Cities are particular examples of ecosystems. They are kept alive by continuous inputs of raw material, primary resources, and energy from the outside environment. Such currencies are exchanged between the various socioeconomic activities that comprise the urban environment and these transfers give rise to a web-like structure that can be described as a collection of boxes connected by arrows.

Accordingly, cities also can be seen as ecological networks. By studying energy/matter exchanges through network architectures, ecologists have been able to unveil how efficiently an ecosystem uses energy and the major constraints which curb the maximization of this efficiency. In addition, they have been able to determine the ecosystem potential for development and ability to maintain structure and functions over time in the face of external stress. Since achieving sustainability critically depends upon making better use of natural resources and creating new patterns of development in which protection and stability are maximized, the ecosystem perspective is more than a simple conceptual analogy. Criteria and tools which form the apparatus of ecosystem analysis can thus be applied to human systems to obtain clues for sustainable strategies. In natural ecosystems, there can be as many networks as currencies in use (carbon, nitrogen, phosphorus, and so forth); in urban ecosystems, this possibility is further amplified as currency are water, energy, wood, metals, glass, paper, and many others. In principle, a complete view of an urban ecosystem can be obtained only by analyzing the whole suite of networks involving all the currencies used in it. However, there are currencies that are essential, such as water, and cannot be surrogated or substituted. The analysis of these types of network becomes essential to grasp the very nature of urban metabolism and can provide clues for sustainability. Figure 7 provides an example of an urban water network.

Recalling from the previous sections, the TST quantifies the entire amount of currency that the system handles.

Figure 7 Urban water network. Boxes are human activities and arrows specify the direction of water exchanges. Flow values

Figure 7 Urban water network. Boxes are human activities and arrows specify the direction of water exchanges. Flow values

Ascendency measures what fraction of this currency that is exchanged efficiently through optimized connections with minimum redundancy to cut maintenance costs. What remains is encumbered complexity, or overhead, that is currency dissipated or channeled through redundant connections, either internal or as import and exports. The summation of ascendency and overhead defines the development capacity, that is the potential of a system to become a completely organized whole. How could one use these indices to assess the propensity of human ecosystem to be sustainable.? With respect to water use in the system of Figure 7, ascendency is 43% of the development capacity. This number seems meaningless per se, but a comparison with ecological systems in nature can shed light on its meaning. In the Chesapeake Bay ecosystem, for example, ascendency reaches 60% of the system's capacity, and in the South Florida Everglades graminoids this index is 55% of the potential for development. The water network, thus, seems less developed than several natural ecosystems and this contradicts the idea that human systems are highly organized. There is room thus for more development in the urban network. However, development in human organization is usually measured on another basis than accounting on natural resources. More development is usually pursued through intensification of human activity (i.e., industrial activity as number or size of enterprises).

From the perspective of the water network this would imply an augmented demand for water, that is, greater pressure on groundwater resources. Accordingly, system size (TST) would increase with repercussions on ascendency, which would augment as well. Nonetheless, this increase would be only in absolute terms. In fact, as more medium becomes available, system potential for development increases, but because the organization of flows remains the same, the excess quota of potential for development would be trapped in the overhead on imports. This highlights the fact that often in human systems, development is confused with a mere increase of size. From an ecosystem perspective, this increase in size would not bring further development. This term in fact measures how well the system performs its functions by keeping external support at a minimum. This, in turn, can be obtained by reducing dissipation (i.e., increasing process efficiency), increasing internal exchanges and recycling, and limiting exports if they are not coupled with some return for the system. So the development capacity establishes only an upper limit to development, and depends on how much medium is made available to the system. Ascendency, on the contrary, measures how well the system builds up into an organized whole to use the medium efficiently.

It may well be that human systems show very high potential for development, because they use vast amount of natural resources, but their ascendency (as fraction of development capacity) remains rather low because such resources are used inefficiently. In the ecosystem realm, sustainability and development are not terms in conflict with one another because development implies a series of mechanisms that reduce system dependence over external resources. In this case, it can be said that sus-tainability is the ultimate goal of development. In human systems, the two terms are often in contradiction because development is intended solely in terms of size. Without a contemporary optimization of processes at the whole system scale, this type of 'development' would inevitably deplete all the resources upon which the system relies.

See also-. Body Size, Energetics, and Evolution; Ecological Network Analysis, Ascendency; Shannon-Wiener Index.

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