Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A): The number of trophic levels in the grazing food chain is restricted.
Reason (R): In the grazing food chain, 10 per cent of the energy is transferred to each trophic level from the higher trophic level.
In the light of the above statements, choose the most appropriate answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (A) is true but (R) is false.
Energy passes up a grazing food chain from the producers to the herbivores and then to the carnivores. At each step only about 10 per cent of the energy of the lower level reaches the level above, so energy runs short after a few steps.
Assertion (A): The number of trophic levels in the grazing food chain is restricted. — True. A grazing food chain usually has only a few levels, such as producer, herbivore, primary carnivore and secondary carnivore.
Reason (R): In the grazing food chain, 10 per cent of the energy is transferred to each trophic level from the higher trophic level. — False. Energy flows only upward: 10 per cent of the energy is transferred to each trophic level from the lower trophic level, not from the higher one. Energy never moves down from a higher level to a lower one.
So A is true but R is false.
NCERT Extract:
Energy Flow – “… The number of trophic levels in the grazing food chain is restricted as the transfer of energy follows 10 per cent law – only 10 per cent of the energy is transferred to each trophic level from the lower trophic level. In nature, it is possible to have so many levels – producer, herbivore, primary carnivore, secondary carnivore in the grazing food chain (Figure 12.3).”
Figure 12.3 Energy flow through different trophic levels