GeographyISC Class 11 · Biosphere and Biodiversity

Ecosystems, Energy Flow and the Value of Biodiversity (ISC Class 11 Geography)

Updated 17 October 2026 · Fact-checked

The biosphere is the thin zone where land, water and air support life. An ecosystem links abiotic parts with producers, consumers and decomposers through energy flow and nutrient cycling, where only about 10 per cent of energy passes to the next trophic level. Biodiversity (genetic, species, ecosystem) is valued ethically, morally, economically and aesthetically.

Notice that each step up keeps only about 10 per cent of the energy, so food chains are short and top consumers are few.

Illustration drawn for QuestionBuddy.

Understand Ecosystems, Energy Flow and the Value of Biodiversity

Biosphere. The layer of the lower atmosphere, hydrosphere and upper lithosphere where life exists, concentrated near the surface and in the upper sea. It depends on sunlight, water, air gases and soil nutrients, and recycles matter in biogeochemical cycles.

Ecosystem. Living organisms interacting with each other and with their non-living surroundings through energy flow and matter cycles.

Abiotic: sunlight, temperature, water, air, soil, minerals.
Biotic: producers (green plants, algae), consumers (herbivores as primary, carnivores as secondary and tertiary, omnivores) and decomposers (bacteria, fungi).
Energy flow: from sun to producers along food chains and webs, such as grass to deer to tiger. About 10 per cent passes to the next trophic level (ten per cent law), so chains are short and top animals are few.
Nutrient cycling: soil, plants, animals, decomposers, soil.
Interdependence: pollination, seed dispersal, predator control; losing one species may disturb the system.

Biodiversity. Genetic (variation within a species, such as many rice varieties), species (variety of species in an area) and ecosystem (forests, wetlands, deserts, reefs). Greatest in warm, wet tropics.

Values. Ethical: every species has a right to exist. Moral: duty to protect life and hand over a rich world; sacred groves and Bishnoi protection of khejri and blackbuck. Economic: food, timber, medicines, fodder, tourism. Aesthetic: beauty and culture. Ecological: pollination, soil, water, climate.

Sustaining mankind. Food and wild crop genes; medicines such as quinine from cinchona and penicillin from a fungus; pollution control by forests, wetlands, mangroves and microbes; soil and water protection; coastal protection by mangroves and reefs; pollination; livelihoods.

Ecosystems, Energy Flow and the Value of Biodiversity: key facts

Ecosystem parts: $A bi o t i c : s u n, w a t er, ai r, so i l ∣ B i o t i c : p r o d u cer s, co n s u m er s, d eco m p oser s$
Ten per cent law: $E n er g y a t n e x tl e v e l = 10$
Levels of biodiversity: $G e n e t i c ∣ S p ec i es ∣ E cosy s t e m$; Richest in the tropics.
Values: $E t hi c a l ∣ M or a l ∣ E co n o mi c ∣ A es t h e t i c ∣ E co l o g i c a l$

How to answer Ecosystems, Energy Flow and the Value of Biodiversity questions

Questions are 3 to 4 marks: components, energy flow, values; sums use the ten per cent law.

1List abiotic then biotic components.
2State the flow of energy along a named chain.
3Apply 10 per cent at each step in sums, writing units.
4Match each value to an example.

Ecosystems, Energy Flow and the Value of Biodiversity in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Energy up the chain

Suppose grass in a field captures 20,000 kJ of energy and the chain is grass, grasshopper, frog, snake.

Find energy reaching the snake.

Try it first, then show the solutionSolution

Grasshopper = 10% of 20,000 = 2,000 kJ.
Frog = 200 kJ; snake = 20 kJ.

Answer: 20 kJ.

Memory hook: Divide by 10 at each step.

Pollinators at risk

Suppose a village notices fruit trees yielding less after insect numbers fall.

Name the ecosystem service lost.

Try it first, then show the solutionSolution

Insects pollinate flowers.
Fewer pollinators give fewer fruits.

Answer: Pollination.

Memory hook: No pollinator, no fruit.

Tricks to remember Ecosystems, Energy Flow and the Value of Biodiversity

Short memory aids. Say each one aloud once, then use it on a question the same day.

PCD
Producers, Consumers, Decomposers.
Use it when: Biotic parts.
10 per cent step
Each trophic step passes about one-tenth.
Use it when: Energy flow.

Ecosystems, Energy Flow and the Value of Biodiversity: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Role · 1 mark · MCQBoard level

Bacteria and fungi in an ecosystem are:
(a) producers
(b) primary consumers
(c) decomposers
(d) tertiary consumers

Try it first, then show the solutionSolution

Break dead matter.

Answer: (c) Decomposers.

Example 2: Levels · 3 marks · Short answerBoard level

Name the three levels of biodiversity with an example of genetic diversity from the chapter.

Try it first, then show the solutionSolution

Genetic, species and ecosystem diversity.
Genetic example: many varieties of rice or mangoes in India.

Answer: Genetic, species, ecosystem; e.g., rice varieties.

Example 3: Energy sum · 4 marks · Long answerHarder board level

Producers in a grassland capture 50,000 kJ. Find the energy at the herbivore, first carnivore and second carnivore levels, and the percentage of the original energy at the top.

Try it first, then show the solutionSolution

Herbivore = 10% of 50,000 = 5,000 kJ.
First carnivore = 500 kJ.
Second carnivore = 50 kJ.
Percentage at top = 50/50,000 x 100 = 0.1 per cent.

Answer: 5,000; 500; 50 kJ; 0.1 per cent.

Example 4: Values explained · 4 marks · Long answerHarder board level

Explain the ethical, moral, economic and aesthetic reasons for conserving biodiversity, with one example each.

Try it first, then show the solutionSolution

Ethical: every species has a right to exist.
Moral: humans must protect life for future generations; sacred groves.
Economic: food, timber, medicines and tourism.
Aesthetic: beauty of forests, birds and flowers that inspires art.

Answer: Right to exist, duty to future, material benefits and beauty.

Example 5: Food chain case · 5 marks · Case-basedHOTS / competency

Case: A pesticide kills most insects in a pond ecosystem, and fish numbers fall. (a) Name the energy loss law that explains chain length. (b) Explain why predators near the top are fewer. (c) Name one decomposer role afterwards.

Try it first, then show the solutionSolution

(a) The ten per cent law.
(b) Only about 10 per cent of energy passes up each level, so little remains at the top.
(c) Bacteria and fungi break down dead matter and return nutrients.

Answer: (a) Ten per cent law (b) little energy reaches the top (c) decomposers recycle nutrients.

Where students lose marks in Ecosystems, Energy Flow and the Value of Biodiversity

The slips examiners see most often, and what to write instead.

Mistake: Forgetting decomposers
Biotic components incomplete.
Do this: List producers, consumers and decomposers.
Usually costs: 1 mark.
Mistake: Using 90 per cent in the energy law
Loss and passage reversed.
Wrong working
50,000 x 0.9 = 45,000
Right working
50,000 x 0.1 = 5,000
Do this: Only about 10 per cent passes on.
Usually costs: 2 marks.
Mistake: Missing examples for values
Values are only named.
Do this: Give one example for each.
Usually costs: 1 mark.

Exam tips

Draw a food chain with kJ values.
Name all three levels of biodiversity.
Give an example for every value.
Say energy is lost as heat at each level.

Practice questions on Ecosystems, Energy Flow and the Value of Biodiversity

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
Green plants are:
(a) consumers
(b) decomposers
(c) producers
(d) omnivores
Hint
Photosynthesis.
Answer
(c) Producers.
Q2 · 3 marks · Short answerHarder board level
In a chain of grass, deer and tiger, grass holds 30,000 kJ. How much energy reaches the tiger?
Hint
10 per cent twice.
Answer
Deer = 3,000 kJ; tiger = 300 kJ.
Q3 · 3 marks · Short answerHarder board level
Explain two ways in which biodiversity helps in pollution control.
Hint
Forests, wetlands, mangroves, microbes.
Answer
Forests absorb carbon dioxide and dust; wetlands and mangroves filter pollutants; microbes break down waste and sewage.
Q4 · 3 marks · Short answerHarder board level
Why does the biosphere need decomposers?
Hint
Recycling.
Answer
Decomposers break down dead plants and animals and return nutrients to the soil, so matter is recycled and producers can grow.
Q5 · 4 marks · Case-basedHOTS / competency
Case: A scientist notes that wild relatives of rice carry resistance to a pest. (a) Name the level of biodiversity. (b) Explain the economic value. (c) Name another source of medicine from nature mentioned in the chapter.
Hint
Genes.
Answer
(a) Genetic diversity. (b) The genes can be used to breed better varieties. (c) Quinine from cinchona, or penicillin from a fungus.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Ten per cent law?Show answer
Only about 10% of energy passes to the next level.
2.Decomposers are?Show answer
Bacteria and fungi.
3.Where is biodiversity greatest?Show answer
Warm, wet tropics.
4.Quinine source?Show answer
Cinchona tree.
5.Sacred protection by Bishnoi?Show answer
Khejri trees and blackbuck.

Ecosystems, Energy Flow and the Value of Biodiversity: frequently asked questions

Why are food chains short?

Energy is lost at each step.

Is the biosphere self-regulating?

Yes, through cycles of matter.

What is an abiotic factor?

A non-living part such as sunlight or water.