AQA GCSE Combined Science: Trilogy Biology Paper 2 (Higher), 2019: Question 5

Part 05.1

Choose two improvements that make the sampling more valid.

Part 05.2

Estimate the population of daisies using the mean number per quadrat and the area of the field.

Part 05.3

Explain why daisies are unevenly distributed using biotic and abiotic factors.

✅ Correct answers

• Repeat for another ten quadrats
• Use a random method to place the quadrats

💡 Key knowledge

• Random sampling avoids bias. If you choose where the quadrats go, you might accidentally place them where daisies are unusually common or rare.
• Repeating with more quadrats makes the estimate more reliable because you get a larger sample size.
• Validity improves when the sample is representative of the whole field.

🧠 Exam technique

• Look for methods that reduce bias and improve representativeness.
• “Use the same person to place all the quadrats” does not improve validity.
• “Use a long tape measure” helps measure distance, but it does not improve the sampling method itself.

❌ Common errors

• Ticking do not put any quadrats near trees — this avoids one area, but it is not a general validity improvement and can still be biased.
• Choosing options that sound scientific but do not improve the sampling process.

📐 Calculation steps

1. Find the area of the field.
   The mark scheme allows:
   area = 62 × 164 + (164 × 68 ÷ 2)

   This gives:

   15 744 m²
2. Work out how many 50 cm × 50 cm quadrats fit into the field.
   A 50 cm × 50 cm quadrat has area:
   0.5 m × 0.5 m = 0.25 m²
   So the number of quadrats is:
   15 744 ÷ 0.25 = 62 976
3. Multiply by the mean number of daisies per quadrat.
   Mean = 7.65
   
   62 976 × 7.65 = 481 766.4
4. Round to 2 significant figures and give standard form.
   
   4.8 × 10⁵

✅ Correct answer

Population of daisy plants = 4.8 × 10⁵

Acceptable intermediate answers in the mark scheme include 481 766.4, 481 766, or 480 000.

💡 Key knowledge

• When estimating a total population, use: mean per quadrat × total number of quadrats.
• You must convert units correctly: 50 cm = 0.5 m.
• Standard form should be written as a × 10ⁿ, where 1 ≤ a < 10.

🧠 Exam technique

• Show the working clearly so you can get method marks even if the final rounding is slightly wrong.
• If your area is wrong, you can still score marks for the later multiplication if your method is correct.
• Always include units for area: m².
• For 2 significant figures, 4.81766 × 10⁵ becomes 4.8 × 10⁵.

❌ Common errors

• Forgetting to convert 50 cm × 50 cm into square metres.
• Using 7.65 as the total population instead of the mean per quadrat.
• Forgetting to multiply by the number of quadrats in the whole field.
• Writing the final answer in standard form incorrectly, for example 48 × 10⁴ instead of 4.8 × 10⁵ .

✅ What a full-mark answer could include

Example answer:

Daisies may be fewer near the trees because the trees block light, so there is less light for photosynthesis. Trees also compete with daisies for water and mineral ions from the soil. The area near the sports pitches may have fewer daisies because trampling can kill plants. Near the school buildings there may be less light, so daisies grow less well. Along the roads, pollution or toxins from vehicles may reduce growth. In some parts of the field, the soil may be too wet, too dry, have a poor pH, or lack ions, which would slow growth and lead to fewer daisies there.

💡 Key knowledge

• Biotic factors are caused by living things, for example:
  • competition for light, water, minerals, and space
  • trees shading the ground
  • trampling by people or animals
  • insects, disease, or animals eating plants
• Abiotic factors are non-living, for example:
  • light intensity
  • soil pH
  • water availability / drainage
  • mineral ion content
  • pollution

🧠 Exam technique

• To reach the top level, make several linked points: cause → effect → fewer daisies.
• Use Figure 3: mention trees, sports pitches, buildings, and roads.
• High-mark answers are clear and connected, not just a list of unrelated facts.
• Try to include both biotic and abiotic factors.

❌ Common errors

• Only naming a factor without explaining how it affects daisies.
• Saying “trees are there so fewer daisies” without mentioning light or competition.
• Not using the diagram evidence.
• Mixing up biotic and abiotic factors.

📐 How marks are awarded here

• 1–2 marks: simple points stated, not well linked.
• 3–4 marks: relevant points with some linking.
• 5–6 marks: several accurate reasons, clearly linked into a logical explanation.

Top-level responses

• Used the diagram as evidence.
• Gave multiple causes for uneven distribution.
• Explained each cause with a chain of reasoning.

Where marks were lost

• Random comments with no explanation.
• Ignoring the link between the factor and photosynthesis, water uptake, or growth.
• Weak calculations caused by unit errors and poor rounding.