(b) Carbon sequestered in forest biomass and agricultural soils

(c) Carbon contained in petroleum and natural gas

(d) Carbon present in atmosphere

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.

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 2021 · Q19

Easy

What is blue carbon?

Answer & explanation

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.

Sources

IUCN Issues Brief: Blue carbon ↗ “Blue carbon is the carbon stored in coastal and marine ecosystems. Coastal ecosystems such as mangroves, tidal marshes and seagrass meadows sequester and store more carbon per unit area than terrestrial forests”
NOAA National Ocean Service: What is Blue Carbon? ↗ “Blue carbon is the term for carbon captured by the world's ocean and coastal ecosystems.”

Question and answer: UPSC's official GS Paper I (2021, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 30 Sept 2026 (how we verify). Permalink ·

Prelims 2021 · Q20

Easy

In the nature, which of the following is/are most likely to be found surviving on a surface without soil?

1.Fern
2.Lichen
3.Moss
4.Mushroom

Select the correct answer using the code given below.

Answer & explanation

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.

📘 Read it in NCERT: Class 11 Fundamentals of Physical Geography, Ch 5 (practise this chapter) · Class 11 Fundamentals of Physical Geography, Ch 5 (practise this chapter) · Class 7 Curiosity, Ch 2 (practise this chapter) · Class 9 Exploration, Ch 12 (practise this chapter) · Class 9 Exploration, Ch 12 (practise this chapter)

Sources

NCERT Class 11 · Fundamentals of Physical Geography, Chapter 5 “First, the weathered material or transported deposits are colonised by bacteria and other inferior plant bodies like mosses and lichens.”
NCERT Class 11 · Fundamentals of Physical Geography, Chapter 5 “The dead remains of organisms and plants help in humus accumulation. Minor grasses and ferns may grow”
NCERT Class 7 · Curiosity, Chapter 2 “Lichens are formed by the association of two living organisms, a fungus and an alga. They grow on rocks and trees”
NCERT Class 9 · Exploration, Chapter 12 “You might have seen them as large green mats on damp rocks or even on the walls of some old buildings”
NCERT Class 9 · Exploration, Chapter 12 “They either absorb nutrients from dead or decaying matter through fine filaments”

Question and answer: UPSC's official GS Paper I (2021, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 30 Sept 2026 (how we verify). Permalink ·

Prelims 2021 · Q22

Medium

Consider the following kinds of organisms:

1.Copepods
2.Cyanobacteria
3.Diatoms
4.Foraminifera

Which of the above are primary producers in the food chains of oceans?

Answer & explanation

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.

📘 Read it in NCERT: Class 8 Curiosity, Ch 2 (practise this chapter) · Class 12 Biology, Ch 11 (practise this chapter)

Sources

NASA Earth Observatory, What are Phytoplankton? ↗ “Among the common kinds are cyanobacteria, silica-encased diatoms, dinoflagellates, green algae, and chalk-coated coccolithophores. … Phytoplankton are the foundation of the aquatic food web, the primary producers, feeding everything from microscopic, animal-like zooplankton to multi-ton whales.”
NCERT Class 8 · Curiosity, Chapter 2 “Microalgae are microscopic plant-like organisms that live in water, soil, air, and even on trees. They make their own food using sunlight.”
NCERT Class 12 · Biology, Chapter 11 “Many marine fish are infested with ectoparasitic copepods.”
University of California Museum of Paleontology, Introduction to the Foraminifera ↗ · reference work “Other species eat foods ranging from dissolved organic molecules, bacteria , diatoms and other single celled phytoplankton, to small animals such as copepods.”

Question and answer: UPSC's official GS Paper I (2021, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 30 Sept 2026 (how we verify). Permalink ·

Prelims 2021 · Q27

Easy

In case of which one of the following biogeochemical cycles, the weathering of rocks is the main source of release of nutrient to enter the cycle?

Answer & explanation

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.

📘 Read it in NCERT: Class 12 Biology, Ch 12 (practise this chapter) · Class 9 Exploration, Ch 13 (practise this chapter)

Sources

NCERT Class 12 · Biology, Chapter 12 “Atmosphere or hydrosphere is the reservoir for the gaseous type of cycle (carbon), whereas Earth's crust is the reservoir for sedimentary type (phosphorus).”
NCERT Class 9 · Exploration, Chapter 13 “The largest reservoir of nitrogen is in our atmosphere.”
University of California Museum of Paleontology, Understanding Global Change: Phosphorus ↗ · reference work “There are no major gaseous forms of phosphorus, so aside from wind distributing small particles of dust containing phosphorus, only very small amounts of phosphorus move through the atmosphere.”

Question and answer: UPSC's official GS Paper I (2021, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 30 Sept 2026 (how we verify). Permalink ·

Prelims 2021 · Q28

Medium

Which of the following are detritivores?

1.Earthworms
2.Jellyfish
3.Millipedes
4.Seahorses
5.Woodlice

Select the correct answer using the code given below.

Answer & explanation

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.

📘 Read it in NCERT: Class 12 Biology, Ch 12 (practise this chapter) · Class 9 Exploration, Ch 12 (practise this chapter)

Sources

NCERT Class 12 · Biology, Chapter 12 “Detritivores (e.g., earthworm) break down detritus into smaller particles. This process is called fragmentation.”
NCERT Class 9 · Exploration, Chapter 12 “Unlike poriferans, cnidarians can catch their prey using tentacles instead of depending on water currents to bring food particles inside through pores.”
USDA Forest Service, International Institute of Tropical Forestry: Direct and indirect effects of millipedes on the decay of litter ↗ “Millipedes are considered to be important organisms involved in decomposition, both for their direct feeding on detritus and their indirect effects on microbial activity.”
Royal Horticultural Society, Woodlice ↗ · reference work “Function - Detritivores, mainly feeding on decomposing plant materials”
Florida Museum of Natural History, Longsnout Seahorse ↗ · reference work “The diet of the longsnout seahorse consists of ghost shrimp, grass shrimp, opossum shrimp, amphipods, and other small marine crustaceans.”

Question and answer: UPSC's official GS Paper I (2021, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 30 Sept 2026 (how we verify). Permalink ·

Prelims 2021 · Q30

Medium

Which of the following have species that can establish symbiotic relationship with other organisms?

1.Cnidarians
2.Fungi
3.Protozoa

Select the correct answer using the code given below.

Answer & explanation

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.

📘 Read it in NCERT: Class 9 Exploration, Ch 12 (practise this chapter) · Class 12 Biology, Ch 11 (practise this chapter) · Class 9 Exploration, Ch 12 (practise this chapter)

Sources

NOAA National Ocean Service, Corals Tutorial: What is Zooxanthellae? ↗ “Most reef-building corals contain photosynthetic cells called zooxanthellae that live in their tissues. The corals and zooxanthellae have a mutualistic relationship.”
NCERT Class 9 · Exploration, Chapter 12 “Cnidarians, such as Hydra (Fig. 12.14), jellyfish (Fig. 12.16b) and corals show a major change in body organisation.”
NCERT Class 12 · Biology, Chapter 11 “Lichens represent an intimate mutualistic relationship between a fungus and photosynthesising algae or cyanobacteria. Similarly, the mycorrhizae are associations between fungi and the roots of higher plants.”
NCERT Class 9 · Exploration, Chapter 12 “Some of the fungi establish a mutualistic (symbiotic) relationship with other organisms while others live as parasites, and cause diseases in plants and animals.”
Kodama et al., 'Effects of the Symbiotic Chlorella variabilis on the Host Ciliate Paramecium bursaria Phenotypes' (PubMed Central) ↗ “The relationship between P. bursaria and symbiotic Chlorella spp. is characterized by mutualism, in which both organisms benefit from this association.”

Question and answer: UPSC's official GS Paper I (2021, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 1 Oct 2026 (how we verify). Permalink ·

Prelims 2021 · Q60

Easy

"Leaf litter decomposes faster than in any other biome and as a result the soil surface is often almost bare. Apart from trees, the vegetation is largely composed of plant forms that reach up into the canopy vicariously, by climbing the trees or growing as epiphytes, rooted on the upper branches of trees." This is the most likely description of

Answer & explanation

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.

📘 Read it in NCERT: Class 11 Fundamentals of Physical Geography, Ch 11 (practise this chapter)

Sources

NCERT Class 11 · Fundamentals of Physical Geography, Chapter 11 “Tropical evergreen forests with dense canopy cover and large biodiversity are found in this climate.”
NASA Earth Observatory: Biome, Rainforest (floor and canopy) ↗ “This material decomposes rapidly in the wet, warm conditions (like a compost pile) sending nutrients back into the soil. … Thick, woody vines are also found in the canopy. They climb trees in the canopy to reach for sunlight.”

Question and answer: UPSC's official GS Paper I (2021, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 30 Sept 2026 (how we verify). Permalink ·

Prelims 2021 · Q61

Hard

The vegetation of savannah consists of grassland with scattered small trees, but extensive areas have no trees. The forest development in such areas is generally kept in check by one or more or a combination of some conditions. Which of the following are such conditions?

1.Burrowing animals and termites
2.Fire
3.Grazing herbivores
4.Seasonal rainfall
5.Soil properties

Select the correct answer using the code given below.

Answer & explanation

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.

Sources

Encyclopaedia Britannica: Savanna (Population and community development and structure) ↗ · reference work “Wildfire is an important ingredient in savanna ecosystems in all regions. … led to an increase in the destruction of woody plants and transformed a heavily wooded savanna into a grass savanna with very few trees.”
Encyclopaedia Britannica: Savanna ↗ · reference work “savanna , vegetation type that grows under hot, seasonally dry climatic conditions and is characterized by an open tree canopy (i.e., scattered trees) above a continuous grass understory”
Encyclopaedia Britannica: Savanna (Environment) ↗ · reference work “An unusually large proportion of dead organic matter—approximately 30 percent—is decomposed through the feeding activities of termites.”
University of California Museum of Paleontology: The grassland biome (Savanna) ↗ · reference work “Frequent fires and large grazing mammals kill seedlings, thus keeping the density of trees and shrubs low. … Climate is the most important factor in creating a savanna. Savannas are always found in warm or hot climates”

Question and answer: UPSC's official GS Paper I (2021, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 1 Oct 2026 (how we verify). Permalink ·

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