Our environment — ecosystem components, food chains, ozone depletion

Question

Draw a food chain in a grassland ecosystem. Explain the 10% law of energy transfer and describe how ozone depletion occurs.

(CBSE Class 10 — Our Environment)

Ecosystem Energy Flow

flowchart LR
    A["Sun (100% energy)"] --> B["Producers / Plants (1% captured)"]
    B -->|"10%"| C["Primary Consumers / Herbivores"]
    C -->|"10%"| D["Secondary Consumers / Small Carnivores"]
    D -->|"10%"| E["Tertiary Consumers / Top Carnivores"]
    B --> F["90% lost as heat"]
    C --> G["90% lost as heat"]
    D --> H["90% lost as heat"]
    E --> I["Decomposers recycle nutrients"]

Solution — Step by Step

A simple food chain in a grassland:

Grass → Grasshopper → Frog → Snake → Hawk

Grass = Producer (makes food by photosynthesis)
Grasshopper = Primary consumer (herbivore)
Frog = Secondary consumer (carnivore)
Snake = Tertiary consumer
Hawk = Top/Quaternary consumer

Each organism depends on the previous one for food. Energy flows in one direction — from producers to consumers.

Proposed by Raymond Lindeman, the 10% law states that only 10% of the energy at one trophic level is transferred to the next. The remaining 90% is used by the organism for its own life processes (respiration, movement, growth) and lost as heat.

If grass captures 10,000 J of solar energy:

Grasshopper gets: 1,000 J (10%)
Frog gets: 100 J (10% of 1,000)
Snake gets: 10 J
Hawk gets: 1 J

This is why food chains rarely have more than 4-5 trophic levels — there is simply not enough energy left to support another level.

The ozone layer (in the stratosphere, 15-35 km altitude) absorbs harmful UV radiation from the Sun.

How depletion occurs:

CFCs (chlorofluorocarbons) from refrigerators, ACs, and aerosol cans rise to the stratosphere
UV radiation breaks down CFCs, releasing chlorine atoms
Each chlorine atom destroys thousands of ozone (O $_3$ ) molecules: Cl + O $_3$ → ClO + O $_2$
The ClO reacts with atomic oxygen to regenerate Cl, which destroys more ozone

Consequences: More UV reaches Earth’s surface, causing skin cancer, cataracts, and damage to crops and marine life.

Solution: The Montreal Protocol (1987) banned CFC production. The ozone hole is slowly recovering.

Why This Works

Ecosystems function because energy flows and nutrients cycle. Producers capture solar energy, consumers transfer it up the food chain, and decomposers recycle nutrients back to the soil. The 10% law limits the chain length. Ozone depletion disrupts this system by damaging producers (plants) and consumers alike through UV radiation.

Alternative Method — Food Web vs Food Chain

A food web is more realistic than a single food chain because most organisms eat multiple things:

Grass is eaten by grasshoppers AND rabbits AND deer
Frogs eat grasshoppers AND other insects
Hawks eat snakes AND rabbits AND frogs

Food webs show interconnections. Removing one species affects many others — this is why biodiversity conservation matters.

For CBSE, always mention the 10% law by name (“Lindeman’s 10% law”) and give a numerical example. The ozone question often asks for the chemical equation: $\text{O}_3 \xrightarrow{\text{UV}} \text{O}_2 + \text{O}$ and the role of CFCs. Including the Montreal Protocol shows awareness of the solution, which earns extra credit.

Common Mistake

Students draw arrows in food chains in the wrong direction. The arrow shows the direction of energy flow, not “who eats whom.” It goes FROM the organism being eaten TO the organism that eats it: Grass → Grasshopper (energy flows from grass to grasshopper). Writing it backwards (Grasshopper → Grass) reverses the meaning entirely.