Tansley (1935):
"an ecosystem includes not only the organism-complex, but the
whole complex of physical factors forming what we call the environment."
Lindeman (1942):
"an ecosystem is any system composed of physical, chemical and biological processes active within any space-time unit."
Whittaker (1962):
"An ecosystem is a functional system that includes an assemblage of interacting organisms (plants, animals and saprobes) and their environment, which acts on them and on which they act."
Odum (1971):
Any unit that includes all of the organisms (i.e. the community) in a given area interacting with the physical environment so that a flow of energy leads to a clearly defined trophic structure, biotic diversity, and material cycles (i.e. exchange of materials between living and nonliving parts within the system) is an ecological system or ecosystem.
National Science Foundation:
Organized assemblage of plants, animals, and microorganisms in complex
association with each other and with the physical environment.