Question
What is the difference between a food chain and a food web? Why does only 10% of energy transfer to the next trophic level? Draw a simple food web for a grassland ecosystem.
(CBSE Class 6 and Class 10 Science)
Solution — Step by Step
A food chain shows ONE straight path of who eats whom:
Each organism is at a trophic level: producers (grass) are at level 1, primary consumers (grasshopper) at level 2, and so on.
In reality, organisms eat more than one thing. A frog eats grasshoppers AND caterpillars. An eagle eats snakes AND rabbits. When we connect multiple food chains together, we get a food web.
A food web is more realistic because it shows the multiple feeding relationships in an ecosystem.
At each trophic level, organisms use about 90% of the energy they get for their own life processes (breathing, moving, body heat). Only about 10% gets stored in their body and is available to the next consumer.
If grass captures 10,000 J of solar energy:
- Grasshopper gets: 1,000 J
- Frog gets: 100 J
- Snake gets: 10 J
- Eagle gets: 1 J
This is called the 10% law (proposed by Lindeman).
Energy Flow Pyramid
flowchart TD
A["Sun provides energy"] --> B["Producers — Grass: 10,000 J"]
B -->|"10% transferred"| C["Primary consumers — Grasshopper: 1,000 J"]
C -->|"10% transferred"| D["Secondary consumers — Frog: 100 J"]
D -->|"10% transferred"| E["Tertiary consumers — Snake: 10 J"]
E -->|"10% transferred"| F["Top predator — Eagle: 1 J"]
B -->|"90% lost as heat"| G["Used in respiration"]
C -->|"90% lost as heat"| G
D -->|"90% lost as heat"| GWhy This Works
Energy enters the ecosystem through photosynthesis — producers convert sunlight into chemical energy (food). This energy then flows upward through consumers. But at each step, most energy is “lost” as heat during respiration.
This is why food chains rarely have more than 4-5 levels — there simply is not enough energy left to support another level. It also explains why there are always more producers than top predators in any ecosystem.
Common Mistake
Students often say energy is “lost” or “destroyed” at each trophic level. Energy is never destroyed — it is converted to heat during respiration and released into the environment. The 90% is not wasted; it was used by the organism to stay alive. Only the 10% stored in body tissue passes to the next consumer.
For CBSE Class 10 board exams: always mention the 10% law by name (Lindeman’s 10% law) and give a numerical example with actual joule values. This scoring pattern has appeared in 3 and 5 mark questions repeatedly.