primary productivity (NPP) can be described as the rate
at which producers store chemical energy
(via photosyntehsis) minus the rate
at which producers use chemical energy
(via respiration). In other words, NPP is the rate
at which energy
for consumption by consumers is stored by producers.
The first graph
displays the average net
primary productivity, expressed in kilocalories per meters squared per year. The second graph
shows the average world net
primary productivity, measured in billions of kilocalories per year. The first graph
represents the actual level of productivity for an ecosystem, while the second graph
represents the level of productivity in relation
to the amount of space
that type of ecosystem
occupies on the earth.
For example, in the first graph, the open ocean ranks toward the bottom in regards to NPP. However, in the second graph, the open ocean is the highest ranked. This is because oceans cover about 71% of the planet's surface, and while their individual NPP may be low, they cover such a large portion of the earth that the total world NPP is much, much higher.