AQA GCSE Biology Foundation (8461), Foundation tier · Ecology › Trophic levels in an ecosystem
Practise Pyramids of biomass. 6 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
A pyramid of biomass shows the relative amount of biomass at each trophic level of a food chain. You need to draw one accurately to scale from data and to describe and interpret its shape. This subtopic is in GCSE Biology only, not Combined Science.
Key facts
Biomass = mass of living material (e.g. in g/m²)
Trophic level 1 (producers) at the bottom
Bar width proportional to biomass; same scale for every bar
Bars equal in height, centred and labelled
Bars get smaller going up the pyramid
Bar width = biomass ÷ biomass per cm of the scale
Notes
What a pyramid of biomass shows
Biomass is the mass of living material. In a pyramid it is usually given as mass per unit area, e.g. g/m².
A pyramid of biomass is a stack of bars, one for each trophic level, showing how much biomass each level of a food chain contains compared with the others.
Trophic level 1 (producers) is at the bottom. The other levels are stacked on top, in order.
The bars get smaller going up, because only a small fraction of the biomass at each level is passed on to the next level.
The shape is a pyramid even when the numbers of organisms are not: one large tree has a far greater biomass than all the insects feeding on it.
Drawing an accurate pyramid
Use a ruler and draw one horizontal bar for each trophic level.
Choose a scale so that the largest bar fits the space, e.g. 1 cm = 100 g/m². Use the same scale for every bar.
The width of each bar is proportional to its biomass. Keep all the bars the same height.
Centre each bar above the one below it.
Label each bar with the organism or trophic level.
The worked example drawn to scale. The frog bar is tiny, and that is correct.
A very narrow top bar is correct, not a mistake. Draw it anyway.
How to answer each type of question
Describe and explain the shape of a pyramid of biomass
2 marksGrade 5
Describe: biomass decreases at each trophic level going up.
Explain: biomass is lost at each level (e.g. in respiration and waste), so less is passed on to the next level.
Example. Explain why the bar for the secondary consumers in a pyramid of biomass is smaller than the bar for the primary consumers.
Show the model answerHide the model answer
Not all the biomass of the primary consumers is passed on to the secondary consumers (1). Much of it is lost, e.g. as carbon dioxide and water from respiration, in faeces and urine, or in parts that are not eaten (1).
Don’t lose marks
Drawing bars that are not to scale, or using a different scale for different bars.
Putting the producers at the top, or the bars in the wrong order.
Leaving bars unlabelled or off-centre.
Changing the height of the bars to show biomass. Only the width changes.
More tips
Memory tricks
Producers are always at the bottom: plants prop up the pyramid.
Check your drawing: measure a bar, multiply by the scale, and you should get back the data value.
With the same scale, the ratio of two bar widths equals the ratio of their biomasses, so you can divide the widths directly.
Exam technique
Use a sharp pencil and a ruler; accuracy is usually worth a mark.
Write your scale next to the pyramid so the examiner can check your widths.
If a bar is tiny, still draw it as a thin bar rather than leaving it out.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
Define biomassThe mass of living material, often given per unit area, e.g. in g/m².
Grade 4
Identify the trophic levels on a pyramidTrophic level 1 (the producers) is always the bottom bar.
Grade 5
Describe the shape of a pyramid of biomassEach bar is smaller than the one below, because biomass decreases at each trophic level.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Complete the sentences. Choose answers from the list. 1 · 3 · biomass · number · water A pyramid of biomass shows the relative amount of ....................... at each trophic level of a food chain. Trophic level ....................... is always at the bottom of the pyramid.
biomass; 1
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
{fig} shows a pyramid of biomass for a food chain.
[object Object]
(a) Which organism in {fig} is the producer?[1]
(b) What does the width of each bar in a pyramid of biomass show?[1]
(c) Why is each bar smaller than the bar below it? Tick (✓) one box.[1]
Biomass is lost between trophic levels
Organisms at higher trophic levels are smaller
There are more predators than prey
Producers do not photosynthesise
(d) At which trophic level are the grass snakes?[1]
Show the answer and mark scheme
(a)Answer: Grass
grass
(b)
the (relative) biomass / mass of living material at that trophic level
(c)Answer: Biomass is lost between trophic levels
(d)Answer: Level 4 (tertiary consumers)
4 / tertiary consumers
Question 2Medium6 marks
Students want to construct a pyramid of biomass for this food chain in a meadow: grass → grasshoppers → spiders
Describe how the students could collect and process the data needed to construct an accurate pyramid of biomass for this food chain, and how they should draw the pyramid.[6]
Show the answer and mark scheme
place several quadrats (e.g. 1 m²) at random positions in the meadow, so the sample is representative
collect the grasshoppers and spiders from the area of each quadrat, e.g. with a sweep net or pooter, before cutting all the grass in the quadrat at ground level
sort the organisms into the three trophic levels
dry each sample in an oven at a low temperature (e.g. about 80 °C) and reweigh it until its mass stops changing (constant mass), to find the dry mass
dry mass is used because the amount of water in organisms varies
calculate the mean dry mass per m² for each trophic level
draw a bar for each trophic level, to scale (bar width proportional to the biomass), with the producer (grass) at the bottom and the spiders at the top
only small samples should be taken, because the organisms are killed when they are dried
Marked with levels of response: the full level descriptors are in the app.