تعیین شبکه زیست محیطی پیوندهای بخشی، مناسبات سودمند و ویژگی های ساختاری در نظام اقتصادی زیست محیطی شهری

Analyzing the structure and functioning of the urban system revealed ways to optimize its structure by adjusting the relationships among compartments, thereby demonstrating how ecological network analysis can be used in urban system research. Based on the account of the extended exergy utilization in the sector of urban socio-economic system, which is considered as the composition of extraction (Ex), conversion (Co), agriculture (Ag), industry (In), transportation (Tr), tertiary (Te) and households (Do) sectors, an urban ecological network model is constructed to gain insights into the economic processes oriented to sustainable urban development. Taking Beijing city as the case, the network accounting and related ecological evaluation of a practical urban economy are carried out in this study in the light of flux, efficiency, utility and structure analysis. The results showed that a large quantity of energy and resources have to be consumed to maintain the structure and function of a city. The thermodynamic efficiencies of individual sector in Beijing remain at a low level. The social system in Beijing is a highly competitive network, and there are 8 competitive relations and only two mutualistic ones. The Domestic and Agricultural sector are the major controlling factors of the system. Moreover, the assessment results of Beijing are compared with the other three socio-economic systems, Norway, UK and Italy, and the ecological network function and structure comparisons are correspondingly illuminated and discussed. The conclusions indicate that the exergy-based network analysis can be refined to become an integrative tool for evaluation, policy-making and regulation for urban socio-economic system management concerning structure and efficiency at urban levels.

The paradigm of urban ecological economic system has been formulated in an attempt to focus thinking towards a balanced co-evolution of the economic, natural and social environments while living within the carrying limits of the supporting ecosystems (World Conservation Union, 1991). Frosch and Gallopoulos (1989) proposed that socio-economic systems could become more sustainable if they were organized like biological ecosystems after considering the natural system as an integral part of the urban system. It immediately follows, since the mid-1980s, a lot of experts and scholars have carried out thorough and meticulous researches into the urban sustainable operation. However, the main urban study fields have been focused on the fluxes of single elements and on ‘nutrition’ with respect to that element, while analysis of urban structure, function and utility are seldom mentioned (Fischer-Kowalski, 1998 and Huang et al., 2006). Analyzing the structure and functioning of the urban system revealed ways to optimize its structure by adjusting the relationships among compartments, thereby demonstrating how ecological network analysis can be used in urban system research. In particular, urban ecology brings into focus the sustainability of physical resource use patterns and how they may drive changes in the system. Relational structures develop and change through the interactions among the urban sectors and exogenous forces acting on the system. Despite criticisms that ecological metaphors are inadequate to understand urban systems, the remarkable similarities between the attributes of the structure of regional economies and those of biological ecosystems render their analogies meaningful (Taylor, 1998). Moreover, for these attributes, an ecological perspective introduces additional concepts or quantitative measures that could improve understanding of the urban systems. No satisfactory tool was available to measure the system function objectively until the adaptation of network analysis to natural systems. One of the popular ways of representing the activities taking place in an ecosystem is to depict them as networks of flows of material and/or energy. The process which began with the quantitative analysis of interconnections was pioneered by Leontief (1941) as input–output (I–O) analysis and since then has also been applied to study energy flows in the economic system (Costanza, 1980 and Costanza and Herendeen, 1984). Network analysis affords an ecosystem researcher or manager several new avenues for extracting heretofore unavailable information about the flows of material or energy in the ecosystem (Wulff et al., 1989). More ecological network properties such as cyclic pathways, link density and connection have revealed new patterns and insights (Patten, 1985 and Ulanowicz, 1986). Structural analysis of ecological food webs has received considerable attention of late (Dunne, 2006 and Fath and Halnes, 2007). Patten and Fath applied this method to develop a paradigm of ecological network analysis including indirect effects, network amplification, network homogenization and network synergism (Patten, 1991, Fath, 2004, Scharler and Fath, 2009 and Patten, 2010). The methods mentioned above construct the theoretical underpinning of ENA, and the related analysis methods and indices have been addressed explicitly (Ulanowicz, 1997, Ulanowicz, 2004, Fath, 2007, Tollner et al., 2009 and Shevtsov et al., 2009). Scholars are paying increasing attention to the social and organizational forces at work in social and economic systems (Fath, 2006 and Fath et al., 2008). Ji (2010) took a network accounting and related ecological evaluation of a practical urban economy based on embodied cosmic exergy. Zhang et al. (2010a) developed an ecological network model for the urban energy metabolic system, and used four Chinese cities as examples of how the integration of throughflow analysis and ecological network utility analysis provides insights into the flows within the system at both high and low levels of detail. Zhang et al. (2010b) developed another ecological network model for the urban water metabolic system. Using a network utility matrix, the relationships and degrees of mutualism among six compartments were determined. This integrated network analysis framework presents an approach for a more holistic understanding of the interaction between natural and socio-economic systems by placing them within a complex systems framework. A reference can be established in urban economic scale to obtain insights into the structure and functioning of urban ecosystems and explore the consequences of various ecosystem change scenarios. Current studies viewed in the direct and indirect structure and function in an interconnected web based upon different inflows (such as flows of materials, energy, population or monetary). To consider the integrated image of interactions and connectedness both proximate and distal of urban socio-economic system, however, a quantitative method is needed to integrate the value of free environment investment, goods, services and labor forces in a common unit based on the principle of holism, where the whole has the greater affects for system behaviour than the sum of the parts. A variety of techniques have attempted to quantify the flux of exchange and the contribution of ecosystems to economic activity. As an indicator of the distance from thermodynamic equilibrium, exergy provides a unified measure of various forms of materials and energy carriers, and thus qualified as a basic medium used in the bookkeeping to qualify ecological networks of exchange (Chen et al., 2008). It can capture various quality aspects of streams as indicated by their mass, energy, concentration, velocity and location. Thus, exergy can characterize both mass and energy streams, and is the truly limiting resource on this planet (Sciubba, 2001 and Wall, 2002). The application of the exergy method for a society was to account for the use and conversion of natural resources including both energy carriers and materials was introduced by Wall (1977) and carried out for many countries (Wall, 1990, Scheffer et al., 2000, Ertesvåg, 2001, Ertesvåg, 2005, Chen and Qi, 2007 and Zhang and Chen, 2010). Extended Exergy Accounting (EEA) proposed by Sciubba determines cumulative exergy consumption associated with not only raw material inputs but also labor and capital inputs and non-energetic externalities (Sciubba, 2001 and Sciubba et al., 2008). The proposed thermodynamic approach is not meant to replace, but to complement an economic approach. The goals of this study were to determine the sectoral linkages, utility relations and structural characteristics of urban ecological economic system, and provide the implications of these attributes for system sustainability. The urban ecological network model was designed to describe the structure of an urban system, collect data addressing the attributes of the framework throughout the desired period and evaluate what processes brought about these characteristics. Finally, the assessment results of Beijing are compared with three other socioeconomic systems, Norway, the UK and Italy. It may present an initial diagnosis of resource utilization efficiency, input/output structure and environmental impact on the urban scale and priority on funding of the whole sectors. As a continuation of our earlier efforts of unified analysis based on extended exergy for quantifying the flux of exchange in Beijing society (Liu et al., 2011) and ecological network model for the urban metabolic system (Zhang et al., 2010a and Zhang et al., 2010b) the present work provides an ecological network determination of sectoral linkages, utility relations and structural characteristics on the urban ecological economic system, with emphasis on a joint application of the extended exergy synthesis and ecological network analysis methods and regional comparison.

To provide an effective avenue to evaluate the capacity of the urban ecological economic system, we incorporate the network analysis into this study to integrate indicators of flux, efficiency, utility and structure. Based on the account of the extended exergy utilization in the society of Beijing, which is considered as the composition of extraction, conversion, agriculture, industry, transportation, tertiary and households sectors, an urban ecological network model is constructed to gain insights into the development processes oriented to sustainable urban development. All the results provide useful information for policy making, which are valid also from the sustainable point of view in seven sectors of Beijing. Moreover, these results are compared with the EEA results of three socioeconomics systems (Norway, UK and Italy) on national scale. Our analysis indicates that Beijing, as a city with a “large and complete” structure, depends on the larger external systems surrounding the city which can supply resources and services that are lacking within the city. The basic system structure is appears as a “barbell” due to the weak contribution rate of the decomposer nodes. Thus, the decomposer potential for self-organizing processes becomes a key variable in environmental. It will be necessary to strengthen the urban metabolic functions described in our model in such a way that the impact on the environment can be significantly reduced, thereafter providing more specific guidance on how to develop ecologically sustainable cities. There are several implications of this study to the application of extended exergy-based ENA for urban sustainable management. A diversity of sub-sectors, functions and relationships is thought to be important for maintaining sustainability (McCann, 2000). Meanwhile, extended exergy provides a unified measure of various forms of natural and socio-economic flux as a basic medium to qualify ecological networks of exchange. Adopting this perspective suggests that urban sustainability would encompass the adaptive capacities and interaction relationship—allowing sub-systems (sectors or regions) to grow, interact and form linkages (including symbiosis) with others in ways that enhance overall system health and function.