 The report goes on to argue that “the industrial world has already used much of the planet’s ecological capital. This inequality is the planet’s main ‘environmental’ problem; it is also its main ‘development’ problem” (WCED 1987, Overview, paragraph 17). In the concept of sustainable development the report combines the resource economist’s notion of “sustainable yield” with the recognition that developing countries of the world are entitled to economic growth and prosperity. The notion of sustainable yield involves thinking of forests, rivers, oceans and other ecosystems, including the natural species living in them, as a stock of “ecological capital” from which all kinds of goods and services flow. Provided the flow of such goods and services does not reduce the capacity of the capital itself to maintain its productivity, the use of the systems in question is regarded as sustainable. Thus, the report argues that “maximum sustainable yield must be defined after taking into account system-wide effects of exploitation” of ecological capital (WCED 1987, Ch. 2, paragraph 11).