(1) A complete, interacting system or unit of organisms in a space considered together with their environment, e.g., a marsh, a watershed, a lake, etc., mentally isolated for purposes of study or management. A flow of energy leads to clearly defined food and feeding relationships, biologic diversity, and biogeochemical cycles (i.e., exchange of materials between living and nonliving parts) operating as an integrated system. "Ecosystem" is the preferred term in English while "biocoenosis" or "biogeocoenosis" is preferred by writers using or familiar with the Germanic and Slavic languages. (2) A nameable natural unit that includes not only the total array of plant and animal species in an environment, but also the energy to power the system. (3) A biotic community plus its abiotic environment. (4) Any complex of living organisms with its environment. (5)An interconnected community of plants and animals, including humans, and the physical environment within which they interact. (USFS 1999)