Life represents a balance between the tendency to increase entropy (Second Law of Thermodynamics) and the decreased entropy through continuous energy inputs necessary to concentrate resources for growth and reproduction. All energy for life on Earth ultimately comes from solar radiation, which powers the chemical storage of energy through photosynthesis. Given the First and Second Laws of Thermodynamics, the energy flowing through ecosystems, including resources harvested for human use, can be no greater, and usually is much less, than the amount of energy stored in carbohydrates.
Organisms have been compared to thermodynamic machines powered by the energy of carbohydrates to generate maximum power output in terms of work and progeny (Lotka 1925, H. Odum and Pinkerton 1955, Wiegert 1968). Just as organisms can be studied in terms of their energy acquisition, allocation, and energetic efficiency (Chapter 4), so ecosystems can be studied in terms of their energy acquisition, allocation, and energetic efficiency (E. Odum 1969, H. Odum and Pinkerton 1955). Energy acquired from the sun powers the chemical synthesis of carbohydrates, which represents storage of potential energy that is then channeled through various trophic pathways, each with its own power output, and eventually is dissipated completely as heat through the combined respiration of the community (Lindeman 1942, E. Odum 1969, H. Odum and Pinkerton 1955).
The study of ecosystem energetics was pioneered by Lindeman (1942), whose model of energy flow in a lacustrine ecosystem ushered in the modern concept of the ecosystem as a thermodynamic machine. Lindeman noted that the distinction between the community of living organisms and the nonliving environment is obscured by the gradual death of living organisms and conversion of their tissues into abiotic nutrients that are reincorporated into living tissues.
The rate at which available energy is transformed into organic matter is called productivity. This energy transformation at each trophic level (as well as by each organism) represents the storage of potential energy that fuels metabolic processes and power output at each trophic level. Energy flow reflects the transfer of energy for productivity by all trophic levels.
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