Ecology and ecosystems: previous-year questions
40 real UPSC Prelims questions (2011–2026) with the official answer and a clear explanation. 8 of them came in the last five papers.
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With reference to agricultural soils, consider the following statements :
- 1. A high content of organic matter in soil drastically reduces its water holding capacity.
- 2. Soil does not play any role in the sulphur cycle.
- 3. Irrigation over a period of time can contribute to the salinization of some agricultural lands.
Which of the statements given above is/are correct ?
Answer (b) & explanation
Only statement 3 is correct (b). Statement 1 is wrong because organic matter (humus) works like a sponge. It raises the soil's water-holding capacity, improves structure and feeds microbes. Statement 2 is wrong because soil is central to the sulphur cycle. Soil bacteria break down organic sulphur, oxidise it into sulphates that plants absorb, and reduce it again in waterlogged soils. Statement 3 is right because heavy irrigation with poor drainage raises the water table. Water then evaporates at the surface and leaves salts behind, causing salinisation and alkalinity, as seen in parts of Punjab, Haryana and the Indira Gandhi Canal area of Rajasthan.
Notice the fixes: gypsum for alkaline soils, sub-surface drainage, and salt-tolerant varieties from CSSRI, Karnal.
Answer as per the official UPSC answer key.
Consider the following statements :
- 1. Most of the world’s coral reefs are in tropical waters.
- 2. More than one-third of the world’s coral reefs are located in the territories of Australia, Indonesia and Philippines.
- 3. Coral reefs host far more number of animal phyla than those hosted by tropical rainforests.
Which of the statements given above is/are correct ?
Answer (d) & explanation
All three statements are correct (d). Statement 1 is right because reef-building corals need warm (about 20–29°C), clear, shallow, salty water, so most reefs lie in the tropics, between about 30°N and 30°S. Statement 2 is right because Australia, Indonesia and the Philippines together hold more than a third of the world's reef area. Indonesia alone has the most, and the Coral Triangle is the richest region. Statement 3 is right because reefs host more animal phyla (major body plans) than rainforests. Many sea groups, such as echinoderms, never moved onto land.
Notice the threat: the fourth global bleaching event (2023–25) hit over 80% of reefs. India's reefs are in the Gulf of Mannar, Gulf of Kachchh, Lakshadweep and the Andamans.
Answer as per the official UPSC answer key.
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. Pollination of some plants could be adversely affected.
- 2. There could be a drastic increase in the fungal infections of some cultivated plants.
- 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 (c) & explanation
Statements 1 and 3 are correct (c). Butterflies are pollinators of many flowering plants, so a big fall in their numbers could hurt pollination (1). Butterflies and their caterpillars are also food for wasps (some lay eggs in caterpillars), spiders and birds. Losing them would affect these predators further up the food web (3). Statement 2 has no link. Butterflies do not control plant fungi, so their loss would not cause a sudden rise in fungal infections of crops.
Butterflies are also bio-indicators, sensitive to pesticides and climate change. Notice the wider idea of a trophic cascade: losing one link in the food web can ripple through the whole ecosystem, much like declines in bees threaten food security.
Answer as per the official UPSC answer key.
Which one of the following is the best description of the term ‘ecosystem’?
Answer (c) & explanation
An ecosystem is a community of organisms together with the non-living environment in which they live, such as soil, water, air and light, all working as one unit (c). Option (a) describes only a community, without the physical environment. Option (b) describes the biosphere, and option (d) describes the flora and fauna of an area.
The term was coined by A. G. Tansley in 1935. An ecosystem has producers, consumers and decomposers, with energy flowing through it and nutrients cycling within it. Notice how a pond, a forest or even an aquarium can be studied as an ecosystem.
Answer as per the official UPSC answer key.
Which of the following adds/add carbon dioxide to the carbon cycle on the planet Earth?
- 1. Volcanic action
- 2. Respiration
- 3. Photosynthesis
- 4. Decay of organic matter
Select the correct answer using the code given below.
Answer (c) & explanation
Volcanic action releases CO₂ stored in the Earth (1 correct). Respiration by plants, animals and microbes releases CO₂ (2 correct), and so does the decay of organic matter (4 correct). Photosynthesis does the opposite: it takes CO₂ out of the air and locks the carbon into plant matter, so options (a) and (d) fail.
On top of these natural flows, burning fossil fuels and clearing forests add extra carbon and drive climate change. Notice how this links biology (respiration and photosynthesis), geography (volcanoes) and environment (carbon sinks such as forests and oceans) in one cycle.
Answer as per the official UPSC answer key.
Lichens, which are capable of initiating ecological succession even on a bare rock, are actually a symbiotic association of
Answer (b) & explanation
A lichen is a symbiotic association of a fungus with an alga or a cyanobacterium (b). The fungus provides shelter and holds water; the alga makes food by photosynthesis. Options with bacteria and fungi (c), algae and bacteria (a), or fungi and mosses (d) do not describe a lichen, though some lichens do include cyanobacteria.
Lichens are the pioneers of primary succession on bare rock: their acids slowly weather the rock into the first thin soil, preparing the way for mosses and then larger plants. They are also sensitive to sulphur dioxide, which makes them useful indicators of air pollution.
Answer as per the official UPSC answer key.
Which one of the following is the correct sequence of a food chain?
Answer (a) & explanation
A food chain always starts with a producer. Diatoms are phytoplankton that make food by photosynthesis. Small crustaceans such as copepods graze on them, and herrings eat the crustaceans. So the order is Diatoms–Crustaceans–Herrings (a). Options (b), (c) and (d) put a consumer before the producer or reverse the order of eating.
Diatoms have glassy silica shells and produce a large share of the oxygen made in the oceans. Notice how energy shrinks at each step (roughly the 10% rule), which is why food chains rarely have more than four or five links.
Answer as per the official UPSC answer key.
Which of the following adds/add nitrogen to the soil?
- 1. Excretion of urea by animals
- 2. Burning of coal by man
- 3. Death of vegetation
Select the correct answer using the codes given below.
Answer (c) & explanation
Items 1 and 3 are correct (c). Animals excrete urea in urine (1), and dead vegetation decays (3); in both cases decomposers convert the nitrogen into ammonia and then nitrates in the soil (ammonification and nitrification). Item 2 is the doubt: burning coal releases nitrogen oxides into the air, and these can reach the soil through acid rain, so some argue it adds nitrogen indirectly; UPSC's key does not count it. So (a), (b) and (d) fail on the key.
Notice the full cycle: fixation by Rhizobium and lightning, nitrification by Nitrosomonas and Nitrobacter, and denitrification by Pseudomonas returning nitrogen to the air. The Haber–Bosch process makes most fertiliser nitrogen. Trap: emissions mainly add nitrogen to the atmosphere, not directly to soil.
Answer as per the official UPSC answer key.
Which one of the following terms describes not only the physical space occupied by an organism, but also its functional role in the community of organisms?
Answer (b) & explanation
Ecological niche is correct (b). A habitat is an organism's address, the physical place it lives, while its niche is its profession: the space it occupies plus its functional role, what it eats, when it is active and how it affects others. (a) is wrong: an ecotone is a transition zone between two ecosystems, such as a forest edge or mangrove, often rich in species (edge effect). (c) is wrong: habitat is only the physical space. (d) is wrong: a home range is the area an animal regularly moves through.
Notice the related principle: Gause's competitive exclusion principle says two species cannot share exactly the same niche for long. The idea was developed by Joseph Grinnell and G.E. Hutchinson. Trap: 'habitat' and 'niche' are often used loosely as synonyms.
Answer as per the official UPSC answer key.
With reference to the food chains in ecosystems, which of the following kinds of organism is/are known as decomposer organism/organisms?
- 1. Virus
- 2. Fungi
- 3. Bacteria
Select the correct answer using the codes given below.
Answer (b) & explanation
Fungi and bacteria are correct (b). Fungi and bacteria break down dead plants, animals and wastes by secreting enzymes and absorbing the products, returning nutrients such as nitrogen and phosphorus to the soil. They are the decomposers of the food chain (2, 3). Item 1 is wrong: viruses cannot feed on dead matter; they have no metabolism of their own and multiply only inside living cells. So (a), (c) and (d) fail.
Notice the related terms: detritivores such as earthworms and termites physically break down litter before decomposers finish the job. The detritus food chain starts from dead matter and is important in forests. Without decomposers, nutrients would stay locked in dead bodies. Trap: viruses are microbes but not decomposers.
Answer as per the official UPSC answer key.
In the grasslands, trees do not replace the grasses as a part of an ecological succession because of
Answer (c) & explanation
Water limits and fire is correct (c). Grasslands get too little rainfall to support a full forest, and frequent fires, together with grazing, kill tree seedlings, while grasses regrow from their underground roots. Succession therefore stops at the grassland stage, a fire- or drought-maintained climax. (a) is wrong: insects and fungi affect both trees and grasses but do not stop succession. (b) is wrong: grasslands get plenty of sunlight, and their soils are often rich, as in the prairies and steppes. (d) fails because (c) is correct.
Notice India's grasslands: the terai and Kaziranga floodplain grasslands, the shola grasslands of the Nilgiris and the Banni grasslands of Kutch. Many are wrongly classed as 'wastelands' and planted with trees. Trap: (b) misreads open, sunny grasslands.
Answer as per the official UPSC answer key.
Which one of the following is the correct sequence of ecosystems in the order of decreasing productivity?
Answer (c) & explanation
Mangroves, grasslands, lakes, oceans is correct (c). Mangroves and estuaries are among the most productive ecosystems per unit area, thanks to nutrients from rivers and tides, warmth and sunlight. Grasslands come next, then lakes. The open ocean has the lowest productivity per unit area because surface waters are poor in nutrients, so it is often called a 'watery desert'. (a), (b) and (d) give the wrong order.
Notice the twist: because it is so vast, the ocean still produces about half of the world's total primary production. Other highly productive systems include coral reefs, swamps and tropical rainforests. Productivity is measured as net primary productivity in g/m²/year. Trap: total output and output per area are different measures.
Answer as per the official UPSC answer key.
With reference to food chains in ecosystems, consider the following statements :
- 1. A food chain illustrates the order in which a chain of organisms feed upon each other.
- 2. Food chains are found within the populations of a species.
- 3. A food chain illustrates the numbers of each organism which are eaten by others.
Which of the statements given above is/are correct?
Answer (a) & explanation
Statement 1 only is correct (a). A food chain shows the order in which organisms feed on each other, who eats whom, in sequence, such as grass → grasshopper → frog → snake → eagle (1). Statement 2 is wrong: a food chain links different species at different trophic levels, not members of one population. Statement 3 is wrong: a food chain shows no numbers; counts of organisms at each level are shown by ecological pyramids of number, biomass and energy. So (b), (c) and (d) fail.
Notice the related ideas: many interlinked chains form a food web. Only about 10% of energy passes to the next level (Lindeman's rule), so chains rarely exceed four or five links. Detritus food chains start from dead matter. Trap: chains show order, pyramids show quantity.
Answer as per the official UPSC answer key.
Consider the following kinds of organisms :
- 1. Bat
- 2. Bee
- 3. Bird
Which of the above is/are pollinating agent/agents?
Answer (d) & explanation
All three are correct (d). Bees are the best-known pollinators of crops and wild plants. Birds, such as sunbirds in India and hummingbirds in the Americas, pollinate many red and tubular flowers (ornithophily). Bats, especially fruit and nectar bats, pollinate night-blooming plants such as banana, durian, mahua and agave (chiropterophily). So options (a), (b) and (c) leave out real pollinators.
Notice the stakes: about 75% of leading food crops benefit from animal pollination. Pollinator decline from neonicotinoid pesticides, habitat loss and climate change worries scientists; the EU restricted neonicotinoids in 2018. India's National Beekeeping and Honey Mission (2020) promotes the 'Sweet Revolution'. Trap: bats are often seen only as pests.
Answer as per the official UPSC answer key.
The Millennium Ecosystem Assessment describes the following major categories of ecosystem services—provisioning, supporting, regulating, preserving and cultural. Which one of the following is supporting service?
Answer (c) & explanation
Nutrient cycling and crop pollination is correct per UPSC's key (c), but pollination's place is doubtful. In the Millennium Ecosystem Assessment (2005), provisioning services give products such as food and water (a), regulating services control processes such as climate and disease (b), and supporting services keep everything running: nutrient cycling, soil formation and primary production. The MEA itself lists pollination under regulating services, so (c) is only half right; UPSC chose it as the best fit. (d) is wrong: 'maintenance of diversity' is a habitat idea, not an MEA supporting service as worded.
Notice the wider idea: the MEA has four categories, not five; 'preserving' in the question is not one of them. Cultural services include recreation and spiritual value. Trap: the stem adds a fake category.
Answer as per the official UPSC answer key.
What would happen if phytoplankton of an ocean is completely destroyed for some reason?
- 1. The ocean as a carbon sink would be adversely affected.
- 2. The food chains in the ocean would be adversely affected.
- 3. The density of ocean water would drastically decrease.
Select the correct answer using the codes given below :
Answer (a) & explanation
Statements 1 and 2 are correct (a). Phytoplankton are the ocean's microscopic plants. Through photosynthesis they absorb CO₂ and, when they die and sink, carry carbon to the deep sea: the biological pump, which makes the ocean a carbon sink (1). They form the base of marine food chains, feeding zooplankton, fish and whales (2). Statement 3 is wrong: seawater density depends on temperature and salinity, not on plankton. So (b), (c) and (d) fail.
Notice the scale: phytoplankton produce about half of the world's oxygen. Ocean warming and acidification threaten them, and blooms can turn harmful. Satellites track them through ocean colour, as India's Oceansat does to forecast fishing zones. Trap: losing plankton would not change water density.
Answer as per the official UPSC answer key.
Human activities in the recent past have caused the increased concentration of carbon dioxide in the atmosphere, but a lot of it does not remain in the lower atmosphere because of
- 1. its escape into the outer stratosphere.
- 2. the photosynthesis by phytoplankton in the oceans.
- 3. the trapping of air in the polar ice caps.
Which of the statements given above is/are correct ?
Answer (b) & explanation
Only statement 2 is correct (b). The oceans absorb roughly a quarter of the CO₂ humans emit, and phytoplankton, microscopic plants in sunlit surface waters, fix large amounts of it through photosynthesis. When they die, some of that carbon sinks to the deep sea, a process called the biological pump. Statement 1 is wrong: CO₂ does not escape to the outer stratosphere in any significant amount; it is well mixed in the atmosphere. Statement 3 is wrong: air bubbles trapped in polar ice are a tiny, ancient store, useful to scientists for studying past climates, not a sink for today's CO₂. So options (a), (c) and (d) fail.
Notice the cost: absorbing CO₂ makes seawater more acidic, called ocean acidification, which harms corals and shellfish. Forests and soils are the other major carbon sinks. Atmospheric CO₂ passed about 425 ppm in 2025. Trap: ice cores are records, not sinks.
Answer as per the official UPSC answer key.
In the context of ecosystem productivity, marine upwelling zones are important as they increase the marine productivity by bringing the
- 1. decomposer microorganisms to the surface.
- 2. nutrients to the surface.
- 3. bottom-dwelling organisms to the surface.
Which of the statements given above is/are correct ?
Answer (b) & explanation
Only statement 2 is correct (b). In upwelling zones, winds push surface water away from the coast, and cold, deep water rises to replace it. This deep water is rich in nutrients such as nitrates and phosphates (2). In the sunlit surface, these nutrients cause phytoplankton blooms, which feed zooplankton and fish, making upwelling zones among the most productive fisheries. Statements 1 and 3 are wrong: upwelling does not specially bring up decomposers or bottom-dwelling organisms. So options (a), (c) and (d) fail.
Notice the famous examples: the coasts of Peru (Humboldt current, anchovy fishery), Namibia (Benguela), California and northwest Africa, and in India the Kerala coast during the monsoon, which supports oil sardine catches. During El Niño, upwelling off Peru weakens, and fish catches collapse. Upwelling zones cover about 1% of the ocean but give a large share of the world's fish. Trap: nutrients, not organisms, are lifted.
Answer as per the official UPSC answer key.
If a tropical rain forest is removed, it does not regenerate quickly as compared to a tropical deciduous forest. This is because
Answer (a) & explanation
The answer is (a). In a tropical rain forest, most nutrients are stored in the living vegetation, not in the soil. Dead matter decomposes and is recycled very fast in the hot, wet climate, and heavy rainfall leaches nutrients out of the thin soil, leaving poor laterite. When the forest is cleared, the nutrients go with the trees, and the remaining soil is too poor to support quick regrowth. Option (b) is wrong because rain forest seeds are not generally of poor viability. Option (c) is wrong because many species grow fast. Option (d) is wrong because the soil is not fertile.
Notice why this matters: slash-and-burn cultivation (jhum in Northeast India) works only with long fallow periods, since cleared land loses fertility within a few years. Deciduous forests, with slower leaching, recover faster. Trap: lush forest does not mean rich soil.
Answer as per the official UPSC answer key.
Consider the following :
- 1. Photosynthesis
- 2. Respiration
- 3. Decay of organic matter
- 4. Volcanic action
Which of the above add carbon dioxide to the carbon cycle on Earth ?
Answer (c) & explanation
Respiration, decay and volcanic action add CO₂, so the answer is (c). Respiration by plants, animals and microbes releases carbon dioxide (2). Decay of organic matter by decomposers releases it too (3). Volcanic eruptions vent CO₂ from deep inside the Earth (4). Statement 1 is wrong: photosynthesis does the reverse; it removes CO₂ from the air and stores the carbon in plants. So (a), (b) and (d) fail.
Notice the wider cycle: the oceans are the biggest carbon sink, forests and soils store a large share, and burning fossil fuels is the main human addition, pushing atmospheric CO₂ above 420 ppm today. Trap: photosynthesis is part of the carbon cycle but it is a sink, not a source.
Answer as per the official UPSC answer key.