The Input of Energy
Tropical regions every day and temperate regions during the growing season 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 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.
(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.
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 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|
What happens to the net productivity of a plant community?
- Some is harvested by plant-eating herbivores, e.g., insects, deer. Often this is only the start of a series of transformations as it passes through a series of heterotrophs — that together make up a food chain.
- Some is consumed by organisms of decay, e.g., fungi and bacteria.
- Some may be stored, e.g., in a growing forest or as peat in a bog.
What about humans?
Humans, like all heterotrophs, depend upon net productivity for their food both directly as we consume plant material (e.g., rice, wheat, corn) and indirectly when we eat animals that have, themselves, fed on plant material (poultry, cattle, sheep, etc.) and/or animal products (e.g., milk, eggs).
We also use the earth's net productivty to meet other needs such as:
- wood for fuel
- wood and fiber (e.g., cotton, flax) to house and clothe us
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 as high as 80% in South central Asia to as low as 6% in South America.
We also reduce the net productivity of our planet by other activities such as
- paving over land for buildings, roads, parking lots, etc.
- burning forests to clear them for agriculture