The tiger is left with 10 J of energy in a grass–deer–tiger food chain. The energy available at grass level is:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 1000 J
By the 10 per cent law, only about 10 per cent of the energy at one trophic level passes to the next. Working backwards, each lower level must hold 10 times the energy of the level above it.
In grass → deer → tiger, the tiger is at the third level with 10 J. The deer must have had 10 × 10 = 100 J, and the grass 10 × 100 = 1000 J.
Option (1) 100 J — Incorrect. 100 J is the energy at the deer (herbivore) level, one step below the tiger, not at the grass level.
Option (2) 2000 J — Incorrect. 2000 J would mean only 5 per cent passes from grass to deer, which does not follow the 10 per cent law.
Option (4) 10,000 J — Incorrect. 10,000 J would fit a chain with one more level between the grass and the tiger. This chain has only three levels.
NCERT Extract:
Energy Flow – “The important point to note is that the amount of energy decreases at successive trophic levels. … 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.”