Question
What is the difference between a food chain and a food web? How does energy flow from producers to top consumers, and why is only 10% transferred at each level?
(CBSE 6 + CBSE 10 Board)
Solution — Step by Step
A food chain shows one path of energy transfer:
Grass Grasshopper Frog Snake Eagle
Each organism is called a trophic level: Producer (grass), Primary consumer (grasshopper), Secondary consumer (frog), Tertiary consumer (snake), Top predator (eagle).
In reality, organisms eat more than one thing. A frog eats both grasshoppers and caterpillars. A snake eats frogs and mice. When we connect all these relationships, we get a food web — a network of overlapping food chains.
A food web shows the real complexity of an ecosystem. Removing one species affects many others through the web.
At each trophic level, only about 10% of the energy is passed to the next level. The rest (90%) is:
- Used for the organism’s own life processes (respiration)
- Lost as heat
- Found in parts not eaten (bones, shells)
So if producers capture 10,000 J of solar energy:
- Primary consumers get 1,000 J
- Secondary consumers get 100 J
- Tertiary consumers get 10 J
This is why food chains rarely have more than 4-5 levels — there is not enough energy left beyond that.
The energy pyramid is always upright (never inverted), because energy decreases at each level. This contrasts with the pyramid of numbers (which can be inverted — think of one tree supporting thousands of insects).
flowchart TD
A["SUN - Energy Source"] --> B["PRODUCERS - Plants, Algae"]
B --> C["PRIMARY CONSUMERS - Herbivores"]
C --> D["SECONDARY CONSUMERS - Small Carnivores"]
D --> E["TERTIARY CONSUMERS - Top Predators"]
B -- "10% energy transferred" --> C
C -- "10% energy transferred" --> D
D -- "10% energy transferred" --> E
F["DECOMPOSERS - Bacteria, Fungi"] --> B
E --> F
D --> F
C --> FWhy This Works
Energy enters an ecosystem through photosynthesis (producers convert sunlight to chemical energy). This energy flows upward through consumers, but at each step, most is lost as heat during respiration. This is the second law of thermodynamics at work in biology — energy transformations are never 100% efficient.
The 10% rule (proposed by Lindeman in 1942) is an approximation, but it captures the essential truth: energy diminishes rapidly through the food chain, which limits how many trophic levels an ecosystem can support.
Alternative Method
To understand food webs better, think of it as a city: there are many roads (food chains) connecting different areas (organisms). If one road is blocked (a species disappears), traffic can take alternative routes — but the system becomes stressed. This is why biodiversity matters — more connections in the food web make the ecosystem more resilient.
For CBSE 10, remember to draw an energy pyramid (always upright) alongside the food chain in your answer. Mention the 10% law by name and give a numerical example. This is a sure-shot 5-mark question structure.
Common Mistake
Students confuse the pyramid of energy with the pyramid of numbers. The energy pyramid is ALWAYS upright (producers have the most energy). But the pyramid of numbers can be inverted — for example, one large tree (1 producer) supports thousands of insects (many primary consumers). Do not say “all pyramids are upright” — only the energy pyramid is always upright.