AQA A-Level Biology Paper 2, June 2025: Question 10

✅ Mark Scheme Points

1. (Large) number/variety of plant species (or plant populations).
2. Creates more/different habitats or niches.
3. Provides a (greater) variety/types/sources of food.

🧠 Exam Technique

Always build the ecological chain of causation for animal biodiversity:

High plant diversity → creates diverse microhabitats/niches + varied food types → supports many different adapted animal species.

❌ Common Errors & Examiner Traps

• Writing "more food": Examiner explicitly notes Ignore 'more food'. It is about variety of food, not sheer volume.
• Vague words for niche: Ignore 'shelter', 'homes', or 'environments'. Use the precise biological terms: niches or habitats .

💡 Key Knowledge

A niche is the role of an organism within its environment, including its interactions with biotic and abiotic factors. Varied plant architectures provide diverse vertical layers (canopy, understorey) creating numerous specialised niches.

✅ Mark Scheme Points (Any Two)

• High/optimum temperature: Increases kinetic energy for faster enzyme-controlled reactions / respiration / ammonification / nitrification.
• Water/moisture: Required for metabolic/hydrolysis reactions or as a medium for enzyme action.
• Oxygen concentration: Required for aerobic respiration in decomposers/nitrifying bacteria.
• Near-neutral/optimum pH: Prevents enzyme denaturation in decomposers.

🧠 Exam Technique

This is a two-part explanation for each factor: Name the abiotic factor + Link it mechanistically to decomposer physiology (enzymes, respiration, or hydrolysis).

❌ Common Errors & Examiner Traps

• "Water for photosynthesis": REJECTED . The question asks about nutrient recycling on the ground (decomposition), not primary production!
• Leaving factor unqualified: Just stating "warmth" or "rain" without linking it to enzyme activity or chemical reactions scores 0.

💡 Key Knowledge

Saprobionts decompose dead organic matter via extracellular digestion: secreting enzymes (hydrolases) directly onto substrates. Both heat (kinetic energy) and water (medium & reactant for hydrolysis) directly control this rate.

✅ Mark Scheme Breakdown (4 Marks)

• Mark 1 (Conversion): Proteins / amino acids / DNA converted into ammonium / ammonia.
• Mark 2 (Bacteria): Carried out by saprobionts (saprotrophic bacteria/fungi).
• Mark 3 (Conversion): Ammonium / ammonia oxidised into nitrite, and then into nitrate.
• Mark 4 (Bacteria): Carried out by nitrifying bacteria.

💡 Key Knowledge: The Two Recycling Steps

1. Ammonification:
   Organic N (proteins/DNA/urea) → NH₄⁺ (by saprobionts)
2. Nitrification:
   NH₄⁺ → NO₂⁻ (Nitrosomonas) → NO₃⁻ (Nitrobacter) (by aerobic nitrifying bacteria)

❌ Major Traps: Irrelevant Nitrogen Stages

• Writing about Nitrogen Fixation or Denitrification: The mark scheme states Ignore nitrogen fixation and denitrification. The stem specifies recycling from dead organic matter.
• Mismatched organisms: Saying "nitrite converted to nitrate by saprobionts" forfeits bacterial marks. Keep conversions strictly matched to organism names.

🧠 Exam Technique

Pair every biological chemical change directly with the specific class of microorganism performing it. State the specific ions: NH₄⁺ , NO₂⁻ , NO₃⁻ .

✅ Mark Scheme Points (Any 4 of 5)

1. Colonisation by pioneer species (or germination of existing seeds/spores).
2. Pioneers / organisms change the abiotic environment (e.g. add organic matter / humus / nutrients / form deeper soil).
3. Environment becomes less hostile / more suitable for new/adapted species (OR less suitable for previous species / previous species outcompeted).
4. Change / increase in biodiversity.
5. Reaches a stable climax community.

🧠 Succession Model: "The Golden Formula"

Always structure succession questions using these 5 sequential steps:

1. Pioneer arrives.
2. Pioneer changes abiotic environment.
3. Environment becomes less hostile for successors / previous outcompeted.
4. Biodiversity increases.
5. Climax community reached.

❌ Common Errors

• Confusing Primary with Secondary: In secondary succession, soil is already present. You do not need lichens breaking down bare rock!
• Vague words: Saying the environment becomes "more hospitable" or "habitable" is ignored. Use the syllabus term: less hostile or more suitable.
• "Genetic diversity": Stating "increase in genetic diversity" will not score for species biodiversity.

💡 Why Secondary Succession is Faster

Because soil, seeds, root fragments, and soil microbes already exist following disturbances like fires, secondary succession bypasses the lengthy soil-formation stage required in primary succession.

✅ Mark Scheme Options (Any Two)

• Growing crops / Monoculture: Continuous harvesting and weeding prevents establishment of native seedlings.
• Animal farming / Livestock grazing: Animals eat young tree shoots/seedlings, trampling ground.
• Ploughing / Tilling: Mechanically destroys roots, seeds, and emergent pioneer plants.
• Application of herbicides / weedkillers: Kills broadleaf colonisers and tree saplings.
• Controlled burning / Clearing fields: Destroys emerging vegetation before trees can establish.
• Erecting farm buildings / structures: Covers soil completely with concrete/barns.

🧠 Exam Technique & Traps

Make sure your suggestions are genuine agricultural actions that continually disrupt pioneer plant survival (deflected succession / plagioclimax).