Ecology questions about food chains almost always come down to one rule: only about 10% of the energy at one trophic level reaches the next one. Everything else is spent staying alive. Muscles working, lungs breathing, cells turning over, heat lost to the surroundings, waste leaving the body. A grasshopper eats a leaf, and the leaf only ever gives up roughly a tenth of itself to the grasshopper that eats it.
That is why an energy pyramid is always the same shape. The producers hold the most energy, and each level above them holds roughly a tenth of the level below. By the top of a long food chain the number is so small that there is rarely enough energy left to support many large animals at all.
Energy pyramids get asked about because the 10% rule is easy to memorise and easy to misapply. The number of organisms can go up as you climb a level even while the energy goes down, and the biomass pyramid can even turn upside down in open water. This builds the energy column level by level and shows you where the energy actually goes.
Leave efficiency at 10% unless your course tells you otherwise. Real values sit somewhere around 5% to 20% and fall lower in cold, wet conditions, which is why so few top-level predators exist. A number pyramid is not an energy pyramid: the two confuse each other in almost every exam that sets this question.
How to use this energy pyramid calculator
Enter the energy available to the producers in kJ per year, pick how many trophic levels you want, and leave transfer efficiency at 10% unless your course says otherwise. The calculator steps the energy down one level at a time and shows you where each level ends up, plus how much energy has been lost by the time you reach the top.
Worked example: if producers capture 10,000 kJ per year, then primary consumers receive 1,000 kJ, secondary consumers 100 kJ, and tertiary consumers 10 kJ. Across three transfers only 0.1% of the original energy is still available, and 9,990 kJ has been spent on staying alive.
Why the three kinds of pyramid are not the same thing
This is where most marks are lost. Three different diagrams get called “pyramids” and they can look completely different.
- Energy pyramid — energy at each level. Always upright, because energy genuinely falls at every transfer.
- Number pyramid — how many organisms are at each level. Often also upright, but not always.
- Biomass pyramid — the total living mass at each level. This one can invert.
A biomass pyramid can turn upside down in open water. A small standing crop of phytoplankton can support a larger mass of zooplankton, because the phytoplankton reproduce fast enough to be eaten and replaced continuously while the zooplankton accumulate. A standing crop is a snapshot, not a lifetime, so a small fast-turning crop can support a big slow-turning one.
None of that contradicts the energy pyramid. Energy still falls by roughly 10% at every level regardless. The energy pyramid is the only one of the three that is genuinely always upright, which is why it is the one worth memorising.
What the 10% figure actually represents
The 10% rule is an average, not a law. Real transfer efficiencies sit roughly between 5% and 20% and they drop in cold, wet conditions, where ectotherms move less and respire less. Plants in particular photosynthesise poorly in cold water, which is one reason aquatic food chains are so short — there simply is not enough energy at the base to support many levels.
Endotherms also sit nearer the lower end, because keeping a constant body temperature is expensive. An insect, being an ectotherm, passes on rather more of its energy than a mouse of similar size. That is why grasshoppers are so energy-efficient prey and mice are not.
Worked questions this calculator answers
- Producers capture 50,000 kJ. What is available to secondary consumers? Answer: 50 kJ, after two transfers.
- Why is a food chain usually limited to four or five levels? Answer: the energy at the top is a small fraction of the original, and there is not enough of it to support many large organisms.
- A pyramid of numbers is inverted but the energy pyramid is not. Explain. Answer: the organisms are small, numerous and fast-turning, but the energy still falls at every level.
- If transfer efficiency rises to 20%, what happens to the top level? Answer: it roughly doubles, which is why high-efficiency chains can support one more level.
Everything here is the standard 10% rule used in first-year ecology, so the numbers should line up with what your course expects. If a question gives you a different efficiency, put it in the efficiency box rather than working around it.
Frequently Asked Questions
What is the 10% rule in ecology?
Only about a tenth of the energy at one trophic level reaches the next one. The rest is spent on movement, breathing, heat loss and waste.
Why is an energy pyramid always upright?
Because energy genuinely falls at every transfer, so there is never more energy at a higher level than below it.
Can a biomass pyramid be inverted?
Yes. In open water a small standing crop of phytoplankton can support a larger mass of zooplankton, because the phytoplankton turn over very fast while the zooplankton accumulate.
What is the difference between a pyramid of numbers and a pyramid of energy?
Numbers counts individuals and energy counts joules. They can disagree, because one small fast-reproducing organism can support a larger standing mass of slow-reproducing ones.
Why do food chains usually stop at four or five levels?
The energy reaching the top is a small fraction of the original, and there is not enough of it left to support many large organisms.
Is 10% always the right figure?
No. Real transfer efficiency sits roughly between 5% and 20%, and it falls in cold, wet conditions. Enter your course figure in the efficiency box if it gives one.
What happens if I raise the transfer efficiency to 20%?
Each level roughly doubles, which is why a more efficient chain can support one extra trophic level before the energy runs out.