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Ecosystem Set-3 MCQ Online Practice Test | Class 12 | Quiz Tweek

Ecosystem Set-3

Practice Important MCQs for Ecosystem — Biology, Class 12.

This chapter, “Ecosystem,” forms the backbone of how living organisms interact with their environment through energy flow, nutrient cycling, productivity, trophic levels, and ecological efficiencies. It is frequently asked in CBSE board exams and also in competitive tests (NEET/JEE) through numerical problems (GPP–NPP–R, production efficiencies, decomposition) and conceptual reasoning (energy vs biomass pyramids, residence time, ecological efficiencies).

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Q1. In a temperate grassland, gross primary productivity (GPP) is measured as $1500\ \text{kJ m}^{-2}\text{yr}^{-1}$ and autotrophic respiration (R) is $600\ \text{kJ m}^{-2}\text{yr}^{-1}$ . Using $NPP = GPP - R$ , what is the net primary productivity (NPP) of the grassland?

(A)

$1500\ \text{kJ m}^{-2}\text{yr}^{-1}$

(B)

$900\ \text{kJ m}^{-2}\text{yr}^{-1}$

(C)

$600\ \text{kJ m}^{-2}\text{yr}^{-1}$

(D)

$2100\ \text{kJ m}^{-2}\text{yr}^{-1}$

Q2. In a small lake the annual net primary productivity (NPP) is $12{,}000\ \text{kJ m}^{-2}\text{yr}^{-1}$ . Zooplankton consume 30% of NPP. Of the consumed energy, assimilation efficiency (AE) is 40% and net production efficiency (NPE, fraction of assimilated energy converted to biomass) is 25%. Calculate the annual secondary production (zooplankton biomass produced) in $\text{kJ m}^{-2}\text{yr}^{-1}$ (use $P = NPP \times C \times AE \times NPE$ ).

(A)

$1080\ \text{kJ m}^{-2}\text{yr}^{-1}$

(B)

$1440\ \text{kJ m}^{-2}\text{yr}^{-1}$

(C)

$300\ \text{kJ m}^{-2}\text{yr}^{-1}$

(D)

$360\ \text{kJ m}^{-2}\text{yr}^{-1}$

Q3. A grassland ecosystem has an annual NPP of $800\ \text{g C m}^{-2}\text{yr}^{-1}$ . The average turnover time of producer biomass is $0.25\ \text{yr}$ . If herbivores consume 40% of NPP uniformly throughout the year, what is the approximate standing crop of producer biomass (use $B = NPP \times T$ ) and can herbivores be sustained for two months using only the standing crop without any new production?

(A)

Standing crop ≈ $200\ \text{g C m}^{-2}$ ; Yes — two‑month consumption ≈ $53\ \text{g C m}^{-2}$ which is less than standing crop.

(B)

Standing crop ≈ $200\ \text{g C m}^{-2}$ ; No — two‑month consumption ≈ $133\ \text{g C m}^{-2}$ which is greater than standing crop.

(C)

Standing crop ≈ $400\ \text{g C m}^{-2}$ ; Yes — two‑month consumption much less than standing crop.

(D)

Standing crop ≈ $66.7\ \text{g C m}^{-2}$ ; No — two‑month consumption is greater than standing crop.

Q4. Assertion (A): The pyramid of energy is always upright in any ecosystem.
Reason (R): Biomass at higher trophic levels is always less than biomass at lower trophic levels.

(A)

Both A and R are true and R is the correct explanation of A.

(B)

Both A and R are true but R is NOT the correct explanation of A.

(C)

A is true but R is false.

(D)

A is false but R is true.

Q5. A temperate forest has soil organic carbon stock $S=10{,}000\ \text{g C m}^{-2}$ . Annual inputs to soil (litter + root exudates) are $I=400\ \text{g C m}^{-2}\text{yr}^{-1}$ and annual outputs (heterotrophic respiration + leaching) are $O=400\ \text{g C m}^{-2}\text{yr}^{-1}$ (steady state). (i) Calculate the initial residence time $\tau$ of soil C using $\tau=\dfrac{S}{O}$ . (ii) If prolonged drought reduces inputs to $I'=250\ \text{g C m}^{-2}\text{yr}^{-1}$ while outputs remain $400$ in the first year, what will be the short-term annual change in soil C stock and will the stock increase or decrease?

(A)

$\tau = 25\ \text{yr}$ ; soil C will increase by $\approx +150\ \text{g C m}^{-2}\text{yr}^{-1}$ .

(B)

$\tau = 25\ \text{yr}$ ; soil C will decrease by $\approx -150\ \text{g C m}^{-2}\text{yr}^{-1}$ .

(C)

$\tau = 40\ \text{yr}$ ; soil C will decrease by $\approx -150\ \text{g C m}^{-2}\text{yr}^{-1}$ .

(D)

$\tau = 25\ \text{yr}$ ; soil C will remain unchanged in the short term.

Q6. In a grassland ecosystem, gross primary productivity (GPP) is measured as $2400\ \text{kcal m}^{-2}\text{yr}^{-1}$ and plant (autotrophic) respiration is $1600\ \text{kcal m}^{-2}\text{yr}^{-1}$ . Using $NPP = GPP - R$ , the net primary productivity (NPP) is:

(A)

$400\ \text{kcal m}^{-2}\text{yr}^{-1}$

(B)

$1600\ \text{kcal m}^{-2}\text{yr}^{-1}$

(C)

$800\ \text{kcal m}^{-2}\text{yr}^{-1}$

(D)

$2400\ \text{kcal m}^{-2}\text{yr}^{-1}$

Q7. In a terrestrial ecosystem annual net primary productivity (NPP) is $1000\ \text{g C m}^{-2}\text{yr}^{-1}$ . Herbivores consume $25\%$ of NPP (consumption efficiency), assimilate $40\%$ of what they consume (assimilation efficiency), and convert $50\%$ of assimilated carbon into secondary production (production efficiency). The annual secondary production by herbivores (in $\text{g C m}^{-2}\text{yr}^{-1}$ ) is:

(A)

$50\ \text{g C m}^{-2}\text{yr}^{-1}$

(B)

$100\ \text{g C m}^{-2}\text{yr}^{-1}$

(C)

$250\ \text{g C m}^{-2}\text{yr}^{-1}$

(D)

$125\ \text{g C m}^{-2}\text{yr}^{-1}$

Q8. During summer stratification in a clear-water temperate lake the measured standing biomass is $2\ \text{g m}^{-2}$ for phytoplankton and $20\ \text{g m}^{-2}$ for zooplankton. Which of the following best explains this apparently inverted pyramid of biomass?

(A)

Phytoplankton are long‑lived and therefore store less biomass per unit time than zooplankton despite low productivity.

(B)

Zooplankton primarily feed on detritus and microbial loops rather than phytoplankton, so their biomass is decoupled from producer standing crop.

(C)

A measurement artefact: phytoplankton were sampled per unit volume while zooplankton were integrated per unit area, producing an apparent inversion.

(D)

Phytoplankton exhibit very high primary productivity and rapid turnover (short residence time); a small standing crop with high flux can support a larger consumer biomass, producing an inverted biomass pyramid.

Q9. Leaf litter on an aerobic forest floor decreases from $100\ \text{g}$ to $40\ \text{g}$ in 2 months. Using the exponential decay model $M_t = M_0 e^{-kt}$ , calculate (i) the decomposition constant $k$ (in month $^{-1}$ ) and (ii) if anaerobic conditions reduce $k$ to $40\%$ of its aerobic value, what mass (in g) would remain after 1 month of anaerobic decomposition starting from $100\ \text{g}$ ? (Choose the closest values.)

(A)

$k \approx 0.183\ \text{month}^{-1};\ M_{1}\approx 83.3\ \text{g}$

(B)

$k \approx 0.458\ \text{month}^{-1};\ M_{1}\approx 83.3\ \text{g}$

(C)

$k \approx 0.916\ \text{month}^{-1};\ M_{1}\approx 40.0\ \text{g}$

(D)

$k \approx 0.458\ \text{month}^{-1};\ M_{1}\approx 33.3\ \text{g}$

Q10. A temperate grassland has monthly gross primary productivity $GPP = 600\ \text{g C m}^{-2}\text{month}^{-1}$ and plant respiration $R = 200\ \text{g C m}^{-2}\text{month}^{-1}$ at $20^\circ\text{C}$ . A heatwave raises temperature to $30^\circ\text{C}$ (+ $10^\circ\text{C}$ ). Assume GPP increases by $15\%$ due to CO $_2$ fertilization/longer growing season, while plant respiration follows a $Q_{10}=2$ response (i.e. respiration doubles with a $10^\circ\text{C}$ rise). Under these assumptions the new NPP (in $\text{g C m}^{-2}\text{month}^{-1}$ ) will be approximately:

(A)

$400\ \text{g C m}^{-2}\text{month}^{-1}$

(B)

$520\ \text{g C m}^{-2}\text{month}^{-1}$

(C)

$290\ \text{g C m}^{-2}\text{month}^{-1}$

(D)

$690\ \text{g C m}^{-2}\text{month}^{-1}$

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