IB Biology HLEnergy & Matter in EcosystemsPaper 1 & 2~11 min read
Energy Losses between Trophic Levels
A rabbit eats a large amount of grass and turns almost none of it into rabbit. Around 90 % of the energy never makes it into the next set of tissues – and knowing exactly where it goes explains the shape of every pyramid and the length of every food chain.
📚 What you need to know
Only around 10 % of the energy in a consumer’s food ends up stored in the consumer’s tissues. Around 90 % is lost to the environment.
Losses come from: not every part being eaten, incomplete digestion (faeces), heat from respiration, excretion, and organisms dying uneaten.
The energy that is left fuels the consumer’s life functions, including being stored in carbon compounds during growth.
Detritivores and saprotrophs are not part of a food chain, but they take up the energy in uneaten parts, undigested waste and dead bodies.
Heat is lost during cellular respiration and when ATP is used, in producers, consumers and decomposers alike, by radiation.
Food chains rarely exceed four or five trophic levels, because too little energy is left to support another predator.
Biomass decreases at each level, giving food chains a pyramid structure.
Where the 90 % goes
When a consumer ingests another organism, not all the chemical energy in its food is transferred to its own tissues. Here are the five reasons, and it is worth learning all five because “explain” questions usually want three or four of them.
Loss
Why it happens
Example
Not every part is eaten
The energy in uneaten tissue is never taken in at all
Roots and woody parts of plants, bones of animals
Incomplete digestion
Some ingested material cannot be digested, so it is egested as faeces
Cellulose in plants, fur on prey
Heat from respiration
Respiration releases energy, and some is always lost as heat
A warm mammal on a cold night
Excretion
Waste products of metabolism carry energy out of the body
Urea in urine
Dying uneaten
Not all individuals are consumed; some die and decompose
An old rabbit that no fox ever caught
Whatever survives all five is available to the consumer to fuel its life functions – and some of it is stored in carbon compounds in its tissues during growth. That stored portion is the only part the next trophic level can ever get.
Only the green sliver at the bottom is passed up the chain. Everything above it has left the food chain for good.
Decay organisms and energy loss
Detritivores and saprotrophs are not considered part of a food chain, but they still have a role in the loss of energy from one. They take up energy that the chain has dropped:
They decompose the parts of organisms that are not eaten, such as the bones and teeth of dead prey.
They break down undigested waste material.
They decompose the bodies of any organisms that die and are not consumed.
This is a fussy little distinction that examiners like. Decomposers are not on the chain, but they are fed by every level of it. If a question asks you to explain their role in energy loss, those three bullet points are the answer.
Heat loss to the environment
The transfer of energy in food chains is not 100 % efficient.
Heat is lost to the environment during cellular respiration and when ATP is used in other cellular processes.
This applies to producers, consumers, detritivores and saprotrophs – everything that respires.
Heat is lost at every trophic level and also during decomposition.
The heat leaves by the process of radiation.
Why this is a one-way street. Heat radiated into the air cannot be picked up and used by an organism to build tissue. That is why energy flows through an ecosystem and leaves, while matter cycles round and round inside it. Do not mix the two verbs up.
Why food chains are short
Food chains rarely have more than around four or five trophic levels. Two reasons, and they are linked:
With energy losses at each level, there is less and less energy available to the consumer as you move up the chain.
Once a chain gets longer than four or five levels, it becomes too difficult for a predator to hunt enough prey to gain the energy it needs to survive.
Nothing stops a sixth level existing in theory. There is simply no energy left to build one.
Biomass falls too
Biomass decreases with each trophic level, either because there are fewer individuals or because the individuals are smaller.
Since only around 10 % of the energy in a producer’s tissues is available to a primary consumer, primary consumers must eat a lot of plant biomass to survive.
The same applies again higher up: secondary consumers must consume a lot of prey biomass to get enough energy.
This large reduction at each level means that, drawn in terms of biomass, food chains have a pyramid structure.
A detail worth knowing. Although the total biomass falls at each trophic level, the energy stored per unit of mass does not change. A gram of fox tissue is not more energetic than a gram of rabbit tissue – there is simply far less fox.
Worked examples
WE 1
Explain the losses
Explain why only about 10 % of the energy in a primary consumer’s food is stored in its tissues. (4 marks)
Loss 1: never eaten
Not every part of the food organism is eaten, e.g. roots and woody stems, so that energy is never taken in.
Loss 2: not digested
Some ingested material such as cellulose cannot be digested and is egested as faeces.
Loss 3: respiration
Energy released by respiration is lost to the environment as heat, by radiation.
Loss 4: excretion
Waste products of metabolism such as urea carry energy out in urine.
Uneaten + undigested + heat + excreted = about 90 %name specific examples – “energy is lost” on its own scores nothing
WE 2
Calculate energy transfer
Producers in a field fix 24 000 kJ m−2 yr−1. Primary consumers store 2 040 kJ m−2 yr−1. Calculate the percentage of energy transferred, and the energy lost. (3 marks)
Step 1: percentage transferred
(2 040 ÷ 24 000) × 100 = 8.5 %Step 2: energy lost
24 000 − 2 040 = 21 960 kJ m−2 yr−1Step 3: as a percentage
100 − 8.5 = 91.5 % lost, close to the usual 90 % figure.
8.5 % transferred, 21 960 kJ m−2 yr−1 lostkeep the units on every line – a bare number loses the final mark
WE 3
Explain the length of food chains
Suggest why food chains rarely contain more than five trophic levels. (3 marks)
Point 1: the losses
Around 90 % of energy is lost at each transfer, as heat, faeces, excretion and uneaten material.
Point 2: what that leaves
So there is less and less energy available to the consumer as you move up the chain.
Point 3: the consequence
Beyond four or five levels it becomes too difficult for a predator to hunt enough prey to gain the energy it needs to survive.
The energy runs out before the ideas dofinish with the consequence for the predator – that is usually the third mark
💡 Exam tips
Learn all five losses. Most questions want three or four named ones.
Use egested for faeces and excreted for urea. They are not the same word.
Say heat is lost by radiation, during respiration and when ATP is used.
Remember decomposers are not part of the chain but still receive its energy.
Link short food chains to energy availability, then to the difficulty of hunting.
Energy flows, matter cycles. Never write “energy is recycled”.
⚠ Common mistakes
Saying energy is “destroyed”. It is transferred to the environment as heat, not destroyed.
Saying only 10 % of the food is eaten. The 10 % figure is about energy ending up in the consumer’s tissues, not the amount eaten.
Forgetting producers lose energy too. Plants respire and lose heat as well.
Claiming energy per unit mass falls up the chain. Total biomass falls; energy density does not.
Putting decomposers on a trophic level. They sit outside the chain.
Up next: Primary and Secondary Production – turning all of this into the numbers ecologists actually measure, and why secondary production can never catch up with primary.
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