Worked Solutions
Module 4: Ecosystem Dynamics — Worked Solutions (Preliminary Biology)
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Worked examples for Preliminary Biology Module 4 — Ecosystem Dynamics. Each shows where the marks are awarded, the key idea, and a full model answer in your choice of tutor — Stella, Ella or Cassie.
How to use these
Attempt each question under exam conditions first, then check your response against the model answers. Use the tutor tabs to read the solution in the style that suits you: Stella is direct and to the point, Ella is warm and explains the why, and Cassie is concise and uses bullet points.
Example 1 — Estimating population with quadrats
Question
A student uses ten 1 m² quadrats placed randomly in a 500 m² field to estimate the number of dandelion plants. The total count across the ten quadrats is 85 plants. Estimate the total population of dandelions in the field, and explain why the quadrats were placed randomly. (4 marks)
Solution
First find the mean density per quadrat: $\frac{85}{10} = 8.5$ plants per m².
Each quadrat is 1 m², so density is 8.5 plants per m². The field is 500 m², so multiply: $8.5 \times 500 = 4250$ plants.
Estimated population ≈ 4250 dandelions.
Random placement removes sampling bias — it stops the student from unconsciously choosing dense or sparse patches, so the sample is more representative of the whole field and the estimate is more reliable. State the figure and justify randomness.
Let's work out the estimate step by step. Across ten 1 m² quadrats the student counted 85 plants, so the average per quadrat is $\frac{85}{10} = 8.5$ plants. Since each quadrat covers 1 m², that's a density of 8.5 plants per square metre.
To scale up to the whole field, we multiply the density by the total area: $8.5 \times 500 = 4250$. So we estimate about 4250 dandelions in the field.
Now, why place the quadrats randomly? If the student chose where to put them, they might lean towards patches that look interesting — clumps of dandelions, or bare ground — and bias the result. Random placement means every part of the field has an equal chance of being sampled, so the sample better represents the whole field and the estimate is more reliable.
- Mean density = $\frac{85}{10}$ = 8.5 plants per m²
- Total area = 500 m²
- Estimated population = $8.5 \times 500$ = 4250 plants
Random placement:
- Avoids sampling bias (no unconscious choosing of dense/sparse areas)
- Makes the sample representative of the whole field → more reliable estimate
Where the marks go
- 1 mark: Calculates the mean density (8.5 plants per m²)
- 1 mark: Multiplies density by total field area (8.5 × 500)
- 1 mark: States a correct estimate of approximately 4250 plants
- 1 mark: Explains random placement avoids bias / gives a representative, more reliable sample
Key idea
Estimate a population by scaling the mean quadrat density up to the total area; random sampling avoids bias so the sample represents the whole habitat.
Example 2 — Energy flow through trophic levels
Question
Explain why the amount of energy available decreases at each successive trophic level in a food chain, and why food chains rarely have more than four or five levels. (4 marks)
Solution
Energy enters the food chain when producers capture light in photosynthesis. It then flows to consumers as they feed.
At each trophic level, only about 10% of the energy is passed on to the next level. The rest is lost — used in respiration (released as heat), lost in movement and metabolic processes, and contained in parts that are not eaten or are excreted (egested) as waste.
Because only a small fraction transfers each time, the energy available falls sharply at each successive level.
After four or five levels, so little energy remains that it cannot support a viable population of another consumer — there isn't enough energy to sustain a higher level. That limits the length of food chains. The key term is energy lost at each transfer.
Energy first enters a food chain through producers, which capture sunlight by photosynthesis. From there it passes along the chain as one organism eats another.
But the transfer is far from complete. At each step, only roughly 10% of the energy makes it into the next level. Where does the rest go? A lot is used in respiration and released as heat, some is used for movement and other life processes, and some is simply never consumed or is lost as waste (egestion and excretion).
Because around 90% is lost at every transfer, the energy available shrinks dramatically as you move up the chain.
This is exactly why food chains are short — usually four or five levels at most. By the time you reach the top, there's so little energy left that it can't support another level of consumers. There simply isn't enough energy to go around, so the chain ends.
- Energy enters via producers (photosynthesis), then flows to consumers
- Only ~10% passes to the next trophic level
- Energy lost at each transfer: respiration (heat), movement/metabolism, uneaten parts and waste (egestion/excretion)
- So available energy decreases sharply at each successive level
- After 4–5 levels too little energy remains to support a further population → limits chain length
Where the marks go
- 1 mark: States that only a small proportion (about 10%) of energy is transferred to the next trophic level
- 1 mark: Identifies how energy is lost (respiration/heat, movement, uneaten parts, waste)
- 1 mark: Links the loss at each transfer to decreasing energy at successive levels
- 1 mark: Explains that after 4–5 levels insufficient energy remains to support a further trophic level
Key idea
Roughly 90% of energy is lost at each trophic transfer (mainly as heat from respiration), so energy decreases up the chain and runs out after four or five levels.