Is it correct to say: Writing NPP = GPP + R?

Respiration uses up part of what is produced, so NPP = GPP − R. NPP is always less than GPP.

Is it correct to say: Saying only 10% of energy is lost at each trophic level?

About 10% is transferred to the next level; the remaining (about 90%) is lost, mostly as heat.

Is it correct to say: Claiming the pyramid of energy can be inverted?

Energy is lost as heat at every step, so the energy at a higher level is always less. The energy pyramid is always upright.

Is it correct to say: Saying the pyramid of biomass is always upright?

In the sea it is generally inverted, because the biomass of fishes far exceeds that of phytoplankton.

Is it correct to say: Mixing up leaching and catabolism?

Leaching: water-soluble inorganic nutrients go down into the soil and precipitate as unavailable salts. Catabolism: bacterial and fungal enzymes break detritus into simpler inorganic substances.

Is it correct to say: Assuming the grazing food chain carries most energy in every ecosystem?

That is true in aquatic ecosystems. In terrestrial ecosystems a much larger fraction of energy flows through the detritus food chain.

📄 Formula sheet (PDF)

Ecosystem

Ecosystem structure and function, primary and secondary productivity, GPP and NPP, decomposition, energy flow, the 10 per cent law, food chains and ecological pyramids — NCERT Class 12 Biology Ch 12

📐 15 formulas✏️ 4 examples🎯 6 practice⚖️ Unit X Ecology and Environment: 10 marks (with Ch 11 and 13)🏫 CBSE📚 Class 12✓ 2025–26 syllabus

💡

Board Exam Tips

→NPP = GPP − R. State the units (g m⁻² yr⁻¹ or kcal m⁻² yr⁻¹) in every productivity answer.
→List the decomposition steps in order — fragmentation, leaching, catabolism, humification, mineralisation — and add one line on what happens in each.
→For energy-flow numericals, multiply by 0.1 for every step up the food chain and divide by 0.1 for every step down. Count the steps carefully.
→Know which pyramids can be inverted: number (insects on a big tree) and biomass (sea), but never energy. Give the reason for the energy pyramid in one sentence.
→Draw pyramids with the producers at the base and label each trophic level (T₁ to T₄).

📐 Formulas(15)

Primary Production and Productivity

Production: g m^{- 2} or kcal m^{- 2}; Productivity: g m^{- 2} yr^{- 1} or kcal m^{- 2} yr^{- 1}

Net Primary Productivity★ Board fav

NPP = GPP - R

Symbol	Meaning
$GPP$	Gross primary productivity (g m⁻² yr⁻¹ or kcal m⁻² yr⁻¹)
$R$	Respiration losses of the producers (same units)
$NPP$	Net primary productivity (same units)

Secondary Productivity

Secondary productivity = rate of formation of new organic matter by consumers

Global Net Primary Productivity

NPP_{biosphere} \approx 170 \times 1 0^{9} tons yr^{- 1}; NPP_{oceans} \approx 55 \times 1 0^{9} tons yr^{- 1}

Steps of Decomposition★ Board fav

Fragmentation \to Leaching \to Catabolism \to Humification \to Mineralisation

Humus

Humus: dark, amorphous, colloidal, very slow to decompose, reservoir of nutrients

Rate of Decomposition

Faster: nitrogen-rich, water-soluble sugars, warm and moist; Slower: lignin, chitin, low temperature, anaerobiosis

Food Chains

GFC: Grass \to Goat \to Man; DFC: begins with dead organic matter (detritus)

Trophic Levels

T_{1} : producers \to T_{2} : primary consumers \to T_{3} : secondary consumers \to T_{4} : tertiary consumers

Photosynthetically Active Radiation

PAR < 50% of incident solar radiation; plants capture 2 to 10% of PAR

Unidirectional Energy Flow

Sun \to producers \to consumers; energy lost as heat at each step

Ten Per Cent Law★ Board fav

E_{n + 1} = 0.1 \times E_{n}

Symbol	Meaning
$E_{n}$	Energy at trophic level n (J or kcal)
$E_{n + 1}$	Energy transferred to the next trophic level

Energy at the nth Trophic Level

E_{n} = E_{1} \times (0.1)^{n - 1}

Symbol	Meaning
$E_{1}$	Energy fixed by producers (J or kcal)
$n$	Trophic level number (producers are n = 1)

Standing Crop

Standing crop = mass of living material at a trophic level at a given time

Ecological Pyramids★ Board fav

Number, biomass: usually upright, can be inverted; Energy: always upright

✏️ Solved Examples

1Solved Exampleeasy2 steps

A grassland has gross primary productivity of 25,000 kcal m⁻² yr⁻¹, and respiration by the plants uses 15,000 kcal m⁻² yr⁻¹. Find the NPP and the percentage of GPP it represents.

Subtract respiration losses from GPP.

NPP = GPP - R = 25000 - 15000 = 10000 kcal m^{- 2} yr^{- 1}

2Solved Exampleboard3 steps

Producers in a food chain fix 10,000 J of energy. Using the 10 per cent law, find the energy available at the primary, secondary and tertiary consumer levels. (2 marks)

Multiply by 0.1 for each step up the chain.

E_{2} = 0.1 \times 10000 = 1000 J

3Solved Exampleboard2 steps

A tertiary consumer (T₄) obtains 15 kJ of energy. Assuming the 10 per cent law, how much energy must the producers have fixed?

T₄ is three steps above T₁, so reverse the 10% law three times.

E_{1} = \frac{E _{4}}{( 0.1 ) ^{3}}

4Solved ExampleHOTS4 steps

1,000,000 J of sunlight falls on a field. Assume 45% of it is PAR, the plants capture 4% of the PAR as GPP, and they use 40% of the GPP in respiration. Calculate the GPP and the NPP, and state what the NPP represents.

Find the PAR (NCERT: less than 50% of incident radiation).

PAR = 0.45 \times 1 0^{6} = 450000 J

⚠️ Traps & Common Mistakes

⚠️Common Mistakes6

1
Writing NPP = GPP + R
✓Respiration uses up part of what is produced, so NPP = GPP − R. NPP is always less than GPP.
2
Saying only 10% of energy is lost at each trophic level
✓About 10% is transferred to the next level; the remaining (about 90%) is lost, mostly as heat.
3
Claiming the pyramid of energy can be inverted
✓Energy is lost as heat at every step, so the energy at a higher level is always less. The energy pyramid is always upright.
4
Saying the pyramid of biomass is always upright
✓In the sea it is generally inverted, because the biomass of fishes far exceeds that of phytoplankton.
5
Mixing up leaching and catabolism
✓Leaching: water-soluble inorganic nutrients go down into the soil and precipitate as unavailable salts. Catabolism: bacterial and fungal enzymes break detritus into simpler inorganic substances.
6
Assuming the grazing food chain carries most energy in every ecosystem
✓That is true in aquatic ecosystems. In terrestrial ecosystems a much larger fraction of energy flows through the detritus food chain.

🎯 Practice Yourself

🎯Practice Yourself6 questions

Q1
GPP = 8000 kcal m⁻² yr⁻¹ and R = 4800 kcal m⁻² yr⁻¹. Find the NPP.
Q2
Producers fix 50,000 J of energy. How much energy reaches the secondary consumers (T₃) under the 10 per cent law?
Q3
Using NCERT's figures (biosphere 170 billion tons, oceans 55 billion tons), find the annual NPP of land and the oceans' share as a percentage.
Q4
Name the steps of decomposition in order.
Q5
Why is the pyramid of energy always upright?
Q6
Give two limitations of ecological pyramids.

📝 Notes

Ecosystem

An ecosystem is a functional unit of nature in which living organisms interact among themselves and with the physical environment. This chapter covers how energy enters (productivity), how it moves (energy flow and food chains), how dead matter is recycled (decomposition) and how all of this is summarised (ecological pyramids).

Productivity: one subtraction

Producers make organic matter at a rate called GPP. They burn part of it in respiration, and what remains is NPP:

$\text{NPP} = \text{GPP} - R$

NPP is what herbivores and decomposers can use. Secondary productivity is the rate at which consumers form new organic matter. Always write units with a per-year term.

Energy flow and the 10 per cent law

Less than half the incident sunlight is PAR, and plants capture only 2–10% of the PAR. After that, roughly 10% of the energy passes from one trophic level to the next:

$E_n = E_1 \times (0.1)^{n-1}$

For numericals, count the number of steps between the two levels before multiplying or dividing. The flow is one-way: energy lost as heat never comes back to the producers.

Decomposition

Fragmentation, leaching, catabolism, humification and mineralisation together turn detritus into humus and inorganic nutrients. Warm, moist, well-aerated conditions and nitrogen-rich, sugary detritus speed it up; lignin, chitin, cold and anaerobic conditions slow it down.

Pyramids and their limits

Pyramids of number, biomass and energy all place producers at the base. Number and biomass pyramids can be inverted (insects on one tree; fishes and phytoplankton in the sea), but the energy pyramid cannot. Pyramids ignore species that feed at more than one level, cannot show food webs, and leave out saprophytes.