Gross primary productivity (GPP):
Gross primary productivity is the amount of organic matter synthesized by the plants in unit time and area. Producers make use of solar energy to synthesize organic matter. Nature tends to provide the plants with enough solar energy and nutrients, most times so that the GPP can increase.
Net primary productivity:
The productivity obtained after subtracting the respiration losses is called net primary productivity. Net primary productivity is therefore the available biomass that can be used by the consumers.
Man uses the plant products for food and by increasing the net primary productivity man increases the food production.