Ecosystem — Questions & Answers

Sample questions from Ecosystem

1. Q1. Producers, the foundational trophic level in most ecosystems, are organisms that:

   • A.Synthesize organic compounds from inorganic sources, typically through photosynthesis✓
   • B.Consume other organisms exclusively
   • C.Decompose dead organic matter exclusively
   • D.Cannot produce any energy-containing compounds
   Solution

   Producers (primary producers), typically photosynthetic plants, algae, and certain bacteria, synthesize organic compounds from inorganic raw materials using external energy sources (usually sunlight), forming the energy base for most ecosystems.

2. Q2. A lake study reports gross primary productivity of 3,200 kJ m-2 yr-1 and producer respiration equal to 35 per cent of GPP. Using the ten per cent law for the next two transfers, the energy reaching the secondary consumers is closest to:

   • A.20.8 kJ m-2 yr-1✓
   • B.208 kJ m-2 yr-1
   • C.32 kJ m-2 yr-1
   • D.112 kJ m-2 yr-1
   Solution

   Respiration is 0.35 x 3,200 = 1,120, so NPP = 3,200 - 1,120 = 2,080 kJ m-2 yr-1. Only NPP is available to herbivores, so primary consumers get 208 and secondary consumers get 10 per cent of that, 20.8 kJ m-2 yr-1. The common slip is to apply the transfer law to GPP instead of NPP.

3. Q3. In a grassland ecosystem, if producers store 5000 kcal/m2/yr and only 10% is transferred at each successive trophic level, the energy available to secondary consumers (third trophic level) is approximately:

   • A.50 kcal/m2/yr✓
   • B.500 kcal/m2/yr
   • C.5 kcal/m2/yr
   • D.5000 kcal/m2/yr
   Solution

   Applying the 10% law twice: primary consumers get 10% of 5000 = 500 kcal/m2/yr; secondary consumers get 10% of 500 = 50 kcal/m2/yr.

4. Q4. A thrush feeds on earthworms living in leaf litter and is in turn eaten by a hawk. The thrush is best described as an organism that:

   • A.Links the detritus food chain with the grazing food chain✓
   • B.Belongs exclusively to the grazing food chain
   • C.Is a decomposer because it eats litter-dwelling animals
   • D.Is a producer because leaf litter is plant material
   Solution

   The earthworm derives its energy from detritus (the DFC), while the hawk is part of a predator chain (the GFC). By consuming earthworms, the thrush transfers energy from the detritus food chain into the grazing food chain; NCERT notes that the DFC may be connected with the GFC at some levels.

5. Q5. In a pond ecosystem, which of the following organisms belongs to the SAME trophic level as phytoplankton?

   • A.Zooplankton feeding on algae
   • B.A rooted submerged plant such as Hydrilla✓
   • C.A bottom-dwelling snail feeding on debris
   • D.A flagellate decomposer at the pond bottom
   Solution

   Trophic level is decided by the source of food, not by size or habitat. Phytoplankton and rooted submerged plants like Hydrilla are both autotrophs that fix solar energy, so both occupy the first trophic level (producers); zooplankton and snails are consumers and flagellate decomposers are saprotrophs.

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