Ecology & ecosystems

Answer: (d) Mankind's over-exploitation/misuse of natural resources, fragmentation/loss of natural habitats, destruction of ecosystems, pollution and global climate change.

The "sixth extinction" is the loss of species happening now, at a pace far above the natural rate, and it is driven by human activity: habitat loss and fragmentation, over-exploitation, invasive species and the like. Option (d) names these human pressures. Earlier mass extinctions were natural events, and the current one is what makes this episode different.

• ✓ (d) Species are now disappearing an estimated 100 to 1,000 times faster than before humans, and human activities are responsible. Habitat loss and fragmentation, over-exploitation, alien species and co-extinctions are the main causes.
• ✗ (b) This describes a feared natural event, a meteorite impact. The sixth extinction is not a fear about the future but an ongoing loss caused by people.
• ✗ (a) Monoculture and chemical use damage ecosystems, but they are only some of the pressures. They do not define the term, which covers the whole human-driven loss of species.
• ✗ (c) Loss of native crop varieties is a narrower concern, agricultural biodiversity. The sixth extinction is about species loss across all ecosystems.

Remember · Five natural mass extinctions came before; the sixth is human-driven, at 100 to 1,000 times the pre-human rate. The four main causes are the 'Evil Quartet'.

📘 Read it in NCERT: Class 12 Biology, Ch 13 (practise this chapter) · Class 12 Biology, Ch 13 (practise this chapter)

Question and answer: UPSC's official GS Paper I (2018, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 30 Sept 2026 (how we verify). Permalink · Report an error

Answer: (a) Increased plant growth due to increased concentration of carbon dioxide in the atmosphere

Carbon fertilization (CO2 fertilization) means plants photosynthesise more, and often grow more, when the CO2 in the air rises. CO2 acts like a nutrient for plants, so the effect is on plant growth. Whether the extra photosynthesis becomes real growth still depends on temperature, moisture and nutrients.

• ✓ (a) The IPCC defines CO2 fertilization as the increase of plant photosynthesis and water-use efficiency in response to higher atmospheric CO2; it can lead to more plant growth and carbon storage.
• ✗ (b) Rising temperature from extra CO2 is the greenhouse effect (global warming), a different process.
• ✗ (c) Falling ocean pH because the sea absorbs CO2 is ocean acidification, a separate consequence of higher CO2.
• ✗ (d) 'Fertilization' refers to boosted plant growth, not to all living things adapting to climate change.

Remember · Carbon (CO2) fertilization = more atmospheric CO2 boosts plant photosynthesis and growth; warming is the greenhouse effect and ocean pH fall is acidification.

Question and answer: UPSC's official GS Paper I (2018, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 30 Sept 2026 (how we verify). Permalink · Report an error

Answer: (b) 3 only

Only statement 3 is correct: repeated irrigation can leave salts in the soil and make farmland saline. Organic matter (humus) helps soil hold water, and soil is where sulphur is stored and converted into a form plants can use.

• ✗ 1. Humus, the dark organic matter of soil, is rich in nutrients and retains moisture. More organic matter raises water-holding capacity; it does not drastically reduce it.
• ✗ 2. Soil is a main site of the sulphur cycle. Sulphur added as elemental S must be oxidised to sulphate in the soil before plants can take it up.
• ✓ 3. NCERT notes that intensive irrigation in Punjab, Haryana and western Uttar Pradesh is increasing soil salinity, because irrigation water leaves dissolved salts behind as it evaporates.

Remember · Organic matter (humus) holds water; soil is a working part of the sulphur cycle; long-term irrigation can make soil saline.

📘 Read it in NCERT: Class 12 India: People and Economy, Ch 4 (practise this chapter) · Class 7 Exploring Society: India and Beyond (Part 2), Ch 1 (practise this chapter)

Question and answer: UPSC's official GS Paper I (2018, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 30 Sept 2026 (how we verify). Permalink · Report an error

Answer: (b) It tends to reduce the biodiversity in the area in which it grows.

Prosopis juliflora (vilayati babul or vilayati kikar) is an invasive alien tree from South America. It spreads fast and pushes out native plants and the animals that depend on them, so it is in the news for removal drives.

• ✓ (b) The Wildlife Institute of India calls it one of the worst invaders affecting ecosystems and local biodiversity in Tamil Nadu. NCERT explains that alien species that turn invasive cause decline or extinction of native species.
• ✗ (a) The plant is not in the news for any cosmetics extract; in India it was introduced for fuelwood and to green degraded land.
• ✗ (c) It is not in the news as a source of pesticide raw material; it is in the news for the harm it does as an invader.

Remember · Prosopis juliflora (vilayati babul) is an invasive alien tree that displaces native species and lowers biodiversity.

📘 Read it in NCERT: Class 11 Understanding Society, Ch 3 (practise this chapter) · Class 12 Biology, Ch 13 (practise this chapter)

Question and answer: UPSC's official GS Paper I (2018, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 30 Sept 2026 (how we verify). Permalink · Report an error