The rate of biomass production is called productivity and it is expressed in terms of _______ to compare the productivity of different ecosystems.

$gm^{-2} yr^{-1}$ or $(\text{kcal}\, m^{-2})yr^{-1}$

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

Productivity represents the rate of biomass production and is expressed as grams per square meter per year (g m–2 yr–1) or kilocalories per square meter per year (kcal m–2 yr–1) to facilitate comparison between different ecosystems.

Primary production can be further divided into two components: gross primary productivity (GPP) and net primary productivity (NPP). Gross primary productivity represents the total rate of organic matter production during photosynthesis in an ecosystem. However, a significant portion of this energy is utilized by plants for their own respiration needs.

Net primary productivity, on the other hand, is the amount of organic matter available for consumption by heterotrophs, such as herbivores and decomposers. It is calculated by subtracting the energy used by plants for respiration (R) from the gross primary productivity (GPP):

GPP - R = NPP.