The rate of biomass production is called productivity and it is expre…

The correct answer is Option (2) → \( gm^{-2} yr^{-1} \) or \( (\text{kcal}\, m^{-2})yr^{-1} \)

A rate of production has to show three things:

• the amount: weight in grams (g) or energy in kilocalories (kcal);

• per unit area: per square metre, m⁻²;

• per unit time: per year, yr⁻¹.

Option (1) \( gm^{-2} yr^{-2} \) or \( (\text{kcal}\, m^{-2})yr^{-2} \) — Incorrect. The area unit is right, but yr⁻² is not a rate per year.

Option (2) \( gm^{-2} yr^{-1} \) or \( (\text{kcal}\, m^{-2})yr^{-1} \) — Correct. Productivity is the rate of biomass production. It is expressed as weight, g m⁻² yr⁻¹, or energy, (kcal m⁻²) yr⁻¹: per unit area per year, so that ecosystems can be compared.

Option (3) \( gm^{-1} yr^{-1} \) or \( (\text{kcal}\, m^{-2})yr^{-2} \) — Incorrect. Area is m⁻², not m⁻¹, and the second unit has time squared.

Option (4) \( gm^{-1} yr^{-2} \) or \( (\text{kcal}\, m^{-1})yr^{-2} \) — Incorrect. Both the area and the time units are wrong: m⁻¹ and yr⁻².

Note: A similar question was also asked in CUET (UG) 2025 on 02-06-2025 (Slot 2), on 30-05-2025 (Slot 1), on 20-05-2025 (Slot 1) and on 15-05-2025 (Slot 2).

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. It is expressed in terms of gm^–2 yr^–1 or (kcal m^–2) yr^–1 to compare the productivity of different ecosystems.”