Energy Transmission in Biological Systems

Excerpts from George M. Woodwell’s “The Energy Cycle of the
Biosphere,” Scientific American, September 1970.

“It is solar energy that moves the rabbit, the deer, the whale, the
boy on the bicycle outside my window, my pencil as I write these

“Only about a tenth of 1 percent of the energy received from the sun
by the earth is fixed by photosynthesis ….  about the equivalent to
the annual production of between 150 and 200 billion tons of dry
organic matter and includes both food for man and the energy that
runs the life support systems of the biosphere, namely the earth’s
major ecosystems : the forests, grasslands, oceans, marshes,
estuaries, lakes, rivers, tundras, and deserts.

“The complexity of ecosystems is so great as to preclude any simple,
single-factor analysis that is both accurate and satisfying. Because
of the central role of energy, however, an examination of the
fixation of energy and its flow through ecosystems yields
understanding of the ecosystems themselves. It also reveals starkly
some of the obscure but vital details of the crisis of environment.”

“The worldwide increase of human numbers not only is shifting the
distribution of energy within ecosystems but also requires that a
growing fraction of the total energy fixed be diverted to the direct
support of man….. ”

“Much of our current understanding of ecosystems has been based on a
paper published in Ecology in 1942 by Raymond L. Lindeman, a young
colleague of Evelyn G. Hutchinson’s at Yale University …. He called
attention to the fixation of energy by natural ecosystems and to the
quantitative relations that must exist in nature between the
different users of this energy as it is divided progressively among
the various populations of an ecosystem.

“Lindeman’s suggestions were provocative. They stimulated a series of
field and laboratory studies, all of which strengthened his
synthesis. One of the most useful generalizations of his approach,
sometimes called ‘the 10 percent law,’ ….  It seems fair to assume
that in the grazing chain perhaps 10 to 20 percent of the energy
fixed by the plant community can be transferred to herbivores, 10 to
20 percent of the energy entering the herbivore community can be
transferred to the first level of carnivores and so on. In this way
what is called a mature community may support three or four levels of
animal populations, each related to its food supply quantitatively on
the basis of energy fixation.”


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