|Index to this page|
How efficient are plants at converting this energy into organic molecules?
The table shows the use of visible sunlight is a cattail marsh. The plants have trapped only 2.2% of the energy falling on them.
(including transpiration and
heating of the surroundings)
However, at least half of this is lost by cellular respiration as the plants run their own metabolism.
One way to determine this is to collect and weigh the plant material produced on 1 m2 of land over a given interval. One gram of plant material (e.g., stems and leaves), which is largely carbohydrate, yields about 4.25 kcal of energy when burned (or respired).
The table shows representative values for the net productivity of a variety of ecosystems — both natural and managed. These values are only approximations and are subject to marked fluctuations because of variations in temperature, fertility, the availability of water, and the variety of species present (species richness is correlated with greater net productivity in grasslands and some forests).
|Estimated Net Productivity of Certain Ecosystems (in kilocalories/m2/year)|
|Temperate deciduous forest||5,000|
|Tropical rain forest||15,000|
|Ocean close to shore||2,500|
|Clear (oligotrophic) lake||800|
|Lake in advanced state of eutrophication||2,400|
|Silver Springs, Florida||8,800|
|Field of alfalfa (lucerne)||15,000|
|Corn (maize) field, U.S.||4,500|
|Rice paddies, Japan||5,500|
|Lawn, Washington, D.C.||6,800|
|Sugar cane, Hawaii||25,000|
|Link to discussion of food chains.|
Added together it is estimated that our species now appropriates some 20% of world's net productivity for our own use. However, this figure obscures large regional variations with estimates running: