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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Questions per year, counted from the official papers.
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Which of the following best explain(s) the rationale for protecting mangrove ecosystems in the context of climate resilience ?
- 1. Mangroves reduce tidal energy and store freshwater, making them ideal sites for paddy cultivation in saline estuarine belts.
- 2. Their salt-sensitive roots filter seawater, making mangroves key to converting coastal land into freshwater aquaculture zones.
- 3. By withstanding tidal surges and offering biomass resources, mangroves function both as natural bio-shields and livelihood bases for rural communities.
Select the answer using the code given below :
Answer (d) & explanation
Only statement 3 explains the rationale (d). Statement 3 is right: by withstanding tidal surges and cyclones, mangroves act as natural bio-shields, and they offer biomass, such as fish, crabs, honey and fuel, for coastal villages. Statement 1 is wrong: mangroves do not store fresh water for paddy. Statement 2 is wrong: they are salt-tolerant, not salt-sensitive. Clearing them for farms or aquaculture ponds is exactly what harms them. So options (a), (b) and (c) fail.
Notice the proof: mangroves cut deaths in the 1999 Odisha super-cyclone and the 2004 tsunami. They also store much blue carbon. India's largest mangrove forest is the Sundarbans; the MISHTI scheme (2023) aims to plant more along the coast.
statement 2 says "salt-sensitive". Mangroves live in salt water because they tolerate it.
Answer as per the official UPSC answer key.
One of the following regions has the world’s largest tropical peatland, which holds about three years worth of global carbon emissions from fossil fuels; and the possible destruction of which can exert detrimental effect on the global climate. Which one of the following denotes that region ?
Answer (b) & explanation
The Congo Basin has the world's largest tropical peatland (b). The Cuvette Centrale peatland in the Congo Basin, spread across the Republic of Congo and the DR Congo, was mapped only in 2017. It is the largest tropical peatland known, storing about 29 billion tonnes of carbon, roughly three years of the world's fossil-fuel emissions. Peat is waterlogged, half-rotted plant matter; if drained or burnt, it releases that carbon. The Amazon (a) has peatlands too, but smaller. The Kikori (c) in Papua New Guinea and the Rio de la Plata (d) are not the largest.
Notice the threats: plans for oil drilling and farming. Indonesia's peat fires of 2015, from forest clearing for palm oil, showed how fast peat can release carbon.
the Amazon is the famous rainforest, but its peat stocks are smaller.
Answer as per the official UPSC answer key.
Which one of the following shows a unique relationship with an insect that has coevolved with it and that is the only insect that can pollinate this tree ?
Answer (a) & explanation
The fig has a unique relationship with its fig wasp (a). Each fig species is pollinated by its own tiny fig wasp. The female wasp enters the closed fig, called a syconium, through a small opening, pollinates the flowers inside, and lays her eggs there. Her young grow inside the fig, and the new females fly off carrying pollen to another fig. Neither partner can complete its life cycle without the other. This one-to-one mutualism is a classic example of coevolution. Mahua (b), sandalwood (c) and silk cotton (d) are pollinated by many animals, such as bats, birds and insects.
Notice why figs matter: they fruit all year and feed many animals, so they are called keystone species in tropical forests. India's banyan and peepal are figs.
Answer as per the official UPSC answer key.
Consider the following trees :
- 1. Jackfruit (*Artocarpus heterophyllus*)
- 2. Mahua (*Madhuca indica*)
- 3. Teak (*Tectona grandis*)
How many of the above are deciduous trees?
Answer (b) & explanation
Two of the three are deciduous (b). A deciduous tree sheds all its leaves in one season, usually the dry summer, to save water. Teak (Tectona grandis) does this; it is the classic tree of India's tropical moist deciduous forests and is prized for its timber. Mahua (Madhuca indica) also sheds its leaves before flowering in spring; its sweet flowers are vital to tribal food and liquor. Item 1 is wrong: jackfruit (Artocarpus heterophyllus) is an evergreen tree of the wetter tracts, such as Kerala's home gardens.
Notice that tropical deciduous forests, also called monsoon forests, are India's most widespread type. Sal, sandalwood, khair and shisham also belong to this group.
Answer as per the official UPSC answer key.
Consider the following statements :
- Statement-I : The soil in tropical rain forests is rich in nutrients.
- Statement-II : The high temperature and moisture of tropical rain forests cause dead organic matter in the soil to decompose quickly.
Which one of the following is correct in respect of the above statements?
Answer (d) & explanation
Statement I is wrong but Statement II is correct (d). Statement II is right: the high heat and moisture of tropical rainforests make dead leaves and organic matter decompose very fast. Statement I is wrong: the nutrients released are taken up at once by the dense plant cover, and heavy rain washes, or leaches, the rest away. Most nutrients sit in the living plants, not the soil. So rainforest soil is poor, often red laterite rich in iron and aluminium.
Notice why clearing rainforest for farming fails: once trees are cut, the thin soil is exhausted in a few years. This explains shifting cultivation (jhum) in such areas. In contrast, cold temperate forests decompose slowly and build thick, fertile soils.
lush forest suggests rich soil. In rainforests the richness is in the plants, not the soil.
Answer as per the official UPSC answer key.
“If rainforests and tropical forests are the lungs of the Earth, then surely wetlands function as its kidneys.” Which one of the following functions of wetlands best reflects the above statement ?
Answer (d) & explanation
Absorbing heavy metals and excess nutrients is the kidney function (d). Kidneys filter waste out of the blood. Wetlands do the same for water: their aquatic plants, microbes and sediments take up heavy metals and excess nutrients, such as nitrates and phosphates, before the water moves on. That is why they are called the "kidneys of the landscape". Options (a), (b) and (c) are true of wetlands, describing the water cycle, the food web and sediment balance, but they are not "filtering" functions.
Notice that East Kolkata Wetlands, a Ramsar site, treat much of the city's sewage naturally while supporting fisheries. Wetlands also control floods and store carbon. India has rules under the Wetlands (Conservation and Management) Rules, 2017.
Answer as per the official UPSC answer key.
Which of the following are nitrogen-fixing plants ?
- 1. Alfalfa
- 2. Amaranth
- 3. Chickpea
- 4. Clover
- 5. Purslane (Kulfa)
- 6. Spinach
Select the correct answer using the code given below:
Answer (a) & explanation
Alfalfa, chickpea and clover fix nitrogen (a). Nitrogen fixing is done by Rhizobium bacteria living in the root nodules of legumes, which turn nitrogen from the air into forms plants can use. Item 1 is right: alfalfa (lucerne) is a legume fodder crop. Item 3 is right: chickpea (gram) is a legume. Item 4 is right: clover is a legume. Items 2, 5 and 6 are wrong: amaranth, purslane (kulfa) and spinach are leafy plants of other families and fix no nitrogen.
Notice why farmers grow legumes in crop rotation: they enrich the soil and cut the need for urea. Nitrogen is also fixed by free-living microbes such as Azotobacter and cyanobacteria, used as biofertilisers in paddy fields.
Answer as per the official UPSC answer key.
The “Miyawaki method” is well known for the :
Answer (c) & explanation
The Miyawaki method creates mini forests in urban areas (c). Developed by the Japanese botanist Akira Miyawaki in the 1970s, the method plants many native species, about three to five saplings per square metre, very close together on small plots, in layers from shrubs to tall trees. The saplings compete for light and grow about ten times faster, so a dense mini forest forms in two to three years, which suits cities. Option (a) is wrong: it is not about commercial farming. Option (b) is wrong: it uses native, not GM, plants. Option (d) is wrong: it has nothing to do with wind energy.
Notice its use in Indian cities such as Chennai, Bengaluru and Hyderabad. Critics say such small plots cannot replace natural forests.
Answer as per the official UPSC answer key.
Consider the following statements :
- Statement 1 : The United Nations Capital Development Fund (UNCDF) and the Arbor Day Foundation have recently recognized Hyderabad as 2020 Tree City of the World.
- Statement 2 : Hyderabad was selected for the recognition for a year following its commitment to grow and maintain the urban forests.
Which one of the following is correct in respect of the above statements?
Answer (d) & explanation
Statement 1 is wrong, statement 2 is right (d). Statement 1 is wrong: Hyderabad was named a Tree City of the World for 2020, but the programme is run by the Food and Agriculture Organization (FAO) with the Arbor Day Foundation, not the UNCDF. Statement 2 is right: Hyderabad was recognised for its commitment to growing and maintaining urban trees and forests, helped by the Telangana government's Haritha Haram greening programme.
Notice the five standards for the title: assigning responsibility for trees, setting rules, keeping an inventory, allotting resources, and holding a celebration of trees. Mumbai later joined the list. Notice too the Nagar Van Yojana (2020), which aims to create urban forests in Indian cities.
the city and year are correct; only the UN body is swapped.
Answer as per the official UPSC answer key.
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 (c) & explanation
Lichens and mosses survive on surfaces without soil (c). Lichens, a partnership of a fungus and an alga or cyanobacterium, and mosses are the pioneer species on bare rock. They cling to the surface, release acids that slowly break it down, and trap dust and dead matter, which starts soil formation. This begins primary succession. Item 1 is wrong: ferns have roots that need some soil. Item 4 is wrong: mushrooms are fungi that need dead organic matter to feed on.
Notice that lichens are indicators of clean air: they disappear where sulphur dioxide pollution is high. The final stable community in succession is the climax community, such as a forest.
Answer as per the official UPSC answer key.
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 (b) & explanation
Cyanobacteria and diatoms are primary producers (b). Primary producers make their own food by photosynthesis, forming the base of the food chain. Item 2 is right: cyanobacteria (blue-green algae) photosynthesise and also fix nitrogen. Item 3 is right: diatoms are phytoplankton with glass-like silica shells, and they produce much of the world's oxygen. Item 1 is wrong: copepods are tiny crustaceans that eat phytoplankton, so they are consumers. Item 4 is wrong: foraminifera are shelled protists that mostly feed on other organisms.
Notice that marine phytoplankton produce about half of Earth's oxygen. Foraminifera shells pile up on the ocean floor and help scientists study past climates.
Answer as per the official UPSC answer key.
Which one of the following is a filter feeder?
Answer (c) & explanation
The oyster is a filter feeder (c). An oyster pumps water through its gills and strains out plankton and fine organic particles, so it is a filter feeder. One adult oyster can filter many litres of water an hour, which also cleans the water and makes oyster reefs valuable to coastal ecosystems. Option (a) is wrong: a catfish is a bottom feeder and scavenger that eats insects, fish and debris. Option (b) is wrong: an octopus is a predator that catches crabs and fish. Option (d) is wrong: a pelican scoops up fish in its throat pouch; it drains the water but does not filter food.
Notice other filter feeders: clams, mussels, sponges, barnacles, flamingos, and the baleen whales and whale shark.
Answer as per the official UPSC answer key.
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 (c) & explanation
The phosphorus cycle depends on rock weathering (c). The phosphorus cycle has no important gas phase. Phosphorus is locked in rocks as phosphate minerals and enters soil and water only as rocks weather. Plants take it up, animals eat the plants, and it returns to soil and the sea when they die, finally settling as sediment that becomes rock again. Option (a) is wrong: carbon cycles mainly through the air as CO₂. Option (b) is wrong: nitrogen enters mainly from the air through fixation. Option (d) is wrong: sulphur has both rock and gas sources, such as volcanoes and SO₂.
Notice that phosphorus is often the limiting nutrient in lakes, and excess phosphate from fertilisers and detergents causes eutrophication.
sulphur also comes from rocks, but it has a large gas phase (SO2, H2S). Phosphorus has none.
Answer as per the official UPSC answer key.
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 (c) & explanation
Earthworms, millipedes and woodlice are detritivores (c). Detritivores feed on detritus: dead plant and animal matter, such as fallen leaves and rotting wood. They break it into small pieces, which helps decomposers like bacteria and fungi finish the job and return nutrients to the soil. Item 1 is right: earthworms eat dead matter in soil. Item 3 is right: millipedes eat decaying leaves on the forest floor. Item 5 is right: woodlice are small crustaceans that eat rotting wood and leaves. Items 2 and 4 are wrong: jellyfish and seahorses are predators that catch small animals.
Notice the difference: detritivores eat detritus, while decomposers break it down chemically. Notice vermicomposting, which uses earthworms to make compost.
Answer as per the official UPSC answer key.
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 (d) & explanation
All three groups have symbiotic species (d). Symbiosis is a close, long-term relationship between two different species. Item 1 is right: corals, which are cnidarians, host algae called zooxanthellae that give them food and colour; losing them causes coral bleaching. Item 2 is right: fungi form mycorrhizae with plant roots, helping them absorb water and phosphorus, and they team up with algae as lichens. Item 3 is right: protozoa in a termite's gut digest the cellulose in the wood it eats.
Notice the types of symbiosis: mutualism (both gain), commensalism (one gains, the other is unaffected) and parasitism (one gains at the other's cost). Rhizobium bacteria in legume roots are another mutualism.
Answer as per the official UPSC answer key.
“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 (d) & explanation
This describes the tropical rain forest (d). Leaf litter decays fastest where heat and moisture are constant all year: the tropical rain forest. Microbes, fungi and insects break down fallen leaves within weeks, so the soil surface is often almost bare. Little light reaches the floor, so many plants climb (lianas) or grow on branches (epiphytes, such as orchids and ferns) to reach the canopy. Coniferous forests (a) are cold, so litter decays slowly. Dry deciduous forests (b) have a long dry season. Mangroves (c) are coastal and waterlogged.
Notice that rain forest soils are poor, because nutrients are quickly taken up by plants and washed out by rain. India's rain forests are in the Western Ghats, the North-East and the Andamans.
Answer as per the official UPSC answer key.
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 (c) & explanation
Fire, grazing and seasonal rainfall keep savannahs open (c). Savannahs are grasslands with scattered trees. Item 2 is right: frequent fires in the dry season kill young trees, while grasses regrow from their roots. Item 3 is right: grazing and browsing animals, such as zebras, antelopes and elephants, eat young trees and shrubs. Item 4 is right: seasonal rainfall, with a long dry season, limits tree growth. Items 1 and 5 can play a part locally, but they are not the standard explanation, and only option (c) groups the three main conditions.
Notice that the largest savannahs are in Africa, such as the Serengeti. In South America they are called llanos and campos. India's tropical grasslands include the Banni of Kachchh.
Answer as per the official UPSC answer key.
What is the use of biochar in farming ?
- 1. Biochar can be used as a part of the growing medium in vertical farming.
- 2. When biochar is a part of the growing medium, it promotes the growth of nitrogen-fixing microorganisms.
- 3. When biochar is a part of the growing medium, it enables the growing medium to retain water for longer time.
Which of the statements given above is/are correct ?
Answer (d) & explanation
All three statements are correct (d). Biochar is a charcoal made by heating crop waste or wood with little oxygen (pyrolysis). Statement 1 is right: biochar can be mixed into the soil-less growing media used in vertical farming, along with cocopeat and others. Statement 2 is right: its porous body gives a home to useful microbes, including nitrogen-fixing bacteria. Statement 3 is right: those same pores hold water and nutrients, so the medium stays moist for longer.
Notice its climate benefit: biochar locks carbon in the soil for hundreds of years, a form of carbon sequestration. Turning crop stubble into biochar is also proposed as an alternative to stubble burning in Punjab and Haryana.
Answer as per the official UPSC answer key.
Consider the following States :
- 1. Chhattisgarh
- 2. Madhya Pradesh
- 3. Maharashtra
- 4. Odisha
With reference to the States mentioned above, in terms of percentage of forest cover to the total area of State, which one of the following is the correct ascending order?
Answer (c) & explanation
The ascending order is Maharashtra, Madhya Pradesh, Odisha, Chhattisgarh (c). By the India State of Forest Report, forest cover as a share of area is about 16–17% in Maharashtra, about 25% in Madhya Pradesh, about 33% in Odisha, and about 41% in Chhattisgarh. The trap is that Madhya Pradesh has the largest forest area in India, but because it is a big state, its share is lower. So options starting with MP (a, b) are wrong, and (d) puts Chhattisgarh before Odisha.
Notice the latest ISFR 2023 (released December 2024): India's forest and tree cover is about 25% of its area. Mizoram has the highest share, while Lakshadweep leads the UTs.
MP tops the list by area, which tempts you to put it last.
Answer as per the official UPSC answer key.
Which of the following has/have shrunk immensely/dried up in the recent past due to human activities ?
- 1. Aral Sea
- 2. Black Sea
- 3. Lake Baikal
Select the correct answer using the code given below :
Answer (a) & explanation
Only the Aral Sea has shrunk drastically (a). From the 1960s, the Soviet Union diverted the Amu Darya and Syr Darya rivers to grow cotton in Uzbekistan and Kazakhstan. The Aral Sea, once the world's fourth-largest lake, shrank by about 90% into small pools. It left behind the salty Aralkum desert and wrecked the fishing towns. The Black Sea (2) faces pollution and low oxygen in its deep water but has not dried up. Lake Baikal (3), the world's deepest lake, faces pollution but is still full.
Kazakhstan's Kok-Aral dam (2005) has partly brought back the North Aral. Notice other lakes shrinking due to people: Lake Chad, Lake Urmia in Iran, and the Dead Sea.
Answer as per the official UPSC answer key.