(i) Diatom ~ Guppies ~ Large fishes ~ Crane
(ii) Grass ~ Rabbit ~ Snake ~ Hawk

In a forest ecosystem the pyramid of number is spindle in shape, it is because the base (T ₁) of the pyramid occupies large sized trees (Producer) which are lesser in number. Herbivores (T₂) (Fruit eating birds, elephant and deer) occupying second trophic level, are more in number than the producers. In final trophic level (T₄) tertiary consumers (lion) are lesser in number than the secondary consumer (T₃) (fox and snake).

Significance of food web
(I) Food web is constructed to describe species interaction called direct interaction.
(II) It can be used to illustrate indirect interactions among different species .
(III) It can be used to study bottom-up or top-down control of community structure.
(IV) It can be used to reveal different patterns of energy transfer in terrestrial and aquatic ecosystems.

Main source of energy for the grazing food chain is the Sun. It begins with the first link, producers (plants). The second link in the food chain is primary consumers (mouse) which get their food from producers. The third link in the food chain is secondary consumers (snake) which get their food from primary consumers. Fourth link in the food chain is tertiary consumers (eagle) which get their food from secondary consumers.
Grass       ➝             Mouse        ➝              Snake         ➝                Eagle
Producers       Primary consumers    Secondary Consumers    Tertiary consumers

A trophic level refers to the position of an organism in the food chain.
The number of trophic level is equal to the number of steps in the food chain.
The green plants (producers) occupying the first trophic level (T₁) are called producers
Green plants
The energy produced by the producers is utilized by the plant eaters (herbivores) they are called primary consumers and occupies the second trophic level (T₂).
Rabbit
Diagrammatic representation of trophic levels
(i) Herbivores are eaten by carnivores, which occupy the third trophic level (T₃).
(ii) snake
(iii) They are also called secondary consumers or primary carnivores.
(iv) Carnivores are eaten by the other carnivores, which occupy the fourth trophic level (T₄). They are called the tertiary consumers or secondary carnivores.
(v) Eagle
(vi) Some organisms which eat both plants and animals are called as omnivores (Crow).
(vii) Such organisms may occupy more than one trophic level in the food chain.