MPSC 2022 Prelims Paper-I · Question 47 of 100

Correct — option (3), all four. The stem is reproducing a standard textbook description of an ecosystem, and the four items listed are precisely the four functional aspects through which the living and non-living components of an ecosystem are held together. All four belong on the list, so the option that names all four is the answer. Statement (a), productivity, is the rate at which the ecosystem produces organic matter. Green plants fix solar energy in photosynthesis, and the rate at which they do so is the gross primary productivity; what remains after the plants have spent part of it on their own respiration is the net primary productivity, the amount actually available to the organisms that eat them. The rate at which consumers build their own biomass from the food they take in is secondary productivity. Productivity is what makes an ecosystem a going concern, because everything else in it is paid for out of this income. Statement (b), decomposition, is the process that returns what productivity has locked up. Decomposers break down the dead remains of plants and animals and their waste through fragmentation, leaching, the chemical attack of enzymes, the formation of dark amorphous humus, and finally mineralisation, in which the humus is broken down to release inorganic nutrients back to the soil; the process runs fastest in warm and moist conditions and slowest where it is cold or dry. Statement (c), nutrient cycling, is what makes the ecosystem self-sustaining. Carbon, nitrogen, phosphorus and the other nutrients move from the physical environment into living tissue and back again, through biogeochemical cycles that are described as gaseous where the reservoir is the atmosphere and sedimentary where it is the rocks and the soil, and because the same atoms are used again and again the ecosystem does not depend on a fresh supply of matter from outside. Statement (d), energy flow, is the one process in the list that is not a cycle. Energy enters as sunlight, passes from producers to herbivores to carnivores in a single direction, and is lost as heat at every step, only a small fraction being transferred from one trophic level to the next; because it cannot be recycled, an ecosystem needs a continuous supply of solar energy in a way that it does not need a continuous supply of matter. The contrast between statements (c) and (d) — matter cycles, energy flows through — is the single most important idea in the whole topic, and the question includes both deliberately. Option (3) is the answer.

• (1)Only (a) — This option accepts productivity alone and rejects the other three, which reduces the ecosystem to its production side and leaves out everything that makes it a functioning system rather than a crop. Productivity puts organic matter and stored energy into the ecosystem, but without decomposition the dead material would accumulate and the nutrients locked in it would never be released; without nutrient cycling the small stock of available nitrogen, phosphorus and other elements would be exhausted; and without energy flow there would be no account of how the energy fixed by plants reaches the animals that live on them. The stem describes the ecosystem as self-regulatory and self-sustained, and no single process could produce either of those properties on its own. Where a stem defines a thing and then lists processes, the safe approach is to test each item against the definition given rather than to look for a reason to shorten the list.
• (2)Only (a) and (b) — This option pairs production with decomposition, which is a coherent picture as far as it goes — matter is built up by plants and broken down again by decomposers — but it stops at the point where the two most heavily examined ideas of the topic begin. Nutrient cycling is what the products of decomposition are for: mineralisation releases inorganic nutrients into the soil, and it is their return to the plants that closes the loop and makes the system self-sustaining. Energy flow is the process that runs alongside and behaves differently, moving in one direction through the trophic levels and being lost as heat rather than returned. Leaving both of them out abandons the distinction between matter, which cycles, and energy, which does not, and that distinction is the heart of the chapter this question comes from.
• (4)(a), (b) and (d) — This is the most dangerous of the three wrong options, because it accepts three of the four items and drops only nutrient cycling, which a candidate may quietly assume is covered by decomposition and therefore not a separate process. It is not covered by it. Decomposition is the breakdown of dead organic matter by decomposers; nutrient cycling is the larger movement of an element through the atmosphere or the soil, into living tissue, and back again, and it includes pathways that have nothing to do with decomposers at all — the fixation of atmospheric nitrogen by bacteria, the weathering of rock that releases phosphorus, the exchange of carbon dioxide between the ocean and the air. Decomposition is one step within nutrient cycling rather than a substitute for it, which is why the standard account lists the two separately, and why an option that names three of the four items is still short of the answer.

An ecosystem is a functional unit of nature in which living organisms interact with one another and with the physical environment around them, and it is studied under two heads. Its structure is the composition of its biotic and abiotic components — producers, consumers and decomposers on one side, and light, temperature, water, soil and the stock of inorganic nutrients on the other — together with the way they are arranged in space, in the vertical layers of a forest or the depth zones of a lake. Its function is described through four processes. Productivity is the rate at which organic matter is produced, measured as gross primary productivity, net primary productivity after the plants' own respiration is deducted, and secondary productivity for the consumers. Decomposition is the breakdown of dead organic matter, proceeding through fragmentation, leaching, enzymatic breakdown, humification and mineralisation, and controlled by temperature, moisture and the chemical quality of the litter. Energy flow is unidirectional: solar energy fixed by producers passes to herbivores and then to carnivores, with a large loss as heat at every transfer, so that the amount reaching each successive trophic level is a small fraction of what was available at the one below. Nutrient cycling is the repeated movement of elements between the environment and living things through biogeochemical cycles, classed as gaseous when the reservoir lies in the atmosphere, as for carbon and nitrogen, and as sedimentary when it lies in the earth's crust, as for phosphorus and sulphur.

The stem of this question is close to a textbook sentence, and the four items are the four headings under which the standard treatment of ecosystem function is organised, so the item rewards a candidate who has read the chapter as a structure rather than as a set of isolated facts. That is characteristic of how MPSC sets ecology: the questions follow the organisation of the source closely, and knowing how a topic is divided is often enough to answer them. In an inclusive statement question of this kind the risk is not that a candidate will accept a false statement but that a true one will be dropped, and option (4) exists to collect exactly that mistake by omitting nutrient cycling on the assumption that decomposition already covers it. The remedy is to ask, for each item left out, what would be missing from the system without it. Ecology and environment carry substantial weight in MPSC papers, both as general studies and through the state's own concerns — the forests of the Western Ghats, the reservoirs and rivers of the Deccan, the protected areas and the biodiversity of the Sahyadris — and the vocabulary established in this chapter, trophic level, food chain, food web, ecological pyramid, biogeochemical cycle, is the vocabulary those questions are written in.

• An ecosystem is a functional unit of nature in which biotic and abiotic components interact, and its functioning is described under four heads: productivity, decomposition, energy flow and nutrient cycling.
• Gross primary productivity is the rate at which producers fix solar energy as organic matter; net primary productivity is what remains after the producers' own respiration and is the amount available to consumers.
• Decomposition proceeds through fragmentation, leaching, enzymatic breakdown, humification and mineralisation, and it runs fastest in warm and moist conditions and slowest where it is cold or dry.
• Energy flow through an ecosystem is unidirectional, entering as sunlight and being lost as heat at every trophic transfer, which is why an ecosystem requires a continuous external supply of energy.
• Nutrients, unlike energy, move in closed loops through biogeochemical cycles, described as gaseous where the main reservoir is the atmosphere, as with carbon and nitrogen, and as sedimentary where it is the earth's crust, as with phosphorus.

• Treating nutrient cycling as part of decomposition and therefore dropping it from the list, which is the mistake option (4) is built to collect
• Confusing energy flow with nutrient cycling, when energy passes through the system in one direction and is lost as heat while matter is used over and over
• Assuming that a statement question with four true items must contain a planted error, and shortening a list that is in fact complete
• Mixing up gross and net primary productivity, the difference between them being the energy the producers spend on their own respiration

Ecology questions in MPSC papers are drawn closely from the standard textbook treatment, which makes the structure of that treatment worth learning explicitly. The Commission asks the four functional aspects as a list, as here; it asks the steps of decomposition in order; it asks for the percentage of energy transferred between trophic levels and the shape of the ecological pyramids; and it asks which cycles are gaseous and which sedimentary. Environment questions with a Maharashtra bearing appear alongside — the biodiversity of the Western Ghats, the state's national parks and sanctuaries, wetland conservation — and they use the same vocabulary. A candidate who can set out the structure and the function of an ecosystem in a single page, with the four processes named and one line of explanation for each, has covered the ground from which most of these questions are built.

No directly related past PYQ was found.

• practice — not a real PYQ

  Which of the following statements correctly distinguishes the movement of energy from the movement of nutrients in an ecosystem ?

  • (a)Both energy and nutrients move in closed cycles within the ecosystem
  • (b)Energy moves in a closed cycle while nutrients pass through in one direction
  • (c)Energy passes through in one direction and is lost as heat, while nutrients move in closed cycles
  • (d)Neither energy nor nutrients move once fixed by the producers

  Answer(c) Energy passes through in one direction and is lost as heat, while nutrients move in closed cycles — energy enters the ecosystem as sunlight, is fixed by producers and passes up the trophic levels with a heavy loss as heat at each transfer, so it can never be reused and must be supplied continuously from outside. Nutrients, by contrast, move repeatedly between the physical environment and living tissue through the biogeochemical cycles, so the same atoms serve the ecosystem again and again.

• practice — not a real PYQ

  The final step of decomposition, in which humus is broken down to release inorganic nutrients into the soil, is called

  • (a)Fragmentation
  • (b)Leaching
  • (c)Humification
  • (d)Mineralisation

  Answer(d) Mineralisation — decomposition proceeds through fragmentation of the litter by detritivores, leaching of water-soluble nutrients into the soil, enzymatic breakdown of the detritus, humification into dark amorphous humus, and finally mineralisation, in which microbes break the humus down and release inorganic nutrients such as nitrogen and phosphorus into the soil where plants can take them up again. It is this last step that connects decomposition to nutrient cycling.