Every Meal on Earth Traces Back to Sunlight
Learn to build a food chain and label producers, consumers and decomposers, see how chains link into a web, follow energy up the trophic levels, and predict what one change does to the whole web.
Where does the energy in your dinner originally come from?
From sunlight. Whether the plate holds rice, dal, vegetables, milk or mutton, that energy was captured by a green plant somewhere and passed along a chain of eating.
No animal can make food from sunlight, so every chain begins with a plant. This page covers everything in the CBSE Class 8 Science chapter's second part: building food chains, food webs and energy flow, decomposers, and predicting change.
No animal can make food from sunlight, so every chain begins with a plant. This page covers everything in the CBSE Class 8 Science chapter's second part: building food chains, food webs and energy flow, decomposers, and predicting change.
How do you build a food chain and label its parts?
A food chain is a sequence showing who eats whom, with the arrow pointing in the direction the food and energy travel.
Arrange a given set of organisms by starting with the plant and asking repeatedly "what eats this?"
Grassland chain: grass, then grasshopper, then frog, then snake, then eagle.
Pond chain: algae, then small fish, then big fish, then heron.
Field chain: crop plants, then rat, then snake, then hawk.
The roles are named by how each organism gets its food:
- Producers — green plants, which make their own food by photosynthesis. Every chain begins with them. Grass, algae, crop plants, trees.
- Consumers — organisms that eat ready-made food. They come in order:
- Primary consumers — herbivores, eating plants directly. Grasshopper, deer, cow, rat, goat.
- Secondary consumers — eating the primary consumers. Frog, small fish, birds.
- Tertiary consumers — eating those in turn. Snake, big fish, eagle, tiger.
- Decomposers — bacteria and fungi, which break down dead plants and animals and their waste.
The arrow direction is what students most often reverse, and it decides whether the chain makes sense. Grass, then grasshopper means the grass is eaten by the grasshopper — energy flowing from grass to grasshopper. Drawing the arrow the other way would claim the grasshopper feeds the grass.
Arrange a given set of organisms by starting with the plant and asking repeatedly "what eats this?"
Grassland chain: grass, then grasshopper, then frog, then snake, then eagle.
Pond chain: algae, then small fish, then big fish, then heron.
Field chain: crop plants, then rat, then snake, then hawk.
The roles are named by how each organism gets its food:
- Producers — green plants, which make their own food by photosynthesis. Every chain begins with them. Grass, algae, crop plants, trees.
- Consumers — organisms that eat ready-made food. They come in order:
- Primary consumers — herbivores, eating plants directly. Grasshopper, deer, cow, rat, goat.
- Secondary consumers — eating the primary consumers. Frog, small fish, birds.
- Tertiary consumers — eating those in turn. Snake, big fish, eagle, tiger.
- Decomposers — bacteria and fungi, which break down dead plants and animals and their waste.
The arrow direction is what students most often reverse, and it decides whether the chain makes sense. Grass, then grasshopper means the grass is eaten by the grasshopper — energy flowing from grass to grasshopper. Drawing the arrow the other way would claim the grasshopper feeds the grass.
How do food chains link into a web, and how does energy flow?
A food web is many food chains interconnected, because in reality most organisms eat more than one thing and are eaten by more than one.
A grasshopper may be eaten by a frog, a bird or a lizard. A snake may eat frogs, rats or birds. Each of those possibilities is a different chain, and together they form a web.
A food web is a truer picture than a single chain, and it is also why an ecosystem is stable: if one food source becomes scarce, a consumer with alternatives can switch, while one with only a single food source cannot.
Energy flow runs one way — from the Sun, through the producers, and up through the consumers.
The feeding steps are called trophic levels:
- First trophic level — producers (green plants)
- Second — primary consumers (herbivores)
- Third — secondary consumers
- Fourth — tertiary consumers
At each step most of the energy is lost — used up in the organism's own life processes, movement and warmth, or released as heat. Only a small part is stored in its body and becomes available to whatever eats it next.
Two consequences follow directly, and both are examined. Food chains are short, usually only four or five links, because so little energy remains after several steps. And the number of organisms decreases as you go up: a grassland supports a great deal of grass, fewer deer and very few tigers.
Energy flow is also one-way, unlike nutrients. It enters as sunlight, passes up the levels and leaves as heat — it is never recycled back to the plants, which is why sunlight must keep arriving.
A grasshopper may be eaten by a frog, a bird or a lizard. A snake may eat frogs, rats or birds. Each of those possibilities is a different chain, and together they form a web.
A food web is a truer picture than a single chain, and it is also why an ecosystem is stable: if one food source becomes scarce, a consumer with alternatives can switch, while one with only a single food source cannot.
Energy flow runs one way — from the Sun, through the producers, and up through the consumers.
The feeding steps are called trophic levels:
- First trophic level — producers (green plants)
- Second — primary consumers (herbivores)
- Third — secondary consumers
- Fourth — tertiary consumers
At each step most of the energy is lost — used up in the organism's own life processes, movement and warmth, or released as heat. Only a small part is stored in its body and becomes available to whatever eats it next.
Two consequences follow directly, and both are examined. Food chains are short, usually only four or five links, because so little energy remains after several steps. And the number of organisms decreases as you go up: a grassland supports a great deal of grass, fewer deer and very few tigers.
Energy flow is also one-way, unlike nutrients. It enters as sunlight, passes up the levels and leaves as heat — it is never recycled back to the plants, which is why sunlight must keep arriving.
What happens to dead matter and waste in nature?
Decomposers break it down, and the nutrients inside it return to the soil.
Decomposers are the bacteria and fungi that feed on dead plants, dead animals and animal waste. They break the complex substances in that material into simpler ones, which dissolve into the soil where plant roots can absorb them again.
The dark, crumbly matter they produce is called humus, and it is what makes soil fertile.
So nature has two very different flows, and the contrast is the key idea of this section:
- Energy flows one way — from the Sun, up the trophic levels, and out as heat. It has to be replaced continuously.
- Nutrients are recycled — they pass from soil to plant to animal, and decomposers return them to the soil to be used again.
A compost pit shows the whole process in weeks. Kitchen peelings and dead leaves, covered and left, are broken down by bacteria and fungi into manure that is dug back into a field.
Without decomposers the consequences would be twofold, and an answer should give both: dead plants and animals would pile up unchanged, and the nutrients locked inside them would never be released, so the soil would gradually become infertile and plants would fail.
That is why decomposers are sometimes called nature's cleaners — but the more important half of their job is not the cleaning. It is the returning, which closes the nutrient cycle and lets the producers start the chain again.
Decomposers are the bacteria and fungi that feed on dead plants, dead animals and animal waste. They break the complex substances in that material into simpler ones, which dissolve into the soil where plant roots can absorb them again.
The dark, crumbly matter they produce is called humus, and it is what makes soil fertile.
So nature has two very different flows, and the contrast is the key idea of this section:
- Energy flows one way — from the Sun, up the trophic levels, and out as heat. It has to be replaced continuously.
- Nutrients are recycled — they pass from soil to plant to animal, and decomposers return them to the soil to be used again.
A compost pit shows the whole process in weeks. Kitchen peelings and dead leaves, covered and left, are broken down by bacteria and fungi into manure that is dug back into a field.
Without decomposers the consequences would be twofold, and an answer should give both: dead plants and animals would pile up unchanged, and the nutrients locked inside them would never be released, so the soil would gradually become infertile and plants would fail.
That is why decomposers are sometimes called nature's cleaners — but the more important half of their job is not the cleaning. It is the returning, which closes the nutrient cycle and lets the producers start the chain again.
How does one change spread through a food web?
By moving along the connections, in both directions — upward to whatever ate the changed organism, and downward to whatever it ate.
Removing a predator. If all the snakes are killed in a field:
- The rats they ate are no longer controlled, so rat numbers rise
- The rats eat more crops and stored grain, so plant numbers fall and the farmer loses more grain than before
- Hawks and eagles that ate snakes lose a food source and decline
- Eventually the rats may outgrow their food supply and crash
Cutting down trees (deforestation).
- Birds, insects and animals lose habitat and shelter, so their numbers fall
- Herbivores lose food, and the carnivores that ate them follow
- Less photosynthesis means less oxygen produced and more carbon dioxide remaining
- Roots no longer hold the soil, so soil erosion increases and the land loses fertility
- Rainfall patterns are disturbed
Removing producers. Take the grass from a grassland and the deer starve; the tigers that eat deer starve after them. The chain collapses from its first link.
Excess use of pesticides kills useful insects such as bees along with the pests, so pollination falls and fruit production drops.
The pattern to carry away is that no change stays local. Because organisms sit in a web rather than a line, a single removal sends effects in several directions at once — which is exactly why conservation protects whole habitats rather than single popular species.
Removing a predator. If all the snakes are killed in a field:
- The rats they ate are no longer controlled, so rat numbers rise
- The rats eat more crops and stored grain, so plant numbers fall and the farmer loses more grain than before
- Hawks and eagles that ate snakes lose a food source and decline
- Eventually the rats may outgrow their food supply and crash
Cutting down trees (deforestation).
- Birds, insects and animals lose habitat and shelter, so their numbers fall
- Herbivores lose food, and the carnivores that ate them follow
- Less photosynthesis means less oxygen produced and more carbon dioxide remaining
- Roots no longer hold the soil, so soil erosion increases and the land loses fertility
- Rainfall patterns are disturbed
Removing producers. Take the grass from a grassland and the deer starve; the tigers that eat deer starve after them. The chain collapses from its first link.
Excess use of pesticides kills useful insects such as bees along with the pests, so pollination falls and fruit production drops.
The pattern to carry away is that no change stays local. Because organisms sit in a web rather than a line, a single removal sends effects in several directions at once — which is exactly why conservation protects whole habitats rather than single popular species.
Exam tip
Exam tip: drawing the arrows the right way round
Food-chain questions are marked on direction and labels, and both are quick to get right.
Draw every arrow pointing from the eaten to the eater, in the direction energy travels — grass, then grasshopper, then frog. A reversed arrow loses the mark however correct the organisms are.
Begin every chain with a producer, and label the roles asked for: producer, primary consumer, secondary consumer, decomposer.
When asked about energy, say it flows one way and that most is lost at each level as heat — which is why chains are short and numbers fall as you go up.
Contrast the two flows explicitly: energy one-way, nutrients recycled. That sentence answers several questions at once.
And for a "what if" question, trace at least two consequences in different directions — what happens to the prey below and to the predator above.
Draw every arrow pointing from the eaten to the eater, in the direction energy travels — grass, then grasshopper, then frog. A reversed arrow loses the mark however correct the organisms are.
Begin every chain with a producer, and label the roles asked for: producer, primary consumer, secondary consumer, decomposer.
When asked about energy, say it flows one way and that most is lost at each level as heat — which is why chains are short and numbers fall as you go up.
Contrast the two flows explicitly: energy one-way, nutrients recycled. That sentence answers several questions at once.
And for a "what if" question, trace at least two consequences in different directions — what happens to the prey below and to the predator above.
Did you know
Why are food chains never very long?
Because so little energy survives each step.
When a deer eats grass, most of that energy goes into the deer's own breathing, movement and body warmth, and escapes as heat. Only a small fraction is stored in its flesh for a tiger to gain.
Repeat that loss four or five times and almost nothing is left — not enough to support another level of hunters. That is why there is no animal that routinely preys on tigers, and why a grassland can carry vast quantities of grass, a smaller number of deer and only a handful of tigers.
When a deer eats grass, most of that energy goes into the deer's own breathing, movement and body warmth, and escapes as heat. Only a small fraction is stored in its flesh for a tiger to gain.
Repeat that loss four or five times and almost nothing is left — not enough to support another level of hunters. That is why there is no animal that routinely preys on tigers, and why a grassland can carry vast quantities of grass, a smaller number of deer and only a handful of tigers.
Key takeaways
Food chains, webs and decomposers: quick revision
- A food chain shows who eats whom, with arrows pointing from the eaten to the eater, in the direction energy travels.
- Producers are green plants and begin every chain; consumers are primary (herbivores), secondary and tertiary; decomposers break down dead matter.
- A food web is many interlinked chains, and it makes an ecosystem stable because consumers with alternatives can switch food.
- Trophic levels are the feeding steps, and most energy is lost as heat at each one — so chains are short and numbers fall as you go up.
- Energy flows one way from the Sun and out as heat, while nutrients are recycled by decomposers into humus for roots to absorb again.
- One change spreads along the connections in both directions: killing snakes raises rat numbers and starves hawks, and deforestation removes habitat, lowers photosynthesis and increases soil erosion.
You will remember all of this far better after answering five questions on it than after reading it twice.
- Producers are green plants and begin every chain; consumers are primary (herbivores), secondary and tertiary; decomposers break down dead matter.
- A food web is many interlinked chains, and it makes an ecosystem stable because consumers with alternatives can switch food.
- Trophic levels are the feeding steps, and most energy is lost as heat at each one — so chains are short and numbers fall as you go up.
- Energy flows one way from the Sun and out as heat, while nutrients are recycled by decomposers into humus for roots to absorb again.
- One change spreads along the connections in both directions: killing snakes raises rat numbers and starves hawks, and deforestation removes habitat, lowers photosynthesis and increases soil erosion.
You will remember all of this far better after answering five questions on it than after reading it twice.