Construction-Land Expansion and the Risk to Degrade Ecological Services of Watershed Ecosystem at the Edge of Urban Areas

Rapid urbanization involves the expansion of constructed land, which changes the land use and landscape pattern in watersheds, degrades ecosystem services, and creates other adverse ecological impacts. Therefore, urban planning must consider ecological risk by focusing on stressors and the adverse effects generated by rapid urbanization. The study area is Houxi Basin, which is a typical Chinese southeastern coastal watershed at the edge of urbanization. Based on ecosystem vulnerability and the interference with the ecosystem, we evaluate how habitat services provided by terrestrial ecosystems risk degradation due to modified landscape patterns. Second, we apply the export coefficient of the non-point source (NPS) pollution model to build an exposure-response relationship between land use and NPS pollution and investigate the risk of degrading the water-purification services provided by the aquatic ecosystem. Third, we analyze scenarios to simulate trends in ecological risk. The results show that (1) a rapid increase occurs in the risk to water-purification services caused by pollution diffusion from modified land. (2) The risk of degrading habitat provision services increases slightly due to ecological loss in the landscape caused by landscape modifications. (3) The integrated optimization scenario of key areas reduces the risk of pollution diffusion and the landscape risk by 2.71% and 10.25%, respectively, compared with the initial scenario of 2030. (4) According to the analysis of the scenarios, the replacement of both forest and grassland more effectively inhibits pollution diffusion than does replacing only forest or only grassland. Optimizing areas with high land use impact coefficients inhibits pollution diffusion more effectively than does optimizing areas with high export coefficients and is thus a way to control risks. In addition, instead of increasing the area of green land, adjusting its spatial layout prove to be more effective to lower the ecological risk to water-purification and habitat-provision service