AQA GCSE Biology Biology Paper 2 (Foundation), June 2024: Question 11

11 marks · Standard Demand difficulty · Short Answer

Analyze an investigation into tadpole population changes over time, including improving experimental design, calculating missing data, plotting a graph, calculating percentage remaining, and explaining population decline.

Practise this question

Question

Question 11 presents a sampling method used to investigate the number of tadpoles in a pond over 8 weeks using a 10 dm³ bucket. Part 11.1 asks for one improvement to the method (1 mark). Table 1 provides sample counts for four samples at 0, 1, 2, 3, 5, and 8 weeks, with sample 4 at 3 weeks marked as X and totals given. Part 11.2 asks to calculate X (1 mark). Part 11.3 provides a grid for Figure 15 with y-axis 'Total number of tadpoles' from 0 to 60, asking students to label the x-axis, scale it, plot the totals, and draw a line of best fit (4 marks). Part 11.4 asks to calculate the percentage of tadpoles present at 4 weeks compared with 0 weeks using Figure 15 (3 marks). Part 11.5 asks for two reasons why the tadpoles died after 4 weeks (2 marks).
Question text

11 Frogs are animals that lay their eggs in water. The eggs hatch as tadpoles.

Students investigated the number of tadpoles in a pond for 8 weeks.

This is the method used.

1. Collect 10 dm3 of pond water in a bucket.

2. Count the number of tadpoles collected.

3. Put the tadpoles back into the pond.

4. Repeat steps 1 to 3 another three times in different parts of the pond.

5. Repeat steps 1 to 4 at intervals for 8 weeks.

11.1 Suggest one improvement to the method.

[1 mark]

Table 1 shows the results.

Table 1

Number of tadpoles in each sample

Sample

number

0 weeks 1 week 2 weeks 3 weeks 5 weeks 8 weeks

1 11 17 8 9 5 0

2 15 11 12 7 0 5

3 23 16 14 10 7 3

4 11 14 16 X 4 4

Totals 60 58 50 32 16 12

11.2 Value X is the number of tadpoles in sample 4, at 3 weeks.

Calculate value X.

[1 mark]

45 Value X =

11.3 Complete Figure 15 to show how the total number of tadpoles changed over the

8 weeks.

[4 marks]

You should:

• label the x-axis

• use a suitable scale for the x-axis

• plot the data for the total numbers of tadpoles from Table 1

• draw a line of best fit.

Figure 15

11.4 After 0 weeks, no more tadpoles hatched in the pond.

Calculate the percentage of the tadpoles that would still be found in the pond at

4 weeks compared with 0 weeks.

Use information from Figure 15.

[3 marks]

Percentage of tadpoles found at 4 weeks =46 %

11.5 After 4 weeks many of the tadpoles had died.

Suggest two reasons why the tadpoles died.

[2 marks]

Mark scheme

Show the mark scheme Mark scheme for Question 11: 11.1 awards 1 mark for improvements such as collecting more samples, collecting more frequently, using a larger bucket, avoiding double counting by not returning tadpoles until finished, sampling at the same time of day, randomising positions, or using a standardised net. 11.2 awards 1 mark for 6. 11.3 awards 1 mark for correct linear scale and x-axis labelled 'weeks', 2 marks for all points plotted correctly (1 mark for 4 or 5), and 1 mark for a curved line of best fit. 11.4 awards 1 mark for reading values at 0 and 4 weeks (e.g. 60 and 22), 1 mark for correct calculation setup (e.g. 22/60 × 100), and 1 mark for 36.7% (or 37%). 11.5 awards 2 marks for any two reasons from: disease/pathogens, predation, lack of food/competition, low oxygen, temperature change, pH change, pond drying out, or toxic chemicals.

Question 11

AO /

Question Answers Extra information Mark

Spec. Ref.

11.1 any one from: 1 AO3

4.7.2.1

• collect more samples each RPA9

time

• collect samples more allow suggested time interval

frequently

• use a bigger bucket / sample

• do not return tadpoles until allow a method to avoid double

after the fourth sample counting tadpoles

• sample at the same time of

day

• randomise collecting positions

• collect at range of depths

• standardised sweeps with a

net instead of a bucket

AO /

Spec. Ref.

11.2 6 if no answer on line, allow 1 AO2

answer in Table 1 4.7.2.1

– – –

RPA9

AO /

Spec. Ref.

11.3 correct linear scale and axis scale must use at least half 1 AO2

labelled weeks available space 4.7.2.1

RPA9

all points plotted correctly allow a tolerance of ½ small 2

square

allow 4 or 5 correct plots for 1

mark

curved line of best fit ignore line drawn point to point 1

AO /

Spec. Ref.

11.4 correct value at 0 and 4 weeks allow a tolerance of ½ small 1 AO2

from line on student’s graph, square 4.7.2.1

eg 60 and 22

correct calculation eg

× 100 1

36.7 allow 37 or 36.6... 1

allow correct calculation using

values from the student’s graph

if no line drawn on Figure allow

a calculation based on values of

60 and 24 for up to full marks

if line drawn on Figure but data

from table used (60 and 24) only–– –

mp2 and mp3 can be awarded

AO /

Spec. Ref.

11.5 any two from: 2 AO2

4.7.1.2

• disease / (named) pathogens 4.7.1.3

• being eaten or predators

• lack of food allow competition for food

ignore competition unqualified

• low oxygen (concentration allow eutrophication

in water)

• change in temperature

• change in pH

• (some of the) pond dried out allow lack of space 29

• toxic chemical allow named example such as

sewage / fertiliser

ignore pollution

ignore waste

Total Question 11 11

How to answer it

Sampling Pond Ecosystems: Tadpole Survival

📋 What this question tests

This question evaluates practical ecology sampling techniques (RPA 9), manipulating tabular data, plotting accurate curved lines of best fit, reading intermediate values from graphs to calculate percentages, and applying knowledge of biotic and abiotic factors that affect population size.

Question 11.1: Evaluating Experimental Method

Suggest one improvement to the method [1 mark]

✅ Acceptable Answers (Any one)

  • Prevent double counting: Do not return tadpoles to the pond until all 4 samples are taken.
  • Increase reliability/sample size: Take more samples each time (e.g. 10 samples instead of 4) or use a larger volume/bucket.
  • Improve sampling method: Use standardised sweeps with a pond net instead of a bucket.
  • Control variables: Sample at the exact same time of day or at a consistent range of water depths.
  • Sample timing: Collect samples more frequently (e.g. every week rather than skipping weeks 4, 6, 7).

🧠 Exam Technique & Examiner Insight

Look at the flaw in step 3: "Put the tadpoles back into the pond... repeat another three times." If you return them immediately, you risk catching the exact same tadpoles in sample 2, 3, or 4!

Examiner tip: Vague answers like "be more careful" or "repeat it more" without qualification gain 0 marks. Always specify how to repeat (e.g. "take more samples per visit").

Question 11.2: Calculating a Missing Table Value

Calculate value X for sample 4 at 3 weeks [1 mark]

📐 Step-by-Step Calculation

The bottom row gives the total of all 4 samples for each time point:

Total at 3 weeks = Sample 1 + Sample 2 + Sample 3 + Sample 4

  1. Sum of known samples: 9 + 7 + 10 = 26
  2. Subtract from Total: 32 − 26 = 6

Value X = 6

❌ Common Errors

  • Adding across the row instead of column totals.
  • Simple arithmetic slip under exam pressure (e.g. writing 8 or 4).
[1 mark] awarded for the single correct value 6.

Question 11.3: Graph Plotting

Complete Figure 15 to show the total number of tadpoles over 8 weeks [4 marks]

💡 What Your Graph Must Include

  • x-axis label: Must state Time in weeks or weeks [1 mark].
  • Scale: Regular linear scale starting from 0, going up to at least 8, occupying more than 50% of the horizontal grid.
  • Points to plot: (0, 60), (1, 58), (2, 50), (3, 32), (5, 16), (8, 12).
    • All 6 plotted correctly within ±½ small square [2 marks]
    • 4 or 5 correct [1 mark]
  • Line of best fit: A smooth, single curved line passing close to the points [1 mark].

❌ Graph Traps that Lose Marks

  • Plotting individual samples: The question asks for the total number, not sample 1 or sample 4.
  • Uneven intervals on the x-axis: Notice that weeks jump from 3 to 5 to 8! You cannot just write 0, 1, 2, 3, 5, 8 with equal spacing. The scale must be linear (e.g., 1, 2, 3, 4, 5, 6, 7, 8).
  • Joining with a ruler (dot-to-dot): Biological population declines follow a smooth decay curve. Do not draw zig-zag straight lines between points.

Question 11.4: Percentage Calculation from Graph

Calculate the percentage of tadpoles remaining at 4 weeks compared with 0 weeks [3 marks]

📐 Step-by-Step Calculation

  1. Step 1: Read values from your line:
    At 0 weeks = 60
    At 4 weeks = 22 (read from line of best fit; tolerance ±½ small square, typically 21–23).
  2. Step 2: Formulate the percentage:
    Percentage = (Value at 4 weeks ÷ Value at 0 weeks) × 100
    = (22 ÷ 60) × 100
  3. Step 3: Calculate final percentage:
    = 36.666...% ≈ 36.7% (or 37%)

🧠 Mark Breakdown & Examiner Insight

  • Mark 1: Correct value extracted from line at 4 weeks (around 22).
  • Mark 2: Correct fraction set up: (4-week value ÷ 60) × 100.
  • Mark 3: Correct answer: 36.7% (or 37%).
Critical Note: If you did not draw a line in 11.3, examiners accept taking an average or estimate between 3 and 5 weeks (e.g., 24), giving (24 ÷ 60) × 100 = 40%. However, if you drew a line, you must read the value from your line!

Question 11.5: Biotic and Abiotic Mortality Factors

Suggest two reasons why the tadpoles died [2 marks]

✅ Acceptable Reasons (Any two)

  • Predation: Eaten by predators (e.g. fish, birds, dragonfly larvae).
  • Food shortage: Lack of food / algae, or intense competition for food.
  • Pathogens / Disease: Bacterial, fungal, or viral infections.
  • Oxygen depletion: Low oxygen levels in the water (due to eutrophication or high temperatures).
  • Abiotic changes: Unfavourable water temperature changes or extreme pH shifts.
  • Habitat loss: Pond dried out / reduced water volume causing overcrowding.
  • Pollution / Toxins: Toxic runoff entering the pond (e.g. pesticides, sewage, or excess fertiliser).

❌ Common Rejections

  • "Competition" on its own: Insufficient! You must state what they compete for (e.g. competition for food).
  • "Pollution" or "Waste" on its own: Too vague. Must specify toxic chemicals, fertiliser runoff, or sewage.
  • "They grew into frogs": The question specifically stated "many of the tadpoles had died", not that they metamorphosed.
[2 marks available: 1 mark for each distinct biological reason.]

Topics

Biology · Required Practicals · B7: Ecology · Required Practicals

Question and mark scheme from the AQA GCSE Biology examination, Biology Paper 2 (Foundation), June 2024. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.