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Project Amazonia: Characterization - Energy Flow

From the solar energy that gets to the Earth, a very small fraction (approximately 0.1% worldwide) is derived to living systems. Even if the light falls on a zone with abundant vegetation, like a rainforest, only between 1 and 3% of that light (calculated on an annual basis) is used in photosynthesis, thus becoming available for the organisms that form part of that ecosystem. However, a very small fraction like this is enough to result in the production -from carbon dioxide, water, and a few minerals- of several million grams (in dry weight) of organic matter per square meter per year. This adds up to a worldwide total of approximately 120 thousand million metric tons of organic matter per year1

All the biotic and abiotic components of an ecosystem maintain the open exchange of matter and energy on which the system is based. The energy flow in an ecosystem is the most important factor in its organization.

Several attributes inherent to an ecosystem are based on the matter and energy exchange:

bulletPrimary productivity – This is defined as the energy fixation rate by primary producers, of which 99.99% are photosynthetic organisms, although chemosynthetic organisms are considered primary producers as well. Primary producers use light to produce carbohydrates and other compounds that can be used as sources of energy.
bulletSecondary productivity – This is the energy and matter fixation rate by consumers.
bullet Decomposition rate – This is the velocity at which organic matter is degraded into chemically and physically simpler elements.
bulletUse of energy efficiency1:
bulletRatio of the rate of the matter and energy that is fixed in an ecosystem by one trophic level to that of some previous level.
bulletNon-Utilized material/Egestion: Food that is excreted and/or not used, i.e., the matter and energy used in metabolism and therefore not passed along the food chain.
bulletIngestion: Ingested and assimilated food, as well as matter and energy used in the production of new biological tissues. It can also be thought of as the rate at which energy is being removed from the previous trophic level.
bulletStanding biomass – This is all the matter of the ecosystem, i.e., the dry weight of the organisms.
bulletVelocity and pattern of circulation of nutrients – This is defined as the velocity and path traveled by the nutrients (matter). 
bulletVelocity and pattern of circulation of energy – This is defined as the velocity and path traveled by which each particle of energy that enters the ecosystem.
bulletTrophic structure – This is given by the trophic and connecting levels in regard to nutrient and energy circulation within the ecosystem. Energy flow from one organism to another occurs along what is known as a food chain, that is, a sequence of organisms related to one another as prey and predator. The second one eats the first one; the third one eats the second one, and so on, in a series of alimentary or trophic levels.
bulletKey species - These species are crucial because their activities determine the pattern of circulation of matter and energy and help maintain the ecosystem's equilibrium.

matter&energy

As we can see, the movement of energy is open and in only one direction. This means that the energy goes through each trophic level one time. As it goes from one level to another, it is lost with metabolism and in the form of heat. This means that, for example, the energy ingested by producers does not pass completely to consumers. After the energy is lost as heat, it cannot be used for work anymore. The following equation makes this easy to understand.

 Pn = Pg – R

 Where2:

·        Net Primary Productivity (Pn) is a measure of the rate at which energy is fixed in the organisms’ biological tissues. This is the amount of energy that then passes to the following trophic level of the ecosystem. Thus, it can be considered as the amount of energy that is available to the ecosystem. It can also be thought of as the rate at which new biomass is being made by the plants. Generally, it is expressed as the amount of energy (in calories) stored in chemical compounds or as the increase in biomass (in grams or metric tons).

·        Gross Primary Productivity (Pg) is the photosynthetic rate or the rate at which energy is being fixed by photosynthetic organisms and stored in simple sugars.

·        Respiration (R) is the sum of all the organisms' metabolic costs. It represents a loss of energy along the chain.
 

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1:  BIOLOGY 5th edition 1994, Worth Publishers Inc., New York

2:  Lang, Ken <http://sorrel.humboldt.edu/~kll1/ecology.html>, 2002