(1) A branch of science dealing with the relationships of organisms and their cycles and rhythms, community development, and environments -- especially as manifested by natural structure, interaction between different kinds of organisms, geographic distributions, and population alterations. (2) The study of the relation of organisms or groups of organisms to their environment. (3) The study of the relations among living organisms and their environment. (4) The scientific discipline that is concerned with the relationships between organisms and their past, present, and future environments. These relationships include physiological responses of individuals, structure and dynamics of populations, interactions among species, organization of biological communities, and processing of energy and matter in ecosystems (ESA 2004 annual report)