These materials were designed to be used by life science students for instruction in the application of physical theory to ecosystem operation. Most modules contain computer programs which are built around a particular application of a physical process. This report describes concepts presented in another module called "The First Law of Thermodynamics for Ecosystems." It focuses on three important ideas: (1) The classical definitions and Laws of Thermodynamics can apply to all biological systems; (2) The First Law of Thermodynamics is a conservation of energy law that allows a researcher to describe mathematically the heat fluxes of any system of interest; and (3) The heat and mass energy balance of organisms have important ecological consequences that can be studied and that offer challenging problems for scientists in the future. A problem set provides examples of the relative importance of work, heat energy exchange, and mass flow. (Author/CS)