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Question
In the food chain grass → rabbit → fox → golden eagle, identify the trophic level of each organism. (4 marks)
Solution
Producer is always level 1.
Each consumer up = next level.
Answer
Grass — Level 1 (producer); Rabbit — Level 2 (primary consumer); Fox — Level 3 (secondary consumer); Golden eagle — Level 4 (tertiary consumer/apex predator).
Question
Explain why decomposers are essential to ecosystems, even though they aren't usually given a trophic level number. (3 marks)
Solution
State that decomposers break down dead organisms and animal waste.
Explain they release nutrients (e.g. nitrogen, phosphorus) back into the soil.
Explain that producers then re-use these nutrients to grow — without decomposers, nutrients would stay locked in dead matter and plants couldn't grow.
Answer
Decomposers (e.g. bacteria, fungi) break down dead organisms and waste (1). This releases nutrients such as nitrogen and phosphorus back into the soil (1). Producers re-absorb these nutrients to grow, so the ecosystem can keep cycling materials (1).
Question
A meadow contains: grass 5000 g/m² (level 1), grasshoppers 250 g/m² (level 2), small birds 25 g/m² (level 3), kestrels 2 g/m² (level 4). Describe how to construct a pyramid of biomass for this data. (4 marks)
Solution
Choose a sensible scale, e.g. 1 cm = 500 g/m².
Calculate bar widths: grass = 10 cm, grasshoppers = 0.5 cm, small birds = 0.05 cm, kestrels = 0.004 cm (label the value).
Draw bars centred on a vertical axis, producer at bottom, all bars the same height.
Label each level (1-4), label organism names, and add the scale.
Answer
Use scale 1 cm = 500 g/m². Bars: grass 10 cm wide, grasshoppers 0.5 cm, small birds 0.05 cm, kestrels approx 0.004 cm (write value next to it). Stack vertically with grass at the bottom, kestrels on top, all bars centred on a midline. Label each trophic level and the scale.
Question
Explain why a pyramid of biomass usually narrows as you go up the trophic levels. (3 marks)
Solution
State that biomass is lost between trophic levels.
Give a specific reason — not all biomass is eaten OR some lost in faeces OR used in respiration OR excreted.
Give a second reason for full marks.
Answer
Biomass is lost at each trophic level (1). Some biomass is not eaten or is lost in faeces (1). Some is used in respiration for movement, body heat and life processes — releasing CO₂ and heat to the environment (1).
Question
In a food chain, producers contain 25,000 g/m² of biomass. Primary consumers contain 2,000 g/m². Calculate the percentage of biomass transferred between these two levels. (2 marks)
Solution
Use the formula: efficiency = (upper / lower) × 100.
Compute.
Answer
8%.
Question
Describe four ways in which biomass is lost between trophic levels in a food chain. (4 marks)
Solution
Recall AQA's four loss categories.
Match each to a specific reason — uneaten parts; faeces; respiration heat; excretion (urine).
Answer
(1) Some material is not eaten (e.g. bones, fur, roots) — passes to decomposers. (2) Some is egested as faeces (undigested material). (3) Energy is used in respiration for movement and body heat, leaving as heat to surroundings. (4) Some biomass is excreted as urine (urea) or CO₂.
Efficiency of biomass transfer
When to use
Given biomass at two consecutive trophic levels, calculate what % transfers. Typical answer is around 10%.
Example
Grass 1000 g/m², rabbits 80 g/m². Efficiency = 80/1000 × 100 = 8%.
Biomass transfer efficiency
When to use
When asked to calculate the percentage of biomass passed between two trophic levels.
Example
Producer 5000 g/m², consumer 350 g/m². Efficiency = 350/5000 × 100 = 7%.
The position an organism occupies in a food chain, numbered starting from the producer at level 1.
An organism (usually a plant or alga) that makes its own food by photosynthesis. The first trophic level.
A herbivore that eats producers — the second trophic level.
An organism (usually bacteria or fungus) that breaks down dead organisms and waste, releasing nutrients back into the environment.
A carnivore at the top of a food chain with no natural predators — usually trophic level 4 or higher.
The mass of living material — usually measured as dry mass to remove variation due to water content.
A scaled bar diagram showing the biomass at each trophic level in a food chain, with the producer at the bottom.
Mass of biological material after all water has been removed by drying at low temperature.
The percentage of biomass from one trophic level that becomes biomass at the next level — typically around 10%.
Removal of undigested food from the body as faeces — distinct from excretion (which removes metabolic waste).
Removal of metabolic waste products from the body — e.g. urea in urine, CO₂ from respiration.
The biomass and energy lost when absorbed glucose is broken down to release energy for movement, body heat and life processes — energy eventually escapes as heat.
Mistake
Drawing food chain arrows from eater to food.
Why it happens
Logical but wrong — arrows show energy flow.
How to avoid it
Arrows always point TO the organism that EATS — same direction as energy moves.
Mistake
Calling the herbivore 'level 1' because it's first in alphabetical order.
Why it happens
Reading the chain wrong.
How to avoid it
Producer = LEVEL 1. Always start counting from the plant/algae.
Mistake
Calling decomposers 'level 5'.
Why it happens
Wanting to give them a level.
How to avoid it
Decomposers work at EVERY level — don't have one number. Describe their role separately.
Mistake
Drawing the pyramid not to scale.
Why it happens
Trying to make all levels visible.
How to avoid it
Bars MUST be drawn to scale. If a level is too small to see, label its value but don't enlarge it.
Mistake
Putting the predator at the bottom.
Why it happens
Confusion about food chain direction.
How to avoid it
Producer (level 1) is ALWAYS at the bottom — the wide base. Predators at the narrow top.
Mistake
Forgetting to write the scale.
Why it happens
Rushing.
How to avoid it
Always write 'Scale: 1 cm = X g/m²' under the pyramid — examiners need this for accuracy marks.
Mistake
Saying 'energy is destroyed' between trophic levels.
Why it happens
Loose language.
How to avoid it
Energy is NEVER destroyed — it's TRANSFERRED to heat in the surroundings. Use 'lost from the food chain', not 'destroyed'.
Mistake
Mixing up egestion and excretion.
Why it happens
Both involve removal.
How to avoid it
Egestion = faeces (undigested food, never absorbed). Excretion = urine, CO₂ (metabolic waste from inside cells).
Mistake
Using exactly 10% in every calculation.
Why it happens
Memorising 'the 10% rule'.
How to avoid it
10% is an average. For calculations, USE THE NUMBERS GIVEN — don't substitute 10%.