Assertion (A) : In terrestrial ecosystem much larger fraction of energy flows through grazing food chain.

Reason (R) : Grazing food chain may be connected to Detritus food chain at some levels.

Q: Assertion (A) : In terrestrial ecosystem much larger fraction of energy flows through grazing food chain. Reason (R) : Grazing food chain may be connected to Detritus food chain at some levels.

**Correct option: (D)** — Assertion is false, but Reason is true. **Why A is false:** - In a **terrestrial** ecosystem, a **much larger fraction of energy flows through the Detritus Food Chain (DFC)**, not the grazing food chain. (In an **aquatic** ecosystem, GFC is the major route.) So the Assertion is wrong. **Why R is true:** - The DFC **can be connected to the GFC** at some levels — some detritivores/decomposers are eaten by GFC animals, and vice versa. This is a correct NCERT statement. Since A is false and R is true, the code is **D**.

Ecosystem

Energy Flow — Grazing vs Detritus Food Chain

Options

(A)Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation for Assertion (A).

(B)Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation for Assertion (A).

(C)Assertion (A) is true, but Reason (R) is false.

(D)Assertion (A) is false, but Reason (R) is true.

Official Answer

Correct option: (D) — Assertion is false, but Reason is true.

Why A is false:

In a terrestrial ecosystem, a much larger fraction of energy flows through the Detritus Food Chain (DFC), not the grazing food chain. (In an aquatic ecosystem, GFC is the major route.) So the Assertion is wrong.

Why R is true:

The DFC can be connected to the GFC at some levels — some detritivores/decomposers are eaten by GFC animals, and vice versa. This is a correct NCERT statement.

Since A is false and R is true, the code is D.

grazing food chaindetritus food chainenergy flowterrestrial ecosystemaquatic ecosystemdecomposersdetritivorestrophic levels

Marking Scheme

11 mark: correct code (D) — Assertion false, Reason true.
2Accept justification that in terrestrial ecosystems the detritus food chain (not GFC) carries the larger fraction of energy.

Hint

In terrestrial ecosystems, which chain carries more energy — grazing or detritus? Recall the aquatic vs terrestrial contrast.

Quick Oral Answer

The Assertion is false because in terrestrial ecosystems the detritus food chain, not the grazing food chain, carries the larger fraction of energy; the grazing chain dominates only in aquatic systems. The Reason, that the grazing and detritus chains connect at some levels, is true, so the answer is code D.

Analysis & Explanation

Concept — Two channels of energy flow:

Energy moves through an ecosystem along two food chains:

Grazing Food Chain (GFC): starts with producers → herbivores → carnivores.
Detritus Food Chain (DFC): starts with dead organic matter (detritus) → decomposers/detritivores → their predators.

The key NCERT contrast:

In aquatic ecosystems, the GFC is the major conduit of energy flow.
In terrestrial ecosystems, a much larger fraction of energy flows through the DFC, because a large amount of plant material is not grazed but ends up as litter/dead matter decomposed by the detritus chain.

The Assertion reverses this, so it is false.

Why R is a true but independent fact:

NCERT states the two chains are not isolated — the DFC may be connected with the GFC at some levels (some DFC organisms are prey to GFC animals). This is correct, but it does not support the false claim in A. So even though R is true, it cannot explain A — and here A itself is wrong.

Exam trap:

Remember the pairing: aquatic → GFC dominates; terrestrial → DFC dominates. Swapping these is the single most common error on this question.

Common Mistakes

1Marking (A) by assuming grazing dominates on land; in terrestrial ecosystems the detritus food chain actually carries the larger energy fraction.
2Swapping the aquatic/terrestrial rule — GFC dominates aquatic systems, DFC dominates terrestrial systems.
3Thinking the two food chains are completely separate; they interconnect at some trophic levels, which is why the Reason is true.

Interesting Facts

In many terrestrial ecosystems only a small fraction of net primary production is eaten by herbivores; most plant matter dies and enters the detritus food chain.

Decomposers can process a very large share of the energy captured by producers, making them central to terrestrial nutrient cycling.

The grazing and detritus food chains together form a food web because organisms of one chain often become prey in the other.

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Frequently Asked Questions

Which food chain carries more energy in a terrestrial ecosystem?

The detritus food chain (DFC). On land, most plant material is not grazed but dies and is decomposed, so a much larger fraction of energy flows through the DFC. The grazing food chain dominates only in aquatic ecosystems.

If the Reason is true, why isn't the answer A or B?

Because the Assertion is false. A true Reason cannot make a false Assertion correct. Since A is false and R is true, the code must be D.