UPSC CSE Prelims
Food Chain and Food Web Previous Year Questions (PYQs)

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:

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:

Fungi and Bacteria are the primary decomposers in ecosystems that break down dead organic matter into simpler inorganic substances, recycling nutrients back into the environment.

Viruses are not decomposers—they are metabolically inactive outside host cells and function as obligate intracellular parasites that require living cells to replicate, playing no role in decomposition.

Detailed Explanation:

✅ Statement 1 – Correct: A food chain illustrates the linear sequence in which organisms feed upon each other, showing the order of energy flow from producers → primary consumers → secondary consumers → tertiary consumers.

❌ Statement 2 – Incorrect: Food chains operate between different species, not within populations of a single species; they represent inter-species feeding relationships across different trophic levels.

❌ Statement 3 – Incorrect: A food chain shows only the feeding relationship and direction of energy flow, not the numbers or biomass of organisms; ecological pyramids illustrate numerical or biomass relationships.

Detailed Explanation:

✅ Statement 1 – Correct: Phytoplankton perform photosynthesis and absorb atmospheric CO₂, converting it into organic carbon through carbon sequestration. Their destruction would severely impair the ocean's role as a carbon sink.

✅ Statement 2 – Correct: Phytoplankton form the base of marine food chains, being the primary producers consumed by zooplankton and small fish, which support larger marine organisms. Their complete destruction would collapse oceanic food webs.

❌ Statement 3 – Incorrect: Ocean water density is determined by temperature and salinity, not by the presence of microscopic phytoplankton. Their removal would have negligible impact on water density.