Study On Developing And Optimizing Urban Ecological Networks

City is an outcome of economic development of human society and urbanization is an inevitable result of productivity promotion and society progress,but it also has lots of adverse effects along with the development of the city.However,owing to rapid socioeconomic development the tendency that the number and scale of cities are on the rise is a catalyst for dramatic changes in urban landscape pattern.Sustainable development of cities is facing daunting challenges,such as increasingly sharp land use contradiction between urban construction and ecological protection,vast stretches of ecological fields being occupied,shrinking ecological space and fragile biodiversity and biological system services.As paramount ecological infrastructure in cities,green space contributes a lot to urban species and the development of human settlements.Thus,it is of great importance to improve the connectivity of green space by reducing green space habitat fragmentation.It has been proved that developing urban ecological networks and improving connectivity between habitats could facilitate energy and matter fluxes,species dispersal,genetic exchange and many other ecological processes,and further relieve the ecological environmental crisis in cities.With rapid urbanization in the western and central of China,natural area fragmentation and habitat loss are inevitable.Taking the central district of Chongqing?one of the four municipalities directly under the central government of China?as the prototype,this paper tries to probe into the network pattern and ecological space pattern that is beneficial to ecological function and process to ensure the ecological security of cities.GIS is adopted in this paper and both landscape pattern analysis and least-cost model are incorporated into the urban ecological networks.Firstly,landscape metrics are capitalized on to evaluate the status quo of ecological space and green space landscape pattern and reveal the spatial pattern characteristics.Secondly,our study combined least-cost modelling and graph-theory based network analysis to simulate,evaluate,identify and prioritise the urban ecological networks.We selected maximum seed dispersal distance of native tree species,Ficus virens Ait.var.sublanceolata?Miq.?Corner as the threshold distance for network analysis in Conefor2.6.The priority of habitat protection and restoration was determined by evaluating the importance grades of patches/connectivities.Additionally,four types of networks are created on the basis of network simulation and evaluation,including alternative network,branch network,ring network,and optimized network to confirm the optimized ecological network plan after quantitative analyses of the network structure indexes are conducted.Finally,urban green space pattern features after networks are optimized are compared with those before.Effectiveness of the optimized ecological space based on simulated ecological network in relieving the urban land use contradictions and improve regional ecological security are verified in this paper.The results and the methods adopted in this paper can be viewed as a scientific,reliable reference for urban ecological networks investigation and construction.The main results are as followes:?1?Ecological space in the central district of Chongqing covers 56% of the total area with farmland and forest being the main type of landscape in urban ecological space,seen from the forms of patches.Status quo of urban green space system?forest,grassland and artificial green land?in 2014 and landscape pattern indexes in 2020 government plan show clearly that the total area of existing green space is much smaller than that of planned green space especially when it comes to artificial green space landscape but changes in natural green space is not so manifest.The density of artificial green patches in 2020 government plan is lower than that of the present green patches among which the largest patch and the average patch is relatively small in size.It indicates that the fragmentation level of planned green space landscape is lower than that of exiting green space.The landscape level showed a similar trend.The landscape diversity index increases from 0.52 in the existing green space to 0.65 in the planned green space system,and the connectivity index increases from 2.97 to 3.74.All these eloquently reflect that the green space landscape in central district of Chongqing is deteriorating.?2?After the ecological quality of patches in the research green space was assessed on their structures and community features,163 standard patches were chosen as the source patches of ecological networks,accounting for 15.44% of the total green area.Under the circumstances of 2000 m dispersal distance of ficus virens,278 paths were recognized.High-quality patches are proved to be mainly large parks and scenic spots in shapes of rectangles or strips and seen from the further analysis,importance of the patches is closely correlated with their areas?Pearson=0.589,P<0.01?,but there are still some small green patches of relatively high rank,which perhaps can be ascribed to their superior spatial positions as the key nodes of the networks.Path of overriding importance is always the sole connection that links the whole ecological network.For example,the path Core₃1—Core₉ links the green space in village Hongyan and Hongen Temple and is the only channel that connects Jialing River-Yuzhong Peninsula and if it is removed,the holistic connectivity of the network will be reduced by 22.16%;Another important path Core₆2—Core₄2 linking Yidu Park and the Huaxi Riverside Park connects the ecological network of both sides of the Yangtze River in Dadukou District and if this path is removed,network connectivity will be lowered by 15.14%.Importance of patches/connectivity in the network and the current underlying surface shall be considered before a holistic approach is taken in the optimization of the recovery strategy and concrete measures must be adjusted to local conditions flexibly and efforts can be intensified to restore the habitats that are significant and given precedence.?3?Four kinds of network planning scenarios were established,including alternative network?A?,branch network?B?,ring network?C?and optimized network?D?respectively based on ecological networks and judgments.Despite many great indexes in scenario A,it involved some redundant links,which means large tracts of land shall be occupied in real construction and land use contradiction exacerbated.Compared with scenario B and C,scenario D is superior in indexes ?,?,?,cost ratio.Although indexes ?,?,?,cost ratio of scenario D are lower than those of scenario A,but scenario D is more cost-effective,closely interconnected and sophisticated with moderate socioeconomic support.After overall comparison are made,scenario D is the best ecological network.?4?After comparison was made among urban green space pattern based on optimized network,status quo of the green space in 2014 and the 2020 government green space plan for downtown of Chongqing,it shows that green space in the optimized network decreases by 50% of that in the government plan and density of patches is further reduced,but the diversity index increases from 0.64 to 0.68 and connectivity index increases to 3.94.It indicates that optimized ecological network is able to alleviate landscape fragmentation,improve landscape connectivity markedly and mitigate the increasingly sharp contradiction between urban construction and ecological protection more efficiently than government green space plan.