General ecology is hierarchically subordinate to evolutionary biology. It deals with the structure and function of living systems and provides insight into the mechanisms of microevolutionary change, particularly that of selection. The ecosystem concept is a leading tool of ecology, but it is understood in different ways by different authorities. For some, it refers to any delimitable area of nature; for others it refers to specific models of energy flow or nutrient cycling. Some definitions attach notions concerning change and stability in ecosystems. All the definitions include the notion of a system whose variables interact in definite ways, including elements of the living and non-living environment. Other papers in the volume have critiqued some uses of the ecosystem concept, e.g., to assume homeostasis without specific evidence, to reify the ecosystem as if it were an organism itself, to confuse different levels of analysis and others. These are weighty criticisms and should be taken into account in any use of the ecosystem concept.