CBSE Class 8 Science Revision Notes Chapter 12 How Nature Works in Harmony

An ecosystem forms when living organisms interact with one another and with non-living components such as air, water, soil and sunlight.
Food relationships, decomposition and ecological interactions help maintain balance and recycle materials in nature.

Nature works through a network of connected living and non-living components. Plants, animals, microorganisms, air, water, soil and sunlight influence one another inside different habitats.

These CBSE Class 8 Science Revision Notes Chapter 12 cover ecosystems, populations, food chains, ecological relationships and conservation. The notes follow the current 2026–27 chapter for quick and structured revision.

Key Takeaways

• Ecosystem: It includes a community of organisms and the abiotic components with which they interact.
• Food chain: It shows a simple sequence of who eats whom in an ecosystem.
• Decomposers: Bacteria and fungi break down dead matter and return nutrients to the soil.
• Ecological balance: Changes in one population can affect several other organisms and resources.

Access Class 8 Science Chapter 12 How Nature Works in Harmony Notes in 30 Minutes

Revise the chapter in three parts:

First 10 minutes: Habitat, biotic and abiotic components, population and community
Next 10 minutes: Producers, consumers, food chains, food webs and trophic levels
Final 10 minutes: Decomposition, ecological relationships, conservation and sustainable farming

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Habitat and Ecosystem in Class 8 Science Chapter 12 Notes

Every organism lives in a place that provides suitable conditions for survival. Different habitats have different living organisms and physical conditions.

What Is a Habitat?

A habitat is the place where an organism lives, grows and carries out its life processes.

A habitat may be:

• A pond
• A forest
• A farm
• A grassland
• A river
• A large tree
• The bark of a tree

A habitat provides food, water, oxygen, shelter and space.

Biotic and Abiotic Components Class 8

All habitats contain living and non-living components.

Fish in a pond depend on plants and small animals for food. They also depend on oxygen dissolved in water.

Pond Habitat

A pond may contain:

Biotic components

• Fish
• Frogs
• Turtles
• Snakes
• Dragonflies
• Mosquitoes
• Snails
• Ducks
• Algae
• Lotus
• Duckweed

Abiotic components

• Water
• Sunlight
• Air
• Temperature
• Soil and nutrients

Forest Habitat

A forest contains trees, shrubs, animals, birds, insects and microorganisms.

Its abiotic components include soil, water, sunlight, temperature and air.

Different organisms may use the same forest at different times. Some are active during the day, while others come out at night.

Population, Community and Ecosystem

Living organisms can be studied at different levels. These levels show how individuals form larger natural systems.

What Is a Population?

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

Examples include:

• All deer in a forest
• All fish of one species in a pond
• All mango trees in a garden
• All frogs in a wetland

Population refers to the number of individuals of the same type in a place.

What Is a Community?

A community includes different populations living together in one habitat.

For example, a pond community may include populations of fish, frogs, algae, insects and microorganisms.

These organisms interact and depend on one another.

What Is an Ecosystem?

An ecosystem forms when a community of living organisms interacts with the non-living components of its surroundings.

It includes:

• Biotic components
• Abiotic components
• Interactions among them

Individual to Ecosystem

The levels can be written as:

Individual → Population → Community → Ecosystem

Types of Ecosystems in Class 8 Science Notes

Ecosystems may be natural or human-made. They may also be aquatic or terrestrial.

Aquatic Ecosystems

Aquatic ecosystems are water-based.

Examples include:

• Ponds
• Rivers
• Lakes
• Seas
• Oceans

Terrestrial Ecosystems

Terrestrial ecosystems occur mainly on land.

Examples include:

• Forests
• Grasslands
• Deserts
• Farms
• Large trees

Overlapping Ecosystems

Different ecosystems may overlap.

A river may pass through a forest or farmland. Plants and animals may move between these areas and connect them.

Human-Made Ecosystems

Humans create and manage some ecosystems.

Examples include:

• Agricultural fields
• Fish ponds
• Parks
• Gardens

Human-made ecosystems need regular care and management.

Interactions in CBSE Class 8 Science Chapter 12 Notes

Living and non-living components interact continuously. These interactions influence nutrition, respiration, reproduction and survival.

Biotic and Abiotic Interactions

Examples include:

• Plants use sunlight, water and carbon dioxide to prepare food.
• Fish use oxygen dissolved in water.
• Earthworms live in moist soil.
• Plants absorb minerals from soil.
• Animals depend on water for survival.
• Plant roots hold soil and reduce erosion.

Interaction Between Abiotic Components

Non-living components also influence one another.

Examples include:

• Sunlight increases water evaporation.
• Temperature affects soil moisture.
• Wind creates waves on water.
• Rain changes the water level of ponds and rivers.

Interaction Among Biotic Components

Living organisms interact through:

• Feeding
• Competition
• Pollination
• Reproduction
• Shelter
• Protection

A frog eats insects, while a snake may eat frogs or fish.

Why Does Every Organism Matter?

A change in one population may affect many other populations.

For example, fish may eat dragonfly larvae. Fewer dragonflies may allow more bees and butterflies to survive.

These insects pollinate flowers, helping plants form seeds. Therefore, fish populations may indirectly affect seed production near ponds.

Direct and Indirect Effects

An organism may have:

• A direct effect on another organism.
• An indirect effect through a chain of relationships.

Removing one organism can disturb several other populations.

Pollination in How Nature Works in Harmony Notes

Pollination is the transfer of pollen grains from the stamen to the carpel of a flower.

It is essential for the formation of fruits and seeds.

Pollination may be carried out by:

• Wind
• Water
• Insects
• Birds
• Bats

Bees and butterflies receive nectar from flowers. At the same time, they help flowers through pollination.

Producers and Consumers in Class 8 Science Revision Notes Chapter 12

Organisms can be grouped according to how they obtain food.

Producers

Plants make their own food through photosynthesis.

They are called:

• Producers
• Autotrophs

Green plants form the first level of most food chains.

Consumers

Organisms that cannot prepare their own food are called consumers or heterotrophs.

They depend on plants or other animals for nutrition.

Herbivores

Herbivores eat plants and plant products.

Examples include:

• Deer
• Hare
• Cow
• Grasshopper

Carnivores

Carnivores eat other animals.

Examples include:

• Leopard
• Snake
• Hawk
• Tiger

Omnivores

Omnivores eat both plants and animals.

Examples include:

• Crow
• Fox
• Mouse
• Humans

Producers and Consumers Table

Food Chains in Class 8 Chapter 12 Science Notes

A food chain shows a simple feeding sequence in an ecosystem.

It tells us who eats whom.

Examples of Food Chains

Grass → Hare → Leopard

Grass → Grasshopper → Frog → Snake → Eagle

The arrow points from the organism being eaten towards the organism that eats it.

Importance of Food Chains

Food chains show:

• Transfer of food
• Dependence among organisms
• Position of an organism in feeding relationships
• Effect of population changes

If frogs disappear, grasshoppers may increase and snakes may decrease.

Trophic Levels Class 8 Notes

Each feeding position in a food chain is called a trophic level.

Producers are usually more numerous than organisms at higher trophic levels.

Food Webs in How Nature Works in Harmony Notes

A food web is a network of interconnected food chains.

An organism may eat more than one type of food. It may also be eaten by several predators.

For example:

• Grass may be eaten by a hare, mouse and grasshopper.
• A mouse may be eaten by an owl, snake or fox.
• A snake may be eaten by an eagle.

Difference Between Food Chain and Food Web

Food webs show the real complexity of feeding relationships in nature.

Decomposers and Nutrient Recycling in Class 8 Science Chapter 12

Dead plants, dead animals and organic waste must be broken down.

Bacteria, fungi and some small organisms perform this work.

What Are Decomposers?

Decomposers are organisms that break down dead matter into simpler substances.

Examples include:

• Bacteria
• Fungi
• Mushrooms

They are also called saprotrophs.

What Is Decomposition?

Decomposition is the breakdown of complex dead material into simpler substances.

During decomposition:

1. Dead matter is broken down.
2. Nutrients return to the soil.
3. Plants absorb these nutrients.
4. Materials are reused in the ecosystem.

Importance of Decomposers

Decomposers:

• Clean the environment.
• Recycle nutrients.
• Support soil fertility.
• Prevent the accumulation of dead matter.
• Help plants obtain nutrients.

Without decomposers, dead material would collect and nutrients would not return to the soil.

Ecological Balance in Class 8 Science Chapter 12 Notes

An ecosystem remains balanced when populations and resources remain within suitable limits.

This balance is dynamic and may change naturally.

What Is Ecological Balance?

Ecological balance is the stable interaction among organisms and their environment.

It depends on:

• Suitable population sizes
• Availability of resources
• Feeding relationships
• Competition
• Decomposition
• Climate and physical conditions

How Does One Change Lead to Another?

A small ecological change can cause a chain of effects.

For example:

Pollution → Plants die → Less oxygen → Fish die → Insects increase → Crop damage increases

This type of chain reaction is called a cascading effect.

Frog Population Example

Frogs eat many agricultural insects.

Large-scale reduction in frog numbers can:

• Increase insect pests.
• Increase pesticide use.
• Harm soil and water.
• Affect human and environmental health.

India banned the export of frog legs after recognising such ecological damage.

Competition in Ecological Interactions Class 8

Organisms compete when they need the same limited resource.

They may compete for:

• Food
• Water
• Sunlight
• Space
• Shelter
• Mates

Competition helps control population sizes.

Without competition, one population may grow excessively and disturb ecosystem balance.

Mutualism, Commensalism and Parasitism

Organisms may live together in different types of relationships.

Mutualism

In mutualism, both organisms receive a benefit.

A bee receives nectar from a flower. The flower gets pollinated.

Commensalism

In commensalism, one organism benefits while the other is neither helped nor harmed.

An orchid receives physical support from a tree branch. The tree remains unaffected.

Parasitism

In parasitism, one organism benefits while the host is harmed.

A tick obtains blood from a dog. The dog may suffer irritation or disease.

Benefits of Ecosystems in Class 8 Science Notes

Humans depend on healthy ecosystems for survival and well-being.

Ecosystems provide:

• Clean air
• Water
• Food
• Fertile soil
• Timber
• Fibres
• Medicines
• Climate regulation
• Recreation
• Protection from natural hazards

Forest Ecosystem Benefits

Forests:

• Release oxygen.
• Absorb carbon dioxide.
• Support biodiversity.
• Hold soil in place.
• Maintain moisture.
• Provide food and medicines.

Aquatic Ecosystem Benefits

Ponds, rivers and lakes provide:

• Water
• Food
• Habitats
• Nutrient cycling
• Recreation
• Support for agriculture

Sundarbans Ecosystem

The Sundarbans contain the world’s largest mangrove forest.

They lie in the region where the Ganges and Brahmaputra river systems meet between India and Bangladesh.

Importance of the Sundarbans

Mangroves:

• Slow down strong winds.
• Reduce the impact of storm waves.
• Protect coastal settlements.
• Absorb carbon dioxide.
• Release oxygen.
• Support endangered plants and animals.

The Sundarbans were declared a UNESCO World Heritage Site in 1987.

Threats to the Sundarbans

The ecosystem faces threats from:

• Cutting mangrove trees
• Fuelwood collection
• Farming expansion
• Illegal hunting
• Pollution
• Untreated sewage
• Overuse of forest resources

Ecosystem Conservation Class 8

Conservation means protecting ecosystems, species and natural resources.

Major threats to ecosystems include:

• Deforestation
• Habitat loss
• Pollution
• Climate change
• Invasive species
• Overuse of resources
• Unsustainable land use

Protected Areas

Protected areas are regions set aside to conserve wildlife and habitats.

They include:

• National parks
• Wildlife sanctuaries
• Biosphere reserves
• Community-conserved areas

Examples include:

• Jim Corbett National Park
• Manas National Park
• Nilgiri Biosphere Reserve
• Chilika Lake
• Eaglenest Wildlife Sanctuary
• Hemis National Park
• Keibul Lamjao National Park
• Pirotan Island Marine National Park

Wildlife Corridors

Wildlife corridors connect separated habitats.

They allow animals such as elephants to move safely between forests and reduce conflict with people.

Human-Made Ecosystems

Human-made ecosystems are created to fulfil human needs.

Examples include:

• Farms
• Fish ponds
• Parks
• Gardens

They may support biodiversity when planned carefully. However, they need regular human management.

Sustainable Farming in Class 8 Science Chapter 12 Notes

Agriculture is a human-made ecosystem. Poor farming practices can damage soil, water and biodiversity.

Green Revolution

The Green Revolution increased food production through:

• Tractors and machines
• Synthetic fertilisers
• Pesticides
• Irrigation
• Improved crop production methods

These methods helped India improve food security.

Problems Caused by Excessive Chemical Use

Long-term overuse of synthetic fertilisers and pesticides may:

• Reduce friendly soil microorganisms.
• Lower humus content.
• Increase soil erosion.
• Harm earthworms and snails.
• Kill natural predators.
• Pollute water.
• Cause pesticide resistance.
• Reduce biodiversity.

What Is Monoculture?

Monoculture means repeatedly growing the same crop on the same land.

It can:

• Reduce biodiversity.
• Increase pest problems.
• Affect pollinators.
• Reduce soil quality.
• Increase dependence on chemicals.

Role of Humus

Humus is organic matter in soil.

It helps:

• Bind soil particles.
• Retain moisture.
• Support microorganisms.
• Improve soil fertility.
• Reduce erosion.

Sustainable Farming Practices

Sustainable farming may include:

• Compost and organic manure
• Crop rotation
• Mixed cropping
• Reduced pesticide use
• Natural pest control
• Protection of pollinators
• Efficient irrigation
• Recycling farm waste
• Maintaining soil organisms
• Avoiding excessive ploughing

Natural Predators in Farming

Some organisms control crop pests naturally.

For example, beetles may feed on harmful insects. Protecting these predators can reduce pesticide use.

Vrikshayurveda and Soil Health

The ancient text Vrikshayurveda emphasised regular soil nourishment.

It described organic preparations such as Kunapa Jala, made through the fermentation of plant and animal materials.

Important Terms in Class 8 Science Chapter 12

Quick Revision of Class 8 Science Chapter 12 How Nature Works in Harmony

• A habitat provides the conditions needed for survival.
• Biotic components are living.
• Abiotic components are non-living.
• A population contains organisms of the same type.
• Different populations form a community.
• A community and abiotic surroundings form an ecosystem.
• Producers make their own food.
• Consumers depend on other organisms.
• Food chains show who eats whom.
• Food webs connect many food chains.
• Trophic levels show feeding positions.
• Decomposers recycle nutrients.
• Mutualism benefits both organisms.
• Parasitism harms one organism.
• One ecological change may cause many other changes.
• Healthy ecosystems provide food, water, air and protection.
• Sustainable farming protects soil and biodiversity.

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