What is primary production?

The correct answer is Option (2) → The amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis.

Every ecosystem runs on the energy that green plants capture from sunlight. Primary production is the amount of biomass or organic matter produced by plants in photosynthesis, per unit area, over a period of time. It is expressed as weight (g m^–2) or energy (kcal m^–2). Its rate is called productivity.

Option (1) The amount of energy consumed by herbivores. — Incorrect. What the herbivores eat comes out of the plants’ production; it is not the production itself. The rate at which consumers form new organic matter is called secondary productivity.

Option (2) The amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis. — Correct. This is the book’s definition: biomass made by plants during photosynthesis, per unit area, over a time period.

Option (3) The amount of energy lost in the ecosystem. — Incorrect. Energy lost, for example in the plants’ own respiration, is subtracted from gross primary productivity to give net primary productivity. It is a loss, not the production.

Option (4) The energy used by decomposers to break down organic matter. — Incorrect. The breakdown of dead organic matter by decomposers is decomposition, another function of the ecosystem.

NCERT Extract:
Productivity – “A constant input of solar energy is the basic requirement for any ecosystem to function and sustain. Primary production is defined as the amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis. It is expressed in terms of weight (g m^–2) or energy (kcal m^–2). The rate of biomass production is called productivity.”