Energy Pyramids & Transfer Efficiency — Print · Grade 7 Science
Chapter: Ecosystems, Matter & EnergyEnergy Pyramids & Transfer Efficiency
Learning goals
- I can explain energy pyramids & transfer efficiency.
- I can use evidence, a model, and stated assumptions to support a biological explanation.
Before this unit
Use the supplied vocabulary, distinguish an observation from an explanation, and identify any units or model assumptions. Revisit a connected earlier unit if needed; new terms are explained here.
Use three flexible 40–45-minute sessions: core background and Parts 1–2 with their checks; Parts 3–4 and the worked application; then remaining practice, a labeled model, revision, and the exit check. Checks completed during reading can count toward the practice set. Read one short part at a time. Investigations and chapter review/quiz have separate sessions, and flexible weeks allow more time.
There is no ecosystem on Earth with a food chain twenty steps long. Chains run to four or five links and then stop, and the reason is arithmetic rather than accident. At every transfer from one feeding position to the next, most of the energy does not arrive. A trophic position is a place in that sequence — producers first, then the organisms eating them, then the organisms eating those — and the transfer efficiency is the fraction of energy at one position that ends up at the next.
Follow where the rest goes, because the losses are not mysterious. Much of the energy an organism takes in is released by its own respiration to power everything it does, and ends up as heat. Some of what it eats is never digested and leaves as waste. Parts of organisms are not eaten at all — bones, bark, roots. And organisms die and go to decomposers rather than to a predator. None of this is energy destroyed; it is energy that went somewhere other than the next feeding position, which is a crucial distinction.
An energy pyramid is the diagram that shows this, with each level's energy drawn to scale, and it is always a pyramid because each level is far smaller than the one below. A figure of ten per cent is often quoted as a rule of thumb, and it is worth knowing that real measured efficiencies range from around one per cent to over twenty, depending on the organisms and conditions. Treating ten per cent as a law rather than a rough average is a mistake this unit is built to prevent. Pyramids can also be drawn for biomass, the total mass of living material, which usually gives a similar shape but can behave differently — in some ocean systems fast-reproducing producers support a greater mass of consumers at any one instant, so the energy pyramid and the biomass pyramid do not look the same. The underlying quantity in an energy budget is energy, and that is the one that never inverts.
Build your understanding
Where this fits
The last unit established that energy flows one way through a web and is lost at every transfer. This unit quantifies that loss, which explains something the web alone cannot: why food chains are short, and why there are so few large predators compared with the plants at the bottom.
Use three flexible 40–45-minute sessions: core background and Parts 1–2 with their checks; Parts 3–4 and the worked application; then remaining practice, a labeled model, revision, and the exit check. Checks completed during reading can count toward the practice set. Read one short part at a time. Investigations and chapter review/quiz have separate sessions, and flexible weeks allow more time.
Part 1 · Saying exactly what the pyramid measures
A pyramid can be drawn for different quantities and they are not interchangeable. An energy pyramid shows the energy passing through each level over a stated period — a flow, with units of energy per time. A biomass pyramid shows the mass of living material present at one moment — a standing stock. A numbers pyramid counts individuals, and can be the wrong shape entirely, since one oak supports thousands of insects.
Stating which one you mean is therefore part of reading it. The period matters too: energy through a level over a year is a different figure from energy over a day, and comparing two pyramids built over different intervals is meaningless.
The energy pyramid is the one that cannot invert. However fast producers reproduce, they cannot pass on more energy than they captured, so each level above must receive less. A biomass pyramid can appear inverted at an instant because standing stock is not the same as throughput — a small mass of algae turning over rapidly can support a larger mass of grazers.
Pause and explain
Why can a biomass pyramid sometimes look inverted while an energy pyramid never can?
Check your reasoning after answering
Because biomass is a standing stock at one moment and energy is a flow over a period. Fast-reproducing producers, such as ocean algae, can be present in small mass at any instant while passing a large amount of energy through over time, supporting a greater mass of consumers. Energy through a level can never exceed what the level below passed on.
Part 2 · Calculating a transfer from stated figures
The calculation is a fraction expressed as a percentage: energy reaching the next level, divided by energy at the current level, times a hundred. The only skill is identifying the two numbers correctly and dividing in the right order.
Work the model. Producers provide 2,000 units; first consumers receive 300; second consumers receive 60, all over the same interval. The first transfer is 300 ÷ 2,000 = 0.15, which is 15%. The second is 60 ÷ 300 = 0.2, which is 20%. Two transfers with different efficiencies, and neither one is ten per cent.
That result is the point of the exercise. The ten per cent figure is an average across many systems, not a law each transfer obeys, and applying it as a rule would have given 200 and 20 instead of the stated 300 and 60. Where a measurement is available, use it; where it is not, say that you are using a rough average and that real values vary from about one per cent to over twenty.
Pause and explain
Producers provide 2,000 units and first consumers receive 300. What is the transfer efficiency, and does it match the ten per cent rule?
Check your reasoning after answering
It is 300 ÷ 2,000 = 0.15, which is 15%. It does not match ten per cent, and it does not need to: ten per cent is a rough average across many systems rather than a law. Measured efficiencies range from about one per cent to over twenty, so a stated figure should always be used in preference to the rule of thumb.
Part 3 · Where the rest of the energy went
If 2,000 units are available and 300 reach the next level, 1,700 went somewhere else, and naming the routes prevents the error this unit most wants to avoid. Most of it was used by the producers' own respiration, powering their growth, transport and maintenance, and released as heat. Some of the plant material was never eaten — roots, woody stems, parts that died and went to decomposers.
At the consumer level the same routes apply plus two more. Not everything eaten is digested, and the rest leaves as waste for decomposers. And consumers that die without being eaten take their energy to decomposers as well. Decomposers are a major destination for energy at every level, which is why they are missing from a simple pyramid and present in a complete energy budget.
None of these routes destroys energy. The 1,700 units were transferred to heat, to decomposers, or to parts of the system the pyramid does not draw, and the total is conserved throughout. The pyramid narrows because energy leaves the chain, not because it stops existing — and saying which routes it left by is what turns a diagram into an explanation.
Pause and explain
2,000 units are available at the producer level and 300 reach the first consumers. Where did the other 1,700 go?
Check your reasoning after answering
To the producers' own respiration, released as heat as they grew and maintained themselves; to parts never eaten, such as roots and woody tissue; and to decomposers, via plant material that died without being consumed. None of it was destroyed — it left the chain by routes the pyramid does not draw, and the total is conserved.
Part 4 · What a pyramid supports, and what it does not
Pyramids explain structural facts well. Chains are short because after four or five transfers at these efficiencies there is too little energy left to support another level. Large predators are rare because each one needs an enormous base beneath it. And a given area supports far more people fed on plants directly than on animals fed on those plants — one transfer rather than two.
What pyramids do not support is prediction about particular populations. They are summaries over a stated interval for a stated system, and they say nothing about which species, how numbers will change next season, or what happens if one link is removed. The web from the previous unit is better for structure and still cannot give magnitudes.
So state the interval, the system and the source of the figures, and keep the claim at the level the evidence supports. "In this modelled system over this interval, 15% of producer energy reached the first consumers" is a finding. "Ecosystems transfer ten per cent" is a rule of thumb being presented as a measurement.
Pause and explain
Why are food chains rarely longer than four or five links?
Check your reasoning after answering
Because only a small fraction of energy passes to each next level — often in the region of ten per cent, though it varies widely. After four or five transfers, the energy remaining from the original producer input is too small to support another feeding position. The limit is arithmetic, which is also why large predators are rare and need a large base beneath them.
Apply the whole unit · Follow two different transfer fractions
Illustrative energy model, not measured data: over the same interval, producers provide 2,000 units, first consumers receive 300, and second consumers receive 60.
Your task
Calculate both transfer percentages, and explain why the ten-per-cent shortcut and any claim that energy was destroyed are both wrong.
What your response should show
- Explain the relevant biological process accurately.
- Use the supplied evidence and show calculation steps with units where needed.
- Connect a labeled model or mechanism with the claim.
- State an assumption, evidence limitation, or useful next observation.
Response guidance — after your attempt
First transfer: 300 ÷ 2,000 = 0.15, which is 15%.
Second transfer: 60 ÷ 300 = 0.2, which is 20%.
Reject the shortcut: the two transfers differ from each other and neither is ten per cent, which is an average across systems rather than a law.
Account for the missing energy: 1,700 units and then 240 went to respiration as heat, to uneaten parts, to undigested waste and to decomposers — transferred elsewhere, not destroyed.
The two transfers are 15% and 20%. The first is 300 ÷ 2,000 = 0.15 and the second is 60 ÷ 300 = 0.2, and the fact that they differ is the first thing worth noticing: a single efficiency does not apply across a chain, and neither value is the ten per cent often quoted. That figure is a rough average across many studied systems, with real measurements ranging from around one per cent to over twenty, so applying it as a rule here would have predicted 200 and 20 units instead of the 300 and 60 the model states. Where a measured or stated figure exists, it should be used; where none does, a calculation should say explicitly that it is using an approximation. The second error to avoid is treating the shortfall as destruction. At the first transfer, 2,000 − 300 = 1,700 units did not reach the consumers, and every one of them went somewhere. Most was used by the producers' own respiration and released as heat as they grew, transported materials and maintained themselves. Some was in parts never eaten — roots, woody stems — and some was in plant material that died and went to decomposers. At the second transfer the same routes apply, plus food that was eaten but not digested and left as waste, and consumers that died without being eaten. Energy is conserved throughout; what the pyramid shows is energy leaving the chain, not ceasing to exist. That is also why the chain is short: at these efficiencies, a fourth or fifth level has too little arriving to sustain it.
Scientific references for this unit
Parent / teacher teaching notes
Ask for a because-statement and a labeled relationship, not just a memorized word. Work through one calculation step at a time, permit oral or diagram responses, and use flexible sessions for difficult reading. Compare the learner’s actual evidence with the supplied model; do not reward copying or agreement with a personal preference. No private family, genetic, or medical records are required.
The key idea
An energy pyramid represents measured or modeled energy at successive feeding positions for a defined area and interval. It commonly decreases upward because organisms use energy in their own processes and some matter is not consumed or assimilated. Much transferred energy eventually spreads as heat rather than becoming food for the next position.
Worked examples
Example 1 · Compare a measured figure with the rule of thumb
- Calculate the actual transfer: 250 ÷ 5,000 = 0.05, which is 5%.
- Apply the rule of thumb for comparison: ten per cent of 5,000 would be 500 units.
- Compare: the rule would have overestimated by a factor of two, because this system's efficiency is half the average.
Always divide the stated figures. Reach for ten per cent only when nothing better exists, and say so.
Example 2 · Argue from the pyramid to a structural fact
- Count the transfers in each case: plants to people is one transfer; plants to livestock to people is two.
- Apply an efficiency at each step: each transfer passes on only a fraction, so two transfers multiply two fractions together.
- Compare the outcomes: at roughly ten per cent per step, one transfer delivers about ten per cent of the crop energy and two deliver about one per cent.
Count the transfers. Each one multiplies the loss rather than adding to it.
Avoid these mistakes
- Mistake: ten per cent is the transfer efficiency. It is a rough average; the modelled transfers here are 15% and 20%, and real measurements range from about one per cent to over twenty.
- Mistake: the missing energy was destroyed. It went to respiration as heat, to uneaten parts, to undigested waste and to decomposers — conserved, but outside the chain.
- Mistake: an energy pyramid and a biomass pyramid are the same diagram. One is a flow over time and cannot invert; the other is a standing stock and sometimes can.
- Mistake: a pyramid predicts population changes. It summarises energy over a stated interval for a stated system and says nothing about which species or what happens next season.
Check your understanding
Why can a few trees support many insects without contradicting energy flow?
Discuss the answer
Individual count differs from energy amount; organisms vary greatly in size and available energy.
If 200 of 1,000 units transfer, what percentage is modeled?
Discuss the answer
200/1,000×100=20%, rather than an assumed universal ten percent.
Words to know
- trophic position
- A place in the feeding sequence — producers, then those eating them, then those eating those.
- transfer efficiency
- The fraction of energy at one position reaching the next, expressed as a percentage; measured values range from about one per cent to over twenty.
- energy pyramid
- A diagram of the energy passing through each level over a stated period; unlike a biomass pyramid, it can never invert.
- biomass
- The total mass of living material, a standing stock at one moment rather than a flow over time.
- energy budget
- A full account of where energy goes, including respiration as heat, uneaten parts, waste and decomposers — the routes a pyramid leaves out.
Extend your scientific explanation
Compare a second supplied example or the linked educational reference. Identify a connection, a model limit, and an additional observation that would test the explanation.
In the real world
Use a clear question, reliable observations, and a biological mechanism to evaluate a claim. State what remains uncertain before applying a classroom model to a different setting.
Exit check
Can a 10% energy transfer identify exact predator count?
Check your explanation after answering
No. Count depends on each organism’s needs and other ecological information not supplied by that fraction.
Explain your reasoning without notes. If a key idea is missing, revisit the lesson and try again.
Problem 1
Answer and explanation
Individual count differs from energy amount; organisms vary greatly in size and available energy.
Problem 2
Answer and explanation
200/1,000×100=20%, rather than an assumed universal ten percent.
Problem 3
Answer and explanation
No. Energy has other transfers and destinations; it is not all captured as next-level food.
Problem 4
Answer and explanation
No. Count depends on each organism’s needs and other ecological information not supplied by that fraction.
Problem 5
Answer and explanation
The two modeled transfers are 15% and 20%. They are not both ten percent. Lower energy reaching the next position does not mean energy was destroyed; organisms use energy and transfer it into other pathways and heat. The model does not give the full separate budget or exact organism counts.
300/2,000×100=15%.
60/300×100=20%.
Identify other energy pathways and the stated boundary.
Response criteria
- Explain the relevant biological process accurately.
- Use the supplied evidence and show calculation steps with units where needed.
- Connect a labeled model or mechanism with the claim.
- State an assumption, evidence limitation, or useful next observation.
Problem 6
Answer and explanation
No. Energy amount and individual count are distinct quantities. A relationship between them requires further biological information.
Response criteria
- Explain the relevant biological process accurately.
- Use the supplied evidence and show calculation steps with units where needed.
- Connect a labeled model or mechanism with the claim.
- State an assumption, evidence limitation, or useful next observation.
Problem 7
- Energy per stated boundary and interval
- Only individual names
- An uncalibrated image width
- A genetic allele letter
Answer and explanation
Energy per stated boundary and interval
Define energy units and comparable spatial and time boundaries.
Problem 8
Answer and explanation
80 energy units
0.10×800=80 under the stipulated rule.