measured by the mobilization, transport, and depositionof organic and inorganic materials by the kinetic andpotential energy of fluids or solids such as water, wind,and sediment. Both biological energy and physical energyare constrained by conservation of mass and energy laws.Ecology as the interaction of biotic and abiotic processeslooks at the interactions of both types of energy. Someenergy is lost at each transformation so while totalentropy increases in accordance with the second law ofthermodynamics, order is locally increased. This has beendescribed as the self-organization feature of ecosystems orexergy. Emergy, an accounting system developed by H. T.Odum, can be used to put all natural and human produc-tion into common units based on solar radiation. Emergymeasures the inputs to make a product or service. It is ameasure of energy used in the past and thus is differentfrom a measure of current energy use. This provides away to evaluate the costs of ecological goods and servicesin the same units as the costs of human production ofgoods and services. Ecological engineering designs seek tomaximize the use of renewable energy (e.g., solar radiation)and minimize the use of nonrenewable energy. ……