BiologyISC Class 12 · Ecosystem

Ecological Pyramids: Number, Biomass and Energy, ISC Class 12

Q: What are the three types of ecological pyramids?

Pyramids of number, biomass and energy.

Q: Why can the pyramid of energy never be inverted?

Because some energy is always lost as heat at each transfer, so each trophic level has less energy than the one below.

Q: When is the pyramid of numbers inverted?

When one large producer, such as a tree, supports many herbivores, or in parasitic food chains.

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

Phytoplankton have a small standing crop at any time but reproduce rapidly, supporting a larger standing crop of zooplankton and fishes.

Q: What are the limitations of ecological pyramids?

They ignore species at more than one trophic level, assume simple food chains instead of webs, and leave out decomposers.

Updated 15 October 2026 · Fact-checked

Ecological pyramids show number, biomass or energy at successive trophic levels, producers at the base. Most are upright. The pyramid of numbers can be inverted (a single tree), the pyramid of biomass is inverted in the sea (phytoplankton less than zooplankton), but the pyramid of energy is always upright because energy is lost as heat at each step.

Producers are the bottom bar in each; numbers and sea biomass can bulge or invert, but the energy pyramid always narrows upward.

Illustration drawn for QuestionBuddy.

Understand Ecological Pyramids

What a pyramid shows. The relationship between organisms at different trophic levels, expressed as number, biomass or energy. The base is always the producers (first trophic level) and the apex the top consumers. Any calculation must include all organisms at a trophic level.

1. Pyramid of numbers. Number of individuals at each level.

Upright in a grassland: NCERT's figure shows nearly 6 million plants supporting only 3 top carnivores.
Inverted or spindle-shaped where one large producer supports many herbivores: one tree → many insects → fewer small birds → still fewer large birds. The base is narrow, the next level wide, then narrowing again.
Inverted in a parasitic food chain (one host → many parasites → still more hyperparasites).

2. Pyramid of biomass. Standing crop (dry weight) at each level.

Upright on land and in most ecosystems: NCERT's figure shows a sharp decrease in biomass at higher trophic levels.
Inverted in the sea (and in many ponds): a small standing crop of phytoplankton supports a large standing crop of zooplankton (and the biomass of fishes far exceeds that of phytoplankton). The paradox is resolved because phytoplankton reproduce and are eaten very rapidly: the standing crop at any moment is small, but the amount produced over a year is large.

3. Pyramid of energy. Energy at each level per unit area in a given time (or annually).

Always upright; can never be inverted: when energy flows from one level to the next, some energy is always lost as heat. Each level has less energy than the one below.
NCERT's ideal energy pyramid notes that primary producers convert only about 1 per cent of the energy in the sunlight available to them into NPP. (The 10 per cent law concerns transfer between trophic levels, not sunlight to NPP.)

Summary table.

Pyramid	Usual shape	Exceptions
Number	upright (grassland)	inverted or spindle: tree ecosystem, parasitic chain
Biomass	upright (land)	inverted: sea, ponds (phytoplankton < zooplankton)
Energy	always upright	none

Limitations of ecological pyramids.
1. They ignore a species that feeds at two or more trophic levels at once (sparrow; humans).
2. They assume a simple food chain, which almost never exists in nature; they do not show food webs.
3. They do not include saprophytes (decomposers), though these play a vital role.
4. Numbers ignore size; biomass ignores the rate of turnover.

Diagram checklist. Draw horizontal bars stacked: label from the base 'producers (T1)', 'herbivores (T2)', 'primary carnivores (T3)', 'top carnivores (T4)'; write units (number per area, g m⁻² dry weight, kJ m⁻² yr⁻¹); draw at least: upright energy pyramid, inverted biomass pyramid of the sea (small phytoplankton bar under a wider zooplankton bar), and the spindle-shaped number pyramid of a tree.

Ecological Pyramids: key facts

Three pyramids: Number individuals at each trophic level
Biomass standing crop (dry weight) per unit area
Energy energy per unit area per unit time; Base = producers.
Always upright: $E_{T 1} > E_{T 2} > E_{T 3} > \dots$; Heat is lost at every transfer.
Shapes and exceptions: Sea biomass inverted, phytoplankton standing crop smaller than zooplankton
Reason phytoplankton turn over rapidly
Tree or parasite numbers inverted or spindle
Energy never inverted; Always name the ecosystem with the shape.
Sunlight to NPP: Sunlight to NPP about 1% of sunlight available to producers
Level to level about 10% (10 per cent law)
Lost at each step mostly as heat; Not 10%.

Number	individuals at each trophic level
Biomass	standing crop (dry weight) per unit area
Energy	energy per unit area per unit time

Sea biomass	inverted, phytoplankton standing crop smaller than zooplankton
Reason	phytoplankton turn over rapidly
Tree or parasite numbers	inverted or spindle
Energy	never inverted

Sunlight to NPP	about 1% of sunlight available to producers
Level to level	about 10% (10 per cent law)
Lost at each step	mostly as heat

How to answer Ecological Pyramids questions

ISC asks to draw or identify pyramids, explain inverted and upright shapes, state why the energy pyramid cannot be inverted, list limitations, and evaluate assertion-reason statements about pyramids.

1Read the data from producers upwards and compare each level with the one below to decide upright, inverted or spindle.
2Name the pyramid type (number, biomass, energy) and the ecosystem; shapes differ by ecosystem.
3For inverted biomass in the sea, give both the observation and the explanation (rapid turnover of phytoplankton).
4For the energy pyramid, give heat loss at every step as the reason it is always upright.
5In assertion-reason items, check each word: 'decrease' versus 'increase', '1%' versus '10%'.
6For limitations, give the three standard ones.

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.

Inverted pyramid of numbers in a single tree

Suppose students count the life on one mango tree in their school: 1 tree, about 1500 leaf-eating insects, 40 small insect-eating birds and 4 hawks that hunt those birds.

Draw the shape of the pyramid of numbers and of biomass for this tree community and explain the difference.

Try it first, then show the solutionSolution

Numbers from the base: 1 → 1500 → 40 → 4. The base is narrow, the second level very wide, then narrower: a spindle-shaped (partly inverted) pyramid.
Biomass: one huge tree weighs far more than all the insects, which outweigh the birds; the biomass pyramid is upright.
Numbers ignore the size of individuals; biomass accounts for it.
The energy pyramid would also be upright.

Answer: Numbers give a spindle (inverted at the base) shape; biomass and energy give upright pyramids.

Memory hook: One big tree, many small bugs.

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
The energy pyramid can never be inverted because heat is lost at every step.
Use it when: The most-asked one-mark fact.
Sea flips biomass
In the sea, tiny fast phytoplankton under heavy zooplankton: biomass flips.
Use it when: Inverted biomass pyramid.
Tree flips numbers
One tree, thousands of insects: numbers flip.
Use it when: Inverted number pyramid.

Ecological Pyramids: solved examples

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

Example 1: Always upright · 2 marks · Short answerBoard level

Which ecological pyramid is always upright? Give the reason.

Try it first, then show the solutionSolution

The pyramid of energy (1 mark).
When energy flows from one trophic level to the next, some is always lost as heat, so each higher level has less energy than the one below (1 mark).

Answer: Pyramid of energy, because energy is lost as heat at each transfer.

Example 2: Inverted biomass · 3 marks · Short answerBoard level

Draw the pyramid of biomass in a sea and explain its shape.

Try it first, then show the solutionSolution

Diagram: a narrow bar for phytoplankton (producers) at the base and a wider bar for zooplankton above (labelled, with units g m⁻²) (1 mark).
The pyramid is inverted: the standing crop of phytoplankton is smaller than that of zooplankton (and fishes) (1 mark).
Phytoplankton reproduce very rapidly and are eaten quickly, so a small standing crop at any moment can support a larger consumer biomass over time (1 mark).

Answer: Inverted, because a small but fast-growing phytoplankton crop supports a larger zooplankton crop.

Example 3: Evaluating an assertion-reason on the sea · 2 marks · Short answerHarder board level

Assertion: In the sea, the pyramid of energy is inverted, just like the pyramid of biomass. Reason: The standing crop of phytoplankton is smaller than that of the zooplankton feeding on them. Evaluate both statements.

Try it first, then show the solutionSolution

Assertion false: a pyramid of energy measures energy flow per unit area over time and loses heat at every transfer, so it is always upright, in the sea as everywhere else (1 mark).
Reason true: phytoplankton have a small standing crop because they are eaten and replaced rapidly; this explains the inverted biomass pyramid, not the energy pyramid. So the assertion is false and the reason is true (1 mark).

Answer: Assertion false, reason true.

Example 4: Identify the shapes · 3 marks · Short answerHarder board level

Classify each pyramid as upright, inverted or spindle-shaped: (i) grassland numbers: five million grass plants, three hundred thousand insects, 4100 birds, 3 hawks; (ii) a host tree's numbers 1, 1200, 35, 4; (iii) a forest's biomass 2600, 90, 8 g m⁻².

Try it first, then show the solutionSolution

(i) Each level smaller than the one below: upright (1 mark).
(ii) Narrow base, wide second level, narrowing above: spindle-shaped (inverted at the base) (1 mark).
(iii) Decreasing upwards: upright (1 mark).

Answer: Upright; spindle; upright.

Example 5: Limitations · 3 marks · Short answerHarder board level

Give three limitations of ecological pyramids, with an example of the first.

Try it first, then show the solutionSolution

They do not account for a species occupying more than one trophic level: a sparrow eats seeds (primary consumer) and insects (secondary consumer) (1 mark).
They assume simple food chains and ignore food webs (1 mark).
They leave out saprophytes (decomposers), though these handle a large share of energy (1 mark).

Answer: Multi-level species, simple chains only, decomposers omitted.

Example 6: Turnover resolves the paradox · 4 marks · Short answerHOTS / competency

Suppose in a coastal sea the standing crop of phytoplankton is 4 g m⁻² and of zooplankton 21 g m⁻². The phytoplankton population is replaced about 50 times a year. (a) What shape is the biomass pyramid? (b) Estimate yearly phytoplankton production. (c) Use this to explain why the zooplankton are not starving, and what an energy pyramid for the same sea would look like.

Try it first, then show the solutionSolution

(a) Inverted (4 < 21) (1 mark).
(b) 4 × 50 = 200 g m⁻² per year (1 mark).
(c) Over a year phytoplankton produce about 200 g m⁻², far more than the zooplankton standing crop; the small standing crop hides a rapid turnover (1 mark).
An energy pyramid (energy per unit area per year) would be upright, because it measures flow over time and energy is lost at each step (1 mark).

Answer: (a) Inverted. (b) About 200 g m⁻² yr⁻¹. (c) Rapid turnover feeds the zooplankton; the energy pyramid is upright.

Where students lose marks in Ecological Pyramids

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

Mistake: Misreading 'decrease' in an assertion
In 2025 examiners reported that candidates did not read the pond assertion carefully, overlooked 'decrease' and confused options (a) and (c).
Do this: Underline key words; in water, biomass pyramids are usually inverted.
Usually costs: 1 mark.
Mistake: Not distinguishing the three pyramids
Examiners (2025) suggested comparing pyramids of biomass, energy and number with diagrams, and terrestrial with aquatic ecosystems.
Do this: Learn the summary table of usual shapes and exceptions.
Usually costs: 1 to 2 marks.
Mistake: Confusing 1% and 10%
The 10 per cent law is for transfer between trophic levels; producers fix only about 1% of sunlight as NPP.
Do this: 1% sunlight to NPP; 10% level to level.
Usually costs: 1 mark.
Mistake: Unlabelled pyramids
Bars without trophic levels or units lose marks.
Do this: Label each bar and give units.
Usually costs: 1 mark.

Exam tips

Draw pyramids with a ruler; keep bar widths in proportion where data are given.
Always name the ecosystem when giving a shape.
Explain inverted biomass with 'rapid turnover' of phytoplankton.
Learn three limitations word for word.
Write 'can never be inverted' for the energy pyramid.

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
The pyramid of biomass in a sea is generally:
(a) upright (b) inverted (c) spindle-shaped (d) absent
Hint
Phytoplankton versus fishes.
Answer
(b) inverted.
Q2 · 2 marks · Short answerBoard level
What forms the base and the apex of an ecological pyramid?
Hint
First and last trophic levels.
Answer
The base is formed by the producers (first trophic level); the apex by the tertiary or top-level consumers.
Q3 · 1 mark · Assertion-reasonHarder board level
Assertion (A): A pyramid of energy can never be inverted.
Reason (R): Every transfer of energy from one trophic level to the next loses part of it as heat.
(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
Heat loss.
Answer
(a).
Q4 · 2 marks · Short answerHarder board level
In NCERT's ideal pyramid of energy, what percentage of sunlight available to producers is converted into NPP? If producers receive 2 000 000 kcal m⁻² yr⁻¹ of usable sunlight, estimate the NPP.
Hint
About 1%.
Answer
About 1%. NPP ≈ 0.01 × 2 000 000 = 20 000 kcal m⁻² yr⁻¹.
Q5 · 2 marks · Short answerHarder board level
Why is the pyramid of numbers in a parasitic food chain inverted?
Hint
One host, many parasites.
Answer
A single host supports many parasites, which in turn support even more hyperparasites, so numbers increase upwards from the base.
Q6 · 2 marks · Short answerHarder board level
An energy pyramid has 15 000, 1500, 150 and 15 kJ m⁻² yr⁻¹ at four levels. Calculate the transfer efficiency between levels and state the law it illustrates.
Hint
Ratio of successive levels.
Answer
1500/15 000 = 150/1500 = 15/150 = 10% at each step; it illustrates the 10 per cent law of energy transfer.
Q7 · 3 marks · Short answerHOTS / competency
Suppose a student draws a pyramid of numbers for a pond and finds it upright, but her biomass pyramid for the same pond is inverted. Is this possible? Explain.
Hint
Tiny producers.
Answer
Yes. Phytoplankton are extremely numerous but tiny, so numbers decrease upwards (upright). Their total standing crop at any moment is small because they are eaten and replaced rapidly, while consumers such as fishes are large and long-lived, so biomass increases upwards (inverted). Number and biomass measure different things.
Q8 · 3 marks · Case-basedHOTS / competency
Read and answer.
Suppose an ecologist records in a grassland: grasses 6 000 000, grasshoppers 250 000, frogs 3000, snakes 50 and an eagle population of 3.
(a) Draw-describe the pyramid of numbers and its shape. (1)
(b) Which pyramid would show the same shape in every ecosystem? (1)
(c) Give one limitation of using this pyramid to describe the grassland. (1)
Hint
Upright; energy.
Answer
(a) Bars decreasing upwards from 6 000 000 grasses to 3 eagles: upright. (b) The pyramid of energy, which is always upright. (c) It ignores that some animals feed at more than one level (e.g. frogs and snakes may eat several prey types), omits decomposers and assumes a simple chain.

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.Which pyramid is always upright?Show answer
Pyramid of energy.
2.Shape of the pyramid of numbers for a single tree?Show answer
Inverted (spindle-shaped).
3.Why is the biomass pyramid of the sea inverted?Show answer
A small, fast-turnover standing crop of phytoplankton supports a larger standing crop of consumers.
4.What forms the base of every ecological pyramid?Show answer
Producers (first trophic level).
5.Percentage of available sunlight converted to NPP in NCERT's energy pyramid?Show answer
About 1%.
6.One limitation of ecological pyramids?Show answer
They ignore species occupying more than one trophic level.

Ecological Pyramids: frequently asked questions

What are the three types of ecological pyramids?

Pyramids of number, biomass and energy.

Why can the pyramid of energy never be inverted?

Because some energy is always lost as heat at each transfer, so each trophic level has less energy than the one below.

When is the pyramid of numbers inverted?

When one large producer, such as a tree, supports many herbivores, or in parasitic food chains.

Why is the pyramid of biomass inverted in the sea?

Phytoplankton have a small standing crop at any time but reproduce rapidly, supporting a larger standing crop of zooplankton and fishes.

What are the limitations of ecological pyramids?

They ignore species at more than one trophic level, assume simple food chains instead of webs, and leave out decomposers.