In the Millennium Ecosystem Assessment, reporting units that are ecosystem-based but at a level of aggregation far higher than that usually applied to ecosystems. Thus the system includes many component ecosystems, some of which may not strongly interact with each other, that may be spatially separate, or that may be of a different type to the ecosystems that constitute the majority, or matrix, of the system overall. The system includes the social and economic systems that have an impact on and are affected by the ecosystems included within it. For example, the Condition and Trend Working Group refers to ‘‘forest systems,’’ ‘‘cultivated systems,’’ ‘‘mountain systems,’’ and so on. Systems thus defined are not mutually exclusive, and are permitted to overlap spatially or conceptually. For instance, the ‘‘cultivated system’’ may include areas of ‘‘dryland system’’ and vice versa.