The flow of energy and the ten per cent idea

Where does the energy come from?

Switch off the Sun, and a forest, a pond and a field would soon run down. Except for the ecosystems of the deep sea hydrothermal vents, the Sun is the only source of energy for all ecosystems. The deep sea ones live on energy from chemicals in the vents.

How much of the sunlight is used?

The light that reaches the Earth is not all of use to a plant. NCERT gives two numbers.

• Of the incident solar radiation, less than 50 per cent is photosynthetically active radiation (PAR). This is the part that plants can use.
• Of the PAR, plants capture only 2 to 10 per cent.

So the plants of an ecosystem hold only a small part of the sunlight that falls on it. This captured energy supports the whole living world of that ecosystem.

The energy moves one way

The producers turn light into chemical energy in food. A herbivore eats a plant, and a carnivore eats the herbivore. At each step the energy moves up. It does not go back.

• The energy captured by the plants does not return to the Sun.
• The energy that passes to the herbivores does not come back to the plants.

At every step, some is lost as heat. Because the energy is lost as it moves through the trophic levels, it is no longer available to the level before it. So the flow of energy is unidirectional, which means one way. An ecosystem needs a constant supply of energy from the Sun to keep going.

The ten per cent idea

How much energy passes from one trophic level to the next? NCERT says that only about 10 per cent of the energy is transferred to each trophic level from the lower trophic level. This is called the ten per cent law, and it is named after R. L. Lindeman, who worked on it in 1942.

The remaining 90 per cent is not lost without a trace.

• Most of it is burned in respiration and leaves as heat.
• Some of it is in the parts that the next level does not eat, and in the faeces and dead bodies. These pass to the decomposers.

In a real ecosystem the share is not always exactly 10 per cent. It can be about 5 to 20 per cent, and the textbooks use ten as the standard value.

Why are food chains short?

Suppose the plants hold $10000\ \text{kJ}$ in each square metre in a year. At 10 per cent, the herbivores get $1000\ \text{kJ}$, the carnivores $100\ \text{kJ}$ and the next level $10\ \text{kJ}$. A fifth level would get only $1\ \text{kJ}$. That is too little to feed a population. So a chain has three or four trophic levels, and seldom five. Move the numbers in the model below.

You should find that a larger share passed on allows more levels, and that the energy at the top level is always very small.

Try it in Python

The program takes $8000\ \text{kJ}$ at the first level and passes ten per cent at each step.

The second program counts how many trophic levels a chain can have if the top level needs at least $5\ \text{kJ}$.

Practise with new numbers

Common mistakes

• Applying the ten per cent law to numbers or to biomass. It is a law of energy. The pyramids of number and biomass can look quite different.
• Saying that the 90 per cent disappears. It leaves as heat or goes to the decomposers.
• Letting the energy flow back to the Sun or to the plants. The flow is one way.
• Saying that plants capture most of the sunlight. They capture only 2 to 10 per cent of the PAR, which is itself less than half of the light.

Remember these

• The Sun is the only source of energy for all ecosystems, except the deep sea hydrothermal ones.
• PAR is less than 50 per cent of the incident solar radiation. Plants capture 2 to 10 per cent of the PAR.
• The flow of energy is unidirectional.
• Only about 10 per cent of the energy passes to the next trophic level. This is why chains are short.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. What is the only source of energy for most ecosystems? Name the exception.

2. What is PAR? What share of the sunlight is it?

3. Why is the flow of energy called unidirectional?

4. $6000\ \text{kJ}$ is at T1. How much reaches T3 by the ten per cent law?

5. Why do food chains have only three or four levels?

References

• NCERT (2023). Biology, Textbook for Class XII, chapter 12, Ecosystem, section on energy flow.
• OpenStax Biology 2e (2018), chapter 46, Ecosystems, section on energy flow through ecosystems.
• Campbell Biology, 12th edition (2021), Pearson, chapter 55, Ecosystems and Energy, section on energy transfer between trophic levels.