Ecology & ecosystems

Answer: (d) 3 only

Mangroves are salt-tolerant forests of tidal creeks and estuaries that absorb the force of storm surges and tsunami waves while supplying fuelwood, fish and other products to coastal people. They are neither freshwater stores for paddy nor salt-sensitive filters for freshwater fish farming; clearing them for such uses actually weakens coastal resilience.

• ✗ 1. Mangroves grow in saline tidal water and do not store freshwater; converting them to paddy fields removes the very buffer that protects the coast.
• ✗ 2. Mangrove species are salt-tolerant, not salt-sensitive, and their value lies in standing forest, not in turning coasts into freshwater aquaculture ponds.
• ✓ 3. Along with beaches and dunes, mangroves absorb the destructive force of storm and tsunami waves, and they provide wood, fuel and fisheries that support coastal livelihoods.

Remember · Mangroves: salt-tolerant tidal forests; natural bio-shields against cyclones and tsunamis, carbon sinks and livelihood sources. India has about 4,992 sq km of mangroves.

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Answer: (b) II only

The oceans, through photosynthesis by plankton and some bacteria, generate about half or more of the oxygen on Earth, so rain forests do not out-produce them: NOAA notes that one marine bacterium alone makes more oxygen than all tropical rainforests combined. Water can hold only a small amount of dissolved oxygen compared with air, so surface water never has several times more oxygen than the atmosphere.

• ✗ I Roughly half of Earth's oxygen production comes from the ocean (IUCN says more than half), and NOAA notes that the marine bacterium Prochlorococcus alone produces up to 20% of the biosphere's oxygen, a higher share than all the tropical rainforests combined.
• ✓ II NOAA says roughly half of the oxygen production on Earth comes from the ocean, mostly from oceanic plankton: drifting plants, algae and some bacteria that can photosynthesize.
• ✗ III NCERT notes that the amount of dissolved oxygen is fairly low compared with the amount of oxygen in air, which is why aquatic organisms breathe much faster than land ones. Water is therefore poorer in oxygen than air, not richer.

Remember · The ocean makes about half or more of Earth's oxygen (mostly phytoplankton); water holds far less oxygen than air.

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Answer: (b) Congo Basin

The Cuvette Centrale peatlands of the Congo Basin, spread across the Republic of Congo and the Democratic Republic of the Congo, form the world's largest tropical peatland. They lock away carbon equal to about three years of global greenhouse gas emissions, which would be released if the peat were drained or degraded.

• ✓ (b) The Congo Basin peat swamp forest stores around 29 billion tonnes of carbon, roughly three years of global emissions; the 2018 Brazzaville Declaration was signed to protect it.
• ✗ (a) The Amazon has peatlands too, but the single largest tropical peatland complex is the Congo Basin's Cuvette Centrale.
• ✗ (c) The Kikori Basin is a river basin in Papua New Guinea, rich in forest but not the world's largest tropical peatland.

Remember · Cuvette Centrale, Congo Basin: world's largest tropical peatland, ~29 billion tonnes of carbon ≈ three years of global emissions; Brazzaville Declaration (2018).

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Answer: (a) Fig

Fig trees and fig wasps are the classic case of coevolved mutualism. Many fig species can be pollinated only by one partner wasp species; the female wasp pollinates the fig inflorescence while looking for a place to lay her eggs, and her larvae feed on some of the developing seeds.

• ✓ (a) NCERT describes a tight one-to-one link between many fig species and their pollinating wasps: a given fig can be pollinated only by its 'partner' wasp, which in turn needs the fig to breed.
• ✗ (b) Mahua's strong-smelling flowers open at night and are a major attraction for bats, which Down To Earth calls essential for its pollination and seed dispersal; it has no single insect partner.
• ✗ (d) The red silk cotton tree (Bombax ceiba) is pollinated by generalist birds, not by one coevolved insect.

Remember · Fig–fig wasp: obligate, coevolved, often one-to-one mutualism. The wasp pollinates the fig inflorescence and lays eggs in the fruit; each depends on the other.

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Answer: (d) Statement-I is incorrect but Statement-II is correct

The soil of a tropical rain forest is poor, not rich, in nutrients. Heat and moisture make bacterial action intense, so dead plant matter is oxidised quickly and very little humus builds up in the soil; heavy rain also leaches nutrients out of tropical soils.

• ✗ Statement-I Fast decay leaves the soil with very low humus content, and heavy rain leaches away nutrients; lateritic soils of heavy-rain tropical areas are generally deficient in plant nutrients.
• ✓ Statement-II In humid tropical and equatorial climates bacterial growth and action is intense, so dead vegetation is rapidly oxidised. This is the reason the humus content of the soil stays low, not a sign of richness.

Remember · Fast decay plus heavy rain leave tropical rain forest soil low in humus and nutrients; in cold climates decay is slow, so humus and peat accumulate.

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Answer: (d) Aquatic plants absorb heavy metals and excess nutrients.

The kidney comparison is about purification: kidneys filter waste out of blood, and wetlands filter pollutants out of water. Option (d), aquatic plants absorbing heavy metals and excess nutrients, is the function that does this cleaning job.

• ✓ (d) Wetlands are called the earth's kidneys because they filter pollutants. Wetland plants take up heavy metals, and plant roots and microbes absorb excess nutrients from fertilizer, manure and sewage.
• ✗ (a) Runoff, percolation and evaporation describe how water moves through a wetland. This is the water cycle, not the removal of wastes.
• ✗ (b) Algae as the base of the food chain is a habitat and food-web function, closer to a 'nursery' role than to a kidney's cleaning role.
• ✗ (c) Sediment balance and soil stabilisation are physical, erosion-control functions. They do not purify water the way kidneys do.

Remember · Wetlands = kidneys of the landscape: they filter and purify water, with plants trapping heavy metals, pesticides and excess nutrients.

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Answer: (a) 1, 3 and 4 only

Only alfalfa, chickpea and clover fix nitrogen, because they are legumes. Legume roots carry nodules housing Rhizobium bacteria that turn nitrogen from the air into a form the plant can use. Amaranth, purslane and spinach are not legumes.

• ✓ 1. Alfalfa (lucerne) is a legume with its own Rhizobium partner, Rhizobium meliloti, in nodules on its roots.
• ✗ 2. Amaranth is a leafy green and grain crop, not a legume, so it has no Rhizobium root nodules.
• ✓ 3. Chickpea (gram) is a pulse and a legume crop, so its root nodules host nitrogen-fixing Rhizobium.
• ✓ 4. Clover is a legume; its roots form nodules with Rhizobium bacteria (the clover group).
• ✗ 5. Purslane (kulfa) is a leafy vegetable, not a legume, and does not fix nitrogen.
• ✗ 6. Spinach is a leafy vegetable, not a legume, and does not fix nitrogen.

Remember · Nitrogen fixers here are legumes: alfalfa, chickpea, clover. Rhizobium in root nodules does the fixing. Leafy greens such as spinach, amaranth and purslane are not legumes.

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Answer: (a) Restoration of damaged coral reefs

Biorock technology is used to restore damaged coral reefs. A low-voltage electric current is passed through seawater between a submerged metal frame and an anode, so that minerals in the water deposit as limestone on the frame, and corals attached to it grow quickly.

• ✓ (a) With an anode and a cathode in sea water and a current between them, calcium and carbonate ions combine and stick to the cathode structure as calcium carbonate; corals adhere to it and grow fast. The UN has cited biorock artificial reefs as successful in certain areas.
• ✗ (b) Biorock is not a plant-residue building material; it is limestone grown from seawater minerals on an electrified frame.
• ✗ (c) Nothing in Biorock concerns locating or extracting shale gas.
• ✗ (d) Biorock has no link to salt licks for forest animals; it is an undersea technology.

Remember · Biorock (mineral accretion): low-voltage current in seawater grows limestone on metal frames, letting corals attach and recover. Used to restore reefs.

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Answer: (c) Creation of mini forests in urban areas

The Miyawaki method is famous for creating dense mini forests in urban areas. Named after Japanese botanist Akira Miyawaki, it plants saplings of many native species very close together so that they grow into a thick forest in a few years.

• ✓ (c) Saplings of various species are planted close to each other and grow into a dense urban forest, ten times faster and thirty times denser than by the traditional method, according to the Prime Minister's Office.
• ✗ (a) The method makes forests, not commercial farms, and it is not a dryland-farming scheme.
• ✗ (b) It uses mixed native tree species, not genetically modified plants.
• ✗ (d) It has no connection to wind energy.

Remember · Miyawaki method (Akira Miyawaki, Japan): close planting of many native species to raise dense mini urban forests within 2-3 years.

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Answer: (a) Ant

Ants are the well-known fungus farmers among these options. The fungus-gardening (attine) ants, including leafcutter ants of the American tropics, grow fungus in their nests as their food.

• ✓ (a) Attine ants, more than 250 species, cultivate fungi as their main food. Leafcutter ants cut leaves, carry them into underground nests and use them to feed the fungus.
• ✗ (b) Cockroaches are not known as fungus cultivators.
• ✗ (c) Crabs are not known as fungus cultivators.
• ✗ (d) Spiders are not known as fungus cultivators.

Remember · Fungus-growing (attine) ants, such as leafcutter ants, farm fungus in their nests as food. This is an example of mutualism between an insect and a fungus.

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Answer: (a) Carbon captured by oceans and coastal ecosystems

'Blue carbon' is the carbon taken up and stored by the ocean and by coastal ecosystems such as mangroves, tidal marshes and seagrass meadows. Per unit area these habitats lock away more carbon than land forests, mostly in their waterlogged soils.

• ✓ (a) The term refers to carbon stored in marine and coastal ecosystems; the colour 'blue' signals water.
• ✗ (b) Carbon held in forests and farm soils on land is usually called 'green carbon', not blue carbon.
• ✗ (c) Carbon in petroleum and natural gas is fossil carbon; burning it releases carbon dioxide rather than storing it.

Remember · Blue carbon = carbon stored by oceans and coastal ecosystems (mangroves, salt marshes, seagrasses); green carbon = land vegetation and soils.

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Answer: (c) 2 and 3

Lichens and mosses are pioneer organisms: they cling to bare rock, walls and bark and are among the first to colonise weathered rock before any soil exists. Ferns arrive only after some soil and humus has built up, and mushrooms need dead organic matter to feed on.

• ✗ 1. Ferns are vascular plants with roots; in soil formation they appear later, once humus has accumulated.
• ✓ 2. A lichen is a partnership of a fungus and an alga that grows directly on rocks and tree bark.
• ✓ 3. Mosses have root-like rhizoids rather than true roots and form green mats on damp rocks and old walls.
• ✗ 4. Mushrooms are fungi that cannot make food; they absorb nutrients from dead and decaying matter, so they grow on soil, litter or wood.

Remember · Pioneers on bare rock: lichens and mosses. They start soil formation; ferns, grasses and trees follow.

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Answer: (b) 2 and 3

Primary producers make their own food by photosynthesis. In the sea these are the phytoplankton, which include cyanobacteria and diatoms. Copepods are tiny crustaceans and foraminifera are shelled protists; both eat other organisms, so they are consumers.

• ✗ 1. Copepods are small crustaceans; they graze on phytoplankton or live as parasites on fish, so they are consumers, not producers.
• ✓ 2. Cyanobacteria (blue-green algae) are photosynthetic bacteria and form part of the ocean's phytoplankton.
• ✓ 3. Diatoms are single-celled, silica-walled algae that make their own food using sunlight; they are among the most important phytoplankton.
• ✗ 4. Foraminifera are single-celled protists with shells that capture food such as bacteria, diatoms and even copepods; they are consumers, even though some species harbour algae inside.

Remember · Ocean food chains start with phytoplankton (cyanobacteria, diatoms, dinoflagellates, coccolithophores). Zooplankton such as copepods and foraminifera are consumers.

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Answer: (c) Phosphorus cycle

Nutrient cycles are either gaseous (reservoir in the air or water) or sedimentary (reservoir in the Earth's crust). Phosphorus has no significant gaseous form, so its supply to soils and water comes mainly from the slow weathering of phosphate-bearing rocks. Carbon, nitrogen and sulphur all have major atmospheric pools.

• ✓ (c) The phosphorus cycle is sedimentary: rocks are its reservoir, and weathering releases phosphate that plants take up from the soil.
• ✗ (a) Carbon cycles mainly through the atmosphere and oceans as carbon dioxide, entering food chains by photosynthesis.
• ✗ (b) The atmosphere is the largest reservoir of nitrogen; it enters living systems mainly through nitrogen fixation, not rock weathering.

Remember · Gaseous cycles: carbon, nitrogen (sulphur has a large gaseous part). Sedimentary cycle: phosphorus, whose reservoir is the Earth's crust and whose source is rock weathering.

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Answer: (c) 1, 3 and 5 only

Detritivores eat detritus, the dead leaves, wood, dead animals and dung on and in the soil, and break it into small fragments, which starts decomposition. Earthworms, millipedes and woodlice all live in leaf litter and feed this way. Jellyfish and seahorses are marine predators that catch live prey.

• ✓ 1. Earthworms are the textbook detritivore: they fragment dead plant matter, the first step of decomposition.
• ✗ 2. Jellyfish are cnidarians that sting and capture live prey with their tentacles.
• ✓ 3. Millipedes feed directly on detritus (dead plant matter) and so speed up decomposition, as a US Forest Service study notes; they are detritivores.
• ✗ 4. Seahorses are ambush predators that suck small live crustaceans such as shrimps and amphipods into their snouts.
• ✓ 5. Woodlice are land crustaceans (isopods) that feed mainly on decomposing plant material, helping recycle nutrients.

Remember · Detritivores fragment dead organic matter: earthworms, millipedes, woodlice, termites, dung beetles, sea cucumbers. Decomposers (bacteria, fungi) then break it down chemically.

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Answer: (d) 1, 2 and 3

All three groups contain species that live in close, often mutually beneficial, partnerships with other organisms. Reef-building corals (cnidarians) host photosynthetic zooxanthellae; fungi form lichens and mycorrhizae; and many protozoa live as symbionts, some carrying algae inside their own cells.

• ✓ 1. Corals, which are cnidarians, house zooxanthellae in their tissues; the algae supply sugars from photosynthesis and the coral gives them shelter and carbon dioxide.
• ✓ 2. Fungi team up with algae or cyanobacteria to form lichens, and with plant roots to form mycorrhizae that help plants absorb phosphorus.
• ✓ 3. Many protozoa live as symbionts; the ciliate Paramecium bursaria, for instance, carries symbiotic Chlorella algae in its cytoplasm. The two benefit each other: the ciliate gives the algae a protected habitat and the algae add to its nutrition through photosynthesis.

Remember · Mutualism examples: coral + zooxanthellae, fungus + alga (lichen), fungus + root (mycorrhiza), Paramecium bursaria + algae, legume + Rhizobium.

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Answer: (d) tropical rain forest

The description fits a tropical rain forest, where constant heat and moisture make dead leaves rot very fast, leaving the ground almost bare, and where climbers and epiphytes reach the tall canopy by using tree trunks and branches.

• ✓ (d) In a rain forest, NASA notes, leaf litter on the floor decomposes rapidly in wet, warm conditions, so the ground stays nearly bare. Woody vines climb the trees to reach the light, and epiphytes such as orchids and ferns grow on the branches. NCERT links tropical evergreen forests with a dense canopy.
• ✗ (a) Coniferous forests grow in cold regions, where decay is slow, so they do not show the fastest leaf-litter breakdown of any biome.
• ✗ (b) Dry deciduous forests have a long dry season, which slows decay, and they lack the dense layered canopy of climbers and epiphytes described here.
• ✗ (c) Mangroves stand in tidal, salty waterlogged mud, so a description of bare forest soil with epiphytes in the canopy does not point to them.

Remember · Tropical rain forest: fast litter decay, thin humus, dense canopy, and climbers and epiphytes reaching the light. Nutrients sit in living plants, not in the soil.

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Answer: (c) 2, 3 and 4

Why not the tempting option · UPSC's key is (c). The general checks on tree growth in savannas are fire, grazing and browsing animals, and the long dry season of a seasonal-rainfall climate — a university biome guide puts it as frequent fires and large grazing mammals killing seedlings, with climate the most important factor. Soil does shape the tree–grass balance locally (thin or poorly drained soils carry no trees in some East African savannas), but the question asks for the general conditions and no option pairs soil with fire and grazing; termites build the mounds on which trees grow rather than checking them. In the exam, pick the option that holds the two universal controls, fire and grazing, together.

Fire, grazing and browsing animals, and the long dry season of a seasonal-rainfall climate are the recognised checks that keep savannas open and stop them turning into forest, so 2, 3 and 4 is the answer.

• ✗ 1. Termites are decomposers that break down about 30 per cent of dead organic matter; in parts of East Africa trees grow only on termite mounds, so termites aid rather than check tree growth. Burrowing animals are not a recognised control.
• ✓ 2. Wildfire is an important part of savanna ecosystems in all regions; annual burning of dry grass and litter helps keep woody plants from taking over.
• ✓ 3. Browsing and grazing animals alter the balance between trees and grass. Rising elephant numbers in Virunga National Park, for example, destroyed woody plants and turned a heavily wooded savanna into grass savanna with very few trees.
• ✓ 4. Savanna is a vegetation type of hot, seasonally dry climates: a long dry season each year limits tree growth and keeps the grass cover open.
• ✗ 5. Soil shapes the tree–grass balance only locally — thin or poorly drained soils carry no trees in some savannas — so it is not one of the general checks, and no option pairs it with fire and grazing.

Remember · Savannas stay open through fire, grazing and browsing animals, and a long dry season; without these, woody plants tend to spread and the savanna turns to forest.

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Answer: (a) 1, 2 and 3 only

Crop diversification, legume intensification and tensiometer use are eco-friendly practices: they cut risk and chemical inputs and save water. Vertical farming, a factory-like indoor system, is not counted among India's eco-friendly farming practices.

• ✓ 1. Diversification means changing the cropping pattern and adding other activities. The Centre's Crop Diversification Programme shifts water-guzzling paddy to pulses, oilseeds and other crops, and restores soil fertility through leguminous crops.
• ✓ 2. ICAR's review calls legumes in cropping systems a pathway to sustainable production, because they fix nitrogen and improve soil fertility while cutting water use and pest pressure.
• ✓ 3. A tensiometer measures soil matric potential, which ICAR research uses to schedule irrigation by the soil's need instead of a fixed interval, so water is not wasted.
• ✗ 4. Vertical farming is a high-technology indoor system whose benefits the IPCC calls unproven so far, so it is not treated as an established eco-friendly practice in India.

Remember · Eco-friendly farming: diversify crops, put legumes in rotation, irrigate by soil-moisture readings (tensiometer); vertical farming is not on this list.

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Answer: (d) 1, 2, 3 and 4

All four statements are correct. The word 'some' in each only requires that at least one species fits, and each has a well-known example: the green turtle, the grass carp, the dugong and the sea snakes.

• ✓ 1. The green turtle is a herbivore, eating mostly seaweed, seagrasses and algae.
• ✓ 2. Many fish are plant eaters; the grass carp, for example, is a basically herbivorous fish that feeds on aquatic weeds.
• ✓ 3. The dugong (sea cow) is a marine mammal that depends largely on seagrass for food.
• ✓ 4. Viviparous means the young develop inside the mother and are born alive. Sea snakes such as the dusky sea snake do this, and so do boas and many vipers.

Remember · 'Some species' statements are usually true: herbivorous green turtle, grass carp and dugong; viviparous sea snakes.

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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'.

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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.

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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.

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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

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)

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 · Report an error

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)

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 · Report an error

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.

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 · Report an error