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Class 10 Science Chapter 13: Our Environment case study question with answers

One case-based question of the kind the board asks in Section E: a passage and a table, then 3 parts worth 4 marks in all, with the answer and the marking scheme for each. Written from the NCERT chapter.

Case study · 4 marks

A team from a fisheries college surveys a reservoir in Odisha. They estimate the energy held each year by the organisms at each level of one food chain, per square metre of water. One value was lost when a page of their notebook got wet. The team also learns that, ten years earlier, a non-degradable pesticide had drained into the reservoir from the surrounding fields. They plan to test tissue from the organisms at every level to see where the pesticide has gone.

Trophic level	Organism	Energy (kJ per m² per year)
I	phytoplankton	12 000
II	zooplankton	1200
III	small fish	120
IV	fish-eating bird	?

Part 1 · 1 mark

The missing value for the fish-eating bird is closest to

(a) 1.2 kJ(b) 12 kJ(c) 60 kJ(d) 108 kJ

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Answer: (b) 12 kJ

Each level holds about 10% of the one below: 12 000 → 1200 → 120, so 10% of 120 = 12 kJ. 108 kJ is 120 less 10%, the reverse slip.

Part 2 · 1 mark

In this chain the secondary consumer is the

(a) phytoplankton(b) zooplankton(c) small fish(d) fish-eating bird

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Answer: (c) small fish

Zooplankton feed on the producers and are primary consumers; the small fish eat them and, at level III, are the secondary consumers.

Part 3 · 2 marks

In which organism will the team find the highest concentration of the pesticide? Give the reason.

Answer and marking scheme

In the fish-eating bird. The pesticide is not degradable, so it accumulates at each trophic level, and the concentration is greatest at the top of the chain: biological magnification.

fish-eating bird (1)
accumulates at each level as it is not degraded (biological magnification) (1)

Using the table, explain why the reservoir supports many fewer fish-eating birds than small fish.

Answer and marking scheme

The birds receive only about 12 kJ against 120 kJ for the small fish: only about 10% of the energy passes from one level to the next. So the higher level can support far fewer individuals, and numbers fall at each step up the chain.

only about 10% of energy passes up (12 kJ against 120 kJ) (1)
less energy supports fewer individuals at the higher level (1)

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