If primary producers are exposed to 1,000,000 Joules of sunlight, the amount of energy transferred to tertiary consumers will be:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 1000 J
Note: The given answer is as per the official NTA Answer Key. However, as per NCERT the correct answer should be Option (4) 10 J. NCERT’s own pyramid of energy (shown below) starts with the same 1,000,000 J of sunlight and gives 10 J to the tertiary consumers.
Energy flows through an ecosystem in one direction: from the sun to the producers, then to the herbivores and then to the carnivores. Two losses happen on the way.
At the producers: plants capture only a small part of the sunlight that falls on them. NCERT’s ideal pyramid of energy takes this as 1 per cent, so 1,000,000 J of sunlight gives 10,000 J at the producers.
At every later step: by the 10 per cent law, only about 10 per cent of the energy at one trophic level passes to the next. The rest is lost as heat in respiration and other life processes.
Working with NCERT’s figures:
Sunlight: 1,000,000 J
Producers (1% of sunlight): 10,000 J
Primary consumers (10% of 10,000 J): 1000 J
Secondary consumers (10% of 1000 J): 100 J
Tertiary consumers (10% of 100 J): 10 J
Why the answer given by NTA is incorrect
The answer given by NTA, 1000 J, is what reaches the primary consumers in NCERT’s pyramid. It comes out at the tertiary consumers only if the whole 1,000,000 J of sunlight is taken as fixed by the producers and the 10 per cent law is then applied three times (100,000 J, 10,000 J, 1000 J). But producers never fix all the sunlight that falls on them.
Option (1) 100 J — Incorrect. In NCERT’s pyramid, 100 J is the energy of the secondary consumers, one level below the tertiary consumers.
Option (2) 1000 J — Incorrect as per NCERT. 1000 J is the energy of the primary consumers. It is the answer given by NTA.
Option (3) 10000 J — Incorrect. 10,000 J is the energy held by the producers themselves, after they fix 1 per cent of the sunlight.
Option (4) 10 J — Correct as per NCERT. After 1 per cent capture by the producers and 10 per cent transfer at each of the next three steps, only 10 J reaches the tertiary consumers.
NCERT Extract:
Ecological Pyramids –
