What is meant by trophic level in a food chain? Construct a terrestrial food chain with four trophic levels. The energy flow in a food chain is always unidirectional. Why?

Trophic Levels – The various steps in a food chain at which the transfer of food takes place are called trophic levels.

In a food chain, each step representing an organism forms a trophic level.

A food chain with four trophic levels can be shown as follows:

$Grass\left(Producer\right)\to Insect\left(Herbivore\right)\to Frog\left(Carnivore\right)\to Bird\left(LargeCarnivore\right)$

 The flow of energy in the ecosystem is said to be unidirectional because the energy lost as heat from the living organisms of a food chain cannot be reused by plants in photosynthesis.

[Extra reference material:

1.  Green plants trap solar energy with the help of their green pigment, chlorophyll, and convert it into chemical energy. They store this energy in the form of carbohydrates. On average, about one percent of the sun's energy falling on the leaves of green plants is used by the plants in the process of photosynthesis and stored as the chemical energy of food.
2. When plants are eaten by herbivores, the chemical energy stored in the plants is transferred to them.
3. The herbivores are then consumed by carnivores, and the chemical energy stored in the flesh of the herbivores is transferred to the carnivores.
4. Some of the energy from producers and consumers is also utilized for the life processes of micro-organisms called decomposers. Decomposers, in turn, release the unutilized energy into the environment as heat. The energy that is not utilized by producers, consumers, and decomposers is lost in the environment as heat. This heat is called community heat.