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.
Charles Sutherland Elton
In 1927, Elton published his now classic book "Animal Ecology". This book outlines the important principles of ecological studies of animal behaviour and life history, such as food chains, the size of food items, the ecological niche and the concept of a pyramid of numbers as a method of representing the structure of an ecosystem in terms of feeding relationships.
In later works on the niche theory, Elton's definition – the Eltonian niche – in terms of functional attributes of organisms (or its position in the trophic web), has been viewed by some authors as opposed to Joseph Grinnell's earlier definition emphasising states of the environment suitable for the species. However, others have argued that there are more similarities than dissimilarities between the two versions of the niche concept.