The rate at which an organism, population, or community assimilates energy (gross productivity) or makes energy potentially available (as body tissue) to an animal that feeds on it (net productivity). The difference between these two rates is due to the rate at which energy is lost through excretion and respiration. Thus gross primary productivity is the rate at which plants (or other autotrophs) assimilate energy, and net primary productivity is the rate at which energy is incorporated as cells or tissue. It is measured in kilojoules per square metre per year (kJ m⁻² y⁻¹). In terrestrial plants, much of the net productivity is not actually available to consumers, e.g. tree roots are not eaten by herbivores. See also energy flow; secondary productivity.