The term ecosystem was first used by A. G. Tansley in 1935 who defined ecosystem as 'a particular category of physical systems, consisting of organisms and inorganic components in a relatively stable equilibrium, open and of various sizes and kinds’. Later on, various definition had been proposed on it. The term 'eco' is related to ’environment' and the "system" means 'interacting groups'. Hence the term "ecosystem" is generally defined as inter-linking between the biotic and abiotic components of earth. Ecosystem can be divided into two types i.e. Natural ecosystem such as forests, grasslands,
deserts, and aquatic ecosystems and Man modified ecosystems include agricultural land and urban or industrial land use patterns.

All the ecosystems are connected to each other, e.g., ecosystem of river is connected with the ocean ecosystem and a small ecosystem of dead logs is a part of large forest ecosystem. A complete self-sustained ecosystem is rarely found in nature.

///STRUCTURE OF ECOSYSTEM -- Ecosystem is composed / made of two components i.e. Living thongs or Biotic Components and Non-living things or Abiotic Component.

//Biotic Components of ecosystem are the living components such as plants, animals etc. From nutrition point of view, the biotic components can be grouped into two. One those can produce their food own, which include all green plants and other, who depends on other components for food which include non-green plants, animals and all micro-organisms. 

But generally biotic components are divided into three types. 

1. Producers or Autotrophs --- Producers are also known as autotrophs, these organisms make their food own by own. They use energy of the sun and with the help of water, they convert that energy into useable energy in the form of sugar or food by the process of photosynthesis. The most common example are green-plants.

2. Consumers or Heterotrophs --- These organisms are also called as heterotrophs because they eat something else (hetero) i.e. synthesized by producer as food. For ex- all kinds of animals like Birds, insects etc. Mushroom, which looks like plants, but they are also heterotrophs.
---Consumers are further sub-divided into five classes such as primary consumers, secondary consumers, tertiary, top consumers and micro consumers.
---- Primary Consumers are herbivorous animals, which are dependent for their food on producers or green-plants. For ex- Elephant, cow, rabbit etc. in the terrestrial ecosystem and in the aquatic ecosystem.
--- Secondary Consumers are primary carnivores, who eats primary consumers as their food and Tertiary consumers are secondary carnivores, who eats secondary consumers as their food. For ex- snakes are secondary consumers whereas eagle is the tertiary consumers, who eats snakes.
--- Top Consumers are the omnivorous animals, who eats all herbivorous and carnivores’ animals as their food. For ex- Human, Pigs etc.

___-Besides that, some micro consumers like parasites, scavengers and saprobes are also included in the consumers. The parasitic plants and animals utilize the living tissues of different plants and animals. The scavengers and saprobes utilize dead remains of animals and plants as their food.

3. Decomposers or Reducers or Saprotrophs ---Decomposers are also called as Reducers or Saprotrophs. These are a group of organisms consisting of small animals like worms, insects, bacteria and fungi, which eats dead organic materials for their food.
These break down dead organic material into smaller particles and finally into simpler substances that are used by plants as nutrition. Decomposition thus is a vital function in nature, as without this, all the nutrients would be tied up in dead matter and no new life could be produced.

///FUNCTION OF AN ECOSYSTEM -- Function of an ecosystem is a very complex process, which can be understood through various process like Energy flow, Ecological succession, Biogeochemical cycle, Ecological Pyramid.

But a simple function of ecosystem can be understood through the lifestyle of members of ecosystem i.e. Producers, Consumers and Decomposers. According to which we divide function as three types

 //1. Primary Function or Productivity -- This is the production of food by producers by using sun light, water and soil nutrients.

 //2. Secondary Function or Energy Flow -- This is the distribution or flow of energy in the form of food from one tropical level to another or among consumers.

 //3. Tertiary Function or Material Cycling -- When living organism became dead, the decomposers eat them as food and in the meantime all the nutrients goes in to the environment, producing a suitable condition for the growth of producers.

-- Like these all the three function are inter-linked and inter-dependent forming a never-ending cycle.




 1. Biotic Components- The living components of pond ecosystem consists of:
   a) Producers- These are autotrophic green-plants and some photosynthesis bacteria. These fix radiant energy and with the help of minerals derived from the water and mud, they produce food. These can be divided into two types.
      i// Macrophytes or Hydrophytes -- These are rooted aquatic plants that grows in or near water partly or completely submerged, floating and include helophytes (a plant that grows in marsh)
          ii// Phytoplanktons --
          b) Consumers -- These are heterotrophs, which depends on macrophytes and phytoplanktons and they can be divided as below:
           i//Primary Consumers (Herbivorous) -- These feed living plants or plants remaining, which are further divided into two types
           ..>> Benthos -- These organisms live on, in or near rooted plants, which includes fish, mites, maluses.

           ..>> Zooplanktons -- These are chiefly the rotifers like Brachionous although some protozoans and crustaceans are also present. Usually these are microscopic but some are visible to our eyes. They feed chiefly on Phytoplanktons.
           ii// Secondary Consumers -- These are the primary carnivores, who eat primary consumers as food. These are mainly insects and fish.
           iii/// Tertiary Consumers -- These are the secondary carnivores, who eat primary carnivores as their food. For ex- large fish, who eats smaller fish.
           Besides that, some mammals like cows, buffalos etc.  also visit the pond feeding the rooted plants and some birds feeding insects and fish. 
          c) Decomposers -- These micro-organisms eat dead materials and play a vital role by returning minerals elements again to the pond. These are chiefly bacteria, fungi and actinomycetes.

  /// FUNCTION OF A POND ECOSYSTEM -- Most ponds become dry after the rains and covered by terrestrial plants. When a pond begins to fill during the rains, its life forms such as the algae and microscopic animals, aquatic insects, snails, and worms come-out of the floor of the pond where they have remained dormant in the dry phase. Gradually more complex animals such as crabs’ frogs and fish return to the pond. The vegetation in the water consists of floating weeds and rooted vegetation on the periphery which grow on the muddy floor under water and emerge out of the surface of the water.
As the pond fills in the monsoon a large number of food chains are formed. Algae is eaten by microscopic animals, which are in turn eaten by small fish on which larger fish depend. These are in turn eaten by birds. Aquatic insects, worms and snails feed on the waste material excreted by animals and the dead or decaying plant and animal matter. They act on the detritus, which is broken down into nutrients, which aquatic plants can absorb, thus completing the nutrient cycle in the pond. The temporary ponds begin to dry after the rains. Animals such as frogs, snails etc. remain dormant in the mud, awaiting the next monsoon.