The sum total of biomass (organism mass expressed as live weight, dry weight,
ash-free dry weight or carbon weight) produced at each trophic level at a given point
in time is termed the standing crop. This needs to be treated with caution; if taken at
the end of an optimum growing period it indicates full potential; if taken during a
drought, cool season, period of agricultural neglect or insect damage, it is an underestimate
of possible production. Primary productivity may be defined as the rate at which
organic matter is created (usually by photosynthesis, although in some situations by
other metabolic processes) at the first tropic level. It may be established in several ways.
The total energy fixed at the first trophic level is termed gross primary production.
Minus the estimated respiration losses, this gives net primary productivity (in g m–2 d–1
or g m–2 y–1). Net primary productivity gives a measure of the total amount of usable
organic material produced per unit time. Most cultivated ecosystems, i.e. efforts to
stretch food and commodity production, are well below the net primary production of
more productive natural ecosystems. There is thus, in theory, potential for the improvement
of existing agriculture.