The available biomass for consumption to herbivores and decomposers is

Net primary productivity is the available biomass for consumption to heterotrophs (herbivores and decomposers).

Gross primary productivity (GPP) of an ecosystem is the rate of production of organic matter during photosynthesis. A considerable amount of GPP is utilized by plants in respiration. Gross primary productivity, minus respiration losses (R), is the net primary productivity (NPP).