Eugene P. Odum and Howard T. Odum, Fundamentals of Ecology (Philadelphia: Saunders, 1951).

In chapter 2 "Principles and concepts pertaining to the ecosystem and biogeochemical cycles", the Odum brothers review the history and significance of two fundamental ecological concepts. Although the term "ecosystem" was proposed by Tansley in 1935, it has older roots in Forbes' "microcosm" (1887), Vernadsky's idea of the "biosphere" (1926). Generally, an ecosystem is 'any area of nature that includes living organisms and nonliving substances interacting to produce and exchange of materials between the living and nonliving parts." These biotic and abiotic realms influence each other and are both necessary for the maintenance of life on Earth. Biogeochemical cycles are the more or less circular paths of chemical elements, including all the essential elements of protoplasm, that circulate in the biosphere from environment to organisms and back to the environment." These "inorganic-organic cycles" have a long history, reaching back to 18th century chemistry and natural history.