In ecosystem dynamics, flow of energy is unidirectional and takes place from one trophic level to the next trophic level and about $50 \%$ of solar energy incident over earth is present in Photosynthetic Active Radiation (PAR) and only $2-10 \%$ of this PAR is utilised by green plants to form chemical energy (Gross Primary Productivity (GPP). $90 \%$ of gross primary productivity is lost, maximum during respiration other vital activities. GPP utilised by plants in respiration minus losses due to respiration is the net primary productivity. This is the energy available to the organism of next trophic level (herbivores, carnivores and decomposers) for consumption.
The rate of production of organic matter during photosynthesis is the Gross Primary Productivity of an ecosystem. Nature tends to increase gross primary productivity by supporting large number of plants (producers) in an ecosystem.
Net primary productivity is the biomass available for the consumption by heterotrophs (human and animals). Man tries to increase net primary productivity by cultivation of crops to fulfill their needs.
Formula to genrate NPP is
$\mathrm{NPP}=\mathrm{GPP}-$ Resperatory loss.