Whole ecosystem budgets and the traps of the chapter

One ecosystem, all the numbers

You now know the parts: the GPP, the plants' respiration, the NPP, the ten per cent law and the decomposers. Now let us join them in one budget.

The model has these rules. They are model numbers, not data of a real ecosystem.

1. The GPP is $10000\ \text{kJ}$ in each square metre in a year.
2. The plants lose $4000\ \text{kJ}$ in respiration. So the NPP is $6000\ \text{kJ}$.
3. Herbivores (T2) eat 10 per cent of the NPP while it is alive: $600\ \text{kJ}$. The other $5400\ \text{kJ}$ dies and goes to the detritus.
4. At each level, 10 per cent passes up. T2 passes on $60\ \text{kJ}$ to T3, and T3 passes on $6\ \text{kJ}$ to T4.
5. The rest of each consumer level (90 per cent) is lost. Seven tenths of that loss is burned in respiration and leaves as heat. Three tenths goes into faeces and dead bodies, and goes to the detritus.
6. The decomposers burn all of the detritus in the end, and it also leaves as heat.

Count it. The total detritus is $5400$ from the plants plus the shares from T2, T3 and T4, which is $5580\ \text{kJ}$. The total heat is $4000$ from the plants, plus the respiration of T2, T3 and T4, plus the $5580$ of the decomposers. The sum is $10000\ \text{kJ}$, the same as the GPP. This is no accident. In a steady ecosystem, all the energy that comes in leaves as heat. Energy is neither made nor lost.

You should find that the energy at T3 and T4 is tiny whatever the other numbers are. Most of the energy of the plants goes to the detritus.

Try it in Python

The program follows the model, and it checks that the heat that leaves equals the GPP.

Practise with new numbers

The usual traps of the chapter

Read each trap and the true statement beside it.

• Trap: the ten per cent law is about numbers or biomass. True: it is about energy.
• Trap: the pyramid of energy can be inverted. True: never. Number and biomass can.
• Trap: herbivores get the GPP. True: they get the NPP, which is $\text{GPP} - R$.
• Trap: productivity and standing crop are the same. True: productivity is a rate. The standing crop is a stock at one time.
• Trap: the grazing food chain is the larger route on land. True: on land the detritus food chain is larger. In water the grazing chain is.
• Trap: the earthworm is a decomposer. True: it is a detritivore, which fragments the detritus. Bacteria and fungi are the decomposers.
• Trap: humus decays fast. True: it decays extremely slowly and is a reservoir of nutrients.
• Trap: the Sun is the source of energy of every ecosystem. True: of every one except the deep sea hydrothermal ecosystem.
• Trap: plants capture most of the light. True: less than 50 per cent of the light is PAR, and plants capture 2 to 10 per cent of the PAR.

Remember these

• Energy budget: $\text{GPP} = R + \text{NPP}$. The NPP goes to the grazing chain and to the detritus.
• In a steady ecosystem, the heat that leaves equals the GPP.
• At 10 per cent, the energy at T3 is one hundredth of the NPP.
• The decomposers burn the detritus of every level.

Check yourself

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

1. A field has a GPP of $9000\ \text{kJ}$ and the plants lose $3000\ \text{kJ}$ in respiration. What is the NPP?

2. In the same field, what energy reaches T3 by the ten per cent law, taking T1 as the NPP?

3. Where does the rest of the energy of the plants go, if the herbivores eat 10 per cent of the NPP?

4. In which pyramid is an inverted shape impossible?

5. Is the earthworm a decomposer?

References

• NCERT (2023). Biology, Textbook for Class XII, chapter 12, Ecosystem, sections on productivity, decomposition and 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.