AQA A-Level Biology Paper 3, June 2025: Question 3

8 marks · Medium difficulty · Short Answer

Use the life cycle and mark-release-recapture data of the tansy beetle (Chrysolina graminis) to identify genera, justify sampling methodology, estimate population size, and suggest control variables.

Practise this question

Question

Question 3 presents information about the tansy beetle (Chrysolina graminis) and two other green beetles: Gastrophysa viridula and Chrysolina herbacea. Figure 2 shows the annual life cycle of C. graminis across months from April through winter. Part 03.1 asks how many different genera of beetles are described. Table 2 provides mark-release-recapture data: 405 marked in first sample, 248 unmarked in second sample, and 124 marked in second sample. Part 03.2 asks for two reasons for sampling in August and one reason for identifying species before marking. Part 03.3 asks to calculate the estimated population size. Part 03.4 asks for two precautions taken each year to obtain comparable data over five years.
Question text

03 Tansy, Tanacetum vulgare, is a plant that grows in dense patches. The tansy beetle,

Chrysolina graminis, feeds on, and completes its life cycle on T. vulgare.

C. graminis is 8–10 mm long and green in colour.

Two other green beetles can also be found in the same habitat.

• The dock beetle, Gastrophysa viridula, which is 4–7 mm long.

• The mint leaf beetle, Chrysolina herbacea, which is 6–10 mm long.

Figure 2 shows the life cycle of C. graminis.

Figure 2

03.1 How many different genera of beetles are there in the information provided?

[1 mark]

Ecologists investigated the population size of C. graminis using the

mark-release-recapture method.

Table 2 shows the ecologists’ results.

Table 2

Number of C. graminis Number of unmarked Number of marked

captured and marked C. graminis captured C. graminis captured

in first sample in the second sample in the second sample

*09* 405 248 124

03.2 The ecologists captured and marked the first sample of beetles during the second

week of August. They identified the species of beetle before marking them.

Use the information provided to suggest:

[3 marks]

two reasons why the first sample of beetles was captured during the second week

of August

one reason why the beetles had to be identified before being marked.

03.3 Use the data provided in Table 2 to estimate the population size of C. graminis.

Show your working.

[2 marks]

Answer = beetles

03.4 The ecologists repeated the investigation over a five–year period to monitor the

population size of C. graminis.

State two precautions they would have taken each year to obtain comparable data.

[2 marks]

Mark scheme

Show the mark scheme Mark scheme for question 3: 03.1 awards 1 mark for '2/two'. 03.2 awards 3 marks maximum (max 2 for capture reasons such as emerged from soil/not in burrows, time before winter burrowing, avoid disturbing mating; max 1 for identification reason such as all three beetles being green/similar size or only investigating C. graminis). 03.3 awards 2 marks for 1215 with intermediate calculation marks for 405 x 372 / 124. 03.4 awards 2 marks for precautions such as same time of year, same habitat/location, same collection method, or ensuring marks do not affect survival.

Question Marking Guidance Mark Comments

If no number is

2/two; 1

present, accept

03.1 (1 x

Gastrophysa and

AO2)

Chrysolina alone

Max 2 marks for MP1-5 and Max 1 mark for

MP6-8

Reasons for capture

1. (So beetles) have emerged (from the soil)

OR

(So beetles) are not still in burrows

OR

(So beetles) are not under the soil/ground;

2. (To allow beetles to) evenly/randomly distribute

(before they burrow underground);

3. To allow time for (re)capture (before they

burrow underground); 3 max

03.2 (3 x

4. To not disturb mating/egg laying (in April); AO3)

5. To ensure no population changes/increase

(due to production of eggs/larvae);

Reason for identification

6. (All three beetles are) same colour/green;

7. (All three beetles are a) similar length; 7.

Accept size

8. Only C. graminis/tansy beetle (population size

being) investigated

OR

Only investigated one/correct species

OR

Did not investigate (the) other/wrong species;

Correct answer of 1215 = 2 marks;;

Evidence of 810 (not calculated total number in 2nd

sample) = 1 mark

OR

Evidence of 608 (divided by unmarked rather than

marked) = 1 mark

OR

150 660 2

Evidence of 124 (correct working but incorrect or

03.3 (2 x

no answer) = 1 mark

AO2)

OR

Evidence of 405 × 372 (correct working but incorrect

or no answer) = 1 mark

OR

Evidence of 405 124 (correct working but incorrect

𝑛𝑛 × 372

or no answer) = 1 mark;

1. Collected samples during the second week of

August

OR

Collected samples at the same time of year;

2. Collected samples from the same

location/area/habitat;

3. Used the same method; 3. Accept any correct

2 max named part of the

03.4 4. Used a different mark (each year); (2 x method used

AO3) being the same

5. Marking does not affect survival/predation/

behaviour/recapture

OR

Mark is not toxic

OR

Mark is not removed (before second capture);

How to answer it

Ecology: Mark-Release-Recapture & Classification of Chrysolina graminis

📋 What this question tests

This question assesses your ability to apply ecological sampling techniques and taxonomic principles to an unfamiliar organism:

  • Binomial nomenclature: Identifying genus from scientific names.
  • Fieldwork timing & life cycles: Interpreting seasonal life-cycle charts to justify sampling windows and sampling validity.
  • Lincoln Index calculation: Calculating estimated population size from mark-release-recapture (MRR) data while avoiding the second-sample sum trap.
  • Standardising ecological investigations: Controlling variables across longitudinal studies to ensure data comparability.

Part 03.1: Beetle Genera Identification

Identifying taxonomic groups from binomial species names [1 Mark]

✅ Correct Answer

2 (or two)

1 Mark: AO2 application. If numerical 2 is absent, examiners credit naming the two genera: Gastrophysa and Chrysolina.

💡 Key Knowledge

In binomial nomenclature, the scientific name consists of two parts:

  • First name (capitalised) = Genus: Chrysolina and Gastrophysa.
  • Second name (lowercase) = Species: graminis, herbacea, and viridula.
  • Tanacetum vulgare is a plant (tansy), not a beetle!

❌ Common Errors

  • Counting 3 beetle species instead of genera (confusing genus with species).
  • Counting 4 genera by incorrectly including the host plant Tanacetum.

🧠 Exam Technique

Highlight the word beetles in the prompt. Cross out plant names immediately so they do not contaminate your count. Notice that two of the beetles share the same genus name: Chrysolina graminis and Chrysolina herbacea.

Part 03.2: Justifying Capture Timing & Species Identification

Applying life-cycle data to sampling design [3 Marks]

✅ Correct Answers (Max 3 marks)

Two reasons for capture in second week of August (Max 2 marks):

  • Adult beetles have emerged from the soil / are not burrowed underground.
  • Allows sufficient time for beetles to disperse / evenly distribute before they burrow in September.
  • Allows time for recapture before beetles burrow underground for winter.
  • Avoids disturbing mating or egg-laying (which occurs in April).
  • Ensures population size does not increase due to births/hatching of larvae.

One reason for identification before marking (Max 1 mark):

  • All three beetle species are green / the same colour.
  • All three beetle species are of similar length / size (overlapping ranges 4–10 mm).
  • Only C. graminis was being investigated / to avoid marking incorrect species.

🧠 Examiner Commentary

This is a split-mark scheme:

  • Maximum 2 marks for explaining the August timing (MP1–5).
  • Maximum 1 mark for the identification reason (MP6–8).
  • Students who gave three capture-timing reasons could only get 2/3! Always check that you answer both sub-prompts in the question.

💡 Key MRR Assumptions Tested

For the mark-release-recapture formula to be valid:

  • Marked individuals must have enough time to mix randomly back into the population.
  • There must be no significant births or deaths between release and recapture (hence avoiding April–July breeding/larval development).

❌ Common Errors

  • Vague statements such as "the weather is warmer in August" or "they are active". Reference the burrowing/emergence shown in Figure 2.
  • Stating "beetles are easy to see" without referencing morphological overlap (shared green colour, overlapping lengths) with the other two cohabiting species.

Part 03.3: Population Estimation Calculation

Lincoln-Petersen Index with Recapture Breakdown [2 Marks]

📐 Step-by-Step Population Calculation

Formula: Total Population (N) = (n₁ × n₂) / m₂

Where:

  • n₁ = Number of beetles captured and marked in the 1st sample = 405
  • n₂ = Total number of beetles captured in the 2nd sample = Unmarked + Marked
  • m₂ = Number of marked beetles recaptured in the 2nd sample = 124

Step 1: Calculate total captured in sample 2 (n₂)

n₂ = 248 (unmarked) + 124 (marked) = 372

Step 2: Substitute into Lincoln Index

N = (405 × 372) / 124

N = 150 660 / 124

Step 3: Solve

N = 1215 beetles

2 Marks: Correct final answer of 1215.
1 Mark (Method): Evidence of (405 × 372) / 124 or 150 660 / 124 if arithmetic fails, OR common trap answers 810 or 608.

❌ Classic Calculation Traps

  • Trap 1 (Yields 810): Forgetting that Sample 2 contains both unmarked and marked individuals. Using only 248 as n₂ gives: (405 × 248) / 124 = 810 (Awards 1 method mark only).
  • Trap 2 (Yields 608): Dividing by unmarked beetles instead of marked: (405 × 372) / 248 ≈ 608 (Awards 1 method mark only).

🧠 Exam Tip

Always inspect the headers in Table 2 carefully: "Number of unmarked" vs "Number of marked". Add them together to find the true size of the second sample!

Part 03.4: Ensuring Comparable Data Over 5 Years

Standardising experimental variables in longitudinal sampling [2 Marks]

✅ Correct Answers (Any two for 2 Marks)

  • Collect samples at the same time of year (or explicitly during the second week of August).
  • Collect samples from the same location / habitat / area.
  • Use the same method of sampling / capture (e.g. sweep netting or hand collection).
  • Use a different mark each year (so previous years' survivors are not counted as sample 1 captures).
  • Ensure the mark does not affect survival / predation / behaviour / recapture probability (e.g. non-toxic, non-conspicuous to predators, waterproof/not easily rubbed off).

🧠 Examiner Commentary

Questions asking for "comparable data" are asking about control variables. High-scoring responses state specific factors to keep identical:

  • "Same time of year" is critical because beetle populations fluctuate massively across the life cycle (underground vs active adults).
  • Using a different colour or mark each year is an excellent A-Level point that shows a true understanding of multi-year mark-release studies.

❌ Common Errors

  • Too vague: "Do it in August" (lacks the control aspect: specify "the same week/time in August each year").
  • Stating sample size: "Capture the same number of beetles each year" is incorrect—population size varies naturally, so you cannot predetermine the number captured!
  • Writing "fair test" or "keep all conditions the same" without specifying at least one actual ecological variable (method, location, or timing).

Topics

Biology · Practical skills · 3.4 Genetic information, variation and relationships between organisms · 3.7 Genetics, populations, evolution and ecosystems (A-level only) · Experimental design · Data analysis

Question and mark scheme from the AQA A-Level Biology examination, Paper 3, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.