Ecosystem — NEET Botany MCQs with answers

Ecosystem is a Class 12 Botany chapter in the NEET (UG) syllabus. NEET720 has 588 reviewed practice questions on it, each with a quick answer and a step-by-step explanation. The 8 questions below are free and fixed, so you can bookmark this page; the full chapter, plus mistake tracking and spaced revision, is in the app.

Topics covered

Ecosystem Structure and Productivity · Decomposition · Energy Flow and Food Chains · Ecological Pyramids · Food Chains · Consumers · Ecology · Primary Productivity · Energy Flow · Ecosystem Structure · Biogeochemical cycles · Ecological succession · Productivity · Food web · Nutrient cycling · Succession · Ecosystem Disturbance · Community Ecology · Ecosystem Function

8 free Ecosystem practice questions with answers

Choose an answer in your head before opening it. Each explanation says why the correct option is right and, where relevant, why the tempting wrong option is wrong.

1. Question 1 · easy · Decomposition

   Decomposition is best defined as the process by which:

   • A.Inorganic nutrients are converted into complex organic compounds by autotrophs alone
   • B.Solar energy is converted into chemical energy stored in organic compounds of green plants
   • C.Complex organic matter of detritus is broken down into simpler inorganic substances that are released back for reuse by producers
   • D.Energy flows unidirectionally from producers to successive trophic levels
   Show answer and explanation

   Answer: C. Complex organic matter of detritus is broken down into simpler inorganic substances that are released back for reuse by producers

   Decomposition is the breakdown of complex organic matter in detritus (dead remains and faecal matter) into simpler inorganic substances like CO2, water and nutrients, which producers reuse.

   Decomposition is a fundamental ecosystem process carried out mainly by bacteria and fungi (decomposers) acting on detritus — dead plant and animal remains including faecal matter. Option A correctly captures this: complex organics are catabolised to simple inorganic substances that re-enter the nutrient pool. Option B describes photosynthesis, the opposite process building organic matter from inorganic CO2 and water using light energy. Option C wrongly reverses the direction, describing synthesis by autotrophs, not breakdown. Option D describes energy flow through trophic levels, a related but distinct ecosystem concept.

   Common mistake: Confusing decomposition (breakdown, releasing nutrients) with photosynthesis (synthesis, fixing carbon).

   Key point: Decomposition converts complex organic detritus into simple inorganic substances, recycling nutrients back to producers.

2. Question 2 · medium · Decomposition

   The organisms chiefly responsible for carrying out decomposition of detritus in an ecosystem are:

   • A.Green algae and cyanobacteria
   • B.Protozoans and nematodes exclusively
   • C.Bacteria and fungi
   • D.Herbivorous insects such as grasshoppers
   Show answer and explanation

   Answer: C. Bacteria and fungi

   Bacteria and fungi are the principal decomposers; their extracellular enzymes catabolise detritus into simpler substances.

   NCERT identifies bacteria and fungi as the main decomposers, secreting digestive enzymes onto detritus to break it down. Detritivores (e.g., earthworms) assist mechanically via fragmentation but the actual enzymatic catabolism is microbial. Option A names producers/nitrogen fixers, not decomposers. Option C exaggerates the role of protozoans/nematodes over the principal microbial agents. Option D describes herbivory on living tissue, unrelated to decomposition of dead matter.

   Common mistake: Crediting detritivores like earthworms as the main decomposers instead of bacteria and fungi.

   Key point: Bacteria and fungi are the main decomposers of detritus in every ecosystem.

3. Question 3 · easy · Decomposition

   Which of the following correctly describes 'detritus', the raw material for decomposition?

   • A.Only the freshly fallen dry leaves of deciduous trees
   • B.Living root exudates released by plants into the rhizosphere
   • C.Dead remains of plants and animals, including faecal matter
   • D.The humus layer formed after complete mineralisation
   Show answer and explanation

   Answer: C. Dead remains of plants and animals, including faecal matter

   Detritus comprises dead plant and animal remains, including faecal matter, and forms the raw material decomposers act upon.

   NCERT defines detritus broadly as dead remains of organisms, plant and animal, together with faecal matter, not just leaf litter. Option A wrongly restricts it to fallen leaves. Option B describes living root exudates, unrelated to dead matter. Option D confuses detritus (starting substrate) with humus (a later, resistant intermediate), inverting the sequence of decomposition.

   Common mistake: Equating detritus only with fallen leaves and ignoring animal remains/faeces.

   Key point: Detritus = dead plant + animal remains + faecal matter, the substrate decomposers work on.

4. Question 4 · medium · Decomposition

   Enzymatic catabolism of detritus by bacteria and fungi ultimately releases which of the following simpler substances?

   • A.Complex proteins and polysaccharides only
   • B.ATP and NADPH exclusively
   • C.Chlorophyll precursors for use by nearby producers
   • D.Carbon dioxide, water and inorganic nutrients
   Show answer and explanation

   Answer: D. Carbon dioxide, water and inorganic nutrients

   Catabolic breakdown of detritus yields simple inorganic substances such as CO2, water and mineral nutrients, which re-enter the ecosystem.

   The catabolic step of decomposition uses bacterial and fungal enzymes to degrade complex organics into CO2, water, and inorganic nutrients (e.g., nitrates, phosphates), which are released back into the soil for uptake by producers. Option A lists the starting complex substrates, not products. Option B names cellular energy carriers unrelated to decomposition output. Option C is fabricated; plants synthesise their own chlorophyll and do not absorb it from soil.

   Common mistake: Listing macromolecules as products instead of the simple inorganic end products.

   Key point: Decomposition end products are CO2, water and inorganic nutrients — the currency of nutrient cycling.

5. Question 5 · hard · Decomposition

   Mineralisation, the final recognised step of decomposition, refers to the process by which:

   • A.Detritivores physically shred detritus into smaller fragments
   • B.Some microbes further degrade humus, releasing inorganic nutrients once again
   • C.Soluble nutrients are washed down into deeper soil horizons by water
   • D.Producers directly absorb humus molecules through their root hairs
   Show answer and explanation

   Answer: B. Some microbes further degrade humus, releasing inorganic nutrients once again

   Mineralisation is the slow, continuing degradation of resistant humus by certain microbes, which liberates inorganic nutrients again for producer uptake.

   After humification, humus is not entirely inert; some microorganisms slowly degrade it further, releasing inorganic nutrients back into the soil in the process called mineralisation. This is a continuous, extremely slow process since humus resists breakdown. Option A describes fragmentation. Option C describes leaching. Option D is biologically incorrect — plant roots absorb simple inorganic ions, not intact organic humus molecules.

   Common mistake: Confusing mineralisation with fragmentation or leaching, which are earlier, distinct steps.

   Key point: Mineralisation is the slow ongoing microbial degradation of humus, continuously releasing inorganic nutrients.

6. Question 6 · easy · Decomposition

   Woody twigs decompose much more slowly than fallen flower petals mainly because woody material contains a higher proportion of:

   • A.Soluble sugars and amino acids
   • B.Free water available for microbial metabolism
   • C.Lignin, a resistant structural compound
   • D.Chlorophyll pigment retained after senescence
   Show answer and explanation

   Answer: C. Lignin, a resistant structural compound

   Lignin-rich woody tissue resists microbial enzymatic attack, so decomposition proceeds slowly compared to soft, nutrient-rich petals.

   The rate of decomposition depends on chemical composition: detritus rich in soluble nutrients like sugars and amino acids (as in petals) decomposes fast, whereas material rich in lignin and chitin (as in wood) resists microbial enzymes and decomposes slowly. Option A names fast-decomposing substances, opposite to the correct reasoning. Option B misattributes the cause to moisture rather than chemistry. Option D is biologically irrelevant since chlorophyll is degraded during senescence.

   Common mistake: Attributing slow wood decomposition to low moisture rather than to lignin content.

   Key point: Lignin and chitin resist microbial breakdown, slowing decomposition; sugars and amino acids speed it up.

7. Question 7 · medium · Decomposition

   If decomposers were experimentally removed from an ecosystem, the most direct and severe long-term consequence would be:

   • A.An immediate increase in atmospheric oxygen due to reduced respiration
   • B.Producers would switch to a fully heterotrophic mode of nutrition
   • C.Progressive accumulation of dead organic matter with blocked recycling of essential nutrients back to producers
   • D.Faster nutrient cycling as detritus bypasses microbial breakdown entirely
   Show answer and explanation

   Answer: C. Progressive accumulation of dead organic matter with blocked recycling of essential nutrients back to producers

   Without decomposers, detritus would accumulate and nutrients locked in dead organic matter could not be released back for producer uptake, disrupting the entire ecosystem's nutrient supply.

   Decomposers are essential for nutrient cycling; removing them halts the conversion of detritus into reusable inorganic nutrients, so dead matter piles up and producers face nutrient starvation over time. Option A misapplies an unrelated respiration argument. Option B is biologically impossible since producers cannot switch modes of nutrition. Option D reverses cause and effect — nutrient cycling would slow, not accelerate, without decomposers.

   Common mistake: Assuming decomposer loss speeds up nutrient availability rather than blocking it.

   Key point: Decomposers are indispensable — their loss blocks nutrient recycling and causes detritus accumulation.

8. Question 8 · medium · Decomposition

   Earthworms breaking down leaf litter into smaller particles primarily benefits subsequent decomposition by:

   • A.Directly converting organic matter into inorganic nutrients through their digestive enzymes
   • B.Increasing the surface area of detritus available for microbial enzymatic attack
   • C.Removing soluble nutrients from detritus through leaching action in their gut
   • D.Producing humus directly as a by-product of digestion
   Show answer and explanation

   Answer: B. Increasing the surface area of detritus available for microbial enzymatic attack

   Fragmentation by detritivores like earthworms breaks detritus into smaller pieces, greatly increasing the surface area exposed to microbial enzymes and speeding subsequent breakdown.

   Fragmentation is the first step of decomposition: detritivores such as earthworms mechanically break detritus into smaller particles. This does not itself convert matter to inorganic nutrients (that is catabolism) nor does it involve leaching or humus formation; its key contribution is increasing surface area, which accelerates the subsequent microbial catabolic step. Options A, C, and D each misassign a later, distinct step to fragmentation.

   Common mistake: Believing detritivores chemically convert detritus to nutrients rather than mechanically fragmenting it.

   Key point: Fragmentation's key contribution is increased surface area, enabling faster microbial catabolism.

Questions about Ecosystem for NEET

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Questions are original NEET720 compositions reviewed for correctness, syllabus fit and option quality. Counts update as the bank grows (588 active practice questions in this chapter today).