Only a Tenth of the Energy Survives Each Step Up the Chain
Learn to build a food chain and name its trophic levels, see why a food web is more resilient than a chain, calculate how energy shrinks at each level, and understand why forests are called a lifeline.
Why are there so few tigers and so much grass?
Because only about a tenth of the energy at one level reaches the next, so each step up supports far less life than the one below it.
Start with units of energy captured by grass. The deer eating it store only about . The tiger eating the deer stores only about . Three steps from the grass, roughly one part in a hundred of the original energy is left.
A tiger therefore needs an enormous area of grass beneath it, which is why large carnivores are always rare and always the first to disappear when a habitat shrinks. This page covers the second part of the ICSE Class 8 Biology chapter on ecosystems: food chains, food webs, the energy pyramid, and forests.
Start with units of energy captured by grass. The deer eating it store only about . The tiger eating the deer stores only about . Three steps from the grass, roughly one part in a hundred of the original energy is left.
A tiger therefore needs an enormous area of grass beneath it, which is why large carnivores are always rare and always the first to disappear when a habitat shrinks. This page covers the second part of the ICSE Class 8 Biology chapter on ecosystems: food chains, food webs, the energy pyramid, and forests.
How do you build a food chain and name its trophic levels?
A food chain is a sequence of organisms in which each is eaten by the next, showing how food and energy pass through an ecosystem.
How to draw one. Start with a producer, then the organism that eats it, then the organism that eats that, and so on. Draw an arrow between each pair pointing in the direction the energy travels — that is, from the eaten to the eater.
The direction of the arrow is worth getting right. Grass deer means the deer eats the grass, not the other way round, and reversing an arrow reverses the whole meaning.
A terrestrial chain:
grass grasshopper frog snake hawk
A grassland chain:
grass deer tiger
An aquatic chain:
phytoplankton zooplankton small fish large fish fish-eating bird
Each position in the chain is a trophic level:
- First trophic level — producers, the green plants and algae. They capture energy from sunlight.
- Second trophic level — primary consumers, the herbivores. Grasshopper, deer, zooplankton.
- Third trophic level — secondary consumers, carnivores that eat herbivores. Frog, small fish.
- Fourth trophic level — tertiary consumers, carnivores that eat other carnivores. Snake, large fish.
- Fifth trophic level — top carnivores, eaten by nothing. Hawk, tiger.
So in the first chain, the frog is at the third trophic level and is a secondary consumer — and being able to give both labels is exactly what the question asks for.
Where the decomposers fit. They act at every level, feeding on dead producers, dead herbivores and dead carnivores alike, and returning minerals to the soil. They are usually drawn to one side of the chain rather than at the end of it.
Why food chains are short. Almost every real chain has only three to five links. The reason is the energy loss of the third section: by the fifth level there is too little energy left to support another consumer above it. Nothing forbids a longer chain — there is simply nothing left to eat.
How to draw one. Start with a producer, then the organism that eats it, then the organism that eats that, and so on. Draw an arrow between each pair pointing in the direction the energy travels — that is, from the eaten to the eater.
The direction of the arrow is worth getting right. Grass deer means the deer eats the grass, not the other way round, and reversing an arrow reverses the whole meaning.
A terrestrial chain:
grass grasshopper frog snake hawk
A grassland chain:
grass deer tiger
An aquatic chain:
phytoplankton zooplankton small fish large fish fish-eating bird
Each position in the chain is a trophic level:
- First trophic level — producers, the green plants and algae. They capture energy from sunlight.
- Second trophic level — primary consumers, the herbivores. Grasshopper, deer, zooplankton.
- Third trophic level — secondary consumers, carnivores that eat herbivores. Frog, small fish.
- Fourth trophic level — tertiary consumers, carnivores that eat other carnivores. Snake, large fish.
- Fifth trophic level — top carnivores, eaten by nothing. Hawk, tiger.
So in the first chain, the frog is at the third trophic level and is a secondary consumer — and being able to give both labels is exactly what the question asks for.
Where the decomposers fit. They act at every level, feeding on dead producers, dead herbivores and dead carnivores alike, and returning minerals to the soil. They are usually drawn to one side of the chain rather than at the end of it.
Why food chains are short. Almost every real chain has only three to five links. The reason is the energy loss of the third section: by the fifth level there is too little energy left to support another consumer above it. Nothing forbids a longer chain — there is simply nothing left to eat.
Why is a food web more resilient than a food chain?
Because it offers alternatives, and a single chain does not.
A food web is a network of many interconnected food chains in an ecosystem, showing all the feeding relationships together.
It exists because real organisms do not eat only one thing. A frog eats grasshoppers, but also beetles, flies and worms. A grasshopper is eaten by frogs, but also by birds, lizards and spiders. Each of those links belongs to a different chain, and drawing them all gives a web.
How the two differ:
- Structure. Food chain: a single straight line. Food web: a branching network.
- Number of options. Chain: each organism has one food source and one predator. Web: several of each.
- Stability. Chain: unstable — remove one link and everything above it starves. Web: stable — an alternative usually exists.
- Realism. A chain is a simplification used for teaching; a web is what actually happens.
Predicting the effect of removing an organism. The effects run in both directions — upward to whatever ate it and downward to whatever it ate.
Suppose the frogs are removed from a field.
- Below them, the grasshoppers and insects they ate lose a predator, so their numbers rise sharply and they damage the crop.
- Above them, the snakes that ate frogs lose a food source. Their numbers fall, or they switch to other prey such as rats — which then decline in turn.
- The effect spreads through the web to organisms that never met a frog.
In a web a snake has alternatives and may survive the loss. In a bare chain with frogs as the snake's only food, the snake and the hawk above it both starve.
Which is the real conclusion of this section. Biodiversity is not decoration. The more species a web contains, the more alternative routes it has, and the better it withstands the loss of any one of them. A web with few species behaves like a chain — which is precisely why an artificial ecosystem such as a crop field is so vulnerable to a single pest.
A food web is a network of many interconnected food chains in an ecosystem, showing all the feeding relationships together.
It exists because real organisms do not eat only one thing. A frog eats grasshoppers, but also beetles, flies and worms. A grasshopper is eaten by frogs, but also by birds, lizards and spiders. Each of those links belongs to a different chain, and drawing them all gives a web.
How the two differ:
- Structure. Food chain: a single straight line. Food web: a branching network.
- Number of options. Chain: each organism has one food source and one predator. Web: several of each.
- Stability. Chain: unstable — remove one link and everything above it starves. Web: stable — an alternative usually exists.
- Realism. A chain is a simplification used for teaching; a web is what actually happens.
Predicting the effect of removing an organism. The effects run in both directions — upward to whatever ate it and downward to whatever it ate.
Suppose the frogs are removed from a field.
- Below them, the grasshoppers and insects they ate lose a predator, so their numbers rise sharply and they damage the crop.
- Above them, the snakes that ate frogs lose a food source. Their numbers fall, or they switch to other prey such as rats — which then decline in turn.
- The effect spreads through the web to organisms that never met a frog.
In a web a snake has alternatives and may survive the loss. In a bare chain with frogs as the snake's only food, the snake and the hawk above it both starve.
Which is the real conclusion of this section. Biodiversity is not decoration. The more species a web contains, the more alternative routes it has, and the better it withstands the loss of any one of them. A web with few species behaves like a chain — which is precisely why an artificial ecosystem such as a crop field is so vulnerable to a single pest.
Formula
How much energy is lost at each trophic level?
About ninety per cent of it. Only about ten per cent of the energy at one trophic level is passed on to the next, which is the ten per cent law.
A worked example. Suppose the producers in an ecosystem capture of energy from sunlight. Then
- Producers (first level):
- Herbivores (second level):
- Primary carnivores (third level):
- Top carnivores (fourth level):
So the fourth level receives
of what the producers captured. Three steps have destroyed of the available energy.
Where the other ninety per cent goes:
- Most is used in respiration and finally leaves as heat, which cannot be reused by anything.
- Some is spent on movement, growth and maintaining body temperature.
- Some is lost undigested, in faeces and urine.
- Not all of a level is even eaten — much of it dies naturally and goes to the decomposers instead.
Only what is left and stored as body tissue is available to the next level, and that is roughly a tenth.
The pyramid of energy is the diagram of this: a stack of bars, one per trophic level, each drawn to the size of the energy it holds. Producers form a wide base and each level above is much narrower, giving the characteristic pyramid shape. An energy pyramid is always upright — never inverted — because energy can only decrease upwards.
Two consequences that follow directly.
First, food chains are short. By the fifth level there is too little energy left to support anything further.
Second, a vegetarian diet feeds more people from the same land than a meat-based one, because eating producers directly skips a step and the ninety per cent loss that goes with it. This is a genuine consequence of the ten per cent law and not a matter of opinion.
The contrast to keep in mind. Energy flows one way — in from the Sun, up through the levels, out as heat — and is never recycled. Materials go round and round, because decomposers return them. That asymmetry is why sunlight must keep arriving, and why an ecosystem is self-sustaining in materials but never in energy.
A worked example. Suppose the producers in an ecosystem capture of energy from sunlight. Then
- Producers (first level):
- Herbivores (second level):
- Primary carnivores (third level):
- Top carnivores (fourth level):
So the fourth level receives
of what the producers captured. Three steps have destroyed of the available energy.
Where the other ninety per cent goes:
- Most is used in respiration and finally leaves as heat, which cannot be reused by anything.
- Some is spent on movement, growth and maintaining body temperature.
- Some is lost undigested, in faeces and urine.
- Not all of a level is even eaten — much of it dies naturally and goes to the decomposers instead.
Only what is left and stored as body tissue is available to the next level, and that is roughly a tenth.
The pyramid of energy is the diagram of this: a stack of bars, one per trophic level, each drawn to the size of the energy it holds. Producers form a wide base and each level above is much narrower, giving the characteristic pyramid shape. An energy pyramid is always upright — never inverted — because energy can only decrease upwards.
Two consequences that follow directly.
First, food chains are short. By the fifth level there is too little energy left to support anything further.
Second, a vegetarian diet feeds more people from the same land than a meat-based one, because eating producers directly skips a step and the ninety per cent loss that goes with it. This is a genuine consequence of the ten per cent law and not a matter of opinion.
The contrast to keep in mind. Energy flows one way — in from the Sun, up through the levels, out as heat — and is never recycled. Materials go round and round, because decomposers return them. That asymmetry is why sunlight must keep arriving, and why an ecosystem is self-sustaining in materials but never in energy.
Why are forests called a lifeline, and how can ecosystems be restored?
Because a forest does several irreplaceable jobs at once, and most of them benefit places far away from it.
What forests provide:
- Oxygen and carbon dioxide balance. Forests release oxygen by photosynthesis and absorb carbon dioxide, which moderates the greenhouse effect and global warming.
- Prevention of soil erosion. Roots bind the soil, and the leaf canopy breaks the force of falling rain so that topsoil is not washed away.
- Rainfall and water balance. Transpiration from a large area of leaves returns a great deal of water vapour to the air, contributing to rainfall.
- Groundwater recharge. Forest floors absorb rain slowly instead of letting it run off, so more water soaks down to the water table — which also reduces flooding downstream.
- Habitat and biodiversity. Forests shelter the greatest variety of plants and animals of any terrestrial ecosystem.
- Products. Timber, fuel wood, fodder, bamboo, medicines, gum, resin, honey, fruits and lac.
- Climate moderation. A forest is cooler and more humid than bare land, by shading and by the evaporative cooling of transpiration.
What deforestation causes, each the reverse of an item above: soil erosion, floods in the rains and drought in the dry season, the spread of desert conditions, loss of biodiversity, and rising atmospheric carbon dioxide.
Conservation and restoration measures:
- Afforestation and reforestation — planting trees on bare land and replanting cleared forest. This is the single most direct measure.
- Preventing illegal felling and overgrazing by livestock, which stops young saplings from ever establishing.
- Preventing forest fires, by clearing firebreaks and controlling the burning of stubble.
- Protected areas — national parks, wildlife sanctuaries and biosphere reserves, where habitat and species are legally protected.
- Using alternatives to wood — and recycling paper, since paper is made from wood pulp.
- Social and community forestry — planting on village common land, roadsides and canal banks, managed by the community that uses it.
- Checking soil erosion on degraded land by contour bunding and terracing, so that replanting can succeed.
- Controlling pollution, treating effluent before it is released, and reducing plastic waste.
- Public awareness and legal protection, since none of the above works without them.
Why restoration is harder than conservation. A mature forest is a community built up over a long period, with soil, decomposers, fungi and a full food web. Planting trees on cleared ground recreates the trees quickly but the rest slowly, and a plantation of one species is not an equivalent replacement — it has the low diversity, and therefore the low stability, of an artificial ecosystem.
Which is the practical reason protecting an existing forest is worth far more than replanting a lost one.
What forests provide:
- Oxygen and carbon dioxide balance. Forests release oxygen by photosynthesis and absorb carbon dioxide, which moderates the greenhouse effect and global warming.
- Prevention of soil erosion. Roots bind the soil, and the leaf canopy breaks the force of falling rain so that topsoil is not washed away.
- Rainfall and water balance. Transpiration from a large area of leaves returns a great deal of water vapour to the air, contributing to rainfall.
- Groundwater recharge. Forest floors absorb rain slowly instead of letting it run off, so more water soaks down to the water table — which also reduces flooding downstream.
- Habitat and biodiversity. Forests shelter the greatest variety of plants and animals of any terrestrial ecosystem.
- Products. Timber, fuel wood, fodder, bamboo, medicines, gum, resin, honey, fruits and lac.
- Climate moderation. A forest is cooler and more humid than bare land, by shading and by the evaporative cooling of transpiration.
What deforestation causes, each the reverse of an item above: soil erosion, floods in the rains and drought in the dry season, the spread of desert conditions, loss of biodiversity, and rising atmospheric carbon dioxide.
Conservation and restoration measures:
- Afforestation and reforestation — planting trees on bare land and replanting cleared forest. This is the single most direct measure.
- Preventing illegal felling and overgrazing by livestock, which stops young saplings from ever establishing.
- Preventing forest fires, by clearing firebreaks and controlling the burning of stubble.
- Protected areas — national parks, wildlife sanctuaries and biosphere reserves, where habitat and species are legally protected.
- Using alternatives to wood — and recycling paper, since paper is made from wood pulp.
- Social and community forestry — planting on village common land, roadsides and canal banks, managed by the community that uses it.
- Checking soil erosion on degraded land by contour bunding and terracing, so that replanting can succeed.
- Controlling pollution, treating effluent before it is released, and reducing plastic waste.
- Public awareness and legal protection, since none of the above works without them.
Why restoration is harder than conservation. A mature forest is a community built up over a long period, with soil, decomposers, fungi and a full food web. Planting trees on cleared ground recreates the trees quickly but the rest slowly, and a plantation of one species is not an equivalent replacement — it has the low diversity, and therefore the low stability, of an artificial ecosystem.
Which is the practical reason protecting an existing forest is worth far more than replanting a lost one.
Exam tip
Exam tip: point the arrows from the eaten to the eater
In any food chain, the arrow shows the direction of energy flow, so it points from the organism eaten towards the organism that eats it. Reversing an arrow is a full error, not a slip.
Give both labels when naming a position: the frog is at the third trophic level and is a secondary consumer.
Always start a food chain with a producer.
Remember food chains have only three to five links, and be ready to say why — the energy loss at each step.
For the ten per cent law, show the working level by level: . State the units.
When asked where the lost energy goes, give at least three destinations — respiration and heat, movement and maintenance, undigested waste, and the part that dies and goes to decomposers.
Say an energy pyramid is always upright and never inverted.
For a removal question, trace the effect in both directions — on what the organism ate and on what ate it — and add that a web is more stable than a chain because alternatives exist.
And distinguish sharply: energy flows one way and is lost as heat, while materials cycle.
Give both labels when naming a position: the frog is at the third trophic level and is a secondary consumer.
Always start a food chain with a producer.
Remember food chains have only three to five links, and be ready to say why — the energy loss at each step.
For the ten per cent law, show the working level by level: . State the units.
When asked where the lost energy goes, give at least three destinations — respiration and heat, movement and maintenance, undigested waste, and the part that dies and goes to decomposers.
Say an energy pyramid is always upright and never inverted.
For a removal question, trace the effect in both directions — on what the organism ate and on what ate it — and add that a web is more stable than a chain because alternatives exist.
And distinguish sharply: energy flows one way and is lost as heat, while materials cycle.
Did you know
Why is the top of a food chain never a comfortable place to be?
A top carnivore looks like the winner of the ecosystem. It is eaten by nothing and has no rival above it. In practice its position is the most precarious in the whole system.
The reason is the ten per cent law. A top carnivore lives on the thin remainder of energy left after three or four transfers, so its numbers must always be small and it needs a large area to feed itself. Shrink the habitat and the animal at the top runs out of support before anything below it does.
And the same arithmetic works against it a second way. Any harmful substance that does not break down — certain pesticides and heavy metals — is passed on with the food and becomes more concentrated at every level, because each predator eats many prey. The animal at the top accumulates the most.
So the top carnivore is the first to vanish when a habitat is damaged and the first to suffer when it is polluted — which is exactly why the presence of a healthy population of large predators is taken as a sign that the whole ecosystem beneath them is intact.
The reason is the ten per cent law. A top carnivore lives on the thin remainder of energy left after three or four transfers, so its numbers must always be small and it needs a large area to feed itself. Shrink the habitat and the animal at the top runs out of support before anything below it does.
And the same arithmetic works against it a second way. Any harmful substance that does not break down — certain pesticides and heavy metals — is passed on with the food and becomes more concentrated at every level, because each predator eats many prey. The animal at the top accumulates the most.
So the top carnivore is the first to vanish when a habitat is damaged and the first to suffer when it is polluted — which is exactly why the presence of a healthy population of large predators is taken as a sign that the whole ecosystem beneath them is intact.
Key takeaways
Food chains, energy and forests: quick revision
- A food chain is a sequence in which each organism is eaten by the next. Arrows point from the eaten to the eater, showing energy flow, and a chain always starts with a producer.
- Examples: grass grasshopper frog snake hawk; grass deer tiger; phytoplankton zooplankton small fish large fish bird.
- Trophic levels: first producers, second primary consumers (herbivores), third secondary consumers, fourth tertiary consumers, fifth top carnivores. Decomposers act at every level.
- A food web is many interconnected chains. Chains are single lines and unstable; webs are networks and stable, because alternatives exist. A web is what actually happens.
- Removing an organism affects both directions: its prey increase, its predators decline or switch. Remove frogs and insects multiply while snakes suffer.
- More species means more alternative routes, so biodiversity gives stability — which is why a crop field is so vulnerable.
- Ten per cent law: only about of the energy passes to the next level. From : herbivores , primary carnivores , top carnivores — just of the original.
- The rest is lost in respiration as heat, in movement and maintenance, as undigested waste, and to decomposers when organisms die uneaten.
- A pyramid of energy is always upright, and the law explains why chains are short and why eating producers directly feeds more people from the same land.
- Energy flows one way and is never reused; materials cycle through the decomposers.
- Forests give oxygen and absorb carbon dioxide, prevent soil erosion, contribute to rainfall by transpiration, recharge groundwater and reduce floods, shelter biodiversity, supply timber, fuel, fodder and medicines, and moderate climate.
- Deforestation causes erosion, floods and drought, desert conditions, loss of biodiversity and rising carbon dioxide.
- Measures: afforestation, stopping illegal felling and overgrazing, preventing fires, national parks, sanctuaries and biosphere reserves, wood alternatives and paper recycling, social forestry, checking erosion, controlling pollution, and public awareness.
- Protecting an existing forest is worth more than replanting a lost one, because a mature community takes far longer to rebuild than its trees.
Try working the ten per cent law down four levels from a starting figure of your own, then tracing what happens to a food web when one organism is removed — those two are the questions this chapter is built on.
- Examples: grass grasshopper frog snake hawk; grass deer tiger; phytoplankton zooplankton small fish large fish bird.
- Trophic levels: first producers, second primary consumers (herbivores), third secondary consumers, fourth tertiary consumers, fifth top carnivores. Decomposers act at every level.
- A food web is many interconnected chains. Chains are single lines and unstable; webs are networks and stable, because alternatives exist. A web is what actually happens.
- Removing an organism affects both directions: its prey increase, its predators decline or switch. Remove frogs and insects multiply while snakes suffer.
- More species means more alternative routes, so biodiversity gives stability — which is why a crop field is so vulnerable.
- Ten per cent law: only about of the energy passes to the next level. From : herbivores , primary carnivores , top carnivores — just of the original.
- The rest is lost in respiration as heat, in movement and maintenance, as undigested waste, and to decomposers when organisms die uneaten.
- A pyramid of energy is always upright, and the law explains why chains are short and why eating producers directly feeds more people from the same land.
- Energy flows one way and is never reused; materials cycle through the decomposers.
- Forests give oxygen and absorb carbon dioxide, prevent soil erosion, contribute to rainfall by transpiration, recharge groundwater and reduce floods, shelter biodiversity, supply timber, fuel, fodder and medicines, and moderate climate.
- Deforestation causes erosion, floods and drought, desert conditions, loss of biodiversity and rising carbon dioxide.
- Measures: afforestation, stopping illegal felling and overgrazing, preventing fires, national parks, sanctuaries and biosphere reserves, wood alternatives and paper recycling, social forestry, checking erosion, controlling pollution, and public awareness.
- Protecting an existing forest is worth more than replanting a lost one, because a mature community takes far longer to rebuild than its trees.
Try working the ten per cent law down four levels from a starting figure of your own, then tracing what happens to a food web when one organism is removed — those two are the questions this chapter is built on.