BiologyCBSE Class 12 · Ecosystem

Ecological Pyramids: Number, Biomass and Energy

Q: What are ecological pyramids?

Graphic representations of the number, biomass or energy at successive trophic levels, with producers at the base and top consumers at the apex.

Q: Why is the pyramid of energy always upright?

Because some energy is lost as heat at every transfer between trophic levels, so each higher level has less energy.

Q: Why is the pyramid of biomass inverted in the sea?

At a given time the biomass of fishes far exceeds that of phytoplankton; phytoplankton have a small standing crop but reproduce and are consumed very quickly.

Q: When is a pyramid of numbers inverted?

When a single large producer, such as a tree, supports a large number of consumers such as insects.

Q: What are the limitations of ecological pyramids?

They ignore species occupying more than one trophic level, assume a simple food chain instead of a web, and leave out decomposers.

Updated 15 October 2026 · Fact-checked

Ecological pyramids compare trophic levels, with producers at the base and top consumers at the apex. Pyramids of number, biomass and energy are usually upright. The pyramid of numbers for one tree and the pyramid of biomass in the sea are inverted, but the pyramid of energy is always upright because energy is lost as heat at each level.

Energy pyramids are always upright, but numbers (one tree, many insects) and sea biomass (few phytoplankton, many fishes) can be inverted.

Illustration drawn for QuestionBuddy.

Understand Ecological Pyramids

What a pyramid shows. A pyramid has a broad base narrowing to the apex. Food or energy relationships between trophic levels take the same shape, expressed as number, biomass or energy. The base is the producers (first trophic level); the apex is the tertiary or top consumers. Three kinds: (a) pyramid of numbers, (b) pyramid of biomass, (c) pyramid of energy.

Rules for building them.

Count all organisms at a trophic level; generalisations from a few individuals are wrong.
A trophic level is a functional level, not a species. A species can occupy more than one level at the same time: a sparrow is a primary consumer when it eats seeds, fruits and peas, and a secondary consumer when it eats insects and worms. Humans too function at several levels.

Upright pyramids (most ecosystems). Producers outnumber and outweigh herbivores, which outnumber and outweigh carnivores; and energy at a lower level is always more than at a higher level. Example: a grassland gives upright pyramids of number, biomass and energy.

Inverted pyramids.

Number, inverted: one big tree supports many insects, which support fewer small birds, which support even fewer larger birds; the bottom bar (1 tree) is narrower than the next (many insects), giving an inverted (or spindle-shaped) pyramid of numbers.
Biomass, inverted (in the sea): the biomass of fishes far exceeds that of phytoplankton at a given time. This is not a paradox: phytoplankton have a tiny standing crop but reproduce and are eaten very fast, so a small standing biomass supports a large consumer biomass over time.

Pyramid of energy: always upright. Each bar shows the energy at each trophic level in a given time or annually per unit area. When energy flows from one trophic level to the next, some energy is always lost as heat, so a higher level can never have more energy than the level below. Hence the energy pyramid can never be inverted.

Limitations of ecological pyramids.
1. They do not account for the same species belonging to two or more trophic levels.
2. They assume a simple food chain, which almost never exists in nature; they do not accommodate a food web.
3. Saprophytes (decomposers) are given no place, though they play a vital role.

Flow chart (deciding the shape): which measure? numbers → can be inverted if a single large producer supports many consumers; biomass → inverted where producers are small and fast-turnover (sea); energy → always upright (heat loss at every transfer).

Draw and label 1: three upright pyramids for a grassland. Checklist: three stacked-bar pyramids side by side, four bars each, widest at the base. Labels: producers (PP), primary consumers (PC), secondary consumers (SC), tertiary consumers (TC); titles: pyramid of numbers, pyramid of biomass, pyramid of energy (with heat-loss arrows on the energy pyramid).

Draw and label 2: inverted pyramids. Checklist: (i) a pyramid of numbers with a narrow base bar (one tree), a wide bar above (insects), and narrowing bars above that (small birds, large birds); (ii) a pyramid of biomass for the sea with a narrow base bar (phytoplankton) and a wider bar above (zooplankton/fish). Labels: tree, insects, small birds, large birds; phytoplankton, zooplankton/fish; 'inverted'.

Ecological Pyramids: key facts

Pyramid order: $base: producers \to P C \to S C \to apex: TC$; PC primary, SC secondary, TC tertiary consumers.
Energy pyramid: $E_{T_{n + 1}} < E_{T_{n}} always (about 10% passes on)$; Never inverted.
Inverted cases: $numbers: tree \to insects; biomass: sea (fish ≫ phytoplankton)$; Biomass inversion is explained by fast turnover of phytoplankton.
Units: $number per area; biomass: g m^{- 2} (dry weight); energy: k J m^{- 2} y r^{- 1}$; Energy is per time; numbers and biomass are at a given time.
Limitations: $multi-level species ignored; simple chain assumed (no web); decomposers left out$; Three standard points.

How to answer Ecological Pyramids questions

Pyramid questions ask you to draw, identify a shape, explain an inversion or list limitations.

1Identify the measure (number, biomass or energy) and the ecosystem (grassland, tree, pond, sea).
2Place producers at the base and arrange levels upwards; draw bar widths in proportion to the values given.
3For numbers, check whether one producer supports many consumers (tree): inverted or spindle-shaped.
4For biomass in the sea, say: small standing crop of phytoplankton with rapid turnover supports a larger biomass of consumers: inverted.
5For energy, explain the heat loss at each transfer, so it is always upright.
6For limitations, list the three NCERT points.

Ecological Pyramids in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Shape of a pyramid of numbers

Suppose a student counts organisms on a single large banyan tree in her school: 1 tree, 3000 caterpillars and other insects, 25 small birds eating the insects, and 2 owls eating the small birds.

Describe the shape of the pyramid of numbers and of the pyramid of energy for this tree. Explain the difference.

Try it first, then show the solutionSolution

Numbers: base 1 (tree) is narrower than 3000 (insects); above, 25 and 2 narrow again; the pyramid is inverted at the base (spindle-shaped).
Energy: the tree fixes far more energy than the insects obtain, the insects more than the birds, and so on; the energy pyramid is upright.
Difference: number counts each organism equally, but one tree is huge; energy reflects what is actually transferred, and with heat loss at each step it always narrows upwards.

Answer: Numbers: inverted/spindle (1 → 3000 → 25 → 2); energy: upright, because energy is lost at each transfer.

Memory hook: Count heads and a tree is one; count energy and it is the biggest.

Tricks to remember Ecological Pyramids

Short memory aids. Say each one aloud once, then use it on a question the same day.

Energy never stands on its head
Heat leaks at every step, so the energy pyramid is always upright.
Use it when: Energy pyramid questions.
Tree flips numbers, sea flips biomass
One tree → inverted numbers; ocean → inverted biomass.
Use it when: Examples of inverted pyramids.
S-S-S limitations
Same species at several levels, Simple chain assumed, Saprophytes left out.
Use it when: Three limitations.

Ecological Pyramids: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Base, apex and counting · 2 marks · Very short answerBoard level

Which trophic level forms the base and which the apex of an ecological pyramid? Why must every organism at a trophic level be counted when a pyramid is built?

Try it first, then show the solutionSolution

Base: producers (first trophic level); apex: tertiary or top-level consumers (1 mark).
A pyramid compares whole trophic levels; generalisations based on only a few individuals at a level will not be true, so all organisms at that level must be included (1 mark).

Answer: Producers at the base, top consumers at the apex; all organisms must be counted because a few individuals do not represent the level.

Example 2: Limitations · 3 marks · Short answerBoard level

State three limitations of ecological pyramids.

Try it first, then show the solutionSolution

A species that feeds at two or more trophic levels at once (e.g. a sparrow) cannot be shown correctly (1 mark).
They assume a simple food chain, which almost never exists in nature; they do not accommodate a food web (1 mark).
Saprophytes (decomposers) are given no place, though they play a vital role (1 mark).

Answer: Multi-level species ignored; simple chain assumed; decomposers left out.

Example 3: Inverted biomass in the sea · 3 marks · Short answerHarder board level

The pyramid of biomass in the sea is generally inverted. Explain why this is not a paradox. Would the pyramid of energy in the sea also be inverted?

Try it first, then show the solutionSolution

At a given time, the biomass of fishes (and zooplankton) far exceeds that of phytoplankton (1 mark).
Phytoplankton are tiny, reproduce very quickly and are eaten quickly, so a small standing crop is renewed many times and supports a large consumer biomass (1 mark).
The pyramid of energy is not inverted: the energy fixed by phytoplankton over time is greater than the energy reaching consumers, because heat is lost at each level (1 mark).

Answer: Fast turnover of a small phytoplankton standing crop supports large consumer biomass; energy pyramid stays upright.

Example 4: Sparrow and humans · 2 marks · Very short answerHarder board level

A sparrow eats seeds and insects. At which trophic levels does it function? At how many trophic levels can a human function, with examples?

Try it first, then show the solutionSolution

Sparrow: primary consumer (T2) eating seeds, fruits, peas; secondary consumer (T3) eating insects and worms (1 mark).
Humans: T2 (eating cereals and vegetables), T3 (eating goat meat or milk from herbivores), T4 (eating fish that eat smaller fish) (1 mark).

Answer: Sparrow T2 and T3; humans T2, T3 and T4 depending on food.

Example 5: Building a pyramid of energy · 3 marks · Short answerHarder board level

In a pond, producers fix 15,000 kJ m⁻² yr⁻¹. Using the 10 per cent law, find the energy at T2, T3 and T4, and draw (describe) the pyramid.

Try it first, then show the solutionSolution

T2 = 10% of 15,000 = 1500 kJ; T3 = 150 kJ (1 mark).
T4 = 15 kJ (1 mark).
Pyramid: four bars with widths in the ratio 15000 : 1500 : 150 : 15, upright, producers at the base (1 mark).

Answer: 1500, 150, 15 kJ m⁻² yr⁻¹; upright pyramid.

Example 6: Which pyramid fits? · 3 marks · Short answerHOTS / competency

Suppose three bar diagrams are shown for the same aquatic ecosystem: P has a narrow base and wider bar above; Q has a wide base narrowing upwards; R has a narrow base, wide middle and narrow top. The values are said to be biomass, energy and numbers (not in order). Assign each and justify.

Try it first, then show the solutionSolution

Q must be energy: energy pyramids are always upright (1 mark).
P (narrow base, wider above) fits biomass in water: small standing crop of phytoplankton, larger consumer biomass (1 mark).
R (spindle) fits numbers: e.g. few large producers (aquatic plants) supporting many small herbivores, then fewer carnivores (1 mark).

Answer: Q energy, P biomass, R numbers.

Where students lose marks in Ecological Pyramids

The slips examiners see most often, and what to write instead.

Mistake: Inverting the energy pyramid in the sea
Biomass and energy are confused.
Wrong working
In the sea both biomass and energy pyramids are inverted.
Right working
Only biomass is inverted; energy is always upright.
Do this: Energy never stands on its head.
Usually costs: 1-2 marks.
Mistake: Drawing producers at the top
Orientation reversed.
Wrong working
Top bar labelled producers.
Right working
Producers form the base; top consumers the apex.
Do this: Base = producers, always.
Usually costs: 1 mark.
Mistake: Explaining marine inversion by 'more fish than plankton in number'
Numbers and biomass mixed.
Wrong working
Fish are more numerous than phytoplankton.
Right working
Fish biomass exceeds phytoplankton biomass at a given time; phytoplankton turnover is fast.
Do this: Talk about standing crop and turnover.
Usually costs: 1 mark.
Mistake: Incomplete limitations
Students give only one point.
Wrong working
Pyramids ignore food webs.
Right working
Also: species at several levels ignored; decomposers have no place.
Do this: S-S-S.
Usually costs: 1-2 marks.

Exam tips

Draw pyramids with labelled bars (PP, PC, SC, TC) and a title; scale widths roughly.
Give one example for each inverted case: tree (numbers), sea (biomass).
Use 'standing crop' and 'turnover' to explain the marine biomass pyramid.
Quote the three limitations exactly.
Link energy pyramids to the 10 per cent law in calculations.

Practice questions on Ecological Pyramids

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
Which ecological pyramid can never be inverted?
(a) Number (b) Biomass (c) Energy (d) All can be inverted
Hint
Heat loss.
Answer
(c) Energy.
Q2 · 1 mark · Assertion-reasonBoard level
Assertion (A): The pyramid of numbers for a single tree is inverted.
Reason (R): One tree supports a very large number of insects.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Base narrower than the next bar.
Answer
(a).
Q3 · 2 marks · Very short answerHarder board level
Why is a trophic level called a functional level and not a species?
Hint
Sparrow.
Answer
A trophic level is defined by the way an organism obtains food, not by what species it is. The same species may occupy more than one level at the same time; e.g. a sparrow is a primary consumer when eating seeds and a secondary consumer when eating insects.
Q4 · 4 marks · Case-basedHarder board level
Read and answer.
Suppose a study of a grassland gives these numbers per hectare: grasses 6,000,000; grasshoppers and other herbivores 700,000; spiders and frogs 350,000; birds of prey 3.
(a) Draw (describe) the pyramid of numbers and name its shape. (1)
(b) Why is the top so small? (1)
(c) Would the pyramid of energy for this grassland have the same shape? What would the pyramid of numbers look like for a single tree instead of grasses? (2)
Hint
Upright grassland.
Answer
(a) Bars of 6,000,000 (base), 700,000, 350,000 and 3: an upright pyramid. (b) Very little energy reaches the top trophic level (about 10% per step), so few top carnivores can be supported. (c) Yes, the energy pyramid is upright. For a single tree, the base bar (1) would be narrower than the herbivore bar (many insects), so the pyramid of numbers would be inverted (spindle-shaped).
Q5 · 2 marks · Very short answerHOTS / competency
Suppose an exam question shows a pyramid of energy that is inverted. Explain why this must be an error, using the laws of thermodynamics.
Hint
Second law.
Answer
Energy cannot be created (first law), so a consumer level cannot have more energy than it receives from the level below; and in each transfer some energy is lost as heat (second law), so it always has less. An inverted energy pyramid would require energy to appear from nowhere.
Q6 · 2 marks · Very short answerHarder board level
Distinguish between a pyramid of biomass and a pyramid of energy on two points.
Hint
Time and shape.
Answer
Biomass: standing crop at a given time (g m⁻²), can be inverted (e.g. in the sea). Energy: energy flow over a time period per unit area (kJ m⁻² yr⁻¹), always upright.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Name the three ecological pyramids.Show answer
Pyramids of number, biomass and energy.
2.Which pyramid is inverted in the sea?Show answer
Pyramid of biomass.
3.Which pyramid is inverted for a single tree?Show answer
Pyramid of numbers.
4.What is at the base of every ecological pyramid?Show answer
Producers (first trophic level).
5.Which organisms get no place in ecological pyramids?Show answer
Saprophytes (decomposers).
6.Can a species occupy two trophic levels?Show answer
Yes, e.g. a sparrow (seeds and insects).

Ecological Pyramids: frequently asked questions

What are ecological pyramids?

Graphic representations of the number, biomass or energy at successive trophic levels, with producers at the base and top consumers at the apex.

Why is the pyramid of energy always upright?

Because some energy is lost as heat at every transfer between trophic levels, so each higher level has less energy.

Why is the pyramid of biomass inverted in the sea?

At a given time the biomass of fishes far exceeds that of phytoplankton; phytoplankton have a small standing crop but reproduce and are consumed very quickly.

When is a pyramid of numbers inverted?

When a single large producer, such as a tree, supports a large number of consumers such as insects.

What are the limitations of ecological pyramids?

They ignore species occupying more than one trophic level, assume a simple food chain instead of a web, and leave out decomposers.