribution changes across the wet-to-dry gradient.
Look for these in your work
- Concept applied
- Limit or assumption stated
A second sample contains 50 animals, 10 of which were marked; the first sample marked 25. Estimate N and state one assumption.
Give me a hint
Apply the concept in a new context and qualify its limits.
Compare my reasoning
- Calculate 25×50÷10.
- Check that marking did not affect the animals.
N≈125 animals. This estimate assumes the population remained closed and that marked animals mixed and had the same chance of capture as unmarked animals.
Look for these in your work
- New context handled
- Conclusion appropriately bounded
Common mistakes
- Using a line transect to estimate a whole mobile-animal population
- Swapping the second sample total and recapture count
What can you explain now?
In the Lincoln index, what does m₂ represent?
Compare with the explanation
Marked animals recaptured in the second sample
It is not the total number caught in the second sample.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
How do quadrats, transects and mark-release-recapture answer different sampling questions?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.