The Impact Of Urban Expansion On Regional Habitat Quality And Ecological Network

Urbanization is the inevitable trend for human development.Urban expansion,as the most direct representation of urbanization in geographical space,is the result of natural,social,economic,policy and other factors.Rapid urban expansion significantly changed the regional landscape pattern,which reflected in the decrease of ecological patch area and the increase of ecological patch fragmentation,resulting in the decline of regional habitat quality and the obstruction of regional ecological network function.It constantly threatened regional ecological security.So we took Nanjing,a city had relatively complete urbanization process,as an example,based on multi-stage(1978,1990,2000,2010 and 2018)land use data,DEM data and road network data,researching urban expansion and its impact.We used standard deviational ellipse,landscape expansion index and boosted regression trees to explore spatiotemporal characteristics and driving mechanism of urban expansion since the reform and opening up.And then,we cleared the impact of urban expansion on region habitat quality and ecological network,in order to provide data support and scientific basis for Nanjing’s high-quality development and land space optimization.The main results were as follows:(1)From 1978 to 2018,the expansion speed and intensity of build-up land in Nanjing increased first and then decreased.Among them,the expansion speed and intensity from 2000 to 2010 were the highest,reaching 48.1 km2 and 13.8%each year.The overall trend of urban expansion was "southeast→northwest".The moving track of the expanded centroid was "north→south→ north",and the moving speed increased first and then decreased;the centroids of the six districts moved close to the city center,and then far away.Three expansion modes(Infilling,Edge-expansion and Leapfrogging)appeared in four research periods,with Infilling and Edge-expansion alternation as the main mode,resulting in the phenomenon of "diffusion→aggregation",the same was true for all districts.In terms of driving force,the impact of natural conditions(altitude,slope and distance to water body)on urban expansion increased slightly,the impact of location(distance to built-up area)decreased,and the impact of economic and social factors(distance to the city center,distance to main roads and distance to railways)increased day by day.(2)The impacts of urban expansion on cropland,green space and water body were different in Nanjing metropolis from 1978 to 2018.Cropland as the main encroachment source of build-up land,the Core area had greatly reduced.Meantime,its landscape was more broken,shape was more complex,and connectivity decreased day by day.The Core area of green space decreased slightly,the Core number increased slightly;the Bridge had less quantity and its area was stable.The area and number of Islet relatively reduced.It showed that the protection of green space in Nanjing was good and the structure was relatively stable.But there had less ecological corridors,which lead to most green space existing in isolation.Except Shijiu Lake and Gucheng Lake,the water body in Nanjing had changed little over the years and had been well protected.Some areas had even been larger(such as Jinniu Lake);there was only a certain fragmentation in the Edge.But its connectivity has decreased significantly after 2000,which from 72.7 in 2000 to 31.0 in 2018.(3)From 1978 to 2018,the average value of landscape quality in Nanjing continued to decline.The area of high landscape quality were concentrated in large green spaces and water bodies(Laoshan Mountain,Yangtze River,etc.),and the low value of landscape quality was mainly distributed in the urban areas of each district.Landscape quality was divided into five grades(1 level,2level,3 level,4 level,5 level)in our paper.The transformation of different levels of landscape quality was mainly from 2 level to 1 level,especially from 2000 to 2010,which was basically consistent with the stage of urban expansion.The Core area was the main component of 4 level and 5 leve,but the proportion of Island area in 4 level was significantly higher than that in 5 level.The centroids shift of 4 level was "south→north",the centroids shift of 5 level moved to south.(4)The number of ecological sources in Nanjing was 52 in 1978,53 in 1990,54 in 2000,61 in 2010 and 55 in 2018.The composition of each source was different.The more complex of composition,the easier it was to change.Ecological corridors were misdistribution.Bounded by the Yangtze River,the corridors in the north of the river were significantly less than those in the south.Over the past 40 years,the total length and total resistance of the corridor had increased year by year,reaching 1041.72 km and 2575067.9 km respectively by 2018.The width of the important corridor was set as 100 m,and the width of the general corridor was set as 60 m.From 1978 to 2010,the main land use type of important corridors was green space,but from 2010 to 2018,it became cropland;the main land use type of general corridors was cropland.The number of strategic ecological nodes in Nanjing would continue to increase;and the area of pinch points in Nanjing would gradually decrease.The maximum value of barrier points increased year by year.The distribution of barrier points were more concentrated,and they were basically the complete barrier.Affected by urban expansion,ecological network became more and more complex.