A metabolism is the sum of chemical and physical processes that are continually occurring within a living organism and are essential to life and well-being. In this organic sense, two kinds of metabolism are often distinguished: constructive metabolism, which is the synthesis of the proteins, carbohydrates, and fats that form tissue and store energy; and destructive metabolism: the breakdown of complex substances and the consequent production of energy and waste matter.
In human settlement development, management, governance and planning, the habitat metabolism is the subject of a holistic vision that addresses and treats a human settlement as a living organism and seeks to sustain it. Infrastructure, resource use and efficiency, production, environment viability and human well-being are key elements of a habitat metabolism. (See Habitatabove.)
In a habitat metabolism approach, infrastructure is understood as directing material flows and, therefore, resource use, productivity and efficiency in an urban or rural context. It enables examining human settlements of any scale from a material-flow perspective, with a dynamic and continuous flow of inputs and outputs as its “metabolism,” while also understanding human habitats within the broader system of flows that make it possible for them to function. The habitat metabolism approach perceives the design, construction and operation of infrastructures, such as for energy, waste, water, sanitation and transport, create a social and technical environment that shapes the “way of life” of inhabitants and how they procure, use and dispose of the resources they require. This approach examines pressures for change within cities that go beyond technical considerations and emphasizes the importance of intermediaries as the dominant agents for change, as well as the fact that social processes and dynamics need to be understood and integrated into any assessment of interventions within a comprehensive strategic vision of the city-region or any other human settlement.
Studies of habitat metabolism involve an analysis of stocks and flows. Stocks include the resources available within the human settlement (e.g., buildings, roads, infrastructures), whereas flows involve resource inputs from within and outside the human settlement and the outputs from it to areas within and beyond its borders. However, a complete study of a habitat metabolism should include civil, cultural, social, economic, financial, political, human rights and ethical issues.
The metabolism of a typical modern city can be described as “linear” in the sense that it extracts resources from beyond its recognized boundaries, makes use of them within those boundaries to support human activities, and then deposits the resulting wastes in high concentrations back onto the external environment. Modern cities require a continuous supply of resource inputs and an unlimited capacity of nature to absorb the concentrated wastes they produce.
In this way, the modern city’s metabolism is fundamentally different to the circular metabolism found in a natural ecosystem, which produces no waste and survives off its immediate environment. The habitat metabolism approach to human settlements helps stakeholders to recognize the relevance of the practices in circular metabolisms, especially the ecological benefits, despite the lack of support from officials, urban producers and/or consumers.
As the understanding of human habitat metabolism grows, this approach likely will shed much greater light on the total material requirements (TMR) of cities, including both direct and indirect flows. This will reveal how dependent cities are on material imported from other localities within and beyond national boundaries, indicating the environmental impact of cities on other localities and habitats.