First 188 words of the document:
Energy enters an ecosystem by photosynthesis where light is
converted so other organisms can use it.
It starts with producers (plants) then consumers (primary,
secondary and tertiary).
Each successive part of the food chain is called a trophic level.
Through the stages more and more biomass is lost through
respiration, faeces and heat energy.
So this process isn't very efficient in terms of how much
energy is available for us to consume.
To calculate energy transfer we use the gross production (GP);
total amount of energy trapped in organic matter, minus
respiration to get net production.
There are difficulties in classifying organisms into trophic levels
in that particular species will appear at several stages in
different food chains. But, also omnivores are difficult to place.
After sun's rays hits the leaf, most
energy is absorbed, reflected or scattered in the atmosphere.
Only small fraction used in photosynthesis.
% efficiency of energy transfer between trophic levels=
net productivity of a level
net productivity of previous level ×100