TutorForAll

Biology 11–12 · Year 11

Measuring populations with random quadrats, with a mark-recapture estimator

Module 4: Ecosystem Dynamics (Population Dynamics)

Practical, model not builtLow risk

This site has no interactive model of its own. Where a step or a material names a Concept Studio model, simulation or tool, it has not been built; an external simulation a step names (for example PhET) is not part of this site.

The idea

A population too large to count is estimated from random samples: quadrats for organisms that stay put, and marking and recapture for ones that move.

What you need

  • Quadrat frames 0.5 m x 0.5 m (0.25 square metre), 1 per group
  • Two 30 m tape measures laid at right angles; random number table or generator
  • Identification key for grassland plants; recording sheets or spreadsheet
  • For the abiotic record: soil thermometer, soil pH kit, light meter or phone lux app, moisture probe
  • For the mark-recapture model: a jar of 400 to 1000 dried beans, a marker pen, a cup as the sampling scoop

How to do it

  1. Map the study area (for example a 10 m x 20 m patch of school lawn) and lay the tapes along two edges.
  2. Generate ten pairs of random coordinates, place the quadrat at each, and count the individuals of a chosen plant species (or estimate percentage cover for a grass) in each quadrat.
  3. At each quadrat record soil temperature, pH, light and moisture, and note shade or trampling.
  4. Calculate mean count per quadrat, density per square metre (mean divided by 0.25) and the population estimate (density times area).
  5. For mark-recapture: scoop a sample of beans, mark and return them, mix, scoop a second sample and count marked among total; estimate N = (marked in first catch x total in second catch) / marked in second catch, and compare with the true count of the jar.
  6. Compare the plant's density across quadrats with the abiotic readings and describe any relationship.

What you should see

Ten quadrats give a mean and spread; a mean of 4.5 plants per 0.25 square metre quadrat is 18 per square metre and 3,600 individuals for a 200 square metre area (computed). In the bean model, 80 marked and returned, then 100 drawn of which 20 are marked, gives an estimate of 400 (the OpenStax worked example), and repeated trials scatter around the true jar count. The learner knows it worked when the plant estimate stabilises as quadrats are added and the average of several bean estimates lies close to the true count.

What changes

What you change
location and its abiotic conditions (shade, moisture, trampling)
What you measure
density of the chosen species per square metre
What you keep the same
  • quadrat size
  • random placement
  • same observer criteria
  • time of year

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

  • Quadrats should be placed where the organism is most common; placement must be random to avoid bias.
  • Marking has no effect on the animal; a mark that changes survival or behaviour biases the estimate.
  • A single sample gives the population; only the distribution of many samples shows the uncertainty.

Safety card

Low riskLearners carry it out

Hazards

  • Sun exposure
  • Insect bites and stings
  • Trips on tapes and uneven ground

Controls

  • Hats, sunscreen and water
  • Check the site for nests and hazards first
  • Keep tapes visible and wash hands afterwards

Note

Fieldwork on school grounds under the school's excursion and outdoor activity procedures; soil pH kits are low hazard (RiskAssess, https://www.riskassess.com.au/).

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

  • Biology Stage 6 Syllabus (2017), current: Year 11 taught to the end of 2026 and Year 12 to Term 3 2027BIO11-11BIO11/12-2BIO11/12-3BIO11/12-4BIO11/12-5
  • Biology 11–12 Syllabus (2025), Year 11 focus area Evolution and ecosystems; new syllabus not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027BI-11-03
  • Biology 11–12 Syllabus (2025), Year 11 Working scientifically; new syllabus not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027BI-11WS-02BI-11WS-03BI-11WS-04BI-11WS-05
  • Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

  1. www.nsw.gov.au/education-and-training/nesa/curriculum/science/biology-stage-6-2017
  2. www.nsw.gov.au/sites/default/files/noindex/2025-03/biology-stage-6-syllabus-2017.docx
  3. curriculum.nsw.edu.au/learning-areas/science/biology-11-12-2025/outcomes
  4. curriculum.nsw.edu.au/learning-areas/science/biology-11-12-2025/content/year-11/fad715a0de
  5. practicalbiology.org/environment/fieldwork-techniques/biodiversity-in-your-backyard.html
  6. openstax.org/books/biology-2e/pages/45-1-population-demography

All Concept Studio activities