UPSC CSE Prelims
Fundamentals of Ecology Previous Year Questions (PYQs)

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

• Marine phytoplankton produce nearly 50% of Earth's oxygen.

• Phytoplankton are microscopic photosynthetic organisms found in oceans.

• Oceans are the largest contributor to global oxygen production.

• Atmospheric air contains about 21% oxygen.

• Dissolved oxygen in water is usually measured in mg/L, much lower than oxygen concentration in air.

• Photosynthesis converts CO₂ + Water → Glucose + Oxygen.

• Healthy oceans are essential for maintaining Earth's oxygen balance and climate system.

Detailed Explanation:

Answer: Option 2 — Only three

Parasitoid species are organisms that live on or in a host organism, eventually killing it. Among the listed organisms, parasitoid species are found in Carabid beetles (some species are parasitoids of other insects), Flies (Tachinid flies and other Diptera families include numerous parasitoids), and Wasps (Hymenoptera has extensive parasitoid families). Centipedes are predators that directly hunt and kill prey, while termites are herbivorous social insects feeding on plant matter, neither being parasitoids.

📝 Short Notes: Parasitoids

• Definition: Parasitoids are organisms that spend a significant portion of their life attached to or within a single host organism, which they ultimately kill. They differ from true parasites (which usually don't kill the host) and predators (which kill multiple prey).
• Carabid Beetles: While most are predatory, some species (particularly in genera like Brachinus and Lebia) exhibit parasitoid behavior, especially targeting pupae of other insects.
• Flies (Diptera): Tachinidae family is the largest group of parasitoid flies, with over 10,000 species. Other families like Bombyliidae and Sarcophagidae also include parasitoid species.
• Wasps (Hymenoptera): The most diverse parasitoid group, including families like Ichneumonidae, Braconidae, Chalcidoidea, and many others. They parasitize various life stages of insects and other arthropods.
• Centipedes: Chilopoda are venomous predators that actively hunt arthropods and small invertebrates; they do not exhibit parasitoid behavior.
• Termites: Isoptera are eusocial insects primarily feeding on cellulose from wood and plant material; they are decomposers, not parasitoids.

Detailed Explanation:

Answer: Option 1 — Fig

Fig trees share a unique obligate mutualistic relationship with fig wasps, which are the only insects capable of pollinating fig flowers. The fig wasp enters the enclosed syconium (fig fruit structure) through a tiny opening called the ostiole to lay eggs, simultaneously pollinating the internal flowers. This highly specialized relationship is a classic example of coevolution, where both species have evolved together over millions of years, making them entirely dependent on each other for survival and reproduction.

Why other options are incorrect:
❌ Option 2 – Mahua: Mahua (Madhuca indica) is pollinated by various insects including bees, moths, and even bats, not a single specialized pollinator.
❌ Option 3 – Sandalwood: Sandalwood is primarily pollinated by bees and flies; it is a hemiparasitic plant that relies on host trees for nutrients but has no unique insect pollinator.
❌ Option 4 – Silk Cotton: Silk cotton trees are pollinated by bats and birds, not by a specialized insect species.

📝 Short Notes: Fig-Wasp Mutualism

• Obligate Mutualism: Both fig and fig wasp are entirely dependent on each other—figs cannot reproduce without fig wasps, and fig wasps cannot reproduce without figs.
• Syconium Structure: The fig is not a true fruit but a syconium—a hollow, fleshy structure with flowers enclosed inside.
• Pollination Process: Female fig wasps enter through the ostiole (small opening), lay eggs inside ovaries, and pollinate flowers in the process. Male wasps are wingless and mate inside the fig.
• Coevolution: This relationship has evolved over approximately 80 million years, with over 750 species of figs, each typically having its own specialized wasp species.
• Ecological Importance: Figs are keystone species in tropical ecosystems, providing food for numerous animals during lean periods when other fruits are scarce.

Detailed Explanation:

Answer: Option 4 — Aquatic plants absorb heavy metals and excess nutrients.

The analogy of wetlands as "kidneys of the Earth" is best reflected by their filtration function. Just as kidneys filter waste products and toxins from blood, wetlands act as natural filters by absorbing pollutants, heavy metals, and excess nutrients from water through aquatic plants and microbes. This phytoremediation and nutrient regulation process purifies water and prevents harmful substances from contaminating downstream ecosystems, paralleling the kidney's role in maintaining bodily health.

Why other options are less appropriate:
Option 1 describes the water cycle, which relates more to circulation than filtration. Option 2 explains the food web, not the purification function. Option 3 addresses sediment control, which is important but doesn't capture the filtering/cleansing aspect central to the kidney analogy.

📝 Short Notes: Wetland Functions

• Filtration & Purification: Wetlands remove pollutants, heavy metals, sediments, and excess nutrients through plant uptake and microbial action (phytoremediation and bioremediation).
• Nutrient Regulation: Absorb excess nitrogen and phosphorus, preventing eutrophication in water bodies downstream.
• Flood Control: Act as natural sponges, absorbing excess rainfall and reducing flood peaks.
• Groundwater Recharge: Facilitate percolation of water into aquifers, maintaining groundwater levels.
• Biodiversity Hotspots: Support diverse flora and fauna, including migratory birds, fish, and amphibians.
• Carbon Sequestration: Wetland vegetation and soils store significant amounts of carbon, helping mitigate climate change.
• Coastal Protection: Mangroves and coastal wetlands buffer against storms, tsunamis, and erosion.

Detailed Explanation:

Answer: Option 2 — 2 and 3

Primary producers in ocean food chains are organisms that produce organic compounds from inorganic substances through photosynthesis or chemosynthesis. Among the given organisms, cyanobacteria and diatoms are photosynthetic organisms that convert sunlight into chemical energy, forming the base of marine food webs. Copepods and foraminifera are consumers (zooplankton) that feed on primary producers.

❌ Statement 1 – Incorrect: Copepods are small crustacean zooplankton that function as primary consumers, feeding on phytoplankton and organic matter.
✅ Statement 2 – Correct: Cyanobacteria are photosynthetic bacteria capable of performing photosynthesis and are important primary producers in marine ecosystems.
✅ Statement 3 – Correct: Diatoms are unicellular algae (phytoplankton) that are among the most significant primary producers in oceans, contributing up to 40% of marine primary productivity.
❌ Statement 4 – Incorrect: Foraminifera are single-celled protists that are primarily consumers, feeding on organic particles and smaller organisms using pseudopodia.

📝 Short Notes: Marine Primary Producers and Plankton

• Primary Producers: Organisms that synthesize organic compounds from carbon dioxide through photosynthesis (using sunlight) or chemosynthesis, forming the base of food chains.
• Phytoplankton: Microscopic photosynthetic organisms drifting in water bodies; includes diatoms, dinoflagellates, cyanobacteria, and green algae.
• Diatoms: Most abundant phytoplankton with silica cell walls (frustules); responsible for 20-40% of Earth's oxygen production and major primary producers in oceans.
• Cyanobacteria: Photosynthetic bacteria (blue-green algae); can fix atmospheric nitrogen; some species form harmful algal blooms.
• Zooplankton: Heterotrophic organisms that feed on phytoplankton and organic matter; includes copepods, foraminifera, krill, and jellyfish larvae.
• Copepods: Small crustaceans that are the most abundant zooplankton; primary consumers linking phytoplankton to higher trophic levels.
• Foraminifera: Amoeboid protists with shells (tests); mostly marine; some are benthic while others are planktonic; feed on bacteria, algae, and detritus.

Detailed Explanation:

Answer: Option 3 — 1, 3 and 5 Only

Detritivores are organisms that feed on dead and decaying organic matter (detritus), playing a crucial role in nutrient cycling and decomposition. Earthworms, millipedes, and woodlice are classic examples of detritivores that break down organic debris in soil and forest floors. Jellyfish are primarily carnivorous filter feeders that consume plankton and small fish, while seahorses are also carnivores feeding on small shrimp and invertebrates—neither are detritivores.

✅ Statement 1 (Earthworms) – Correct: Earthworms are classic detritivores that consume and break down organic matter in soil, enhancing soil fertility through decomposition.

❌ Statement 2 (Jellyfish) – Incorrect: Jellyfish are carnivorous organisms that primarily feed on plankton and small fish through filter feeding, not on detritus.

✅ Statement 3 (Millipedes) – Correct: Millipedes are detritivores that feed on decaying leaves and organic debris on forest floors, aiding in decomposition.

❌ Statement 4 (Seahorses) – Incorrect: Seahorses are carnivores that feed on small shrimp, plankton, and other small invertebrates, not on dead organic matter.

✅ Statement 5 (Woodlice) – Correct: Woodlice (also called pill bugs or roly-polies) are terrestrial crustaceans that feed on decaying plant and animal matter as detritivores.

📝 Short Notes: Detritivores and Decomposers

• Detritivores: Organisms that feed on dead organic matter (detritus), including dead plant material, animal carcasses, and feces. They physically break down organic matter into smaller pieces.
• Common Examples: Earthworms, millipedes, woodlice, dung beetles, termites, some snails, and certain types of flies and beetles.
• Role in Ecosystem: Detritivores accelerate decomposition by fragmenting organic matter, increasing surface area for microbial action, and mixing organic matter with soil.
• Difference from Decomposers: Decomposers (bacteria and fungi) chemically break down organic matter at molecular level, while detritivores physically consume and fragment it.
• Nutrient Cycling: Detritivores play a critical role in nutrient cycling by releasing nutrients locked in dead organic matter back into the soil for plant uptake.
• Habitat: Most detritivores are found in soil, leaf litter, forest floors, and aquatic sediments where organic debris accumulates.

Detailed Explanation:

Answer: Option 4 — 1, 2 and 3

All three groups—cnidarians, fungi, and protozoa—have species capable of establishing symbiotic relationships with other organisms. This demonstrates the widespread occurrence of symbiosis across different taxonomic groups in nature.

✅ Statement 1 – Correct: Cnidarians like corals form symbiotic relationships with zooxanthellae (photosynthetic algae), which is essential for coral reef ecosystems.
✅ Statement 2 – Correct: Fungi form diverse symbiotic associations including mycorrhizae with plant roots and lichens with algae or cyanobacteria.
✅ Statement 3 – Correct: Protozoa establish symbiotic relationships, such as those living in termite guts that help digest cellulose.

📝 Short Notes: Symbiotic Relationships in Different Organisms

• Mutualism: Both organisms benefit from the relationship
• Symbiosis: Close and long-term biological interaction between different species
• Zooxanthellae: Single-celled dinoflagellates crucial for coral bleaching studies
• Ecological importance: Symbiotic relationships are fundamental to ecosystem functioning and biodiversity

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.

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.

Detailed Explanation:

Ecosystem = Biotic community (living organisms) + Abiotic environment (non-living factors like soil, water, air, sunlight) functioning together.

❌ Option 1: Describes only a biological community (lacks abiotic factors).
❌ Option 2: Defines the biosphere (entire zone of life on Earth).
✅ Option 3: Correctly captures both organisms and their environment as an integrated functional unit.
❌ Option 4: Refers to biota/biodiversity of a region (only living components).

Detailed Explanation:

A food chain represents the linear flow of energy from producers to consumers in an ecosystem.

Diatoms are microscopic algae and act as primary producers through photosynthesis, forming the base of aquatic food chains.

Crustaceans (like krill, copepods) are primary consumers that feed on diatoms and other phytoplankton.

Herrings are secondary consumers (small fish) that feed on crustaceans and zooplankton.

The correct energy flow follows: Diatoms → Crustaceans → Herrings.

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.

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.

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

Detailed Explanation:

Ecological niche describes both the physical space occupied by an organism and its functional role in the community, including its interactions with biotic and abiotic factors.

Habitat refers only to the physical location where an organism lives, while ecotone is a transition zone between two ecosystems, and home range is the area an animal travels during normal activities.