http://en.wikipedia.org/wiki/Energy_flow_(ecology)
http://www.learner.org/courses/envsci/unit/text.php?unit=4&secNum=3
http://www.colfinder.net/materials/Green_Teacher/m1/unit_6.pdf
http://www.setju.ac.in/Environmental_Studies/Text/Ecosystems_Unit_3/ENER...
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