IB Biology SLTopic 3 — Energy & MatterPaper 1 & 2Core idea~9 min read
Energy Losses
About ninety per cent of the energy at one trophic level never reaches the next. That is not a rounding error — it is the single most important fact in ecology, and it explains why food chains are short and why top predators are rare.
📘 What you need to know
Only about 10 % of the energy at one trophic level is stored in the tissues of the next.
Energy is lost because not all of the organism is eaten, and not all individuals are eaten at all.
Energy is lost in faeces when food cannot be digested.
Energy is lost as heat during respiration, and in excretion of metabolic waste such as urea.
These losses limit food chains to about four or five trophic levels.
Biomass falls up the chain for the same reason, producing a pyramid shape.
Following one hundred units of energy
Imagine 100 units of energy stored in the plants of a meadow. Here is where it all ends up.
Only the green sliver on the right is available to the next trophic level.
The four routes energy escapes
1. Not everything is eaten
A rabbit grazing a field does not swallow the roots. A fox eating a rabbit leaves the bones, the fur and most of the skin. Trees keep energy locked in wood and bark that almost nothing consumes.
On top of that, plenty of individuals are never eaten by anything. They die of disease, cold or old age, and their energy goes to decomposers instead of up the chain.
2. Not everything eaten can be digested
Herbivores swallow large amounts of cellulose, and most animals cannot digest it. Carnivores swallow fur and feathers made of keratin. Whatever cannot be digested passes straight through and leaves the body as faeces. This is egestion, and the energy in it never entered the animal’s tissues.
3. Respiration releases heat
Every cell in every organism respires, and respiration is not perfectly efficient. A large share of the energy released ends up as heat, which radiates away into the surroundings and cannot be recaptured. Mammals and birds lose especially large amounts because they maintain a constant body temperature.
4. Excretion of metabolic waste
When surplus amino acids are broken down, the nitrogen-containing part becomes urea, which still holds chemical energy. Excreting it in urine removes that energy from the organism for good.
Only the green arrow on the right is passed to the next trophic level.
Learn the pair of words that examiners police: egestion is getting rid of food that was never absorbed, and excretion is getting rid of waste made by the body’s own metabolism. Faeces are egested. Urea is excreted.
What the decomposers pick up
Detritivores and saprotrophs are not usually counted as a link in the food chain, but they collect a huge share of the energy that leaks out of it. They break down:
the parts that were never eaten, such as bones, bark and roots;
undigested material in faeces;
the bodies of organisms that died without being eaten.
And because decomposers respire too, that energy also eventually escapes as heat.
Why food chains are short
Start with 10 000 kJ in the producers. After one transfer you have about 1 000, then 100, then 10, then 1. By the fifth level there is so little energy left that no animal could catch enough prey to cover the cost of hunting.
That is why food chains rarely exceed four or five trophic levels, and why apex predators are always rare compared with the herbivores below them.
WORKED EXAMPLE
A caterpillar population takes in 620 kJ m−2 yr−1 of energy. Of this, 265 kJ is egested in faeces, 248 kJ is lost as heat in respiration and 19 kJ is excreted. Calculate the energy stored as new biomass, and the percentage of ingested energy this represents. [3]
Step 1: add up the losses265 + 248 + 19 = 532 kJStep 2: subtract from the energy ingested620 − 532 = 88 kJ m−2 yr−1Step 3: express as a percentage88 ÷ 620 × 100 = 14.2 %note this is a percentage of energy ingested, not of the energy in the whole plant
💡 Exam tip
Learn four named routes of loss: not eaten, faeces, heat from respiration, excretion.
Say heat is lost during respiration. “Energy is wasted” earns nothing.
Use egestion for faeces and excretion for urine. Mixing them up is a guaranteed lost mark.
When explaining short food chains, link it explicitly to insufficient energy at higher levels.
Watch whether a percentage is of energy ingested or of energy available. They give different answers.
Remember decomposers lose energy as heat too, so nothing is recycled back into the chain.
⚠ Common mix-up
Energy is not destroyed. It leaves the food chain, mostly as heat. Never write that energy disappears.
Egestion is not excretion. Faeces never entered the body’s cells.
The 10 % figure is approximate. Do not present it as an exact law.
Losses are not only respiration. Students who only mention heat throw away easy marks.
Biomass and energy are different quantities, even though both fall up the chain.
Endotherms lose more heat than ectotherms, so transfer efficiency differs between species.
Up next: Primary & Secondary Production — we have counted what is lost. Now let us put a proper name and a proper unit on what is actually gained at each level.
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