Energy Flow and Ecological Pyramids: Quick Revision

A brief note on Energy Flow and Ecological Pyramids. Read it before a test. Try each question at the end before you open its answer.

Energy flow

• Source: the Sun, except in the deep sea hydrothermal ecosystem.
• PAR: less than 50 per cent of the incident solar radiation. Plants capture 2 to 10 per cent of it.
• Direction: unidirectional. Energy does not return to the Sun or to the lower level.
• Ten per cent law: about 10 per cent of the energy passes to the next trophic level. The rest is lost as heat or goes to the decomposers.
• Chains have three or four levels, seldom five, because so little energy is left.

Pyramids

• Base: producers. Apex: top consumer.
• Number: upright in a grassland. Inverted at the base for one large tree.
• Biomass: upright on land. Generally inverted in the sea.
• Energy: always upright, drawn for a unit area and a unit time.
• Limits: a species at more than one level, a simple chain, no web, no saprophytes.

A budget

• $\text{GPP} = R + \text{NPP}$. The NPP goes to the grazing chain and to the detritus.
• In a steady ecosystem, all the energy leaves as heat. The heat equals the GPP.

A worked example

A field has 5000 kJ of NPP in each square metre in a year. What energy reaches the third trophic level by the ten per cent law?

1. The NPP is the first level. T2 is $5000 \times 0.1 = 500$ kJ.
2. T3 is $500 \times 0.1 = 50$ kJ for each square metre in a year.

Practice with new numbers

A program to try

The program passes ten per cent of the energy up four levels.

Mistakes to avoid

• Using the ten per cent law for numbers. It is about energy.
• Inverting the pyramid of energy. It is always upright.
• Starting a chain from the GPP. Start from the NPP.
• Forgetting the time in a pyramid of energy. It is a rate.

Check yourself

1. How much of the light is PAR?

2. Why is the pyramid of energy never inverted?

3. In which ecosystem is the pyramid of biomass generally inverted?

4. Name one limitation of the pyramids.

Sources

• NCERT (2023). Biology, Textbook for Class XII, chapter 12, Ecosystem.
• OpenStax Biology 2e (2018), chapter 46, Ecosystems.
• Campbell Biology, 12th edition (2021), Pearson, chapter 55, Ecosystems and Energy.