In ecology, it is usually IMPOSSIBLE to count every individual of a species in a habitat — the area is too large or the organisms too numerous.
Instead, ecologists take a SAMPLE — they count organisms in a smaller, representative section of the habitat and use the results to estimate the total population.
For a sample to be valid:
It must be RANDOM — to avoid bias (e.g. choosing only the easiest areas to access).
It must be REPRESENTATIVE — reflect the full range of conditions in the habitat.
A sufficient NUMBER of samples must be taken — to get a reliable mean.
Three main sampling techniques:
1. QUADRATS — for slow-moving or stationary organisms.
2. TRANSECTS — to show how organisms change across a habitat.
3. MARK-RECAPTURE — for mobile animals.
Quadrats
A QUADRAT is a square frame placed on the ground to define a sample area.
Typically 0.5 m × 0.5 m (0.25 m²) or 1 m × 1 m for vegetation.
How to use quadrats:
1. Place the quadrat RANDOMLY in the habitat (use random number tables or throw the quadrat over your shoulder).
2. Count or estimate the abundance of the target species within the quadrat.
3. Repeat many times across the habitat.
4. Calculate the MEAN count per quadrat.
5. SCALE UP: multiply the mean count by the total number of quadrat-sized areas in the whole habitat.
Formula:
Estimated population = (mean count per quadrat) × (total habitat area ÷ quadrat area)
QUADRATS WORK BEST FOR:
Plants, mosses, lichens.
Slow-moving animals: limpets, snails, woodlice.
NOT suitable for fast-moving animals — they escape before being counted.
Transects and Mark-Recapture
TRANSECTS:
A transect is a LINE drawn across a habitat — organisms are recorded at regular intervals along the line.
Used to show how species DISTRIBUTION changes across a habitat (e.g. from sea to land on a rocky shore, or from open field to shaded woodland).
BELT TRANSECT: a strip (e.g. 0.5 m wide) along the line — quadrats placed at regular intervals. Records abundance.
LINE TRANSECT: simply records which species touch the line — presence/absence only.
MARK-RECAPTURE (Lincoln Index):
Used for MOBILE ANIMALS that would escape quadrats.
Method:
1. Capture a sample of the animal (n₁).
2. Mark each individual (e.g. paint a small spot on a snail shell, attach a leg ring to a bird, clip a fin on a fish).
3. Release marked individuals back into the habitat.
4. Allow time for marked individuals to mix randomly with the population.
5. Capture a second sample (n₂).
6. Count the number of MARKED individuals in the second sample (m).
Formula: N = (n₁ × n₂) ÷ m
ASSUMPTIONS for the formula to be valid:
The mark does not affect survival (does not make animals more visible to predators or less able to move).
Marked animals mix randomly with the rest of the population.
No significant immigration, emigration, births or deaths between the two captures.
All individuals are equally likely to be captured.
⚠️ Common Mistake
In the mark-recapture formula N = (n₁ × n₂) ÷ m — students often confuse what n₁, n₂ and m represent. n₁ = first catch (all marked). n₂ = second catch (total number caught). m = marked individuals IN the second catch. Do NOT divide by n₂ or mix up m and n₂.
📐 Variables
NEstimated population size (N) is measured in individuals ()
n₁Number caught and marked in first sample (n₁) is measured in individuals ()
n₂Total number caught in second sample (n₂) is measured in individuals ()
mNumber of marked individuals in second sample (m) is measured in individuals ()
📐 Key Equations
N = (n₁ × n₂) ÷ m
📌 Key Note
Quadrats: random placement, count organisms, scale up. Transects: show distribution change across habitat. Mark-recapture: N = (n₁ × n₂) ÷ m — for mobile animals. Assumptions: random mixing, no population changes, mark doesn't harm animal.
🎯 Matching Activity — Match the Sampling Technique
Match each situation to the best sampling technique. — drag the symbols on the right to match the component names on the left.
Quadrats
Drop here
Transect
Drop here
Mark-recapture
Drop here
Quadrats
Drop here
Transect
Drop here
Showing how plant species change from an open beach to a sheltered dune
Estimating the population of great crested newts in a pond — mobile animals
Counting the abundance of limpets on different zones of a rocky shore
Estimating the population of daisies in a field — stationary plants, random placement
Recording how lichen cover changes from a roadside to open countryside
⚽ FIFA Worked Examples
Mark-Recapture Calculation
A student catches 40 woodlice, marks them and releases them. The next day, they catch 30 woodlice and find that 6 are marked. Estimate the population.
F
N = (n₁ × n₂) ÷ m
I
N = (40 × 30) ÷ 6
F
N = 1200 ÷ 6
A
N = 200 woodlice
🧪 Required Practical
🔬 RP6 — Use quadrats or transects to estimate population size or distribution of a species in a habitat. Place quadrats randomly and calculate mean count. Scale up to total habitat area.
Know the method, variables, equipment and how to analyse results.
🎯 Test Yourself
Question 1 of 3
1. A student places 10 quadrats (each 1 m²) randomly in a field (total area 500 m²). Mean dandelion count = 4 per quadrat. Estimate the dandelion population.
2. In a mark-recapture study: 25 snails marked (n₁), 20 caught second time (n₂), 5 were marked (m). What is the estimated population?
3. Why must quadrats be placed RANDOMLY in a habitat?
⭐ How Well Do You Understand This Topic?
Be honest with yourself — this helps you know what to revise!
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