AQA GCSE Combined Science: Trilogy Biology Paper 2 (Higher), 2023: Question 3

✅ Correct answer

• Lay a line transect from the edge of the river at right angles to the river.
• Place a quadrat at set intervals along the transect.
• At each quadrat, count the number of different plant species.
• Repeat with 2 more transects at different places and compare results / calculate a mean.

💡 Key knowledge

• A transect is used to study changes across an area.
• A transect should go perpendicular (at right angles) to the river so distance from the river changes fairly.
• A quadrat is used to sample plants in a small area.
• The question asks for number of different species, not total number of plants.

🧠 Exam technique

For full marks, your method must include:

• Where the transect goes
• How quadrats are used
• What is counted
• Repeats for reliability

Best exam wording: at regular intervals , at right angles to the river , count different species .

❌ Common errors

• Saying the transect just goes “from the river into the field” without saying at right angles.
• Placing quadrats at random intervals. The mark scheme wants intervals along the transect.
• Counting the number of plants instead of the number of different species.
• Forgetting repeats. Repeats help make the results more reliable.

✅ Correct answer

The mean value students can be most certain about is 5 metres from the river.

💡 Key knowledge

• The graph shows maximum, mean and minimum.
• You are most certain about a mean where the spread/range is smallest.
• At 5 m, the difference between maximum and minimum is the smallest, so results are most consistent there.

🧠 Exam technique

When asked what a graph shows, check the error bars or spread:

• Small spread = more consistency
• Large spread = less certainty

This is not asking for the highest mean. It is asking where the mean is most certain.

❌ Common errors

• Choosing 6 m because it has a high mean. High mean does not mean high certainty.
• Saying fewer species are always recorded nearer the river. The graph does not prove “always”.
• Calling the 6 m result anomalous when it fits the general pattern.

✅ Correct answer

Moisture levels in the soil

💡 Key knowledge

• Abiotic means non-living factors.
• Examples include light intensity, temperature, soil pH, mineral content, and soil moisture.
• Near a river, soil is often wetter, so moisture is a sensible factor affecting which plants grow there.

🧠 Exam technique

If you see a multiple-choice question on abiotic factors, quickly sort options into:

• Living = biotic
• Non-living = abiotic

Only one option here is clearly a non-living environmental condition.

❌ Common errors

• Microorganisms near the roots are living, so biotic.
• Primary consumers are living, so biotic.
• Oxygen concentration in the air is abiotic, but it is not the factor identified by the mark scheme for this river question.

✅ What a top-level answer includes

• More cows eat and trample more plants, so there are fewer plants.
• With fewer plants, there is less photosynthesis.
• Less photosynthesis means less CO₂ removed from the atmosphere.
• More cows also release CO₂ by respiration.
• Cows release methane, which is a greenhouse gas.
• This can increase the greenhouse effect / global warming.
• Land may be cleared for grazing, causing deforestation and reduced biodiversity.
• More cow faeces may enter rivers, causing water pollution and harming aquatic organisms.

💡 Key knowledge

• Carbon dioxide is a greenhouse gas.
• Methane is also a greenhouse gas.
• Deforestation can reduce habitats and therefore biodiversity.
• Animal waste entering water can cause pollution and damage ecosystems.

🧠 Exam technique

To reach Level 3 (5–6 marks), do more than list points. You must link cause and effect.

Strong chain style:

more cows → more grazing/trampling → fewer plants → less photosynthesis → more CO₂ in atmosphere → greater global warming

Try to give two or three linked chains rather than lots of separate bullet points.

❌ Common errors

• Only saying “it is bad for the environment” with no explanation.
• Listing methane or CO₂ without saying they are greenhouse gases or what this leads to.
• Not linking cow numbers to a consequence, for example less biodiversity or more global warming.
• Writing only about farming benefits. The question asks for environmental implications.

What gained marks

• Using the exact practical terms transect and quadrat.
• Recognising that certainty in graph data depends on small spread.
• Knowing that soil moisture is an abiotic factor near a river.
• Writing a 6-mark response with clear linked environmental consequences.

Where students lost marks

• Describing random sampling instead of a proper transect method.
• Counting plant numbers or percentage cover instead of different species.
• Choosing graph answers based only on the mean, not the spread.
• Giving unlinked points in the final question instead of explained chains.