The rate of biomass production is called productivity. It is expressed in terms of g^–2 yr^–1 or (kcal m^–2)yr^–1 to compare the productivity of different ecosystems. It can be divided into gross primary productivity (GPP) and net primary productivity (NPP). 
Net primary productivity is the available biomass for the consumption to heterotrophs (herbiviores and decomposers).
Primary productivity depends on the plant species inhabiting a particular area. It also depends on a variety of environmental factors, availability of nutrients and photosynthetic capacity of plants. Therefore, it varies in different types of ecosystems. The annual net primary productivity of the whole biosphere is approximately 170 billion tons (dry weight) of organic matter. Of this, despite occupying about 70 per cent of the surface, the productivity of the oceans are only 55 billion tons.