medium difficulty7 min

Energy Flow and Food Chains

Energy flow is the transfer of energy through trophic levels of an ecosystem, beginning with producers and moving through consumers and decomposers, with loss of energy at each transfer.

In one line: Energy is like a one-way stairway: it enters through producers, decreases at each trophic step, and does not climb back as recycled energy.

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Understand it

Energy enters most ecosystems as sunlight captured by producers.
Herbivores feed on producers, carnivores feed on herbivores or other carnivores, and decomposers act on dead organic matter.
Energy flow is unidirectional because energy lost as heat cannot be recycled back to producers.
According to the 10 percent law, only about one-tenth of energy at one trophic level is transferred to the next.
Food chains may be grazing food chains, starting with living green plants, or detritus food chains, starting with dead organic matter.
Food webs provide stability because organisms often have more than one food source.

Example

Grass to grasshopper to frog to snake is a grazing food chain. Fallen leaves to decomposer microbes to detritus-feeding organisms represents a detritus-based pathway.

Rule to remember

10 percent law: approximately 10 percent of energy at one trophic level is transferred to the next trophic level; the rest is used in life processes or lost mainly as heat.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

If producers have 10,000 units of energy, primary consumers may receive about 1,000 units, secondary consumers about 100 units and tertiary consumers about 10 units according to the 10 percent law.

Method you can reuse

1Begin with producers capturing solar energy.
2Place organisms into trophic levels.
3Draw arrows from food to feeder to show energy transfer.
4Apply the 10 percent law if values are given.
5State that energy decreases and is not recycled.

Sketch it in your answer

Use a food chain or food web diagram. Labels should include producer, primary consumer, secondary consumer, tertiary consumer, decomposers, trophic levels, direction of arrows and heat loss.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

Why is energy flow in an ecosystem unidirectional?

Think first, then show answer

Energy flow is unidirectional because energy enters mainly as sunlight and is transferred through trophic levels, but much of it is lost as heat at each step. This lost energy is not recycled back to producers.

Common wrong answer

Drawing arrows from consumer to food is a common error; arrows should show the direction of energy transfer from eaten organism to eater.

Don't mix this up

Students often say energy is recycled like nutrients. Energy flows in one direction, while nutrients are cycled.

Exam tip

Whenever the 10 percent law is asked, mention trophic levels and energy loss as heat during transfer.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

Commonly asked through energy transfer calculations, food chain diagrams, comparison of grazing and detritus food chains, assertion-reason on unidirectional flow, and food web stability questions.

Answer plan

Define energy flow. Draw or describe a food chain with correct arrow direction. State trophic levels. Apply the 10 percent law if needed. Add that energy flow is unidirectional and decreases at successive trophic levels.

1
Enough for 1 mark
Energy flow is the transfer of energy through trophic levels of an ecosystem, beginning with producers and moving through consumers and decomposers, with loss of energy at each transfer.
2
Add for 2 marks
Example: Grass to grasshopper to frog to snake is a grazing food chain. Fallen leaves to decomposer microbes to detritus-feeding organisms represents a detritus-based pathway.
3
Add for 3 marks
Energy enters most ecosystems as sunlight captured by producers.

Practice

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