Productivity

Productivity is the rate at which biomass is produced in an ecosystem. It tells us how fast energy is being captured and stored by organisms, and it is the foundation on which the entire food web rests. Without productivity, there would be nothing for consumers to eat.

The textbook defines two key types of productivity: primary and secondary.

Primary productivity refers to the rate at which producers (plants and other autotrophs) capture solar energy and convert it into chemical energy (organic matter) through photosynthesis. This is the entry point of energy into the ecosystem. Primary productivity is further divided into two parts:

• Gross Primary Productivity (GPP): This is the total rate of photosynthesis, including the energy used by the plants themselves for respiration. In other words, it is the total organic matter produced by plants in a given area over a given time.
• Net Primary Productivity (NPP): This is the organic matter that remains after plants have used some of the GPP for their own respiration. NPP is the actual biomass available to heterotrophs (herbivores and decomposers) in the ecosystem.

Secondary productivity is the rate at which consumers (herbivores, carnivores, decomposers) produce new biomass from the food they eat. It is always much lower than primary productivity because energy is lost at each trophic level (through respiration, excretion, and heat).

The textbook gives a specific, exam-important figure for the global average:

This figure is not just a number — it represents the total food base for all life on Earth, excluding the energy used by plants themselves. The oceans' low share despite their huge area is a genuine puzzle worth thinking through: a vast surface covered in water still produces less than a third of the planet's organic matter each year. …