Explain energy flow in ecosystem.

Except for the deep sea hydro-thermal ecosystem, sun is the only source of energy for all ecosystems on Earth.

Of the incident solar radiation less than 50 per cent of it is photosynthetically active radiation ($PAR$). Plants and photosynthetic and chemosynthetic bacteria (autotrophs) fix Sun's radiant energy to make food from simple inorganic materials.

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, either directly or indirectly. So the energy flow is from sun to producers and then unidirectional to consumers.

Further, ecosystems are not exempt from the second law of thermodynamics. They need a constant supply of energy to synthesis the molecules they require, to counteract the universal tendency toward increasing disorderliness.

The green plant in the ecosystem terminology are called producers.

In a terrestrial ecosystem, major producers are herbaceous and woody plants.

Likewise primary producers in aquatic ecosystem are various species. Like phytoplankton, algae and higher plants.

Food chains and webs exist in nature.

Starting from the plants (or producers) food chains or rather webs are formed such that are animal feeds on a plants or on another animal and in turn is food for another.

The chain or web is formed because of this interdependency.

- No energy that is trapped into an organism remains in it for ever.

The energy trapped by the producer, hence is either passed on to a consumer or the organism dies.

$\Rightarrow$ Death of organisms is the beginning of the detritus too chain/web.

- All animals depend on plants (directly or indirectly) for their food needs. They are hence called consumers and also heterotrophs.

- If they food on the producers, the plants, they are called primary consumers and if the animals eat other animals which in turn eat the plants (or their produce) they are called secondary consumers.

Likewise you could have tertiary consumers too.

Obviously the primary consumers will be herbivores. Some common herbivores are insects, birds and mammals in terrestrial ecosystem and molluses in aquatic ecosystem.

The consumers that feed on these herbivores are carnivores or more correctly primary carnivores.

$-$ Those animals that depend on the primary carnivores for food are labelled as secondary carnivores.