“Everything  that surrounds an organism affects is  called environment.”

The environment includes everything that surrounds an organism and which affects or may affect in the future the life of that organism. The environment is composed of non-living and living components. The non-living or abiotic parts of the environment include such factors as light, water, soil, and temperature. The living or biotic factors include plants, animals, and microorganisms as well as human beings.


The type of environment in which an organism life is called its habitat. Many birds make nests on the branches of trees. The branches of trees are the habitat of these birds. The common Rohu fish lives in our freshwater streams and rivers. The stream or a river is the habitat of Rohu.

Population: A group of organisms of the same species located in the same place or habitat at a particular time is called population.

Community: No specie or population lives

alone. More often, a place is a common habitat of organisms belonging to different species of plants and animals. Different species or populations living together and interacting with one another constitute a community.


Ecosystems are communities interacting with their physical environment. Living organisms (biotic components) interact among themselves as well as with their physical environment (abiotic components). As a result of this interaction, a balanced environmental or ecological system that we call an ecosystem, comes into existence. Ecosystems vary in size. A small pond, a patch of grass, and even a tree can be regarded as an ecosystem. Oceans, forests, rivers, and lakes are examples of large ecosystems.

Classification of Ecosystems:

Natural ecosystems may be classified as aquatic ecosystems and land ecosystems. Aquatic ecosystems may be freshwater ecosystems (streams, rivers, ponds, and freshwater lakes) or marine (seas and oceans). Land ecosystems include forests, deserts, and grasslands.


Living and non-living things in an ecosystem are linked with each other, where a continuous exchange of materials takes place between them. Non-living materials such as water, carbon dioxide, oxygen, and nitrogen are taken up by organisms and are made a part of their bodies.

Carbon Cy. Nitrogen

As these materials are taken up and made part of living matter (biomass) in the bodies of organisms, it is passed from one organism to the other when organisms consume one another as food. Ultimately, these materials are replaced in nature through respiration, combustion, and decay by microorganisms. This flow of materials in the ecosystem continues in the form of cycles.


Like all other systems, whether natural or artificial, energy is required to maintain an ecosystem. The Sun is the main source of energy. Sunlight is used by plants to make food. This food is used by the plants themselves and also by all other living organisms.

Without plants, humans and all other animals would starve to death. This is because they cannot make their own food. The only way animals can obtain energy is by eating, or consuming plants or other animals. Animals are called consumers while plants are called producers.


Food Chain:

Major interactions among organisms in the ecosystem involve feeding relationships. Organisms depend upon one another for food. You would have realized that one organism is eaten by a second, who is eaten by a third, and so on. This series of steps of eating and being eaten is called a food chain. For example:


Example: Every plant or animal in a food chain is called a link. It must be evident from the above two chains that the number of links may vary from one food chain to the other. Most food chains start with green plants.

A herbivore is a primary consumer. It feeds directly on plants. An animal that feeds on primary consumers is called a secondary consumer. Animals consuming secondary consumers are called tertiary consumers. Some animals and humans feed at different levels in the food chain and are therefore called omnivore.


When a food chain is shown in a diagram in the form of feeding levels, it takes

the shape of a pyramid. The lowest level in the food chain or food pyramid is occupied by the producers (plants). The higher levels are successively occupied by primary, secondary, and tertiary consumers. A food pyramid also shows how consumers can get larger in size but smaller in number as we go along the food chain.


Most animals feed one or more than one kind of animals. Therefore, many food chains exist in the ecosystem and form a kind of network or a ‘food web’. For example, a snake does not feed on frogs alone. It also eats lizards, rats, birds and even rabbits. Birds eat grains. They also consume insects, spiders, and worms. If we put all this in the form of a figure, it takes the form of a network or a web.



Energy flows from one link to the next along food chains as producers are eaten
by consumers, and consumers eat each other. Only about 10 percent of energy is
transferred from one level to the next. Upto 90 percent of energy is lost as heat when plants and animals respire and perform, life activities. Since there is less energy available • at each feeding level in a food chain, fewer consumers can live at successively higher levels.


Under normal conditions, natural ecosystems maintain an equilibrium between plants and animals and their non-living environment. We call this as ‘Balance in nature’., Whenever this equilibrium is disturbed, the whole ecosystem is disturbed.

An ecosystem has the ability to withstand changes of low magnitude and may return to its original state: However, if the disturbance is large, the ecosystem may not be able to absorb it and .consequently bear irreparable damages. Imbalance in nature and pollution are closely related. For instance, changes caused by pollution in the composition of air, soil, or water of an area will have harmful effects on the life of plants and animals.

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