Class 12 Biology Ecosystem Energy flow

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.

All organisms are dependent for their food on producers.

Flow of energy in the ecosystem is unidirectional.

The green plant in the ecosystem are called producers. Example-In a terrestrial ecosystem, major producers are herbaceous and woody plants and in aquatic ecosystem phytoplankton, algae and higher plants are producers.

All animals depend on plants for their food needs are called consumers.

Consumers which feed on the producers, they are called primary consumers or herbivores. Example- grass

The animals eat herbivores, they are called secondary consumers or primary carnivores. Example- goat.

The animals which eat the primary carnivores are called tertiary consumers or secondary carnivores. Example- man.

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.

Producers belong to the first trophic level, herbivores (primary consumer) to the second and carnivores (secondary consumer) to the third trophic level.

Food chain is the flow of energy from one trophic level to another trophic level by eating and being eaten.

Food chain if is two types-

  • Grazing food chain
  • Detritus food chain

Food chain which starts from producers and ends on carnivores through herbivores is called grazing food chain.

Grass --> goat --> man.


Fig. grass is eaten by goat and goat is eaten by men  

Food chain which starts from dead organic matter and passes through detrivores to organisms feeding on detrivores is called detritus food chain.

Detrivores are decomposers which meet their energy and nutrient requirements by degrading dead organic matter or detritus, these are also known as saprotrophs.

Decomposers secrete digestive enzymes that breakdown dead and waste materials into simple, inorganic materials.

The interconnected matrix of food chain is called food web. For example- specific herbivore of one food chain may serve as food of carnivores in another food chain.

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.


Fig. A food chain, where grass is eaten by insects and frogs eat the insects which in turn is eaten by the snakes and eagles eat the snakes

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