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Ecosystem Productivity

The Input of Energy

receive some 8,000 to 10,000 kilocalories (kcal) of energy each day on each square meter (1 m2) of surface. A kilocalorie is the amount of heat needed to warm 1 kg of water 1 degree Celsius (°C). Because all of the light trapped in photosynthesis is ultimately released as heat, it makes sense to follow the flow of energy through ecosystems in units of heat.

How efficient are plants at converting this energy into organic molecules?

Gross Productivity

Gross productivity is the amount of energy trapped in organic matter during a specified interval at a given trophic level.

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.

Photosynthesis 2.2%
Reflection 3.0
(including transpiration and
heating of the surroundings
Total 100.0%

However, at least half of this is lost by cellular respiration as the plants run their own metabolism.

Net Productivity

Net productivity is the amount of energy trapped in organic matter during a specified interval at a given trophic level less that lost by the respiration of the organisms at that level.

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, and availability of water.

Estimated Net Productivity of Certain Ecosystems (in kilocalories/m2/year)
Temperate deciduous forest5,000
Tropical rain forest15,000
Tall-grass prairie2,000
Coastal marsh12,000
Ocean close to shore2,500
Open ocean800
Clear (oligotrophic) lake800
Lake in advanced state of eutrophication2,400
Silver Springs, Florida8,800
Field of alfalfa (lucerne)15,000
Corn (maize) field, U.S.4,500
Rice paddies, Japan5,500
Lawn, Washington, D.C.6,800
Sugar cane, Hawaii25,000

What happens to the net productivity of a plant community?

What about humans?

Humans, like all heterotrophs, depend upon net productivity for their food both We also use the earth's net productivty to meet other needs such as:

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:

We also reduce the net productivity of our planet by other activities such as
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25 February 2011