CBSE Class 10 Science 2020 · Paper 31/5/1 · Question 18(a)

Trophic level: each step or level of a food chain, at which the transfer of food (energy) takes place, forms a trophic level; producers form the first level, herbivores the second, and carnivores the third and fourth. Terrestrial food chain with four trophic levels: Grass $\to$ Grasshopper $\to$ Frog $\to$ Snake (any other correct chain, such as Grass $\to$ Insect $\to$ Frog $\to$ Hawk, is acceptable). The energy flow is unidirectional because the energy captured by the autotrophs does not revert to the solar input, and the energy that passes to the herbivores does not come back to the autotrophs; as it moves progressively through the various trophic levels, it is no longer available to the previous level. A large part of it is also lost as heat at every level.