Assertion (A): Gross primary productivity is the amount of organic matter available for consumption by heterotrophs.
Reason (R): Net primary productivity (NPP) is calculated by subtracting the energy used by plants for respiration (R) from the gross primary productivity (GPP).
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → A is false, but R is true.
Assertion (A): Gross primary productivity is the amount of organic matter available for consumption by heterotrophs. — False. Gross primary productivity (GPP) is the rate of production of organic matter during photosynthesis. Plants use a large part of it in their own respiration. What is left, and so available to heterotrophs (herbivores and decomposers), is the net primary productivity (NPP), not the GPP.
Reason (R): Net primary productivity (NPP) is calculated by subtracting the energy used by plants for respiration (R) from the gross primary productivity (GPP). — True. NPP = GPP – R, where R is the respiration loss of the plants.
So A is false and R is true.
NCERT Extract:
Productivity – “… Gross primary productivity of an ecosystem is the rate of production of organic matter during photosynthesis. A considerable amount of GPP is utilised by plants in respiration. Gross primary productivity minus respiration losses (R), is the net primary productivity (NPP).
GPP – R = NPP
Net primary productivity is the available biomass for the consumption to heterotrophs (herbiviores and decomposers).”