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Environment & ecology

Prelims · Environment & ecology · 27 questions

Ecology & ecosystems

Every UPSC Prelims question on this topic, 2016–2026, newest first. Tap an option to check yourself; the answer and explanation open below it.

Ecology & ecosystems questions per year: 2016: 0, 2017: 1, 2018: 0, 2019: 0, 2020: 0, 2021: 7, 2022: 3, 2023: 0, 2024: 2, 2025: 1, 2026: 1 Asked in 6 of 11 years · most in 2021 (7)

UPSC syllabus: “General issues on Environmental ecology, Bio-diversity and Climate Change - that do not require subject specialization.” See the full syllabus →

Prelims 2017 · Q21

Easy

Due to some reasons, if there is a huge fall in the population of species of butterflies, what could be its likely consequence/consequences?

  1. 1.Pollination of some plants could be adversely affected.
  2. 2.There could be a drastic increase in the fungal infections of some cultivated plants.
  3. 3.It could lead to a fall in the population of some species of wasps, spiders and birds.

Select the correct answer using the code given below:

Answer & explanation

Answer: (c) 1 and 3 only

Butterflies are pollinators and also food for many predators, so their decline would hurt pollination and the animals that eat them. They play no role in keeping fungal diseases of crops in check, so statement 2 does not follow.

  • ✓ 1. Butterflies are among the common animal pollinators of flowering plants, so plants that depend on them could set fewer seeds.
  • ✗ 2. Butterflies and their caterpillars feed on nectar and leaves; they do not control plant-infecting fungi, so their loss gives no reason for fungal infections to rise.
  • ✓ 3. Birds, spiders, wasps and dragonflies prey on butterflies or their caterpillars; when a prey species crashes, predators that rely on it can decline too.

Remember · Removing one species ripples through the web: butterflies = pollinators + prey. Their crash hits pollination and predator populations, not fungal disease.

📘 Read it in NCERT: Class 12 Biology, Ch 1 (practise this chapter) · Class 12 Biology, Ch 11 (practise this chapter) · Class 8 Curiosity, Ch 12 (practise this chapter)

Sources

Question and answer: UPSC's official GS Paper I (2017, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 30 Sept 2026 (how we verify). Permalink ·

Prelims 2017 · Q59

Easy

Biological Oxygen Demand (BOD) is a standard criterion for

Answer & explanation

Answer: (c) Pollution assay in aquatic ecosystems

BOD is the amount of oxygen that bacteria would use up to break down the organic matter in a sample of water, so it is a standard test of how polluted water is. A higher BOD means more organic pollution.

  • ✓ (c) BOD measures the oxygen consumed by microbes in decomposing the organic matter in water, so it works as an indirect measure of that matter. The greater the BOD of waste water, the greater its polluting potential.
  • ✗ (a) BOD is a test on water samples. It has nothing to do with the oxygen carried in blood.
  • ✗ (b) BOD says nothing about oxygen in forests. It tells us how much oxygen microbes need in water that carries organic waste.

Remember · BOD is oxygen bacteria need to decompose the organic matter in water. Higher BOD means more pollution; sewage treatment aims to bring it down.

📘 Read it in NCERT: Class 12 Biology, Ch 8 (practise this chapter) · Class 12 Biology, Ch 8 (practise this chapter)

Sources

Question and answer: UPSC's official GS Paper I (2017, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 30 Sept 2026 (how we verify). Permalink ·

Prelims 2017 · Q71

Medium

Which of the following practices can help in water conservation in agriculture?

  1. 1.Reduced or zero tillage of the land
  2. 2.Applying gypsum before irrigating the field
  3. 3.Allowing crop residue to remain in the field

Select the correct answer using the code given below:

Answer & explanation

Answer: (c) 1 and 3 only

Leaving the soil undisturbed (reduced or zero tillage) and leaving crop residue on the field both conserve water, because the residue cover cuts evaporation and lets more rain soak in. Gypsum is a soil amendment for sodic soils, not a water-saving practice.

  • ✓ 1. No-till or reduced tillage, together with residue mulch, is the basis of conservation farming because it conserves water and prevents erosion.
  • ✗ 2. Gypsum supplies soluble calcium to replace exchangeable sodium in sodic soils. It is used to reclaim such soils and is not a way of saving irrigation water.
  • ✓ 3. Crop residue left on the soil surface reduces evapotranspiration (water loss to the air), raises infiltration rates and suppresses weeds.

Remember · Water-saving conservation farming: minimum or no tillage plus a residue mulch. Gypsum is for reclaiming sodic soils, not for saving water.

Sources

  • FAO: Conservation tillage for increased crop production (Chapter 8) ↗ “the no-till system of cultivation with crop residue mulches forms a basis for conservation farming because it conserves water, prevents erosion, maintains organic matter content at a high level, and sustains economic productivity. … retaining crop residues on the soil surface with conservation tillage reduces evapotranspiration, increases infiltration rates, and suppresses weed growth.”
  • FAO Soils Bulletin: Sodic soils and their management ↗ “Soil amendments are materials, such as gypsum or calcium chloride, that directly supply soluble calcium for the replacement of exchangeable sodium”

Question and answer: UPSC's official GS Paper I (2017, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 30 Sept 2026 (how we verify). Permalink ·

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