Ecosystem
The term ecosystem was first used in 1935 in a publication by British ecologist Arthur Tansley. Tansley devised the concept to draw attention to the importance of transfers of materials between organisms and their environment. He later refined the term, describing it as "The whole system, ... including not only the organism-complex, but also the whole complex of physical factors forming what we call the environment". Tansley regarded ecosystems not simply as natural units, but as "mental isolates". Tansley later defined the spatial extent of ecosystems using the term ecotope.
Howard T. Odum
In his 1950 Ph.D. thesis, H.T.Odum gave a novel definition of ecology as the study of large entities (ecosystems) at the "natural level of integration". Hence, in the traditional role of an ecologist, one of Odum's doctoral aims was to recognize and classify large cyclic entities (ecosystems). However another of his aims was to make predictive generalizations about ecosystems, such as the whole world for example. For Odum, as a large entity, the world constituted a revolving cycle with high stability. It was the presence of stability which, Odum believed, enabled him to talk about the teleology of such systems. Moreover, at the time of writing his thesis, Odum felt that the principle of natural selection was more than empirical, because it had a teleological, that is a "stability over time" component. And as an ecologist interested in the behavior and function of large entities over time, Odum therefore sought to give a more general statement of natural selection so that it was equally applicable to large entities as it was to small entities traditionally studied in biology.