AQA GCSE Biology Biology Paper 2 (Higher), June 2024: Question 3

✅ Acceptable Improvements (Any one)

• Prevent double counting: Do not return tadpoles to the pond until all 4 samples are taken.
• Increase sample size: Collect more samples each time (e.g. 10 samples instead of 4) or use a larger volume/bucket.
• Increase frequency: Sample more often (e.g. every week rather than jumping from 3 to 5 to 8 weeks).
• Standardise technique: Use standardised sweeps with a pond net instead of scooping with a bucket.
• Reduce spatial bias: Sample at a variety of different depths, or use random coordinates to choose sampling locations.
• Control variables: Sample at the exact same time of day each week.

🧠 Exam Technique & Examiner Insights

Read the method carefully! Step 3 says: "Put the tadpoles back into the pond" before step 4: "Repeat... another three times in different parts".

If you immediately release mobile animals back into the pond, you risk catching the same individuals again (double counting). Highlighting this is an examiner favourite for aquatic sampling questions.

📐 Step-by-Step Calculation

Look at the column for 3 weeks in Table 1:

• Sample 1 = 9
• Sample 2 = 7
• Sample 3 = 10
• Sample 4 = X
• Total = 32

1. Sum the known samples: 9 + 7 + 10 = 26

2. Subtract from total: X = 32 - 26 = 6

Value X = 6

✅ Awarding Marks

Answer: 6

• Full mark awarded if written on the answer line or clearly written into Table 1.
• No working required for full credit, but showing working prevents simple addition slip-ups.

💡 Mark Breakdown (4 Marks Total)

• Mark 1 (Scale & Label): Correct linear scale using at least half the available grid space, and axis clearly labelled with units ("Time in weeks").
• Marks 2 & 3 (Plotting):
  • All 6 points plotted correctly within ±0.5 small square = 2 marks.
  • 4 or 5 points plotted correctly = 1 mark.
• Mark 4 (Line of Best Fit): Smooth, clean curved line of best fit reflecting the trend.

❌ Common Errors That Cost Marks

• Plotting individual samples: Students accidentally plot sample 1, 2, 3, or 4 instead of the Totals row.
• Non-linear scales: Spacing weeks unevenly (e.g. writing 0, 1, 2, 3, 5, 8 with equal gaps). Intervals must reflect the actual numbers of weeks!
• Ruler join (point-to-point): Connecting points with straight zigzag lines instead of drawing a smooth curve.
• Feathered lines: Sketchy, multiple overlapping lines rather than a single sharp pencil stroke.

✅ Mark Allocation

• Mark 1: Correct readings from student's own graph at 0 and 4 weeks (e.g., 60 and 22).
• Mark 2: Correct method: (value at 4 weeks ÷ 60) × 100 .
• Mark 3: Final answer (e.g. 36.7% or 37%).

🧠 Error-Carried-Forward (ECF)

Examiners award full marks if your calculation is completely correct based on your own line drawn in 03.3.

Warning: If you didn't draw a line, examiners allow values between 60 and 24 (midpoint estimate) for working marks.

✅ Acceptable Reasons (Choose any two)

• Biotic Factors:
  • Predation / eaten by predators (e.g. fish, water beetles, birds).
  • Lack of food / starvation (or competition for food).
  • Infection / disease / named pathogen (e.g. fungus or virus).
• Abiotic Factors:
  • Low dissolved oxygen concentration in pond water (e.g. due to eutrophication).
  • Change in temperature (water got too hot/cold).
  • Change in pH of the water (water became too acidic/alkaline).
  • Pond dried out / reduced water volume (lack of space).
  • Presence of toxic chemicals / named pollutant (e.g. pesticide runoff, fertiliser, sewage).

❌ Common Rejections (Examiner Traps)

• "Competition" alone: You must state what they are competing for (e.g. "competition for food" is accepted; just "competition" is rejected).
• "Pollution" or "Waste" alone: Too vague! Specify toxic chemicals, fertiliser runoff, or sewage.
• "Metamorphosed into frogs": The question specifically states the tadpoles died, so metamorphosis is not accepted.