The Ecosystem chapter is central to both board and competitive exams because it links energy flow and nutrient cycling to real ecological outcomes. Concepts like productivity (GPP, NPP), trophic transfers, trophic cascades, and decomposition-driven impacts on oxygen and SOC are frequently asked, often with calculation-based reasoning.
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10
Questions
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Marking
Q1. In a grassland ecosystem gross primary productivity is measured as and autotrophic respiration is . What is the net primary productivity (NPP) per year per square metre?
(A)
(B)
(C)
(D)
Q2. In a terrestrial ecosystem net primary productivity is . Herbivores consume of NPP; of the energy they ingest they convert into herbivore biomass (herbivore production). Carnivores consume of herbivore production and convert of what they consume into carnivore biomass. Estimate annual carnivore production (in ).
(A)
(B)
(C)
(D)
Q3. Microbial decomposition in a temperate grassland has a temperature sensitivity . If mean annual temperature rises from to , the decomposition rate at will be approximately times the rate at (call the original rate ). Which option best estimates the new decomposition rate and the likely short-term effect on soil organic carbon (SOC) if litter input remains unchanged?
(A) New rate ; SOC will decrease
(B) New rate ; SOC will increase
(C) New rate ; SOC will decrease
(D) New rate ; SOC will decrease
Q4. A shallow lake of area and mean depth has an initial dissolved oxygen (DO) concentration of . An algal bloom deposits of organic carbon over the whole lake surface; microbial decomposition will oxidize this organic carbon. Given that oxidation of organic C consumes O, assess whether decomposition will deplete DO below the hypoxic threshold of .
(A) No — DO will remain above
(B) Yes — decomposition will consume far more O than is present; DO will fall below (likely anoxia)
(C) No — DO will drop only to about due to rapid reaeration
(D) No — decomposition will not significantly lower DO because much of the deposited carbon is incorporated into microbial biomass (little net O use), so DO stays above
Q5. Assertion (A): In many aquatic ecosystems, removal of top predators can cause an increase in phytoplankton biomass (algal bloom).
Reason (R): Top predators suppress populations of mesopredatory fish that feed on herbivorous zooplankton; removal of top predators increases mesopredators, reduces zooplankton grazing and thereby allows phytoplankton to accumulate.
Which option is correct?
(A) Both A and R are true but R does not explain A
(B) A is true but R is false
(C) Both A and R are true and R correctly explains A
(D) A is false and R is true
Q6. In a terrestrial ecosystem the gross primary productivity (GPP) is and autotrophic respiration is . Using , calculate net primary productivity (NPP) in .
Q7. In an aquatic ecosystem and autotrophic respiration . Herbivores consume of NPP; assimilation efficiency (AE) is and production efficiency (P/A) of herbivores is . Using , what is the daily secondary production of herbivores (in )?
Q8. If net primary productivity (NPP) of an ecosystem is and mean trophic transfer efficiency (TE) between successive trophic levels is , energy available at trophic level (producers ) is . If a viable population at the top trophic level requires at least , what is the maximum number of trophic levels that can be supported?
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Q9. Assertion (A): In a forest ecosystem, litter with a C:N ratio of will initially cause net immobilization of inorganic nitrogen by soil microorganisms, reducing plant-available inorganic N.
Reason (R): Soil microbes have a much lower C:N (≈ ); to build microbial biomass from high-C litter they take up inorganic N from the soil, causing immobilization.
Both A and R are true but R does not correctly explain A
A is true but R is false
Both A and R are true and R correctly explains A
Both A and R are false
Q10. A harvested population follows , where is intrinsic growth rate, carrying capacity and constant harvest (individuals yr). The maximum constant harvest that still allows a positive equilibrium equals . For and individuals, what is (individuals yr)?