The net primary productivity (NPP) in an ecosystem can be represented…

The correct answer is Option (2) → GPP - R = NPP

Option (1) GPP + R = NPP — Incorrect. R is the energy plants lose in respiration. A loss must be taken away, not added, so NPP is always less than GPP.

Option (2) GPP - R = NPP — Correct. Plants use part of what they make for their own respiration. What is left after this loss is NPP, the biomass available to herbivores and decomposers.

Option (3) GPP + PP = NPP — Incorrect. GPP is itself the primary productivity of the plants, so PP (primary productivity) is not a separate term. Adding more productivity to it cannot give the smaller NPP.

Option (4) GPP - SP = NPP — Incorrect. Secondary productivity (SP) is the rate of formation of new organic matter by consumers. It belongs to the next trophic level and is not a loss from the plants’ own production.

Note: A similar question was also asked in CUET (UG) 2025 on 03-06-2025 (Slot 2).

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).”