(a) The various steps in a food chain at which the transfer of food and energy takes place are known as trophic levels.
A food chain with four trophic levels can be shown as follows.
Grass Insect Frog Eagle
(b) If we kill all the organisms in one trophic level, there will be no food for the organisms of the next trophic level, and they will die of starvation. Also, the organisms of the previous trophic levels will increase in number as there will be no organism to consume them as food.