Mark and recapture at Higher Level: The same method as SL, with different real cases: whale sharks at Ningaloo, shore crabs and Mekong River dolphins.
Practise this as you read
- Calculate the Lincoln index, formula first.
- Explain which way a broken assumption moves the estimate.
Free preview
This is the free notes preview
You're reading the free notes. Aimnova Pro unlocks the full study experience — and you can try it with your first topic free to keep:
- FlashcardsLock in vocabulary and key terms with spaced repetition.
- Practice questionsAnswer exam-style questions and get instant AI marking.
- Mock exams & past-paper vaultSit full mocks and see exactly how examiners award marks.
- Personalised study planA daily plan built around your exam date and weak areas.
Catch, mark, let go, catch again: Mobile animals cannot be counted in quadrats. Catch some, mark them, let them go, then catch a second sample and see how many are marked.
Capture
Catch a sample with traps or nets.
Mark
Mark each one harmlessly and count them (M).
Release
Let them go and wait for them to mix.
Recapture
Catch a second sample (N) and count the marked ones (R).
The points to remember
- Capture a sample, using the same method both times: live traps, nets, pitfall traps, bait.
- Mark each animal so the mark is harmless and lasts: paint dot, leg ring, tag, or a photo of its pattern.
- Release them where they were caught, and leave time for them to mix back in.
- Recapture a second sample and count how many are marked.
- Use the Lincoln index to estimate the population size.
Remember it as: Catch, mark, let go, catch again, count the marked.
Real example: at Ningaloo Reef in Australia, whale sharks are identified from photographs of the spot pattern behind their gills, a natural mark that never needs a tag.
See how examiners mark answers
Access past paper questions with model answers. Learn exactly what earns marks and what doesn't.
The Lincoln index: Population size = (M × N) ÷ R
How to use it
- Write the formula first: population size = (M × N) ÷ R.
- M = number caught and marked in the first sample.
- N = total number caught in the second sample.
- R = number of marked animals recaptured in the second sample.
- Multiply M by N, divide by R, and round to a whole number of animals.
R goes on the bottom: Divide by R, the marked animals caught again, not by N. A small R gives a large population.
Remember it as: marked first, times caught second, over marked again.
Worked example: Ecologists tag 60 shore crabs (M = 60). A week later they catch 50 (N = 50), of which 12 are tagged (R = 12). Population = (60 × 50) ÷ 12 = 3000 ÷ 12 = 250 crabs.
Why it works: the share of marked animals in the second sample (R out of N) should match the share of marked animals in the whole population (M out of the total).
The Lincoln index gives a fair estimate only if these assumptions hold. A trap-happy or trap-shy animal breaks the last one.
The assumptions
- The population is closed: no births, deaths, arrivals or departures between the samples.
- Marks are not lost before the second sample.
- Marks do not harm the animal or make it easier for predators to find.
- Marked animals mix evenly back into the population.
- Every animal has the same chance of being caught: none become trap-happy or trap-shy.
- Both samples use the same method and effort.
| If this goes wrong | Marked recaptured (R) | The estimate is |
|---|---|---|
| Marks rub off | Too few | Too high |
| Marked animals are eaten more | Too few | Too high |
| Animals become trap-shy | Too few | Too high |
| Animals become trap-happy | Too many | Too low |
Remember it as: Closed, marks stay, no harm, mixed, same chance, same method.
Example: a bright paint mark on a snail's shell can make it easier for thrushes to spot, so marked snails are eaten, R is too small, and the estimate is too high.
Study smarter, not longer
Most students waste 40% of study time on topics they already know. Our AI tracks your progress and optimizes every minute.
Planning a reliable study: Each step of the method can be done in a way that keeps the assumptions true.
The points to remember
- Mark enough animals in the first sample.
- Spread the traps evenly across the whole habitat.
- Allow enough time to mix, but not so long that births and deaths change the population.
- Use harmless, durable marks, and release animals where they were caught.
- Avoid making animals trap-happy (bait) or trap-shy (rough handling).
- Repeat the recapture several times and average the estimates.
- Use it only for mobile species that stay in the study area.
Other ways to estimate mobile animals: Count them in aerial or satellite images, or with camera traps, over a representative area, then scale up to the whole area.
Real example: in the Mekong River, Irrawaddy dolphins are counted from photographs of their dorsal fins, whose nicks and scars act as natural marks, so no dolphin has to be caught.
Remember it as: Enough, spread, time, harmless, no bait trap, repeat, stays put.
Counting a food chain: A pyramid of numbers needs the number of organisms at each level, in the same area. Each level needs its own method.
The points to remember
- Producers (plants): random quadrats, then mean × total area.
- Small, slow animals (snails, insects): quadrats, sweep nets or pitfall traps, or mark-recapture.
- Mobile animals (birds, mice, fish): mark-recapture and the Lincoln index.
- Large or shy animals: aerial photos, bird song counts or camera traps, then scale up.
- Draw each level as a bar whose width is proportional to its number.
Remember it as: Plants: quadrats. Movers: mark-recapture. Big and shy: photos.
Example: for algae, then water snails, then ducks in a pond: water samples for the algae, mark-recapture for the snails, and direct counts or photographs for the ducks.
Every level is an estimate: Repeat each method and sample in the same season, so the pyramid compares like with like.
Practice with real exam questions
Answer exam-style questions and get AI feedback that shows you exactly what examiners want to see in a full-marks response.
How this comes up: Calculate a Lincoln index estimate [1]; describe the method [2-3]; ways to make it reliable or factors that affect accuracy [2-4]; collect data for a pyramid of numbers [7]; how reliable sampling can be [9].
Ecologists plan to estimate the number of hedgehogs in a city park using capture-mark-release-recapture.
Identify four ways of ensuring that the study produces a reliable estimate of population size.
Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.