Ecosystem – Productivity and Decomposition: NEET MCQs with Answers

Explanation: Since NPP = GPP – R, where R is the energy consumed by the plants in their own respiration, and R is always a positive quantity, GPP must always exceed NPP. The net primary productivity is the biomass that is actually available for consumption by herbivores and decomposers.

Explanation: Gross primary productivity is the total rate at which producers fix solar energy into organic matter during photosynthesis, before any deduction for respiration. Subtracting respiratory losses from GPP gives net primary productivity, while secondary productivity refers to the rate of formation of new organic matter by consumers.

Explanation: Coral reefs have an exceptionally high gross primary productivity because of the symbiotic relationship between coral polyps and photosynthetic zooxanthellae, extremely efficient internal nutrient recycling, shallow sunlit warm waters and very high biodiversity — which is why they are called the 'rainforests of the sea'.

Explanation: Plants trap solar energy and incorporate it into organic compounds during photosynthesis. The rate at which this light energy is converted into the chemical energy of organic molecules by the producers of an area is the gross primary productivity of the ecosystem.

Explanation: Gross production is the total rate of capture of energy or total organic material produced by the producers, whereas net production is what remains after meeting the producers' respiratory costs. Since GP = NP + R, gross production always has the highest value; secondary and tertiary production are far smaller because only about 10% of energy is transferred at each step.

Explanation: Tropical rainforests have a warm, wet climate throughout the year, which provides optimal conditions for plant growth and photosynthesis. Abundant photosynthesis and continuous plant growth result in the highest annual net primary productivity of all terrestrial ecosystems.

Explanation: Fallen logs decompose very slowly mainly because of their low moisture content; in addition they are rich in cellulose and lignin, which are resistant to microbial breakdown. The environment around fallen logs is generally aerobic, so oxygen deficiency is not the limiting factor.

Explanation: Net primary productivity is equal to gross primary productivity minus the rate of respiration and other losses of the producers. It is precisely the biomass that remains available for consumption by heterotrophs — the herbivores and the decomposers.

Explanation: A rabbit is a consumer (herbivore), so the rate at which it forms new organic matter is secondary productivity. Any productivity by heterotrophs — herbivores, carnivores or decomposers — is secondary productivity; only producers contribute to primary productivity.

Explanation: Only fragmentation is described correctly — detritivores such as earthworms break down detritus into small fragments. Humification produces humus that is highly RESISTANT to microbial action and decomposes at an extremely slow rate; catabolism is the enzymatic degradation of detritus into inorganic substances (not the anaerobic last step); and in leaching water-soluble nutrients GO DOWN into the soil horizons and get precipitated as unavailable salts.

Explanation: Secondary productivity is the rate of formation of new organic matter by consumers (heterotrophs), which assimilate a part of the food they ingest and convert it into their own biomass. Primary productivity, in contrast, is the organic matter produced by the autotrophs of a given area.

Explanation: Gross primary productivity (GPP) is the rate of total capture of energy, or the rate of total production of organic material (biomass), by producers during photosynthesis per unit area per unit time. The amount left after the producers' own respiratory losses is the net primary productivity.

Explanation: GPP is the total rate of production of organic matter during photosynthesis. NPP = GPP − R, where R is the respiratory loss of the producers. Since a plant always respires, R is always greater than zero, and therefore GPP is always more than NPP.

Explanation: Lignin and chitin are complex organic compounds of plant cell walls and of the exoskeleton of insects and fungi respectively, and they are highly resistant to microbial attack. Detritus rich in them therefore decomposes very slowly. Warmth, moisture, aerobic microbes and a sugar-rich detritus all accelerate decomposition.

Explanation: Statement I is correct. Statement II is incorrect — lignin and chitin are complex, resistant compounds, so detritus rich in them decomposes SLOWLY; decomposition is faster when the detritus is rich in nitrogen and water-soluble substances like sugars. Decomposition is also largely an oxygen-requiring process, favoured by a warm and moist environment.