Ecosystems | ICSE Class 8 Biology Notes

This note covers ecosystems, living and non-living components, forest life, producers, consumers, decomposers, food chains, food webs, feeding levels, pyramids of numbers, energy flow, nutrient recycling, symbiosis, parasitism and predation.

What is an ecosystem, and how is it organised?

An ecosystem is a system in which living organisms interact with one another and with their non-living surroundings. An organism is an individual living thing. A forest ecosystem includes its plants, animals and microorganisms together with air, water, soil, sunlight and other physical conditions.

Microorganisms, also called microbes, are living organisms too small to see individually with the unaided eye; many bacteria are examples.

Biotic components are the living components of an ecosystem. Abiotic components are its non-living components. These groups interact: plants use water and sunlight, animals depend on food, and living roots help hold soil in place. An ecosystem therefore includes relationships as well as components.

Definition: Ecology is the study of relationships among living organisms and between organisms and their physical environment. The ecosystem is a basic unit for studying these interactions.

How do habitat, population and community differ?

A habitat is the place where an organism lives. Even the bark of a tree can be a habitat. A population is a group of organisms of the same kind living in a particular habitat at a given time.

A community comprises different populations sharing a habitat. Its plants, animals and microorganisms interact and depend on one another.

The community is the living part of an ecosystem. To describe the whole ecosystem, include the physical environment and its interactions with that community. A list containing trees and birds describes living components, but leaves out the water, soil and other conditions supporting them.

How large can an ecosystem be?

Ecosystems can be large or small. A large tree and a forest can both be studied as ecosystems. Terrestrial means land-based, while aquatic means water-based. A forest is a terrestrial ecosystem; its organisms and their relationships provide a useful way to study how an ecosystem functions.

How do abiotic factors influence forest life?

An abiotic factor is a non-living feature or condition that influences organisms. Forest life depends on air, soil, water and climatic factors. Climate means the weather conditions of a place considered over a long period. Sunlight, temperature, humidity and wind form part of the physical surroundings.

Temperature describes how hot or cold something is. Humidity refers to water vapour in the air. Wind is moving air. These terms describe different conditions, so humidity should not be used as another name for temperature or for water present in soil.

What do non-living components provide?

Nutrients are substances organisms need for growth and other life processes. Respiration is the process by which cells release energy from food. Air and soil are therefore more than empty surroundings: they supply materials needed for living processes.

Decomposition is the breakdown of dead matter into simpler substances. This process links materials in organisms with materials in soil, and its rate depends partly on physical conditions.

Can organisms change their physical surroundings?

The relationship works in both directions. Plants release oxygen while making food, roots hold soil in place and help prevent erosion, the removal of soil. Plants also retain soil moisture and help cool the atmosphere. Living components can influence non-living conditions.

Conditions within a habitat need not remain the same throughout the day. A forest might be warm during the day and cool at night. Different organisms in that habitat may use resources at different times and experience different conditions.

Temperature and soil moisture also affect the forest floor. Warm and moist conditions favour decomposition, the breakdown of dead matter into simpler substances. Low temperature inhibits this process. Thus physical conditions influence the return of nutrients as well as the growth of living plants.

What roles do producers, consumers and decomposers perform?

Producers are organisms that make their own food. Green plants use photosynthesis, the process of making food from carbon dioxide and water using light energy. Chlorophyll, their green pigment, helps capture that light. Trees and grasses supply food to a forest community.

Producers are also called autotrophs, meaning organisms that make their own food. Consumers depend on food made by other organisms. Consumers obtain food by eating producers or other consumers; they are heterotrophs, organisms dependent on other organisms for food.

How are consumers grouped?

A herbivore eats plants and plant products. Deer and hares are examples. A carnivore eats animals, as a leopard does. An omnivore eats both plants and animals; examples include crows, foxes and mice. These names describe feeding habits.

Decomposers, especially bacteria and fungi, break down dead remains and waste into simpler substances. Fungi are organisms that include mushrooms and obtain food from other material rather than making it by photosynthesis. Some fungi act as decomposers, while others have different relationships with living organisms.

Why are all these roles connected?

Producers make food available to other organisms. Consumers transfer that food through feeding relationships. Decomposers return nutrients from dead material to the surroundings, making them available to plants again. These roles link the living community with the soil and other non-living components.

A mushroom growing on dead plant material is obtaining food from that material. It is not a green producer simply because it remains in one place. Classification by ecological role depends on how an organism obtains food, rather than whether it moves about.

How does a food chain show feeding relationships?

A food chain is a sequence showing who eats whom in an ecosystem. A forest example is green plants eaten by deer, followed by deer eaten by a tiger. Food, and the energy stored in it, passes along this feeding sequence.

Write the sequence as green plants → deer → tiger. The arrow symbol, →, means that food and energy pass from the organism on its left to the organism on its right. It points from the food organism towards the organism eating it.

What is a trophic level?

A trophic level is a feeding position in a food chain. Producers occupy the first trophic level. Primary consumers eat producers and occupy the second level. Secondary consumers eat primary consumers and occupy the third level.

Tertiary consumers feed on secondary consumers and occupy the next level in such a chain. These labels describe the feeding relationships being considered. In the plant, deer and tiger chain, the deer is a primary consumer and the tiger is a secondary consumer.

The drawing shows green vegetation below a deer and a tiger above it. Arrows lead from the vegetation to the deer and from the deer to the tiger.

See Fig. 13.1a in your NCERT textbook

How should a food chain be constructed?

1. Identify the producer that supplies food at the start of the feeding sequence.
2. Place the animal that eats that producer after it as the primary consumer.
3. Place the animal that eats the primary consumer next, using a feeding relationship actually given or observed.
4. Join the organisms with arrows pointing towards the eater, then label their trophic levels.

A chain is not just a list of organisms found in one place. Its neighbours must be linked by feeding. The arrow from deer to tiger records the tiger eating the deer, not the direction in which either animal moves through the forest.

Why are food webs more complex than food chains?

A food web is a network of interconnected food chains. Organisms can have more than one feeding relationship. Each organism is generally eaten by two or more other kinds of organisms, which in turn are eaten by several others. Branching connections represent these relationships.

A food chain follows one route through feeding relationships. A food web combines routes that share organisms. Both diagrams concern food and energy transfer, but a web shows more of the community's connections than a single straight chain.

How should a food web be read?

Begin with the producers and follow an arrow towards a consumer. Continue along linked arrows to trace a food chain within the web. If an organism has several feeding connections, examine each separately. Do not assume every animal eats every other organism shown.

The drawing places green plants along the bottom, with several animals above them and a tiger at the top. Many branching arrows connect the organisms, showing a network rather than one feeding sequence.

See Fig. 13.3 in your NCERT textbook

Can an organism have more than one feeding position?

A trophic level describes a function in a feeding relationship. A sparrow feeding on seeds, fruits or peas is a primary consumer. When it eats insects and worms, it acts as a secondary consumer. Its position depends on the food being considered.

This explains why a species need not have one fixed trophic level in every connection of a food web. Species here means a particular kind of organism. A web can represent several feeding roles that a simple chain, showing one route, leaves out.

Food-web connections also explain interdependence. A change involving one organism can affect others linked to it. However, a feeding diagram alone does not supply exact future population sizes. Those require information beyond the names and arrows in the diagram.

How does energy flow through an ecosystem?

In a forest, energy enters through sunlight captured by green plants. Photosynthesis converts this light energy into chemical energy, energy stored in food. Consumers obtain it by feeding, and decomposers obtain energy from dead material. The organisms are linked by these transfers.

Energy flow is unidirectional, meaning it moves in one direction. Energy captured by producers does not return to the Sun. Energy passed to herbivores does not flow back to producers. At successive feeding levels, the amount of available energy decreases.

What happens during energy transfer?

1. Green plants capture light energy and use it to produce food, introducing food energy into the forest community.
2. Herbivores eat plant material and obtain energy for their own activities, growth and reproduction.
3. Carnivores obtain energy by feeding on other animals, transferring some of the food energy to a further feeding level.
4. At each transfer, energy is used in living processes and some is lost as heat to the environment.
5. When organisms die, their dead material provides an energy source for decomposers.

An average of 10%, meaning ten per cent, can be taken for the organic matter passed from one feeding level to the next. Organic matter means material making up or derived from organisms. This is an average, not an exact transfer for every individual feeding event.

Why does a food chain have limited length?

With much energy lost at each step, less remains available to the next consumer. Food chains generally consist of only three or four steps. Keep the word generally: the statement describes a common pattern, rather than banning every longer chain.

The forest therefore needs an ongoing supply of energy. Decomposition returns nutrients, but it does not turn heat lost to the environment back into food energy for plants. Distinguishing energy flow from nutrient recycling is essential for understanding how the ecosystem continues to function.

What does a pyramid of numbers show?

A pyramid of numbers represents the number of organisms at successive trophic levels. Producers are represented at its base, with consumers at higher levels. Each level describes the organisms occupying that feeding position, rather than the size of one organism's body.

In most ecosystems, pyramids of numbers are upright: producers are more numerous than herbivores, and herbivores are more numerous than carnivores. Upright means broad at the base and narrower towards the top. The word most matters because exceptions occur.

Why can a tree-based pyramid have a different shape?

A big tree can support many feeding insects. The producer level can therefore contain fewer individuals than the insect consumer level above it. Small birds may depend on those insects, while larger birds eat the smaller birds. This arrangement need not form a simple upright pyramid.

A tree counts as an individual producer even though it is much larger than an individual insect. Counting organisms does not measure the food present in their bodies or the energy available at each level. A narrow producer bar need not mean a small producer organism.

How should numbers be represented?

Identify the feeding levels first. Then use the numbers supplied for those levels to represent their relative sizes. Include all organisms counted at each level, rather than selecting a few convenient individuals. Do not invent counts merely to make the outline look like a pyramid.

Note: Keep producers at the base even when consumers are more numerous. A pyramid of numbers shows counts organised by feeding position; it is not an instruction to rearrange feeding levels from largest count to smallest.

The crucial comparison is between number and feeding position. The tree example shows why a statement about energy decreasing along a chain cannot simply be replaced with “the number of organisms always decreases”. These are different measurements of an ecosystem.

How do decomposers recycle nutrients on the forest floor?

Fallen leaves, bark, flowers, dead animals and animal waste provide material for decomposition. Such dead remains and waste are called detritus. Bacteria and fungi break down complex substances in this material into simpler substances, including nutrients that return to the soil.

Plants can take up nutrients from the soil again. Nutrient cycling is this repeated movement of nutrients between organisms and their surroundings. It connects the forest floor with new plant growth and with the consumers that depend on that growth.

What links a fallen leaf to later plant growth?

1. A tree produces leaves, and a leaf eventually falls to the ground, becoming part of the dead material on the forest floor.
2. Organisms acting on dead material help break it down; bacteria and fungi change complex substances into simpler ones.
3. Nutrients released during decomposition enter the soil, contributing to the materials available around plant roots.
4. Plants absorb nutrients from the soil and use them in growth, returning those materials to the living community.

These steps trace the return of materials. They do not imply that every action during decomposition waits for the previous action to finish. Different breakdown processes can operate simultaneously on dead material.

The diagram shows a tree, a fallen green leaf, partly decomposed leaves and organic-rich soil. Branching arrows connect feeding, decomposition, nutrients entering soil and the soil supporting the growing tree.

See Fig. 12.1 in your NCERT textbook

What is humus, and what affects breakdown?

Humus is dark material formed during decomposition in soil. It is highly resistant to microbial action and decomposes at an extremely slow rate. It serves as a reservoir of nutrients. Some microbes break it down further and release nutrients.

Decomposition is largely an oxygen-requiring process. Temperature and soil moisture strongly influence it through their effects on microorganisms. Warm and moist conditions favour decomposition; low temperature and conditions without oxygen inhibit it. This links decomposer activity with the abiotic conditions of the forest floor.

How do symbiosis, parasitism and predation differ?

Organisms interact in ways that benefit both partners or benefit one while harming another. In the mutually useful association called symbiosis here, two different organisms live closely together and help one another. Lichens provide an example of this relationship.

A lichen is an association between an alga and a fungus. Algae are simple organisms that include forms capable of photosynthesis. The alga makes food, while the fungus provides shelter and absorbs water and mineral nutrients for its partner.

What is mutualism?

Mutualism is an interaction in which both organisms benefit. A lichen is a close mutualistic association. Honeybees and flowers also show mutualism: the bee receives nectar, a sugary floral liquid, while the flower is pollinated.

Pollination is the transfer of pollen from the male part of a flower to its female receiving part. Pollen is the dust-like material produced by the male flower parts. Pollination helps flowering plants form seeds, linking animal activity with plant reproduction.

How does parasitism affect the partners?

Parasitism is a relationship in which a parasite benefits while its host is harmed. A parasite obtains nourishment from a living organism called its host. Ticks feeding on a dog's blood benefit, while the dog suffers skin irritation.

Cuscuta is a parasitic plant that derives nutrition from a host plant. A parasite is therefore not necessarily an animal. Many parasites have specialised relationships with their hosts, but their feeding role differs from making food through photosynthesis.

What happens in predation?

Predation is a feeding interaction in which a predator captures and eats prey. The predator is the eater; the prey is the organism eaten. A tiger feeding on deer illustrates predation. Predators transfer energy and help control prey populations.

The illustrations pair a flower with a bee, an orchid with a tree branch, and a dog with a tick. Their labels identify pollination and nectar, physical support without harm to the branch, and skin irritation with blood feeding.

See Fig. 12.16 in your NCERT textbook

The orchid example illustrates commensalism: one organism benefits while the other is unaffected. The orchid receives physical support from the tree. This differs from both mutualism and parasitism, so sharing a living space does not by itself identify the relationship.

How does a forest function as an interconnected ecosystem?

Flora means the plant life of an area; fauna means its animal life. A forest's flora and fauna interact with each other, with microorganisms and with physical conditions. Studying the trees alone therefore leaves out much of the functioning ecosystem.

How is forest life arranged?

Plants occupy different vertical layers. Trees occupy the upper layer, shrubs the next, and herbs and grasses the lower layers. Shrubs are woody plants smaller than trees; herbs are plants with soft stems. This layered arrangement is called stratification.

Deer feed on grass and leaves, while predators such as tigers and leopards depend on prey. Decomposers act on fallen plant material and dead animals. Air, soil, water, light and temperature affect these living components, which in turn influence their surroundings.

What happens when forest conditions change?

Changes in rainfall and temperature affect vegetation. Cutting down trees for roads and buildings contributes to the drying and shrinking of forests. Loss of habitat affects animals as well as plants, because food, water and living space are connected with the forest environment.

When vegetation is scarce and waterholes dry up, elephants may wander into nearby farms or plantations in search of food. This can damage crops and, at times, harm people and domestic animals. These movements connect habitat conditions with interactions between wildlife and people.

Wildlife corridors connect forest habitats and allow animals to move between larger forest areas. They help provide routes for movement without conflict with human settlements. Their importance follows from the need for connected habitats, rather than from considering each forest patch in isolation.

Does ecological balance mean nothing changes?

Ecological balance involves interactions that keep populations and resources stable. It is dynamic, not fixed: natural changes and human activities can disrupt it. Forests also provide people with fresh air, fertile soil, food, fibres, timber and medicines, making these connections important to human well-being.

Glossary

• Ecosystem — A system of living organisms interacting with one another and with their non-living surroundings.
• Habitat — The place where an organism lives and finds conditions supporting its survival.
• Population — A group of organisms of the same kind living in a habitat at a given time.
• Community — Different populations sharing a habitat and interacting as its living components.
• Biotic components — The living parts of an ecosystem, including plants, animals and microorganisms.
• Abiotic components — The non-living parts of an ecosystem, including air, water, soil and physical conditions.
• Producer — An organism that makes its own food, as green plants do through photosynthesis.
• Consumer — An organism obtaining food by feeding on producers or on other consumers.
• Decomposer — An organism that breaks down dead material into simpler substances, returning nutrients to the surroundings.
• Food chain — A sequence of organisms linked by feeding, showing who eats whom.
• Food web — A network of interconnected food chains showing several feeding relationships within an ecosystem.
• Trophic level — The feeding position occupied by an organism in a particular food chain.
• Pyramid of numbers — A representation of the numbers of organisms occupying successive trophic levels.
• Symbiosis — A close, mutually useful association between different organisms, illustrated by the partners in a lichen.
• Parasitism — A relationship in which a parasite benefits through association with a host that is harmed.

Common errors and misconceptions

• Misconception: An ecosystem is just a collection of animals. Correct: It includes plants, animals and microorganisms interacting with one another and with non-living components.
• Misconception: A food-chain arrow points from the hunter towards its food. Correct: It points from the food organism towards the consumer, showing the direction of food and energy transfer.
• Misconception: Every organism occupies one fixed trophic level. Correct: An organism can occupy different levels according to what it eats, as a sparrow does when eating seeds or insects.
• Misconception: Every pyramid of numbers is upright. Correct: Most are upright, but a big tree supporting many insects gives a different relationship between producer and consumer numbers.
• Misconception: Energy is recycled into plants by decomposers. Correct: Nutrients are recycled. Energy moves through feeding relationships and becomes progressively less available because energy is lost at each level.
• Misconception: All fungi are producers because they do not move about. Correct: Fungi obtain food from other material. Decomposer fungi break down dead remains rather than making food by photosynthesis.
• Misconception: Organisms living together must both benefit. Correct: Mutualism benefits both, parasitism benefits one while harming the host, and commensalism benefits one while leaving the other unaffected.
• Misconception: Ecological balance means populations and surroundings never change. Correct: Balance is dynamic. Interactions maintain it, and natural changes or human activities can disrupt it.

Exam-style questions with model answers

Q1. In a forest, deer eat leaves while tree roots absorb water from soil. Identify one interaction between living components and one between living and non-living components from these observations. [2 marks]

1. The deer eating leaves is an interaction between biotic components: the deer and the plant are living organisms.
2. The roots absorbing water from soil show interaction between biotic and abiotic components: living roots use non-living water in soil.

Q2. Green plants are eaten by deer, and the deer are eaten by a tiger. Write this food chain with arrows and explain the feeding role of each organism in three points. [3 marks]

1. Green plants → deer → tiger is the food chain. Green plants are producers and occupy the first trophic level; they make food through photosynthesis.
2. The deer eats the producers, so it is the primary consumer at the second trophic level. Food and energy pass from the plants to the deer.
3. The tiger eats the deer, so it is the secondary consumer at the third trophic level in this chain. The arrows point towards the organism receiving food and energy.

Q3. A sparrow eats seeds in one feeding route and acts as a secondary consumer when eating insects in another. Identify its trophic level in each case and explain why a food web can show this better than one food chain. [3 marks]

1. When eating seeds, the sparrow obtains food directly from a producer. It therefore acts as a primary consumer at the second trophic level.
2. When acting as a secondary consumer by eating insects, it consumes primary consumers. It therefore acts as a secondary consumer at the third trophic level in that feeding route.
3. A food web combines interconnected chains and can show both routes. A single chain follows one sequence, so it need not display both of the sparrow's feeding roles.

Q4. A big tree supports many plant-eating insects; small birds feed on these insects, and larger birds eat the small birds. Without inventing counts, explain in four points how to interpret a pyramid of numbers for this arrangement. [4 marks]

1. Place the tree at the producer level at the base. Its large physical size does not make it count as many individual producers.
2. The many plant-eating insects belong to the primary-consumer level. Their number exceeds the number of trees specified in the question.
3. The small birds are secondary consumers and the larger birds are tertiary consumers. Their exact numbers are not supplied, so exact bar widths cannot be calculated.
4. The producer level is narrower in number than the insect level. Thus the diagram need not be a simple upright pyramid, although the feeding levels retain their order.

Q5. On a forest floor, fallen leaves are broken down by bacteria and fungi; nutrients enter the soil and are absorbed by tree roots. Explain nutrient recycling in five separate points, and distinguish it from energy flow. [5 marks]

1. The fallen leaves are detritus, meaning dead organic material. They provide material on which decomposers act, linking the living tree with processes on the forest floor.
2. Bacteria and fungi act as decomposers. They break down complex substances in the leaves into simpler substances, rather than producing food by photosynthesis.
3. Nutrients released through this breakdown enter the soil. Decomposition therefore returns materials from dead organisms to the non-living surroundings of the forest.
4. Tree roots absorb nutrients from the soil, and plants use them in growth. The movement from plants through dead material and soil back to plants is nutrient cycling.
5. Energy behaves differently: it moves in one direction through the ecosystem, with losses as heat. Nutrient recycling does not restore lost heat as food energy for producers.

Q6. In a lichen, the alga makes food and the fungus provides shelter, water and mineral nutrients. A tick feeds on a dog's blood and irritates its skin. A tiger captures and eats a deer. Classify and explain these three relationships. [3 marks]

1. The lichen shows symbiosis in the mutually beneficial sense: the alga supplies food and the fungus supplies shelter, water and mineral nutrients, so both partners benefit.
2. The tick and dog show parasitism. The tick is the parasite obtaining nourishment, while the dog is the host and is harmed by skin irritation.
3. The tiger and deer show predation. The tiger is the predator that captures and eats the deer; the deer is the prey supplying food.

Q7. Sunlight reaches green forest plants; deer eat the plants, and tigers eat the deer. Organisms use energy in living processes, lose some as heat, and eventually leave dead material. Explain energy movement in this ecosystem in five points. [5 marks]

1. Green plants capture light energy during photosynthesis and convert it into chemical energy in food. They introduce food energy into the living forest community as producers.
2. Deer obtain energy by eating the plants. They are primary consumers in this sequence, receiving food energy that was first captured by the producers.
3. Tigers obtain energy by eating the deer. They are secondary consumers in the given chain, so energy passes to them through another feeding step.
4. Organisms use energy for living processes, and some energy is lost as heat at each transfer. Consequently, less energy remains available at successive feeding levels.
5. Dead material supplies energy to decomposers. Energy flow remains unidirectional: energy transferred to consumers does not return to producers, and captured energy does not return to the Sun.

Q8. In a forest, vegetation becomes scarce and waterholes dry up. Elephants may enter nearby farms in search of food. Explain one abiotic change and one resulting connection with animal behaviour. [2 marks]

1. Drying waterholes reduce available water, changing an abiotic component that animals depend on in their forest habitat.
2. Scarce vegetation and water can lead elephants to search for food in nearby farms; this is a possible response, not an inevitable outcome.

Key takeaways

• An ecosystem includes living organisms, non-living surroundings and the interactions linking them into a functioning system.
• Producers make food, consumers obtain food from other organisms, and decomposers return nutrients from dead material to the surroundings.
• Food-chain arrows point from food towards the eater, showing the direction in which food and energy pass.
• Food webs connect several chains, and an organism's trophic level can change with the food it eats.
• Energy flows in one direction and becomes less available at successive feeding levels; nutrients move through repeated cycles.
• A pyramid of numbers counts organisms at feeding levels; a large tree supporting many insects can give an exception to the upright pattern.
• Mutualism benefits both partners, parasitism benefits a parasite while harming its host, and predation links a predator with its prey.
• A forest's plants, animals, decomposers and physical conditions are interdependent, and changes in habitat can affect animal behaviour.

Test yourself

How does a community differ from an ecosystem?

A community consists of interacting populations. An ecosystem includes that living community, its non-living surroundings and the interactions between them.

What does humidity mean?

Humidity refers to the presence of water vapour in the air, an abiotic condition of the surroundings.

In plants → deer → tiger, which organism is the primary consumer?

The deer is the primary consumer because it feeds directly on the plants, which are producers.

Why can the base of a tree-based pyramid of numbers be narrow?

A big tree is one producer but can support many feeding insects. The diagram represents individual numbers, not body size.

Why is nutrient cycling different from energy flow?

Nutrients return to the surroundings and can be taken up again. Energy flows in one direction, with losses as heat.

What does each partner contribute to a lichen?

The alga prepares food, while the fungus provides shelter and absorbs water and mineral nutrients for its partner.

Why are decomposers important to plants?

Decomposers release nutrients from dead material into the soil, where plants can absorb and reuse them in growth.

Does ecological balance mean a forest never changes?

No. Ecological balance is dynamic and depends on interactions. Natural changes and human activities can disrupt it.