Characteristics of Tianjin's material metabolism from the perspective of ecological network analysis

Cities are primary sites of resource consumption and pollution discharge. As a city with an urbanization rate far higher than the global average, Tianjin is facing severe resource and environmental problems caused by its material consumption and emission. In this study, we calculated the material flows between Tianjin and its external environment, as well as among sectors within the city, and constructed an ecological network model of the city to quantify and analyze changes in its flows from 2000 to 2015. We identified the external characteristics of the flows (size and structure of the inputs and outputs) and the internal characteristics (key nodes and ecological relationships), and their changes over time. Tianjin's material inputs and outputs were 750 and 256 Mt, respectively, in 2015 and have increased greatly since 2000. Tianjin's dependence on external inputs of metallic minerals was the city's biggest dependence. The network's key nodes were Internal Environment, Manufacturing, and Recycling, which participated most in the system's key links and must therefore be prioritized to optimize Tianjin's metabolic processes. The most important ecological relationship was exploitation/control (47.9% of all relationships), followed by mutualism (30.9%) and competition (21.2%). These proportions remained stable throughout the study period, but the proportions varied greatly among the nodes. Despite being a key node, the Manufacturing sector only had one mutualism relationship; increasing this sector's mutualism will become the key to making Tianjin's urban metabolism more sustainable. Ecological network analysis' ability to identify key nodes and ecological relationships demonstrates its ability to support efforts to regulate a city's urban material metabolism. (C) 2019 Elsevier Ltd. All rights reserved.