The overall productivity of a system can be found in an equation, where the Net Primary Production, or NPP, is equal to the Gross Primary Production, or GPP, minus the Carbon respiration, or R. The formula is the NPP = GPP - R. The NPP is the overall efficiency of the plants in the ecosystem.Through having a constant efficiency in NPP, the ecosystem is then more sustainable. The GPP refers to the rate of energy stored by photosynthesis in plants. The R refers to the maintenance and reproduction of plants from the energy expended.