There are different types of living organisms in different environment of the world. In different regions of each habitat, there exists a vast dissimilarity in their climatic condition, weather and other abiotic and biotic factors. For these dissimilarity, the earth is inhabited by various different types of organisms. The organisms you see in the forest are different from those inhabited in the pond. There is a close relationship between biotic and abiotic components of these environments. Further, the animals and plants of an environment are dependent on one another for their survival. In this way, the living component of a particular environment is functioning and interacting with the abiotic components to form a relatively stable system, called ecosystem.

If you observe environment, you will see there is a continuous action-reaction among different components of the ecosystem. The garden near your house or school is the example of a small ecosystem.

Components of ecosystem

As you know, ecosystem consists of abiotic (non -living) and biotic (living)components.

Abiotic components: All non-living components of an ecosystem is abiotic components. These are classified into two categories: (a) Inorganic or physical components and (b) Organic substances. Inorganic component consists of minerals, soil, light, water, air, temperature, humidity, etc. Dead and decomposed organic materials of living organisms are known as organic substances. These inorganic and organic components are very essential for the survival of the living organism.

Biotic components: Biotic components include plants, animals and micro-organisms. As you have learnt in paragraph one, there is an interrelationship between biotic and abiotic components. To keep ecosystem functional, these living organisms play certain role and on the basis of their contribution biotic components are of three types, for example - (a) Producer (b) Consumer and (c) Decomposer.

(a) Producers: The green plants can produce food hence called producers of the ecosystem. They manufacture carbohydrate food in presence of sunlight by the process of photosynthesis. All the animals of an ecosystem are directly or indirectly dependent on this food.

(b) Consumers: The animals which live on eating organic matters obtained from plants or on other animals are known as consumers. These consumers are again divided into three groups-

Primary consumers: All animals which consume plants or plant parts are known as primary consumers. They are also called herbivores. From minute insect to many large animals are included in the group of herbivores. For example- cow, goats etc.

Secondary consumers: Those which live on eating primary consumers. For example- frog, bird, man etc. They are also called carnivores.

Tertiary consumers: Animals which feed on secondary consumers are called tertiary consumers. For example- frog, tortoise, stork, man etc. Some of these animals take food from more than one food level (trophic level). They are called omnivores. When we take pulse, rice, potato etc. we are primary consumers or herbivore. But when we eat fish and meat, we are secondary or tertiary consumers.

(c) Decomposers: These are wide range of microorganisms including bacteria and fungi. After the death of plants and animals these micro-organisms act on their bodies and decompose. Finally, the decomposed body forms different organic and inorganic components. Some of the nutrients are used by them and the rest are mixed up with the soil and air. The green plant again consumes those as a natural process. These action and reaction, between the biotic and abiotic components of the nature, has created the ecosystem as a potential self -supporting ecological unit.

Types of ecosystem

In natural environment, there are two types of ecosystem. They are ecosystem of land and ecosystem of water. Now we will learn about ecosystem of land and ecosystem of water.

Land ecosystem: Land ecosystem may be of different types. For example - ecosystem of forest, ecosystem of desert, etc. The forest region of Bangladesh may be mentioned as the example of forest ecosystem. The forest of Bangladesh is divided into two main regions.

(a) The forest of Sylhet and Chittagong hill tracts and (b) the Sundarbans forest of coastalregion of Khulna. A brief description of the ecosystem of Sundarbans are given below-

The Sundarbans is a single large forest of Bangladesh. The vegetation of Sunderban is different from the vegetation of other forests. The Sundarbans is situated in the southern region of Khulna district skirting the coastal belt of the Bay of Bengal. In the ecosystem of the Sunderbans the soil is of high salt density. As salinity increases coastward in the tidal and sub-tidal areas, there is a transition of dense mangrove vegetation. The soil of the Sundarbans is very muddy. So, it is not suitable for air passing. So the branch roots of the plants of this region grow erect and spread along the upper layer of the soil instead of inward. Root tip of these plants bear numerous spores through which atmospheric oxygen enters into the plant body for respiration. The sundari, garan, gewa, kewra, golpata, etc. are the major plants of the forest. They are the producers of the ecosystem. Insects, birds, deer, etc are primary consumers. Jackals, tortoises, cranes etc. are secondary consumers. Tiger, hogs, etc. are among the tertiary consumers. Among them hogs are omnivorous. The Royal Bengal Tiger, chita, monkey, spotted deer, wild hogs, crocodiles, different types of snakes, birds and insects are the major animals of the Sundarbans.

Aquatic ecosystem

Aquatic ecosystem is divided into three main types, i.e.-

(1) Ecosystem of ponds.

(2) Ecosystem of rivers.

(3) Marine ecosystem.

For your understanding ecosystem of a pond is described here. A classic example of an aquatic ecosystem is the ecosystem of a small pond. In fact, a pond is an ecologically independent and self-regulating unit. In a pond there exist abiotic and biotic components. The abiotic components are water, dissolved oxygen, carbon dioxide and some organic matters. Organisms can use these elements directly. The elements that constitute the biotic components of a pond ecosystem are producers, primary consumers, secondary consumers, tertiary consumers and decomposers of different types. In a pond ecosystem producers are minute floating or suspended small plants, these are known as phytoplankton. Water lily (Shapla), Eichhornia etc. are among floating macrophytes. Like minute floating plants, there are also some microscopic animals too. Those are known as zooplankton. Aquatic insects, small fish, mussels snails etc. feed on producer and is known as primary consumers. Medium sized fishes those live on eating the primary consumers are called secondary consumers. Again, big fish, stork etc. who eat secondary consumers are tertiary consumers. Bacteria, fungi decompose dead organisms. The decomposed substances again used by the producer of the pond ecosystem.