GCSE Biology - How to Solve: Sampling and Population Size (Quadrats, Transects, Capture-Recapture) – Complete Guide

How to Solve: Sampling and Population Size (Quadrats, Transects, Capture-Recapture) – Complete Guide

? Introduction Mastering sampling techniques unlocks 10-15% of your Ecology exam marks—and helps you answer real-world questions like "How many fish are in a lake?" or "Is pollution killing plant species?" in GCSE/A-Level Biology. One wrong step = lost marks. This guide gives you exact steps, formulas, and traps to ace every question.

? WHAT YOU NEED TO KNOW FIRST

Before diving in, you must understand:
1. Random vs. systematic sampling – Why we avoid bias.
2. Mean, median, mode – How to calculate averages from data.
3. Percentage cover – Estimating how much of a quadrat is covered by a species.

If you’re shaky on these, pause and review them first.

? KEY TERMS & FORMULAS

? Key Terms

? Formulas

1. Quadrat Sampling (Mean Density)

Formula: [ \text{Estimated population} = \text{Mean density per quadrat} \times \text{Total area} ] Variables: - Mean density per quadrat = (Total organisms counted) ÷ (Number of quadrats) - Total area = Area of the whole habitat (e.g., 100m²)

MEMORISE THIS – You’ll use it for plants, slow-moving animals, or stationary species.

2. Capture-Recapture (Lincoln-Petersen Index)

Formula: [ N = \frac{M \times C}{R} ] Variables: - N = Estimated total population - M = Number of individuals marked in first capture - C = Total number captured in second sample - R = Number of recaptured (marked) individuals in second sample

MEMORISE THIS – Examiners love testing this!

Assumptions (write these in answers!): ✅ No births/deaths/migration between captures. ✅ Marking doesn’t affect survival. ✅ Marks aren’t lost.

3. Percentage Cover (Quadrat)

Formula: [ \text{Percentage cover} = \left( \frac{\text{Number of squares covered}}{\text{Total squares in quadrat}} \right) \times 100 ] Example: If 12 out of 25 squares have grass → ( (12 ÷ 25) × 100 = 48\% ).

GIVEN ON EXAM SHEET (but practice it anyway).

? STEP-BY-STEP METHOD

? Method 1: Quadrat Sampling (Plants/Stationary Species)

When to use: Estimating population size of plants, barnacles, or slow-moving animals.

Steps:
1. Choose your quadrat size (e.g., 0.5m × 0.5m = 0.25m²).
2. Randomly place quadrats (use a random number generator for coordinates). - Why? Avoids bias (e.g., only sampling "nice" areas).
3. Count organisms in each quadrat (or estimate % cover).
4. Calculate mean density per quadrat = Total organisms ÷ Number of quadrats.
5. Multiply by total area = Mean density × (Total habitat area ÷ Quadrat area). - Example: If mean density = 5 daisies/quadrat, quadrat = 0.25m², field = 100m² → ( 5 × (100 ÷ 0.25) = 2000 ) daisies.

Pro Tip: If asked for percentage frequency, count how many quadrats contain the species, then: [ \text{Percentage frequency} = \left( \frac{\text{Number of quadrats with species}}{\text{Total quadrats}} \right) × 100 ]

? Method 2: Transect Sampling (Distribution Along a Gradient)

When to use: Studying how species change along an environmental gradient (e.g., light, pollution, shore height).

Steps:
1. Lay a tape measure (transect line) across the area.
2. Place quadrats at regular intervals (e.g., every 2m).
3. Record species in each quadrat (count or % cover).
4. Plot results (e.g., bar chart of species vs. distance).
5. Look for patterns (e.g., "Species X decreases as distance from shore increases").

Pro Tip: If asked for belt transect, place quadrats continuously along the line.

? Method 3: Capture-Recapture (Mobile Animals)

When to use: Estimating population size of animals (e.g., fish, insects, mammals).

Steps:
1. First capture (M): Catch animals, mark them (e.g., paint, tags), release. - Record: M = number marked.
2. Wait (long enough for mixing, but not too long for births/deaths).
3. Second capture (C): Catch another sample. - Record: C = total caught in second sample. - Record: R = number of marked animals in second sample.
4. Apply formula: ( N = \frac{M × C}{R} )
5. Check assumptions (write them in answers!).

Example: If M=50, C=40, R=10 → ( N = (50 × 40) ÷ 10 = 200 ).

✏️ WORKED EXAMPLES

? Example 1 – Basic Quadrat Sampling

Question: A student uses 10 quadrats (each 0.5m × 0.5m) to count daisies in a 50m² field. The counts are: 3, 5, 2, 4, 6, 3, 5, 4, 2, 6. Estimate the total daisy population.

Solution:
1. Total daisies counted = 3+5+2+4+6+3+5+4+2+6 = 40
2. Mean per quadrat = 40 ÷ 10 = 4 daisies/quadrat
3. Quadrat area = 0.5 × 0.5 = 0.25m²
4. Number of quadrats in field = 50 ÷ 0.25 = 200
5. Estimated population = 4 × 200 = 800 daisies

What we did and why: - Averaged counts to reduce random error. - Scaled up from small quadrats to the whole field.

? Example 2 – Medium Capture-Recapture

Question: A biologist catches 60 snails, marks them, and releases them. A week later, she catches 80 snails, of which 12 are marked. Estimate the snail population.

Solution:
1. M = 60 (marked first time)
2. C = 80 (total in second sample)
3. R = 12 (marked in second sample)
4. Apply formula: ( N = \frac{60 × 80}{12} = 400 )

What we did and why: - Used the Lincoln-Petersen formula to estimate total population. - Assumed no births/deaths/migration (always state this in answers!).

? Example 3 – Exam-Style Transect Question

Question: A student investigates how seaweed cover changes with distance from the shore. She places a 20m transect and records % cover in 1m² quadrats every 2m. Her results:

a) Plot a graph of % cover against distance. b) Describe the trend. c) Suggest why this pattern occurs.

Solution: a) Graph: - X-axis: Distance (m) - Y-axis: % Cover - Plot points: (0,80), (2,65), (4,40), (6,20), (8,5), (10,0) - Draw a smooth curve (not straight line).

b) Trend: - Seaweed cover decreases as distance from shore increases.

c) Explanation: - Less water further from shore → seaweed dries out. - More exposure to air → desiccation (drying). - Wave action is stronger near shore → seaweed adapted to survive there.

What we did and why: - Plotted data to visualise the trend. - Linked pattern to abiotic factors (water, air, waves) for full marks.

❌ COMMON MISTAKES

? EXAM TRAPS