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How Nature Works in Harmony

NCERT Class 8 · Science Based on NCERT Class 8 Science textbook · Free CBSE study kit

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Chapter Notes

Overview
This chapter explores how all living and non-living things in nature are interconnected and depend on each other. It explains the concepts of habitat, population, community, ecosystem, food chains, food webs, and the different types of interactions among organisms. The big idea a student must take away is that every organism, from the smallest decomposer to the largest predator, plays a vital role in maintaining the balance of nature, and any human interference can disrupt this harmony.
Key Concepts (24)
Habitat
A place where an organism lives, which provides the right conditions for its survival.
e.g. A pond is a habitat for fish, frogs, and lotus plants.
Biotic components
The living beings in a habitat, such as plants, animals, and microorganisms.
e.g. In a forest, trees, deer, and birds are biotic components.
Abiotic components
The non-living things in a habitat, such as air, water, sunlight, temperature, and soil.
e.g. In a pond, water, sunlight, and soil are abiotic components.
Population
A group of organisms of the same kind living together in a habitat at a given time.
e.g. A group of 20 mango trees in a school garden is a population of mango trees.
Community
Different populations of different species living together and interacting in the same habitat.
e.g. In a pond, the community includes fish, frogs, dragonflies, and lotus plants.
Ecosystem
The interaction between biotic components (living) and abiotic components (non-living) in a habitat.
e.g. A forest ecosystem includes trees, animals, soil, sunlight, and water interacting with each other.
Producers (Autotrophs)
Organisms that make their own food using sunlight, water, and carbon dioxide through photosynthesis.
e.g. Green plants like grass, mango trees, and millet are producers.
Consumers (Heterotrophs)
Organisms that cannot produce their own food and depend on other organisms for food.
e.g. Deer, frogs, and eagles are consumers.
Herbivores
Consumers that eat only plants and plant products.
e.g. Deer and hare eat only grass and leaves.
Carnivores
Consumers that eat only animals.
e.g. Leopards and snakes eat other animals.
Omnivores
Consumers that eat both plants and animals.
e.g. Crows, foxes, and mice eat both plants and animals.
Decomposers (Saprotrophs)
Organisms that break down dead plants and animals into simpler substances, returning nutrients to the soil.
e.g. Fungi (mushrooms) and bacteria grow on dead logs and animal dung.
Food chain
A simple linear sequence showing who eats whom in an ecosystem.
e.g. Grass → Grasshopper → Frog → Snake → Eagle
Food web
A network of interconnected food chains in an ecosystem.
e.g. In a grassland, grass is eaten by hare and grasshopper, both of which are eaten by different predators like fox and snake.
Trophic level
The specific position an organism occupies in a food chain.
e.g. Producers are at the first trophic level, herbivores at the second, and carnivores at the third or fourth.
Pollination
The process by which pollen grains are transferred from the stamen (male part) to the carpel (female part) of a flower, leading to fruit and seed formation.
e.g. Bees, butterflies, and wind help carry pollen from one flower to another.
Mutualism
A type of interaction where both organisms benefit.
e.g. Honeybees get nectar from flowers, and flowers get pollinated.
Commensalism
A type of interaction where one organism benefits and the other is neither helped nor harmed.
e.g. Orchids grow on tree branches for physical support; the tree is unaffected.
Parasitism
A type of interaction where one organism benefits and the other is harmed.
e.g. Ticks feed on the blood of dogs, causing skin irritation to the dog.
Decomposition
The process by which decomposers break down complex substances in dead matter into simpler ones, recycling nutrients.
e.g. Mushrooms growing on a dead tree break it down and return nutrients to the soil.
Elephant corridor
A marked safe passage connecting forest habitats that allows elephants and other wildlife to travel between large forest areas without coming into conflict with human settlements.
e.g. In Odisha and Jharkhand, elephant corridors help elephants move safely between forests.
Green Revolution
A period in the mid-20th century when the use of tractors, synthetic fertilisers, and pesticides increased food production in India.
e.g. Between 1950 and 1965, India faced a food crisis, but the Green Revolution helped increase crop yields.
Monoculture
The practice of growing the same type of crop repeatedly on the same land.
e.g. Growing only wheat year after year in the same field is monoculture.
Protected areas
Parts of land or water set aside to conserve wildlife and their habitats, such as national parks, wildlife sanctuaries, and biosphere reserves.
e.g. Jim Corbett National Park and Sundarbans are protected areas in India.
How It Works
Formation of a Food Chain
  1. Step 1: Identify the producers (green plants) in an ecosystem. They make their own food using sunlight.
  2. Step 2: Identify the herbivores (primary consumers) that eat the producers.
  3. Step 3: Identify the carnivores (secondary consumers) that eat the herbivores.
  4. Step 4: Identify the top carnivores (tertiary consumers) that eat other carnivores.
  5. Step 5: Draw arrows from the eaten organism to the eater, showing the flow of energy.
💡 Arrows always point from the organism being eaten to the organism that eats it, showing the direction of energy flow. Students often draw arrows backwards.
Pollination
  1. Step 1: Stamens (male part) of a flower produce pollen grains.
  2. Step 2: Pollen grains are released into the air or carried by agents like wind, water, insects, bats, or birds.
  3. Step 3: Pollen grains land on the carpel (female part) of the same or a different flower.
  4. Step 4: This transfer leads to fertilisation, forming fruits and seeds.
💡 Pollination is essential for fruit and seed formation. Without pollinators like bees, many plants cannot reproduce.
Decomposition
  1. Step 1: Plants and animals die, producing dead organic matter.
  2. Step 2: Decomposers like fungi and bacteria grow on this dead matter.
  3. Step 3: They break down complex substances into simpler ones.
  4. Step 4: Nutrients are released back into the soil, where they are used by plants.
💡 Decomposers are essential for recycling nutrients. Without them, nutrients would remain locked in dead bodies and the soil would become infertile.
How Fish in a Pond Affect Seed Production (Cascading Effect)
  1. Step 1: Fish eat dragonfly larvae, so ponds with fish have fewer dragonflies.
  2. Step 2: Fewer dragonflies mean more bees, butterflies, and flies survive (since dragonflies eat them).
  3. Step 3: More pollinators visit flowers near the pond, leading to more pollination.
  4. Step 4: More pollination results in more seeds being produced by plants near the pond.
💡 This shows how a change in one population (fish) can indirectly affect another (plants) through a chain of interactions. This is called a cascading effect.
How One Change Leads to Another (Pollution in a Pond)
  1. Step 1: Pollution causes many plants in a pond to die.
  2. Step 2: Fewer plants produce less oxygen in the water.
  3. Step 3: Fish die due to lack of oxygen.
  4. Step 4: With fewer fish, insects increase in number.
  5. Step 5: Insects spread to nearby farmlands.
  6. Step 6: Farmers use more pesticides to control pests.
  7. Step 7: Pesticides harm the environment further.
💡 This chain shows that a single human action (pollution) can trigger a series of harmful effects across different ecosystems.
Exam Traps — Don't Make These Mistakes
⚠ Confusing 'Population' with 'Community'
✗ A population is all the different kinds of organisms living together in a habitat.
→ A population refers to only one species (same kind of organism). A community includes multiple populations of different species.
✓ A population is a group of organisms of the same kind living together in a habitat at a given time. A community is made up of different populations of different species living together.
⚠ Drawing Food Chain Arrows Backwards
✗ Eagle → Snake → Frog → Grasshopper → Grass
→ Arrows in a food chain show the direction of energy flow, i.e., from the eaten to the eater. The arrow points from the organism being eaten to the one that eats it.
✓ Grass → Grasshopper → Frog → Snake → Eagle
⚠ Calling All Consumers 'Carnivores'
✗ All consumers are carnivores because they eat other organisms.
→ Consumers include herbivores (eat only plants), carnivores (eat only animals), and omnivores (eat both plants and animals). Not all consumers eat animals.
✓ Consumers are classified into herbivores (plant-eaters), carnivores (animal-eaters), and omnivores (both plant and animal eaters).
⚠ Thinking Decomposers Are the Same as Scavengers
✗ Decomposers are animals like vultures that eat dead animals.
→ Vultures are scavengers (consumers), not decomposers. Decomposers are microorganisms like bacteria and fungi that break down dead matter into simpler substances.
✓ Decomposers are microorganisms like bacteria and fungi that break down dead plants and animals into simpler substances, recycling nutrients. Scavengers like vultures eat dead animals but do not break them down into simpler substances.
⚠ Confusing 'Habitat' with 'Ecosystem'
✗ A habitat is the same as an ecosystem.
→ A habitat is simply the place where an organism lives. An ecosystem includes the interactions between biotic and abiotic components in that area.
✓ A habitat is the place where an organism lives. An ecosystem is formed when biotic and abiotic components interact with each other in that habitat.
⚠ Thinking All Interactions Are Feeding Relationships
✗ The only interaction in an ecosystem is who eats whom.
→ Organisms also interact through competition (for food, space, sunlight), mutualism (both benefit), commensalism (one benefits, other unaffected), and parasitism (one benefits, other harmed).
✓ Besides feeding relationships, organisms interact through competition, mutualism, commensalism, and parasitism, all of which help maintain balance in the ecosystem.
Compare & Contrast
Mode of obtaining food
Producers (Autotrophs)
Consumers (Heterotrophs)
Producers make their own food using sunlight, water, and carbon dioxide (photosynthesis). Consumers cannot make their own food and depend on other organisms for food.
Complexity of feeding relationships
Food Chain
Food Web
A food chain is a simple, linear sequence showing one path of 'who eats whom'. A food web is a network of many interconnected food chains, showing multiple feeding relationships in an ecosystem.
Effect on the organisms involved
Mutualism
Commensalism
In mutualism, both organisms benefit (e.g., honeybee and flower). In commensalism, one organism benefits and the other is neither helped nor harmed (e.g., orchid on a tree).
Exam Focus
Marks pattern: This chapter typically appears with 2 MCQs (1 mark each), 1 short answer question (3 marks) on food chain or interactions, and 1 long answer question (5 marks) on ecosystem balance or human impact. Total: approximately 10 marks.
Most asked: Definition and examples of biotic and abiotic components · Food chain and food web diagrams · Classification of organisms as producers, consumers, herbivores, carnivores, omnivores, and decomposers · Types of interactions: mutualism, commensalism, parasitism · Role of decomposers in nutrient recycling · Effects of human activities on ecosystems (elephant corridors, frog leg export, pollution)
Don't skip: Draw a food chain from a given list of organisms and label trophic levels (5 marks).; Explain how the removal of one organism (e.g., frogs or fish) affects the entire ecosystem (5 marks).; Differentiate between biotic and abiotic components with examples (3 marks).
One-Line Revision (read aloud before the exam)
  1. A habitat is the place where an organism lives, providing food, water, shelter, and space.
  2. Biotic components are living things; abiotic components are non-living things like air, water, and soil.
  3. A population is a group of the same kind of organisms in a habitat at a given time.
  4. A community is made up of different populations living together in the same habitat.
  5. An ecosystem is formed by the interaction of biotic and abiotic components.
  6. Producers (autotrophs) make their own food; consumers (heterotrophs) depend on others for food.
  7. Herbivores eat only plants, carnivores eat only animals, and omnivores eat both.
  8. Decomposers like bacteria and fungi break down dead matter and recycle nutrients.
  9. A food chain shows a single path of 'who eats whom'; a food web shows interconnected food chains.
  10. Pollination is the transfer of pollen from stamen to carpel, essential for fruit and seed formation.
  11. Mutualism benefits both organisms; commensalism benefits one without affecting the other; parasitism benefits one and harms the other.
  12. Human activities like deforestation, pollution, and overfishing can disrupt the balance of ecosystems.
  13. Elephant corridors are safe passages that connect forests to reduce human-animal conflict.
  14. Decomposers are essential because without them, nutrients would not be returned to the soil.
  15. The Sundarbans mangrove forests protect coastal areas from storms and floods.

MCQs — 10 Questions with Answers

Q1. Which of the following is a non-living component of a pond habitat?

  • A. Fish and frogs
  • B. Water and soil ✓
  • C. Algae and lotus plants
  • D. Dragonflies and snakes

Answer: B — Water and soil are abiotic (non-living) components; fish, frogs, algae, lotus, dragonflies, and snakes are all biotic (living) components.

Q2. A group of sparrows living together in one school garden is called a _____.

  • A. Community
  • B. Population ✓
  • C. Habitat
  • D. Ecosystem

Answer: B — A population is specifically a group of the same species (sparrows) living in the same place at the same time.

Q3. Which statement correctly describes a community?

  • A. All sparrows living in a garden
  • B. All plants in a forest
  • C. All living organisms (plants, animals, microbes) sharing the same habitat ✓
  • D. The weather and climate of a place

Answer: C — A community includes all different populations of various species living together in one habitat, not just one species.

Q4. Why do fish in a pond indirectly help plant reproduction nearby?

  • A. Fish produce oxygen that plants need
  • B. Fish eat dragonfly larvae, leaving more bees and butterflies to pollinate flowers ✓
  • C. Fish fertilize the water for plants
  • D. Fish carry pollen from flower to flower

Answer: B — The research study showed that fish → fewer dragonflies → more bees/butterflies survive → more pollination → more seeds; this is an indirect positive effect.

Q5. In Indian states like Odisha and Jharkhand, why are elephant corridors created?

  • A. To allow elephants to walk in straight lines
  • B. To provide elephants with food and water daily
  • C. To connect forest habitats so elephants can move safely without entering human settlements ✓
  • D. To protect elephants from predators like tigers

Answer: C — Elephant corridors enable safe travel between large forest areas, reducing human-wildlife conflict by preventing elephants from entering farms and villages.

Q6. When forests are cut down, animals like elephants move to human farms. This happens because _____.

  • A. Elephants prefer eating cultivated crops
  • B. Loss of habitat removes food sources, waterholes, and shelter that animals need to survive ✓
  • C. Elephants are forced by humans to leave forests
  • D. Temperature in forests increases due to tree cutting

Answer: B — Habitat loss directly removes the abiotic (water, shelter) and biotic (food from vegetation) requirements elephants need, forcing them to search elsewhere.

Q7. You observe two ponds: Pond X with many fish and Pond Y without fish. Predict which pond will have more flowering plants around it and explain why.

  • A. Pond Y because fish eat flowers
  • B. Pond X because fish produce nutrients for plants
  • C. Pond X because fish control dragonfly predators of pollinators, allowing more pollination ✓
  • D. Pond Y because there are no animals to damage plants

Answer: C — Based on the research study, fish control dragonflies which eat pollinators; fewer dragonflies mean more bees and butterflies to pollinate nearby flowers.

Q8. A mango tree in an Indian village serves as a habitat for birds, insects, and small mammals. Which of the following is a biotic interaction in this habitat?

  • A. Sunlight helping the tree make food
  • B. Water from soil being absorbed by roots
  • C. A bird eating insects living on the tree's bark ✓
  • D. Temperature changes during day and night affecting the tree

Answer: C — A bird eating insects is interaction between two biotic (living) components; A, B, and D involve abiotic components.

Q9. In a forest community, if all organisms were suddenly given unlimited food, water, and space, what would still limit their population growth?

  • A. Lack of sunlight
  • B. Predation and disease from other organisms in the community ✓
  • C. Soil quality
  • D. Air pollution

Answer: B — Even with unlimited resources, biotic interactions like predators hunting prey and diseases spreading would control population size within a community.

Q10. If dragonflies were completely removed from a pond ecosystem, which chain of effects would most likely occur?

  • A. More fish, more bees, fewer plants
  • B. Fewer bees and butterflies, less pollination, fewer flowering plants nearby ✓
  • C. More water, cleaner air, healthier plants
  • D. Fewer insects, more oxygen, faster plant growth

Answer: B — Without dragonflies to eat bees and butterflies, initially more pollinators would seem to help plants; however, removing dragonfly larvae means fish would eat other insect larvae, reducing overall insect diversity and pollinator health long-term—but the direct effect is fewer dragonflies eat more bees, so fewer dragonflies removed = more bees = more pollination seen in the study.

Flashcards

What is a habitat?

A habitat is the place or environment where an organism lives and finds food, shelter, water, and space to survive.

Define biotic component with one example.

Biotic components are all living organisms in a habitat such as plants, animals, and microorganisms.

Name four abiotic components of a habitat.

Abiotic components are non-living things such as water, soil, air, sunlight, and temperature.

What is a population in ecological terms?

A population is a group of the same type of organisms living together in the same habitat at a given time.

How does a community differ from a population?

A community consists of different populations of various species sharing the same habitat, whereas a population is just one species.

Why do elephants enter farms in Odisha and Jharkhand?

Elephants enter farms when vegetation is scarce and waterholes dry up in their natural forest habitat due to loss of forest cover and changes in rainfall.

What is an elephant corridor and why is it made?

An elephant corridor is a marked pathway connecting forest habitats that allows safe movement of animals between large forest areas without entering human settlements.

In the pond study, why did ponds with fish have more flowering plants nearby?

Fish ate dragonfly larvae, reducing dragonflies; fewer dragonflies meant more bees and butterflies survived to pollinate nearby flowers, increasing seed production.

Give one example of an indirect interaction between biotic components.

Fish controlling dragonfly larvae indirectly helps plants by allowing pollinators like bees to survive and pollinate flowers.

What happens to a habitat when trees are cut down?

When trees are cut, the forest shrinks, water sources dry up, animals lose their home and food sources, forcing them to move to human areas.

Important Board Questions

What is the difference between biotic and abiotic components of a habitat? Give one example of each. [1 mark]

Biotic = living organisms (plants, animals, microbes); Abiotic = non-living things (water, soil, air, sunlight, temperature). Provide one specific example of each from any habitat.

Define population and community. How are they different? Give one example of each based on a forest habitat. [2 marks]

Population = one species group (e.g., all sparrows); Community = all species together (e.g., all birds, insects, plants, microbes in forest). State the key difference clearly and provide clear examples.

Explain with the help of the pond fish study how fish indirectly help flowering plants near the pond produce more seeds. Draw a flow diagram showing the chain of effects. [3 marks]

Chain: Fish eat dragonfly larvae → fewer dragonflies → more bees and butterflies survive → more pollination → more seeds. Draw boxes for each organism and use arrows with labels (eats, controls, helps) to show direct and indirect effects.

Why do elephants enter human farms and villages in states like Odisha and Jharkhand? Explain how habitat loss and changes in rainfall affect this. What is an elephant corridor and how does it help? Draw and label a simple diagram showing an elephant corridor connecting two forest areas separated by human settlements. [5 marks]

Causes: Loss of forest cover → shrinking habitat; Rainfall changes → dry waterholes and vegetation loss → elephants search for food (bananas, sugarcane) in farms. Corridor: marked safe pathway connecting forest habitats allowing animal movement without human conflict. Diagram should show: two forest areas (shaded), human settlement in middle (houses, crops), and marked corridor path with arrow showing elephant movement, labeled with: Forest A, Forest B, Village, Crops, Elephant Corridor, Direction of movement.

Assertion-Reason Questions for How Nature Works in Harmony

Q1. Assertion (A):

Fish in a pond obtain oxygen from water, which is an abiotic need for their survival.

Reason (R):

Water is a non-living component of the pond habitat that provides essential physical conditions for organisms.

Answer: A — Both A and R are true, and R is the correct explanation of A — Both statements are true, and the reason correctly explains why oxygen from water is an abiotic need—because water itself is non-living (abiotic). Abiotic components provide the non-living requirements organisms need to survive.

Q2. Assertion (A):

A population consists of all different types of organisms living together in a habitat.

Reason (R):

Different populations of various organisms sharing the same habitat form a community.

Answer: D — A is false but R is true — The assertion is false. A population is specifically one type of organism, not all different types. The reason is true—a community does include different populations. Since the assertion is false, the answer is D, regardless of the reason's truth value.

Q3. Assertion (A):

Elephants enter human farms and villages because they prefer crop foods over forest vegetation.

Reason (R):

Loss of forest habitat and changes in rainfall patterns reduce available food and water in their natural environment.

Answer: D — A is false but R is true — The assertion is false—elephants enter farms out of necessity due to habitat loss, not preference. The reason is true and accurately explains the real cause. The correct answer is C: assertion false, reason true.

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Case-Based Questions (CBQ) for How Nature Works in Harmony

Case Study:

A team of wildlife researchers studied two identical ponds in the same region over one year. Pond A contained a healthy population of small fish, frogs, and aquatic plants. Pond B was similar in size and received the same amount of rainfall and sunlight. However, in Pond B, all fish were removed by local fishermen at the beginning of the study. By the end of the year, researchers found that Pond B had much denser growth of algae and duckweeds, while Pond A maintained a balanced mix of aquatic life. Fish-eating birds visited Pond A regularly but rarely came to Pond B.

Q1. What role do fish play in Pond A that affects the overall ecosystem?

  • A. They increase rainfall in the pond
  • B. They control algae and plant populations by consuming them and their associated organisms ✓
  • C. They prevent water from evaporating
  • D. They produce oxygen for other organisms

Answer: B

Q2. Why did fish-eating birds prefer Pond A over Pond B?

  • A. Pond A had better sunlight
  • B. Pond A had more algae for birds to eat
  • C. Pond A had fish, their natural food source ✓
  • D. Pond B had too much water

Answer: C

Q3. The overgrowth of algae in Pond B demonstrates which ecological principle?

  • A. Removing one organism has no effect on the rest of the community
  • B. All organisms in a community are interconnected and removal of one affects others ✓
  • C. Algae naturally grows without needing fish in the pond
  • D. Fish prevent all plant growth in ponds

Answer: B

Q4. Which biotic component's population increased in Pond B after fish were removed?

  • A. Frogs
  • B. Fish-eating birds
  • C. Algae and duckweeds ✓
  • D. Decomposers

Answer: C

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