A trophic level holds 50\,000 kJ m^-2 and the next holds 5\,000 kJ m^-2. Calculate the percentage energy transfer. [2 marks]

WalesBiologySyllabus dot point

What controls the size of populations and the flow of energy through ecosystems?

Ecosystems, energy flow and nutrient cycles, population growth and the factors that limit it, and succession.

A focused answer to WJEC A-Level Biology Unit 3, covering ecosystems and trophic levels, energy flow and the carbon and nitrogen cycles, population growth and limiting factors, and succession.

Generated by Claude Opus 4.811 min answerUpdated 2026-06-02

Reviewed by: AI editorial process; not yet individually human-reviewed

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What this dot point is asking
Ecosystems and energy flow
Nutrient cycles
Population growth and succession
Examples in context
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What this dot point is asking

WJEC wants you to describe ecosystems and trophic levels, explain energy flow and the carbon and nitrogen cycles, explain population growth and the factors that limit it, and describe succession.

Ecosystems and energy flow

Energy transfer efficiency is calculated as the energy in one trophic level divided by the energy in the level below, times one hundred. Knowing this lets you explain pyramids of energy, which always narrow upwards because so much energy is lost at each step.

Nutrient cycles

The carbon cycle moves carbon between the air, organisms and fossil stores through photosynthesis (removing $\text{CO}_2$ ), respiration and decomposition (returning it), and combustion of fuels (releasing stored carbon rapidly).

Population growth and succession

A population grows through a lag phase, then an exponential phase while resources are plentiful, then levels at the carrying capacity when limiting factors (food, space, predation, disease, competition, accumulation of waste) balance births and deaths. Succession is the directional change in a community over time: a pioneer species colonises bare ground, alters the conditions (adding soil and organic matter), and is replaced by successive communities until a stable, self-sustaining climax community forms.

Calculating energy transfer efficiency

A field receives $1\,000\,000 \text{ kJ m}^{-2}\,\text{yr}^{-1}$ of light. The grass (producer) stores $40\,000 \text{ kJ m}^{-2}\,\text{yr}^{-1}$ ; the rabbits (primary consumers) store $4\,000$ .

step Find the producer efficiency (light captured)

\frac{40\,000}{1\,000\,000} \times 100 = 4\%

Only a few percent of sunlight is fixed because much light misses the chloroplasts, is the wrong wavelength, or is reflected.

step Find the transfer to primary consumers

\frac{4\,000}{40\,000} \times 100 = 10\%

step Interpret

About $10\%$ of the grass energy reaches the rabbits; the rest was lost in respiration, faeces and uneaten or indigestible material. This explains why the pyramid of energy narrows so sharply moving up.

Examples in context

Example 1. Lichen succession on bare rock. Lichens are the pioneer species on bare rock; they trap moisture and, as they die, build a thin soil that mosses and then grasses can colonise, eventually leading to shrubs and woodland. This primary succession is the classic WJEC example of how pioneers change conditions for later communities.

Example 2. Crop rotation with legumes. Farmers plant legumes such as clover, whose root nodules house nitrogen-fixing Rhizobium, to restore soil nitrate naturally. This applies the nitrogen cycle directly, reducing the need for fertiliser and the eutrophication that comes with it.

Try this

Q1. Name the bacteria that convert ammonium ions to nitrate. [1 mark]

Cue. Nitrifying bacteria.

Q2. Explain what is meant by the carrying capacity of a population. [2 marks]

Cue. The maximum population size an environment can support over time, set by limiting factors such as food, space and predation.

Q3. A trophic level holds $50\,000 \text{ kJ m}^{-2}$ and the next holds $5\,000 \text{ kJ m}^{-2}$ . Calculate the percentage energy transfer. [2 marks]

Cue. $\frac{5\,000}{50\,000} \times 100 = 10\%$ .

Exam-style practice questions

Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

WJEC 20184 marksExplain why only a small percentage of energy is transferred between successive trophic levels.

Show worked answer →

Not all of the organism is eaten or digestible, so some energy is lost in faeces and in uneaten parts such as bone or root.

Organisms use energy in respiration, which is released as heat to the surroundings and is not passed on to the next level.

Energy is also lost in excretion (for example in urea), so only the energy stored in new biomass is available to the next level, typically around 10 percent.

Markers reward losses in respiration as heat, in faeces, and in excretion, leaving little for the next level.

WJEC 20224 marksA producer fixes 80000 kJ per square metre per year and the primary consumers store 6400 kJ per square metre per year. Calculate the percentage efficiency of energy transfer, and suggest one reason it is not higher.

Show worked answer →

Percentage efficiency $= \frac{\text{energy in the next level}}{\text{energy in the previous level}} \times 100$ .

Efficiency $= \frac{6400}{80000} \times 100 = 8$ percent.

It is not higher because much of the producer's energy is lost before or during transfer: not all the plant is eaten or digestible (lost in faeces), and the consumers use most of the energy they do absorb in respiration, releasing it as heat, with further loss in excretion.

Markers reward the correct 8 percent and at least one valid energy loss (respiration as heat, faeces or excretion).

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Sources & how we know this

WJEC A-level Biology specification — WJEC (2015)