Ecosystem Set-2

Q1. In a grassland ecosystem gross primary productivity is measured as $3000\ \text{kcal}\ \text{m}^{-2}\ \text{yr}^{-1}$ and autotrophic respiration is $1800\ \text{kcal}\ \text{m}^{-2}\ \text{yr}^{-1}$. What is the net primary productivity (NPP) per year per square metre?

Q2. In a terrestrial ecosystem net primary productivity is $2000\ \text{g C m}^{-2}\ \text{yr}^{-1}$. Herbivores consume $20\%$ of NPP; of the energy they ingest they convert $30\%$ into herbivore biomass (herbivore production). Carnivores consume $40\%$ of herbivore production and convert $20\%$ of what they consume into carnivore biomass. Estimate annual carnivore production (in $\text{g C m}^{-2}\ \text{yr}^{-1}$).

Q3. Microbial decomposition in a temperate grassland has a temperature sensitivity $Q_{10}=2$. If mean annual temperature rises from $10^\circ\text{C}$ to $16^\circ\text{C}$, the decomposition rate at $16^\circ\text{C}$ will be approximately $2^{(16-10)/10}$ times the rate at $10^\circ\text{C}$ (call the original rate $k$). Which option best estimates the new decomposition rate and the likely short-term effect on soil organic carbon (SOC) if litter input remains unchanged?

Q4. A shallow lake of area $1\ \text{km}^2$ and mean depth $2\ \text{m}$ has an initial dissolved oxygen (DO) concentration of $8\ \text{mg L}^{-1}$. An algal bloom deposits $50\ \text{g C m}^{-2}$ of organic carbon over the whole lake surface; microbial decomposition will oxidize this organic carbon. Given that oxidation of $1\ \text{g}$ organic C consumes $\approx 2.67\ \text{g}$ O$_2$, assess whether decomposition will deplete DO below the hypoxic threshold of $2\ \text{mg L}^{-1}$.

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?

Q6. In a terrestrial ecosystem the gross primary productivity (GPP) is $8000\ \text{kJ m}^{-2}\text{yr}^{-1}$ and autotrophic respiration is $2500\ \text{kJ m}^{-2}\text{yr}^{-1}$. Using $NPP = GPP - R_a$, calculate net primary productivity (NPP) in $\text{kJ m}^{-2}\text{yr}^{-1}$.

Q7. In an aquatic ecosystem $GPP = 12\ \text{g C m}^{-2}\text{day}^{-1}$ and autotrophic respiration $R_a = 5\ \text{g C m}^{-2}\text{day}^{-1}$. Herbivores consume $30\%$ of NPP; assimilation efficiency (AE) is $60\%$ and production efficiency (P/A) of herbivores is $20\%$. Using $NPP = GPP - R_a$, what is the daily secondary production of herbivores (in $\text{g C m}^{-2}\text{day}^{-1}$)?

Q8. If net primary productivity (NPP) of an ecosystem is $6000\ \text{kJ m}^{-2}\text{yr}^{-1}$ and mean trophic transfer efficiency (TE) between successive trophic levels is $10\%$, energy available at trophic level $n$ (producers $n=1$) is $E_n = \text{NPP}\times(\text{TE})^{\,n-1}$. If a viable population at the top trophic level requires at least $1\ \text{kJ m}^{-2}\text{yr}^{-1}$, what is the maximum number of trophic levels that can be supported?

Q9. Assertion (A): In a forest ecosystem, litter with a C:N ratio of $80:1$ 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 (≈ $10:1$); to build microbial biomass from high-C litter they take up inorganic N from the soil, causing immobilization.

Q10. A harvested population follows $\dfrac{dN}{dt} = rN\!\left(1-\dfrac{N}{K}\right) - H$, where $r$ is intrinsic growth rate, $K$ carrying capacity and $H$ constant harvest (individuals yr$^{-1}$). The maximum constant harvest that still allows a positive equilibrium equals $H_{\max} = \dfrac{rK}{4}$. For $r = 1\ \text{yr}^{-1}$ and $K = 4000$ individuals, what is $H_{\max}$ (individuals yr$^{-1}$)?