An ecosystem can be defined as a functional unit of nature, where living organisms interact among themselves and also with the surrounding physical environment.

The term ecosystem was coined by Sir A.G. Tansley (1935).

The entire biosphere is referred to as global ecosystem which consists of several local Ecosystems of earth.

An ecosystem is divided into two basic categories :-

  • Terrestrial ecosystem. Example – forest, desert.
  • Aquatic ecosystem. Example- pond, lake, wetland,

Ecosystem- structure and functions

  • The various components of the environment are abiotic and biotic.
  • Interaction of biotic and abiotic components result in a physical structure that is characteristic for each type of ecosystem.
  • Biotic components are the living beings and abiotic components are the nonliving things of the ecosystem.
  • Vertical distribution of different species occupying different levels is called stratification.
  • For ex :- trees occupy the level of the forest, shrubs the second and herbs and grasses the bottom layer.

The Functional Components Of Ecosystem Are :-

  • Productivity 
  • Decomposition 
  • Energy flow 
  • Nutrient cycling 
  • The inorganic and organic materials are converted with the help of the radiant energy of sun by the autotrophs.
  • Heterotrophs consume autotrophs.
  • Decomposers decompose the dead organic matter to release them back for reuse by the autotrophs.
  • The matter and minerals are recycled between biotic and abiotic components.
  • The energy flow is unidirectional.


  • The rate of biomass production per unit area, by plant during photosynthesis is called productivity.



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    Primary productivity

    Secondary productivity.

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    Primary productivity is defined as the 

    amount of biomass or organic matter 

    produced per unit area over a time period 

    by plants during photosynthesis.

    Secondary productivity is defined as the rate

    Of new organic matter by consumers.

Primary Productivity Can Be Divided Into:-

*gross primary productivity (GPP)

*Net primary productivity (NPP)

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

Gross primary productivity minus respiration losses ( R ) , is the net primary productivity(NPP).


Factors Affecting Primary Productivity

1. Sunlight:- Sunlight directly regulates primary productivity because plants perform photosynthesis with the help of sunlight. As tropical region receives maximum sunlight so it exhibit higher productivity.

2. Temperature:- it regulates the activity of enzymes So, the optimum temperature is required for the proper functioning of enzymes.

3. Moisture:- rain (humidity) is required for higher primary productivity. Deserts have the lowest primary productivity as the soil is deficient in moisture.

4. Availability of nutrients:- greater nutrients ensure greater primary productivity.

5. Photosynthesis efficiency:- some plants have more efficiency to trap sunlight (sugarcane). So they accumulate more primary productivity.

6. Plant species inhabiting a particular area.



    • The process of breaking down complex organic matter into inorganic substances like Co2, water, and nutrients is called decomposition.

      The raw material for decomposition including dead plant and animal remains and animal remains and their faecal matter are called Detritus.

      Step In Decomposition:-

      1. Fragmentation:- The process of breaking down of detritus into smaller particles is called fragmentation.
      2. Leaching:-     The process by which water-soluble inorganic nutrients go down the soil horizon and get precipitated as unavailable salts is called leaching.
      3. Catabolism:-  the enzymatic process by which degraded detritu is converted into simpler inorganic substances is called catabolism.
      4. Humification:- The process of accumulation of a coloured amorphous substance called humsthat is highly resistant to microbial action and undergoes decomposition at an extremely slow rate is called humification.
      5. Mineralisation:- The process by which humus is further degraded by some microbes to release inorganic nutrients is called mineralisation.

      Factors affecting decomposition 

      1. Chemical composition of detritus:-

      • The decomposition rate is slow if detritus is rich in lignin and chitin.
      • The decomposition rate is higher when detritus is rich in nitrogen and water substances like sugars.


      1. Climatic factors:-       

      • A warm and moist environment favours decomposition.
      • Low temperature and anaerobiosis inhibit decomposition.
    • Food chain and Food web :-


      Food chain 

      Food web


      The transfer of energy from producer to top consumer through a series of organisms is called food chain.

      A number of food chain inter -connected with each other forming a web-like pattern is called food web. 


      One organism holds only one position.

      One organism can hold more than one position.


      The flow of energy can be easily calculated.

      The flow of energy is very difficult to calculate.


      It is always straight and proceed in a progressive straight line.

      Instead of straight line it is a series of branching lines.


      Competition is limited to members of same trophic level.

      Competition is amongst members of same and different trophic levels.

Types Of Food Chain

    • Grazing chain (GFC),              e.g., Grass —  Rabbit —–  Lion 
    • Detritus food chain (DFC),       e.g., dead leaves ——- woodlouse ——- blackbird

Graining Food chain (GFC)

Detritus food chain (DFC)


It starts with green plants called producers as first trophic level.

It begins with dead organic matter and decomposers called saprophytes as first trophic level.


As much less fraction of energy flows through this type of food chain.

A much larger fraction of energy flows through this type of food chain.


Energy for food chain comes from sun.

Energy for the food chain comes from organic remain or detritus.

Trophic Level

        1. In an ecosystem, an organism occupies a specific place on the food chain called a trophic level.

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

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

          Energy Flow :

          • Sun is the only source of energy for all ecosystems on Earth and except for the deep sea hydro-thermal ecosystem.
          •  Less than 50 per of incident solar radiation is Photosynthetically active radiation (PAR).
          •  Plants capture only 2-10 per cent of the PAR and this small amount of energy sustains the entire living world.
          •  All organisms are dependent for their food on producers. Flow of energy in the ecosystem is unidirectional.

          The energy is transferred in an ecosystem in the form of food which is degraded and lose a major part of food energy as heat during metabolic activities and only a very small fraction becomes stored as biomass.

          This is correlated to the second law of thermodynamics.

          The green plants in the ecosystem which can trap solar energy to convert it into chemical bond energy are called Producers.

          All the animals that depend for food on plants are called Consumers.

Consumer are divided into the following Categorie :-

Primary consumers :- Animals which feel directly on ants, e.g., Herbivores.

Secondary consumers :- Consumers that feed on primary consumers, e.g., carnivores.

Tertiary consumer :- Consumers that feed on secondary consumers,

Lindeman’s 10 per cent Law :-

At each step of food chain, when food energy is transferred from one trophic level to the next higher trophic level, only about 10 percent of energy is passed on to the next trophic level. This is known as Lindeman’s 10 percent law given by lindeman in 1942.

Ecological pyramid :-

Ecological pyramid is the graphical representation of various trophic levels of food chain designed to show their number, biomass and energy.


 The base of a pyramid is broad and it narrows down at the apex. The base of each pyramid represents the producers or the first trophic level while the apex represents tertiary or top level consumer.

A given species may occupy more than one trophic level in the same ecosystem at the same time. For example, a sparrow is a primary consumer when it eats seeds, fruits, peas, and a secondary consumer when it eats insects and worms.

The three ecological pyramids that are usually studied are :-

  • Pyramid of number
  • Pyramid of biomass
  • Pyramid of energy

Pyramid of number:-

The relationship between producers and consumers in an ecosystem can be represented in the form of a pyramid in terms of number called pyramid of number.

Pyramid of biomass :-

The relationship between producers and consumers in an ecosystem can be represented in the form of a pyramid in terms of biomass  is called pyramid of biomass.

It can be :-

  1. Upright    e,g., in case of  grassland ecosystem.
  2. Inverted   e,g., in case of pond ecosystem.

The pyramid of biomass in sea is also generally because the biomass of fishes far exceeds that of phytoplankton.

Pyramid of energy:-

The relationship between producers and consumers in an ecosystem can be represented in the form of a pyramid in terms of flow of energy, pyramid of energy.

The pyramid of energy is always upright because when energy flows from a particular trophic level to the next trophic level, some energy is always lost as heat at each step.

Limitations of Ecological Pyramids :-

  • It does not take into account the same species belonging to two or more trophic levels.
  • It assumes a simple food chain, something that almost never exists in nature.
  • It does not accommodate a food web.
  • Saprophytes are not given any place in ecological pyramids.

Ecological Succession:-

The Sequential, gradual and predictable changes in the species composition in an area are called succession or ecological succession.

  •  The entire sequence of communities that successively changes in a given area are called sere.
  •  The individual transitional communities are termed as seral stage seral communities.
  •  The community that is in near equilibrium with the environment is called a climax community.
  •  The species that invade a bare area are called pioneer species.
  • The changes that occur in successive seral stages to reach a climax community are:-
  1.  Changes in the diversity of species of organisms.
  2.  Increase in the total biomass.
  3.  Increase in the number of species and organisms.

Ecological succession is of two types:

Primary succession:

 Starts where no living organisms are there or these could be areas where no living organisms ever existed. It can occur in newly cooled lava, bare rock, newly created pond or reservoir.

Secondary succession:-

 Succession that starts in areas where all the living organisms somehow lost that existed there is called secondary succession. It can occur in abandoned farmlands, burned or cut forests, and lands that have been flooded.

Secondary succession is faster than primary succession because some soil or sediment is present.

Succession of Plants :

  • Plant succession is of two types:-

    1) Hydrach Succession:-

     The plant succession takes place in wet areas or water, leading to successional series progress from the hydric to the mesic condition.

    2) Xerarch succession:-

     The plant’s succession takes place in a dry area, leading to a successional series from xeric to mesic condition.

    Both hydrarch and xerach successions lead to medium water conditions (mesic) neither too dry (xeric) nor too wet (hydric).

    In primary succession on rocks, the pioner species is lichens are able to secrete acids to dissolve.

Primary Succession rock, helping in weathering and soil formation and the little soil leads to the growth of bryophytes.

 in water :-

  • The pioneer species are phytoplanktons
  • The phytoplanktons are replaced by free floating angiosperms
  • Then rooted angiosperms invade sedges, grasses and finally the trees.
  • At last,a stable climax forest is formed.
  • An aquatic habitat is conver

Primary succession on rocks:-

  • Lichens are the pioneer species on a bare area.
  • The lichens secrete some acids to dissolve rock and help in weather in weathering and soil formations.
  • Later, some small bryophytes invade and hold the small amount of soil.
  • The bryophytes are succeeded by herbs, shrubs, and ultimately big trees.
  • At last, a stable climax forest is formed.
  • The xerophytic habitat gets converted into a mesophytic one.