UPSC CSE Prelims
Structure and Functions of Ecosystem Previous Year Questions (PYQs)
Practice solved questions for Structure and Functions of Ecosystem with detailed step-by-step solutions, key insights, and trend analysis for UPSC CSE PRELIMS.
Solved Previous Year Questions
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With reference to the planet Earth, consider the following statements:
I. Rain forests produce more oxygen than that produced by oceans.
II. Marine phytoplankton and photosynthetic bacteria produce about 50% of world's oxygen.
III. Well-oxygenated surface water contains several folds higher oxygen than that in atmospheric air.
Which of the statements given above is/are correct?
Detailed Explanation:
Correct Answer: ✅ Option 2 (II only)
Most people associate oxygen production with forests, but the oceans are actually the largest source of Earth's oxygen. Marine phytoplankton, algae, and photosynthetic bacteria contribute roughly half of the global oxygen production. Also, atmospheric air contains much higher oxygen concentrations than water.
❌ Statement I is Incorrect: Oceans produce more oxygen than rainforests. Marine phytoplankton are responsible for a major share of global oxygen production.
✅ Statement II is Correct: Marine phytoplankton and photosynthetic bacteria produce about 50% of the world's oxygen through photosynthesis.
❌ Statement III is Incorrect: Atmospheric air contains about 21% oxygen, whereas even well-oxygenated water contains only a small amount of dissolved oxygen. Water does not contain several times more oxygen than air.
Short Notes: Oxygen Cycle and Marine Phytoplankton
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Marine phytoplankton produce nearly 50% of Earth's oxygen.
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Phytoplankton are microscopic photosynthetic organisms found in oceans.
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Oceans are the largest contributor to global oxygen production.
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Atmospheric air contains about 21% oxygen.
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Dissolved oxygen in water is usually measured in mg/L, much lower than oxygen concentration in air.
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Photosynthesis converts CO₂ + Water → Glucose + Oxygen.
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Healthy oceans are essential for maintaining Earth's oxygen balance and climate system.
Which of the following leaf modifications occur(s) in the desert areas to inhibit water loss?
- Hard and waxy leaves
- Tiny leaves
- Thorns instead of leaves
Select the correct answer using the code given below:
Detailed Explanation:
Answer: Option 4 — 1, 2 and 3
Desert plants have evolved multiple leaf modifications to minimize water loss through transpiration in arid environments. All three modifications mentioned—hard and waxy leaves, tiny leaves, and thorns instead of leaves—serve this purpose effectively.
✅ Statement 1 – Correct: Hard and waxy leaves have a thick cuticle layer and reduced stomata, which significantly limit water evaporation from the leaf surface.
✅ Statement 2 – Correct: Tiny leaves reduce the total surface area exposed to the atmosphere, thereby decreasing the potential for water loss through transpiration.
✅ Statement 3 – Correct: Thorns replacing leaves (as seen in cacti and other xerophytes) eliminate the primary transpiring surface, drastically reducing water loss while providing protection from herbivores.
📝 Short Notes: Xerophytic Adaptations in Desert Plants
- Leaf Modifications: Desert plants (xerophytes) exhibit structural adaptations to conserve water in water-scarce environments.
- Thick Cuticle: A waxy, waterproof coating reduces cuticular transpiration.
- Reduced Leaf Surface: Small or needle-like leaves (e.g., Acacia) minimize the area for water loss.
- Leaf Transformation to Spines: In cacti, leaves are modified into thorns, and photosynthesis occurs in green stems.
- Sunken Stomata: Stomata located in pits or grooves reduce air movement and water vapor loss.
- CAM Photosynthesis: Crassulacean Acid Metabolism allows stomata to open at night, reducing daytime water loss.
- Deep Root Systems: Extensive roots access groundwater, complementing above-ground water conservation strategies.
Due to some reasons if there is a huge fall in the population of species of butterflies, what could be its likely consequence/consequences?
- Pollination of some plants could be adversely affected.
- There could be a drastic increase in the fungal infections of some cultivated plants.
- 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:
Detailed Explanation:
Answer: Option 3 — 1 and 3 only
Butterflies play a crucial role as pollinators and as prey in food webs. A significant decline in butterfly populations would adversely affect pollination of many flowering plants and reduce food availability for their predators like wasps, spiders, and insectivorous birds, leading to cascading ecological effects.
✅ Statement 1 – Correct: Butterflies are important pollinators for numerous flowering plants, and their population decline would negatively impact pollination success, affecting fruit and seed production.
❌ Statement 2 – Incorrect: Butterfly population decline is not directly linked to increased fungal infections in cultivated plants; fungal diseases are primarily influenced by environmental factors like humidity, temperature, and plant health rather than butterfly populations.
✅ Statement 3 – Correct: Butterflies serve as a critical food source for various predators including wasps, spiders, and insectivorous birds; their decline would disrupt the food chain and negatively impact these predator populations.
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Which of the following adds/add carbon dioxide to the carbon cycle on the planet Earth?
- Volcanic action
- Respiration
- Photosynthesis
- The decay of organic matter
Select the correct answer using the code given below.
Detailed Explanation:
✅ Statement 1 – Correct: Volcanic eruptions release CO₂ trapped in the Earth's mantle and crust into the atmosphere.
✅ Statement 2 – Correct: Respiration by living organisms releases CO₂ as a byproduct of cellular metabolism.
❌ Statement 3 – Incorrect: Photosynthesis removes CO₂ from the atmosphere; plants use it to produce glucose and oxygen.
✅ Statement 4 – Correct: Decomposition of dead organic matter by decomposers releases CO₂ back into the atmosphere.
Lichens, which are capable of initiating ecological succession even on a bare rock, are actually a symbiotic association of
Detailed Explanation:
Lichens are a symbiotic association of algae (or cyanobacteria) and fungi.
The fungal partner (mycobiont) provides shelter, water, and minerals, while the algal partner (phycobiont) performs photosynthesis to produce food. Lichens are pioneer species in ecological succession on bare rocks.
Which one of the following is the correct sequence of ecosystems in the order of decreasing productivity?
Detailed Explanation:
Ecosystems ranked by Net Primary Productivity (NPP) from highest to lowest:
Mangroves (high NPP) – receive nutrient inputs from both terrestrial runoff and tidal flushing, warm tropical climate, high photosynthetic rates
Grasslands (moderate NPP) – abundant herbaceous vegetation with productivity dependent on rainfall, soil fertility, and grazing intensity
Lakes (moderate to low NPP) – productivity varies with nutrient status (eutrophic > oligotrophic), generally lower than terrestrial ecosystems
Oceans (lowest NPP) – particularly open ocean zones have limited nutrient availability in photic zones, lowest productivity despite covering largest area
Related Topics in Environment & Ecology
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