Ecosystem is a large community of living organisms like plants, animals and microbes in a particular area. 

Terrestrial and the aquatic

• Examples of terrestrial ecosystems are grassland,Forest, and desert.
• Examples of aquatic ecosystems are lake,wetland,pond, river.
• Crop fields and an aquarium may also be considered as man-made ecosystems.
• Vertical distribution of different species occupying different levels is called stratification.

Ecosystem of different components are:

(i) Productivity; 

(ii) Decomposition; 

(iii) Energy flow; and 

(iv) Nutrient cycling.  

∙ The consumers are represented by the zooplankton, the free swimming and bottom  dwelling forms. 

∙ The decomposers are the fungi, bacteria and flagellates especially abundant in the  bottom of the pond. 

∙ This type of system performs all functions of any ecosystem and of the biosphere as a  whole, i.e., conversion of inorganic into organic material with the help of the  radiant energy of the sun.

Primary type production:

It is called as the amount of organic matter  produced per unit area over a time period by plants during photosynthesis. ∙

∙ It can be divided into gross primary productivity (GPP) and net primary  productivity (NPP). 

∙ Gross primary productivity of an ecosystem is the rate of production of organic  matter during photosynthesis. 

∙ A considerable amount of GPP is utilised by plants in respiration. ∙ Gross primary productivity minus respiration losses (R), is the net primary  productivity (NPP). 

∙ Net primary productivity is the available biomass for the consumption to  heterotrophs (herbiviores and decomposers). 

∙ Secondary productivity is defined as the rate of formation of new organic matter  by consumers. 

∙ Primary productivity depends on the plant species inhabiting a particular area. ∙ It also depends on a variety of environmental factors, availability of nutrients and  photosynthetic capacity of plants.  

∙ Of this, despite occupying about 70 per cent of the surface, the productivity of the  oceans are only 55 billion tons. 

∙ The important steps in the process of decomposition are fragmentation, leaching,  catabolism, humification and mineralisation. 

∙ Detritivores (e.g., earthworm) break down detritus into smaller particles. This  process is called fragmentation. 

∙ The process of leaching, water soluble inorganic nutrients go down into the soil  horizon and get precipitated as unavailable salts. 

∙ The catabolism is called Bacterial and fungal enzymes degrade detritus into simpler inorganic substances.  

∙ Being colloidal in nature it serves as a reservoir of nutrients. 

∙ Decomposition is largely an oxygen-requiring process. 

∙ Moist and Warm environment favour decomposition whereas low temperature and  anaerobiosis inhibit decomposition resulting in build up of organic materials. ∙ The detritus food chain (DFC) begins with dead organic matter.

∙ It is made up of decomposers which are heterotrophic organisms, mainly fungi and  bacteria. 

∙ They meet their energy and nutrient requirements by degrading dead organic  matter or detritus. 

∙ These are also known as Saprotrophs (sapro:to decompose). 

∙ As against this, in a terrestrial ecosystem, a much larger fraction of energy flows  through the detritus food chain than through the GRAZING FOOD CHAIN. ∙ Detritus food chain may be connected with the grazing food chain at some levels:  some of the organisms of DFC are prey to the GFC animals, and in a natural  ecosystem, some animals like cockroaches, crows, etc., are omnivores. ∙ These natural interconnection of food chains make it a food web. ∙ The consumers that feed on these herbivores are carnivores, or more correctly  primary carnivores (though secondary consumers). 

∙ Those animals that depend on the primary carnivores for food are labeled  secondary carnivores. 

∙ Organisms occupy a place in the natural surroundings or in a community  according to their feeding relationship with other organisms. 

∙ Based on the source of their nutrition or food, organisms occupy a specific place  in the food chain that is known as their trophic level.  

∙ When any organism dies it is converted to detritus or dead biomass that serves as  an energy source for decomposers. 

∙ Organisms at each trophic level depend on those at the lower trophic level for their  energy demands. 

∙ Each trophic level has a certain mass of living material at a particular time called  as the standing crop. 

∙ The standing crop is measured as the mass of living organisms (biomass) or the  number in a unit area. 

∙ The biomass of a species is expressed in terms of fresh or dry weight.

∙ Biomass measurement of dry weight is more accurate.

∙ Each bar in the energy pyramid indicates the amount of energy present at each  trophic level in a given time or annually per unit area. 

∙ These changes lead finally to a community that is in near equilibrium with the  environment and that is called a climax community. 

∙ The individual transitional communities are termed seral stages or seral  communities. 

∙ Succession is hence a process that starts where no living organisms are there – these could be areas where no living organisms ever existed, say bare rock; or in  areas that somehow, lost all the living organisms that existed there. 

∙ The former is called primary succession, while the latter is termed secondary  succession. 

∙ Examples of areas where primary succession occurs are newly cooled lava, bare  rock, newly created pond or reservoir. 

∙ The establishment of a new biotic community is generally slow. ∙ Before a biotic community of diverse organisms can become established, there  must be soil. 

∙ Depending mostly on the climate, it takes natural processes several hundred to  several thousand years to produce fertile soil on bare rock.  

∙ Since some soil or sediment is present, succession is faster than primary  succession. 

∙ Based on the nature of the habitat – whether it is water (or very wet areas) or it is  on very dry areas – succession of plants is called hydrach or xerarch, respectively.

∙ Hydrarch succession takes place in wetter areas and the successional series  progress from hydric to the mesic conditions. 

∙ As against this, xerarch successiontakes place in dry areas and the series progress  from xeric to mesic conditions.

∙ In primary succession on rocks these are usually lichens which are able to secrete  acids to dissolve rock, helping in weathering and soil formation. 

∙ These later pave way to some very small plants like bryophytes, which are able to  take hold in the small amount of soil. 

∙ They are, with time, succeeded by bigger plants, and after several more stages,  ultimately a stable climax forest community is formed. 

∙ The climax community remains stable as long as the environment remains  unchanged. 

∙ With time the xerophytic habitat gets converted into a mesophytic one. succession,  particularly primary succession, is a very slow process, taking maybe thousands of  years for the climax to be reached. 

∙ Another important fact is to understand that all succession whether taking place in  water or on land, proceeds to a similar climax community – the mesic. ∙ The movement of nutrient elements through the various components of an  ecosystem is called nutrient cycling. 

∙ Another name of nutrient cycling is biogeochemical cycles (bio: living organism,  geo: rocks, air, water). 

∙ Nutrient cycles are of two types: (a) gaseous and (b) sedimentary. 

Phosphorus Cycle

Phosphorus is a important constituent of biological membranes, nucleic acids and  cellular energy transfer systems. 

∙ Many animals also need large quantities of this element to make shells, bones and  teeth. 

∙ The natural reservoir of phosphorus is rock, which contains phosphorus in the  form of phosphates. 

∙ When rocks are weathered, minute amounts of these phosphates dissolve in soil  solution and are absorbed by the roots of the plants. 

∙ Herbivores and other animals obtain this element from plants. 

∙ The waste products and the dead organisms are decomposed by phosphate solubilizing bacteria releasing phosphorus.