The term for the productivity of an ecosystem, usually expressed in terms of biomass, is called primary productivity. It refers to the rate at which energy is converted by photosynthetic and chemosynthetic autotrophs to organic substances in an ecosystem. Primary productivity is significant because it establishes the base of the food web, supporting various trophic levels in the ecosystem. Biologists measure both gross primary productivity (total energy captured) and net primary productivity (energy remaining after plant respiration) to determine ecosystem health and efficiency.