Ecological Network Construction Based on Red, Green and Blue Space: A Case Study of Dali City, China

Abstract

Rapid urbanization leads to fragmentation and reduced connectivity of urban landscapes, endangering regional biodiversity conservation and sustainable development. Constructing a red, green, and blue spatial ecological network is an effective way to alleviate ecological pressure and promote economic development. Using circuit theory, hydrological analysis, and suitability analysis, this study constructs a composite ecological network under urban–rural integration. The results show the following: (1) A total of 22 ecological corridors with a length of 349.20 km, 22 ecological pinch points, and 22 ecological barrier points are identified in the municipal area, mainly distributed in Haidong Town. There are 504 stormwater corridors, which are more evenly distributed, 502 riverfront landscape corridors, and 130 slow-moving landscape corridors. (2) A total of 20 ecological corridors, with a length of 99.23 km, 19 ecological pinch points, and 25 barrier points were identified in the main urban area, and most of them are located in the ecological corridors. There are 71 stormwater corridors, mainly located in the northwestern forest area, 71 riverfront recreation corridors, and 50 slow-moving recreation corridors. (3) Two scales of superimposed ecological source area of 3.65 km², and eleven ecological corridors, are primarily distributed between Erhai Lake and Xiaguan Town. There are two superimposed stormwater corridors and fourteen recreational corridors. The eco-nodes are mostly distributed in the east and south of Dali City; wetland nodes are mainly situated in the eighteen streams of Cangshan Mountain; and landscape nodes are more balanced in spatial distribution. The study results can provide a reference for composite ecological network construction.

1. Introduction

Urban–rural ecological contrasts, fragmentation of the landscape, and other ecological issues have been brought about by human activity and continuous urban expansion, which stress the ecosystem [1]. Because of this, nations all over the world have placed a strong emphasis on sustainable development, and the idea of an “eco-city” has been proposed [2]. An eco-city is a perfect city model that has a reasonable structure, stable function, and socioeconomic development that does not exceed the ecological environment’s carrying capacity [3]. The relationship between China’s urban and rural areas has entered a new phase of growth. This is due to the promotion of new urbanization, rural revitalization, and other national programs. The concept of “urban-rural integration and development” was first introduced in the 19th CPC National Congress Report in 2017. It emphasizes that urban and rural areas as a whole, rather than a single town or rural area, are the center of gravity of urban–rural development, focusing on multi-level penetration and integration [4]. It is centered on penetration and intermingling, which is multi-level, multi-domain, and all-around [5]. A key strategy for ecological integration of urban and rural areas is the construction of composite socio-ecological networks [6]. These networks link disparate habitat patches into a cohesive network of landscapes and habitats. This ensures regional ecological security on a limited amount of ecological land, enhances natural ecosystem health, and safeguards biodiversity.

Various administrative levels, the extent of ecological space, project management, planning concepts, and the meaning of ecological land are all significant considerations in eco-city design. Various ecological space levels provide distinct functions and need to be taken into account for various planning and value considerations. While regional differences are evident at the macro level, specific attention should be paid to the network relationship between nature, humans, and structures at the micro level, with the partitioning of ecological space based on the region as a whole [7]. In order to connect urban and rural areas organically and preserve urban ecosystem stability, it is necessary to integrate urban and rural ecological space in a way that is tailored to the local environment. This will also ensure ecological security.

Red, green, and blue space is a collection of water, green space, and recreational resources. Blue space includes natural waters, such as rivers, lakes, mudflats, and wetlands, as well as man-made reservoirs and ditches; green space refers to public green space, protective green space, and ancillary green space integrated with urban construction land [8]. Red space, on the other hand, is a tourism resource that provides slow walking and leisure recreation for the public. Currently, there is no uniform definition of ecological networks in the academic world; in general, ecological networks, in the eyes of different experts and scholars, help to maintain ecological processes’ consistency [9]. In 1939, Troll [10], a leading German biogeographer, proposed the formal introduction of landscape into ecology as the totality of the environmental space in which human beings live and the whole of everything they can see visually, and emphasized that landscape ecology is an integrated study combined with recently developed aerial photogrammetry, ecology, and geography [11]. Ecological networks are network systems of natural ecological space and artificial green space with ecological significance within the city, connected by linear ecological corridors [12].

To date, a large number of scholars have examined ecological networks from the angles of geography, urban planning, and landscape ecology [13]. The main studies include the structure [14] and function [15,16] of ecological networks and the construction [17], evaluation [18], and optimization [19] of ecological networks. They have done this using techniques such as minimum cumulative resistance (MCR) [20], graph theory [21], morphology spatial pattern analysis (MSPA) [22], circuit theory [23], etc. to form the “comprehensive ecological source area identification, landscape resistance surface construction, ecological corridor extraction, ecological network.” Formally, the main paradigm has been formed as “identification of ecological sources, construction of landscape resistance surfaces, extraction of ecological corridors, and construction of ecological networks” [24,25,26]. Researchers have studied several types of region at several scales, and conducted a number of optimization studies in recent years of the determination the width of ecological corridors [27,28], extraction of ecological nodes [29], and multi-objective [30,31] composite ecological networks. Nonetheless, the distinctive ecological requirements of cities and the objective of offering recreational services to citizens have been partially disregarded by the current research, which has also not adequately taken into account the nested relationships of ecological networks at various scales and the paucity of studies on the ecological abundance of low-county units. Given that ecological restoration and protection are major carriers of new urbanization, more focused and methodical research is required to increase the efficacy of ecological networks in these kinds of areas. Weighing the hierarchy of multi-objective demands in red, green, and blue networks, and optimizing the spatial coordination of multiple networks, are also difficult problems to solve in ecological network optimization.

As the only international wetland city in Yunnan Province, Dali City is a popular destination for tourists in China, the rapid urbanization of the region has harmed the ecological environment, causing the issue of urban and rural landscapes to become more serious. In view of this, this study takes Dali City as an example, and the research objectives include the following: (1) identifying the ecological source in the municipal and main urban areas in terms of biological processes, hydrological processes, and humanistic processes; (2) constructing the landscape resistance surfaces of the municipal and the main urban areas; (3) applying the circuit theory to identify the nested and connected ecological corridors, pinch points, and barrier points of the municipal and the main urban areas, and constructing the multi-scale composite ecological network of the red, green, and blue spaces; and (4) proposing the “point, line, and surface” combination of the optimization scheme to coordinate the ecological protection and restoration of natural elements, and make suggestions for further ecological management. The results of the study will alleviate the contradiction between ecological protection and economic development in highland wetland cities and provide the necessary basis and reference for exploring the ecological environment and construction mode of highland wetland cities.

2. Materials and Methods

2.1. Study Area

Dali City, located in the western part of Yunnan Province, in the heart of Dali Bai Autonomous Prefecture, is the political, economic, and cultural center of the state and also the capital of Dali Prefecture. Located between 99–58° to 100–27° east longitude and 25–25° to 26–02° north latitude, the city has jurisdiction over nine towns, one township, and three streets, as well as the Dali Economic Development Zone and Dali Haidong Development Committee. The terrain is high in the northwest and low in the southeast. It is situated in the low-latitude and high-altitude zones and the subtropical plateau monsoon climate zone, and the average annual temperature is 15.2 °C. Erhai Lake within the territory covers an area of 250 km², accounting for 13.7% of the total area. In recent years, due to urban development, habitat patches have been fragmented and ecological pressure has intensified, and there is an urgent need to resolve the conflict between economic development and the ecological environment. Therefore, ecological protection cannot be neglected. To ensure the ecological security of Dali City, the multi-scale network structure provides an effective solution. The overview of the study area is shown in Figure 1.

2.2. Data Sources and Pre-Processing

The land use data and vegetation cover involved in this study were obtained from remote sensing image decoding; the data were obtained from the Geospatial Data Cloud Platform “http://www.gscloud.cn (accessed on 21 March 2024)”; the slope was obtained from the (digital elevation grid) DEM through ArcGIS “https://desktop.arcgis.com/ (accessed on 21 March 2024)” slope analysis; the road network vector data were obtained from the 2006 and 2016 two-phase forest resources type II survey data; and the nighttime light data, (point of interest) POI data, and administrative boundary data were obtained from the BIGEMAP GIS Office “http://www.bigemap.com/ (accessed on 21 March 2024)”. All images and data are unified in the WGS_1984_UTM_Zone_47N coordinate system.

2.3. Research Methodology

In this study, the main urban and Dali city municipal scales are used to form the ecological network. At the highest level of building, municipal works can support the stability of the surrounding ecological environment, thereby maintaining the overall ecological security of the city. In the “Dali City Provincial Forest City Construction Master Plan (2017–2026),” the town development boundary is defined as a small-scale study object, defined as the urban area, and Dali City is defined as a large-scale study object, defined as the municipal area, according to the “Dali City Territorial Spatial Master Plan (2021–2035).” When building an urban network, attention is paid to creating open ecological spaces that serve the needs of the local population while also enhancing and optimizing the network’s structural integrity.

Therefore, according to the actual situation of the study area, this study considers the functional positioning and spatial needs of different scales; takes bio-hydrology–humanities as the goal orientation; adopts the methods of ecological source identification, landscape resistance surface construction, and ecological element identification to construct a habitat network; adopts the methods of river network extraction, stormwater corridor construction, and wetland node construction to construct a water–green network; and constructs recreation and leisure corridors according to the suitability analysis and accessibility analysis. The recreation corridor is constructed based on suitability analysis and accessibility analysis, and the multi-scale nesting of the network and the construction and optimization of the multi-objective ecological network are carried out. The specific research framework is shown in Figure 2.

2.3.1. Habitat Network Construction

(1)

Ecological source area identification

MSPA (Morphological Spatial Pattern Analysis) is a customized sequence of mathematical morphological operators targeted at the description of the geometry and connectivity of the image components. Based on geometric concepts only, this methodology can be applied at any scale and to any type of digital image in any application field [32]. MSPA constitutes an analytical method that can identify essential patches that have a significant influence on the ecological connectivity of the study area. In this study, by taking forest and grassland as the foreground and other land types as the background, seven landscape types (core, isolated, bridging, edge, pore, loop, and spur) were obtained by setting the width of the edge of the municipal area to 5 pixels and the urban area to 3 pixels using the eight-neighborhood method in the MSPA analysis module in Guidos Toolbox [33] “https://forest.jrc.ec.europa.eu/en/activities/lpa/gtb/ (accessed on 22 March 2024)”.

As the selection range of the source region, 2 km and 1 km buffer zones were established for the municipal area and urban area, respectively, in order to maintain range continuity and lessen the influence of each administrative entity on ecosystem services [34,35]. Landscape connectedness illustrates the impact of many types of landscapes, modifying the dispersion resistance of a species [36]. Regional landscape connectivity and the importance of each ecological patch were measured by overall connectivity (IIC) and probable connectivity (PC) [37,38], and the larger the value of dPC, the more important the patch is to the overall connectivity of the landscape. According to the range of patch area in the core area (≥2 km² in the municipal area and ≥0.1 km² in the urban area), Conefor software “www.conefor.org (accessed on 22 March 2024)” was used to set the connectivity distance threshold to 500 m [39], and the probability of connectivity to 0.5, and the patch connectivity index (dPC) was calculated so as to select the ecological source in the municipal area and the main urban area.

$$\mathrm{I}\mathrm{I}\mathrm{C}={\displaystyle \frac{{\sum }_{i-1}^n\sum _{j=1}^n\frac{a_i{a}_j}{1+{nl}_{ij}}}{A_L^Z}}$$

(1)

where n is the total number of patches in the landscape, $a_i$and $a_j$ represent the areas of patch $i$ and $j$, respectively, ${nl}_{ij}$ is the number of connections between patch $i$ and patch $j$, and $A_L^Z$ is the area of the landscape. 0 $\le$ IIC $\le$ 1; a value of IIC of 0 indicates that there is no connection between the habitat patches. Conversely, when the value of IIC is 1, it means that the entire area involved in the calculation is a habitat patch.

$$\mathrm{P}\mathrm{C}={\displaystyle \frac{{\sum }_{i=1}^n{\sum }_{j=1}^{n}1\ne j{P}_{IJ{a}_i{a}_j}^{*}}{A_L^Z}}$$

(2)

where $\mathrm{n}$ is the total number of patches in the landscape, $\mathrm{P}\times ij$ represents the maximum probability of species spreading between patch $i$ and patch $j$, $a_i$ and $a_j$ represent the areas of patch $i$ and $j$ respectively, and $A_L^Z$ is the area of the landscape. 0 < PC < 1.

$$\mathrm{d}\mathrm{P}\mathrm{C}={\displaystyle \frac{PC-{PC}_{remove}}{PC}}\times 100\%$$

(3)

where $\mathrm{P}\mathrm{C}$ indicates the possible connectivity index of a patch in the landscape and ${PC}_{remove}$ indicates the possible connectivity index of the landscape after the patch is removed.

(2)

Ecological resistance surface construction

The resistance surface illustrates both the challenge of relocating between ecological patches of various habitats and the significance of landscape variety in the ecological flow process. The resistance factors to be constructed in this study are land use type, elevation, slope, normalized difference vegetation index (NDVI), distance from roads, distance from rivers, distance from settlements, and nighttime lighting index (the selection and weighting of specific indicators are shown in

Table 1. Resistance factors and resistance values.

Scale	Drag Factor	Resistance Value Rating					Weight
		1	2	3	4	5
Municipal	Land use type	Forest, waters	Grasslands	Cultivated land	Unutilized land	Building land	0.28
	Elevation (m)	<1500	1500–2000	2000–2500	2500–3000	>3000	0.19
	Slope (°)	0–6.20	6.20–14.63	14.63–23.20	23.20–33.33	33.33–70.99	0.14
	NDVI	−0.21	0.02–0.16	0.16–0.25	0.25–0.34	0.34–0.56	0.06
Main urban	Land use type	Forest	Water	Cultivated land	Unutilized land	Building land	0.24
	Elevation (m)	<2000	2000–2100	2100–2200	2200–2300	>2300	0.15
	Slope (°)	0–6.05	6.05–18.45	18.45–31.69	31.69–45.87	45.87–75.22	0.15
	NDVI	−0.14	0.01–0.11	0.11–0.20	0.20–0.29	0.29–0.55	0.06
	Distance from river (m)	<500	500–1000	1000–1500	1500–2000	>2000	0.07
	Distance from road (m)	<500	500–1000	1000–1500	1500–2000	>2000	0.07
	Distance from residential area (m)	<1000	1000–2000	2000–3000	3000–4000	>4000	0.12
	Nighttime Light index	<5	10	15	20	>20	0.14

). These selections are based on previous research, as well as the actual conditions of the study area [40,41] . The integrated resistance surface of the municipal area and the main urban area was obtained using the hierarchical analysis method (AHP) “http://ahpman.metadecsn.com/ (accessed on 21 March 2024)” and the raster calculator tool (Figure 3). Each resistance factor was categorized according to a five-level system; the lower the level, the lower the resistance, and vice versa.

(3)

Ecological network construction and optimization

By giving various ecological significances to physical parameters like resistance, current (which indicates the probability of a species spreading along a given path during movement), and voltage (used to predict the likely probability that a species will leave any one focus and successfully reach the next given focus), circuit theory—which focuses on the random walk properties of wildlife—allows the discovery of ecological corridors [42]. The main principle of the MCR is that the spatial components need to overcome a certain landscape resistance when flowing; the stronger the landscape ecological service function, the more perfect the landscape function, and the smaller the resistance. It minimizes data redundancy and combines the benefits of graph theory and random migration theory in comparison to the MCR model [43]. Circuit theory was used to identify ecological corridors at both scales in Dali City, taking into account the variations in ecological sources and ecological resistance at various scales.

Based on the construction of ecological source and integrated resistance surfaces, the Linkage Mapper toolbox “https://linkagemapper.org/ (accessed on 22 March 2024)” in the ArcGIS platform was used to extract the ecological corridors at the municipal and main urban scales, i.e., ecological circulation paths, and the weighted cost distances of the municipal and main urban corridors were set to 20,000 m [44]. Using Circuitscape software “https://circuitscape.org/ (accessed on 22 March 2024)” in the many-to-one and pairwise modes, the cost-weighted distance of the corridor was determined to be 1000 m based on the construction of the corridor. Next, the ecological pinch points were identified by identifying the locations with greater current density (which indicates the likelihood that a species will migrate or spread along a particular path). The natural breakpoint method was utilized to classify them into five categories, and their maximum values were superimposed with the key ecological corridor to obtain key ecological pinch points, with the rest as general ecological pinch points. Ecological pinch points are critical nodes in the movement of organisms, while barrier points are areas of high resistance to the movement and migration of organisms [45]. The ecological barrier points were searched with 300 m and 100 m as two scales of radius, and the maximum value was used to obtain the simulation results of biological activity barrier areas. The results were then divided into five categories using the natural breakpoint method, and the maximum value was superimposed with the key corridor to extract key ecological barrier points; the rest were general ecological barrier points. The migration patterns of animals within a research area can affected by ecological corridors, pinch points, and barrier points. This information can be used to inform the design and optimization of multi-scale synergistic ecological networks.

Key corridors [46] can be identified and targeted for protection by creating an interaction matrix linking source sites. The gravity model, sometimes referred to as the gravitational model, can be utilized as a standard for determining the significance of corridors and the priority of protection. The formula for the gravity model is as follows:

$$G_{ij}={\displaystyle \frac{N_i{N}_j}{D_{ij}^m}}={\displaystyle \frac{\left[\frac{1}{P}\times \ln \left(S_i\right)\right]\left[\frac{1}{P}\times \ln \left(S_j\right)\right]}{{\left(\frac{L_{ij}}{L_{max}}\right)}^2}}={\displaystyle \frac{L_{max\ln \left(S_i{S}_j\right)}^2}{L_{ij}^2{P}_i{P}_j}}$$

(4)

where $G_{ij}$ is the interaction force between source patch $i$ and patch $j$; $N_i$ and $N_j$ are the weight values of patches $i$ and $j$. $D_{ij}$ is the standardized corridor resistance value between the two patches. $P_i$ denotes the resistance value of patch $i$;$P_j$ denotes the resistance value of patch $j$; $S_i$ and $S_j$ are the areas of the two patches; $L_{ij}$ denotes the cumulative resistance value of the corridor between patch $i$ and patch $j$. $L_{max}$ refers to the potential maximum cumulative resistance value of the corridor.

2.3.2. Water Green Network Construction

The river network was extracted using the ArcGIS hydrologic analysis tool to study the stormwater catchment process in the study area. The level of the river network is different for different cumulative amounts of catchment, and when the cumulative amount is larger, the level is higher [47]. Referring to the related research and after many attempts at debugging, this study selected the thresholds of 1000 and 4000 for the two scales of municipal area and main urban area, respectively, and divided the river network through Strahler’s grading method so as to obtain the stormwater corridor of this study. The wetland node at the catchment node can achieve the tasks of flood storage and flood abatement. The catchment node is the primary hydrological node of the water cycle in the stormwater ecological network [48]. By capturing the pouring point, the catchment node is obtained based on the extraction of the stormwater corridor.

Based on previous research [49], the municipal area chooses forest land and grassland areas larger than 2 km², as well as lakes and reservoirs larger than 0.1 km², in the land use data. Meanwhile, the main urban area extracts forest land larger than 0.002 km² and lakes and reservoirs larger than 0.01 km² as the origin of the water–green network. The multi-level stormwater corridors connect wetland nodes, greenland patches, and hilly patches to form the water–green ecological network.

2.3.3. Landscape Network Construction

The primary goal of constructing a landscape network is to improve the connection between natural and humanistic landscape (townships and characteristic villages) nodes [7]. Based on Dali City’s spatial shape and resource distribution, this article uses the expert scoring method and the hierarchical analysis method for the hierarchical division and weight evaluation. Afterwards, using the three levels of accessibility, landscape, and service, it builds the appropriateness evaluation system. The landscape nodes are connected by the water system, banded green areas, historical and cultural neighborhoods, and featured highways as the main line and the traffic arteries as the link to form a three-dimensional landscape network.

Table 2. Landscape network construction suitability evaluation system.

Category	Indicator Layer	Evaluation Criteria	Weight
Accessibility	Resource accessibility	Assign 4, 3, 2, 1, and 0 points for distance from landscape nodes	0.18
	Accessibility to transportation facilities	4, 3, 2, 1, 0 points for distance from important transportation terminals	0.29
	Existing linear landscape resources	Assigned 4, 3, 2, 1, and 0 points for distance from existing linear landscape resources	0.12
Scenic	Characteristic of the place	4, 3, 2, 1, 0 points according to whether it is a historical and cultural district under key protection and has good landscape and windscape	0.12
	River	Assign 4, 3, 2, 1, 0 points for distance from the river	0.08
	NDVI	4, 3, 2, 1, 0 points were assigned according to the vegetation cover class	0.05
Serviceability	Distribution density of visitor services	4, 3, 2, 1, 0 points according to space heat level	0.08
	Distribution density of civic service facilities	4, 3, 2, 1, 0 points based on space heat level	0.08

demonstrates the indicators, evaluation methods, and weights selected for constructing the landscape network.

2.3.4. Recreation Network Construction

The goal of the recreation network is to meet the needs of human leisure and transportation [50]. Based on prior research and the actual conditions of the study area [51], 11 evaluation indicators were chosen from 4 categories in this study: ecological landscape, natural resources, humanistic landscape, and transport resources. The weights were then categorized and assessed using the AHP hierarchical analysis method (the selection and weighting of specific indicators are shown in

Table 3. Appropriateness evaluation system for recreational network construction.

Level 1 Element		Secondary Element		Evaluation Methodology
Element	Weight	Element	Weight
Ecological background	0.16	Elevation	0.1	4, 3, 2, 1, 0 points are assigned according to the slope classification
		NDVI	0.06	4, 3, 2, 1, 0 points were assigned according to the vegetation cover class
Natural resources	0.4	River	0.12	Assign 4, 3, 2, 1, 0 points for distance from the river
		City parks and squares	0.18	Distance from city parks and squares are given 4, 3, 2, 1, 0 points
		Forest parks, scenic areas	0.1	4, 3, 2, 1, 0 points for the popularity, grade and area of forest parks and scenic areas
Human resources	0.24	Unit of cultural relic protection	0.08	4, 3, 2, 1, 0 points for distance from the heritage conservation unit
		Characteristic Historical Neighborhoods	0.08	4, 3, 2, 1, and 0 points for distance from the Distinctive Historic District
		Museums	0.08	Assigned 4, 3, 2, 1, 0 points for distance from the museum respectively
Transportation facilities	0.2	Road traffic network	0.06	4, 3, 2, 1, 0 points given for distance from road traffic network
		Major transportation terminal	0.04	4, 3, 2, 1, 0 points for distance from important transportation terminals
		Slow-moving road network	0.1	Assign 4, 3, 2, 1, 0 points for distance from the slow-moving road network

). Natural and humanistic recreational resources are two types of resources that can be used to construct an ecological network for recreational purposes. In contrast to humanistic recreational resources, which mostly consist of museums, memorials, urban historic districts, and cultural relic preservation units, natural recreational resources in the city include river systems, urban parks, squares, and other natural landscape features.

Based on the findings of the suitability assessment, point and linear regions that are appropriate for the building of greenways are linked, while inappropriate regions are avoided throughout the linking process. The linear resources for connections are divided into rivers and roads. Rivers are natural ecological recreation corridors, so rivers are prioritized as the main recreation corridors in the city. The city road network is a potential recreational corridor in the leisure and recreational ecological network, and the road linear connectivity resources adopt the principle of being slow moving, in view of which, roads with high value and potential for slow-moving recreational transformation are selected as slow-moving recreational corridors in terms of accessibility and resource landscape.

2.3.5. Corridor Width Optimization

Corridor width directly affects organisms’ migration and energy flow. Many scholars have also proposed the appropriate corridor width in terms of landscape structure and function [52]. Combined with the current situation of the study area, 30, 60, 90, 150, 300, and 600 m were selected for buffer analysis to obtain the area proportion change values of land type composition under different corridor widths, and the change inflection points were used to determine the appropriate width of each type of corridor.

2.3.6. Multi-Scale Nesting and Multi-Target Integration

The construction of a multi-scale ecological network focuses on the integration and expression of ecological elements at each scale. The multi-scale ecological environment is constructed through different indicator evaluation systems, and direct superposition will lead to a complex ecological environment with high construction costs and the inability to meet multiple objectives at a later stage. Therefore, it is necessary to use a multi-scale nested model to realize the organic connection between the municipal area and the main urban area to ensure that the large scale dominates and regulates the small scale, as well as to convey ecological validity by sharing the important elements across the scales.

Using the ArcGIS overlay analysis method, a “triple-networked” red, green, and blue spatial ecological network was built at the biological, hydrological, and humanistic levels. It consisted of a habitat network intended to protect biodiversity, a water green network intended to improve the ecological environment, and a recreation network intended to promote leisure and open space.

3. Results

3.1. Municipal Network Construction

(1)

Habitat network

The core class has the greatest patch in the front background, with a municipal area of 807.6 km², or 46.44% of the entire research area, according to the MSPA identification results. Figure 3 illustrates how the municipal area’s core is more widely distributed, with the largest patches mostly found in Xiaguan Town and the Cangshan Nature Reserve. The remaining patches are smaller, more dispersed areas, and the overall spatial distribution is fractured. Patch 1 (Cangshan Mountain Reserve) had the greatest impact on the connectivity of ecological networks in Dali City, followed by patches 2 and 3, which had importance indices greater than 10, and were primarily located in the mountainous areas or forested lands in the regions of Xizhou Town, Fengyi Town, Shuanglang, and Xiaguan Town. Based on the analysis of dPC values, 13 ecological sources were identified, with a total area of 759.78 km² and an evenly distributed but low overall connectivity. These ecological sources are mostly the study area’s mountains, nature reserves, or forest parks, which are important habitats.

Figure 3 depicts the municipal’s ecologically resistant surface. The aforementioned findings suggest that the south is mostly home to higher resistance levels, whereas Erhai Lake is home to lower resistance values. Using the Linkage Mapper tool, twenty-two ecological corridors totaling 349.20 km were located. According to the distribution of the corridors, there were corridors connecting each source patch, and they were primarily located in Haidong Town, Wase Town, Fengyi Town, and Shuanglang Ancient. In the backdrop of a karst fissure environment, the eastern region of the country has developed a unique ecosystem. This is the result of special geological formations. This ecosystem is extremely fragile, with a high degree of soil erosion sensitivity, making ecological recovery and reconstruction difficult. In the south, the original forest vegetation has been destroyed, and the functions of water conservation and soil conservation are weak, increasing soil erosion. The degradation and sanding of meadows brought about by rocky desertification are prominent, threatening Erhai Lake’s ecological security. Therefore, the conservation and restoration of mountain forest vegetation is the focus of forest city construction in the region.

We numbered the 13 ecological sources and calculated the interaction strength between different ecological source locations in the study area using the gravity model (Figure 4), and the stronger the interaction force between ecological source locations, the more meaningful the construction of intersource corridors. In the municipal area, corridors with an interaction force greater than 200 were selected as key ecological corridors, and the remaining corridors were treated as general ecological corridors. This resulted in 13 key ecological corridors and 9 general ecological corridors. Among them, sources 5 and 6 have the largest interaction force of 3029.13, indicating that the spatial correlation between the two is the strongest, and the lower the resistance encountered by species when migrating and spreading between the two patches, the more favorable it is for regional ecological protection. The interchange of materials is challenging at sources 8 and 11, which also have the lowest contact force and the strongest resistance to organism migration between the two sites. Therefore, to encourage the interchange of materials and biological exchanges between the north and the south, the number and area of ecological nodes in the urban built-up region can be expanded during the planning of possible corridors based on the extracted corridors.

The Pinchpoint Mapper program utilizes Circuitscape to identify ecological pinch points at both sizes. A total of 22 ecological pinch points were found by merging the two modes (Figure 5), of which 9 were general ecological pinch points and 13 were key ecological pinch points in the municipal territory. The eastern and southern sections are more thickly distributed and are primarily centered around the boundaries of the source region and corridor crossings, with the exception of the western and northern sections. With six ecological pinch points, the town of Haidong has the most, making up 27% of all the pinch points.

The analysis of the barrier points using Linkage Mapper yielded two scenarios for the nodes: unselected improvement scores relative to (least-cost distance) LCD percentage and selected improvement scores relative to LCD percentage (Figure 6). Out of the 22 ecological barrier points extracted within the municipal area, 9 were general ecological barrier points and 13 were key ecological barrier points (Figure 7). The geographical distribution of barrier points is as follows: The majority of the eastern and western locations are found at the intersection of urban road networks and ecological corridors, where the ecological environment is more delicate. The southeast region is home to a concentrated number of barrier points; the area is part of Fengyi Township’s development and construction, and the land used for transportation has impeded the flow of ecological elements.

(2)

Water Green Network

Based on the ArcGIS 10.8 hydrological analysis tool used to simulate the stormwater natural confluence process, the municipal’s river network is divided into five levels according to the runoff level, and the river situation is extracted into three levels of stormwater corridors, of which the first level, second level, and third level of stormwater corridors are 40, 78, and 386, respectively. The stormwater corridors are evenly distributed, and the Cangshan area is densely distributed, forming an interconnected situation.

According to the analysis of catchment nodes, wetland node construction is carried out at river confluence nodes and watershed outlet nodes; 112 wetland nodes are identified, and 29 greenland patches are selected, including 14 lakes and reservoirs and 15 woodland meadows, with a total area of 1016.14 km², accounting for 58% of the total area of the region. The stormwater ecological network was supplemented to form a water and green ecological network combining points, lines, and surfaces (Figure 8).

(3)

Landscape networks

Figure 9 illustrates that a high distribution in the center and a low dispersion around indicate the suitability of landscape network design in the study area. The northeast and southwest regions have few recreational opportunities and low landscape value, making them unsuitable for the establishment of landscape ecological networks. The highest areas are primarily found around Erhai Lake, Dali Ancient City, and Xiaguan Town, where selective placement is required.

The municipality has identified 130 slow-moving landscape routes and 502 riverfront landscape corridors. Landscape corridors generally have a distribution pattern “more in the south and less in the north”. Erhai Lake flows from the northeast to the southwest, with the southwestern end being the outlet, forming a river convergence. The study area’s northeastern region has fewer corridors due to its natural woodland landscape type, which is not conducive to recreational activities. Additionally, the region has fewer water systems and roads, which contribute to its low spatial accessibility. This makes it difficult to engage in experiential and recreational activities there. The study area’s natural landscape resources include parks, rivers, and squares; its humanistic landscape resources are mostly museums, distinctive historical neighborhoods, and units for cultural relic safeguarding. The municipal region yielded 27 natural landscape nodes and 21 humanistic landscape nodes.

3.2. Network Construction in Main Urban

(1)

Habitat networks

The landscape area of the main urban core category is 16.90 km², which is 15.69% of the total area of the study area. There is more room for improved ecological network construction in the main urban area because there are fewer core green space patches, fewer bridging-type patches than other types of patches, and insufficient spatial connectivity between the large and medium-sized core-type patches (Figure 3). With a combined size of 5.31 km², 13 patches with dPC > 1 were chosen as ecological source locations within the municipal area. Small and sparsely distributed peri-urban woodlands and villages, with low intensity of human development, are a good ecological source. The area is well connected and mostly situated on the outskirts of the city, and is primarily made up of low-intensity human development communities and forest land.

In the main urban area, twenty ecological corridors totaling 99.23 km in length were found. Of these, those with interaction forces larger than 1000 were designated as major ecological corridors (Figure 4), with the remaining corridors being considered general ecological corridors. Figure 7 displays the 7 general ecological corridors and the 13 key ecological corridors that were identified. Key ecological corridors are primarily found around the towns of Fengyi and Haidong, where the district has a straightforward forest structure and little to no forest cover. Building extensive forest ecological corridors and forest protection green spaces within towns, between urban districts, between towns and cities, on the outskirts of cities, and along transportation routes is a priority for urban construction in the district with the goal of improving the quality of the human environment on a large scale. Of these, patch 1 and patch 8 had the weakest interactions between patches because they straddled built-up land, had the greatest resistance to flow from the source site, made it more difficult for species to migrate, and thus had the weakest interactions between patches, while the greatest interactions were found between patches 12 and 13 in the main urban area, indicating the strongest connectivity in the source area. In order to improve the ecological corridor and maximize the ecological network, the ecological network should be built by adding ecological pinch points or altering the migration paths in the source area, where there is greater flow resistance and greater difficulty for species migrating.

In the main urban area, 19 ecological pinch points were extracted, of which 8 were key ecological pinch points and 11 were general ecological pinch points. There are 25 ecological barriers, of which 10 are key ecological barriers and 15 are general ecological barriers. The majority of ecological barriers are found inside ecological corridors or on the periphery of ecological sources at both scales. Some of the barriers can be restored by strengthening the creation of main urban green space and taking steps like converting farmland back to forests in order to improve and restore the isolation area and maximize the connectivity between ecological resources, as indicated by the remote sensing image map, which shows that some of the barriers are located on town roads.

(2)

Water Green Network

The main urban river network was split into three tiers based on the hydrological analysis in order to identify the secondary stormwater corridors, of which 31 and 40 were primary and secondary stormwater corridors, respectively. A total of 80 wetland nodes and 140 greenland patches—along with 11 lakes and reservoirs and 333 forests and grasslands—were chosen based on the analysis of catchment nodes. The main urban water and green network was created by connecting the corridors, which were primarily located in the northwest section of the city, with one another via woods (Figure 8).

(3)

Recreation Network

In the main urban region, Xiaguan Town and Dali Old Town are the most suited, while Fengyi Town is the least suited because of its lack of recreational opportunities and difficult access to transportation. Combining leisure, recreation, and planning into one linear space is known as a recreational corridor. Thirty-one slow-moving recreational routes and seven riverfront recreational corridors are identified based on the utilization of the available resources and the people’ recreational demands. Nineteen natural recreation nodes and fifteen humanistic recreation nodes were extracted (Figure 9). The construction of the recreation ecological network should be close to and connect the recreation nodes with the existing corridors to ensure the spatial continuity of the recreation resources and improve the overall connectivity of the recreation network.

3.3. Complex Ecological Network Construction

3.3.1. Appropriate Widths for Different Corridors

In the municipal biological migration corridor (Figure 10), in the secondary corridor, the area of forest land and water area gradually decreased, the area of cultivated land, unutilized land, and constructed land continued to increase, and the inflection point of the rate of change was 150 m. In the main urban corridor, the area of forest land in the range of 30–90 m remained stable, the area of constructed land decreased and then increased, and the inflection point of the rate of change was 60 m. In the municipal stormwater corridor, the area of forest land, cultivated land, constructed land, and unutilized land decreased and then increased, and the inflection point of the change was 60 m. In the main urban corridor, the area of forest land, cultivated land, construction land, and unutilized land decreased and then increased, and the inflection point of the change rate was 90 m. The area of forest land in the corridor in the main urban remained stable within the range of 60–300 m, the construction land and water areas decreased, and the inflection point of the change rate was 60 m. According to the current land use characteristics of the city, the suitable widths of different biomigration corridors are 150 and 60 m, respectively; the suitable widths of stormwater corridors are 90 and 60 m; and the suitable widths of open space corridors are 150 and 60 m. Rivers and roads are selection lines, so the width is a fixed value and cannot be set.

3.3.2. Nested Multi-Scale Ecological Networks

Regarding overlapping and intersecting ecological sources, corridors, and nodes at both municipal and main urban scales (Figure 11), the number of common ecological sources at both scales was obtained to be 5, with an overlapping area of 3.65 km², and there were 11 overlapping ecological corridors with an overlapping length of 12.93 km, which accounted for 3.7% and 13.03% of the length of ecological corridors in the municipal area and the main urban area, respectively. The overlapping ecological corridors are mainly located around the built-up area. There are 3 overlapping stormwater corridors with a length of 2.73 km and a total of 2.73 km² of forested land, and 17 overlapping recreational corridors with a length of 38.39 km. These overlapping ecological sources and corridors support ecological processes between the two scales and need to be protected and optimized. The absolute contribution of the shared source sites and corridors in the regional ecological network is low, but the fact that the shared source sites overlap most of the source sites in the main urban area within the municipal suggests that the nested substrate of habitats between the two scales is good and the biological processes may be effective.

Overlapping ecological pinch points and barrier points at both scales (Figure 11) yielded two overlapping pinch points, accounting for 4.44% and 9.52% of the number of pinch points in the municipal and main urban areas, and a total of three overlapping barrier points, accounting for 13.64% and 12% of the total number of barrier points in the municipal and main urban areas, respectively. There are six overlapping wetlands nodes. These represent 5.36% and 7.59% of the number of wetland nodes in the municipal and main urban areas, respectively. There are seven overlapping recreation nodes, accounting for 14.58% and 20.59% of the number of recreation nodes in the municipal and main urban areas, respectively. The low degree of overlap of ecological nodes across scales suggests that future optimization of the ecological network needs to pay attention to the transfer of ecological processes between multiple scales to facilitate the flow of matter and energy. The control of the ecological security boundary and the solution of important ecological problems in Dali City need the support of the main urban area, while the protection and restoration of key ecological nodes in the main urban area is an important link in constructing and connecting the overall ecosystem.

3.3.3. Multi-Targeted Ecological Network Integration

Red–green–blue ecological networks, primarily defined by rivers, biology, stormwater, and recreation, are created by superimposing single-objective ecological networks (Figure 9). Protecting biodiversity, controlling urban runoff, enhancing the urban environment, and offering inhabitants leisure and recreation are just a few of the roles that the river biological corridor plays. After careful examination, it can be seen that the three functional corridors—which are oriented around Erhai Lake and Xiaguan Town—have a very balanced design. There are more recreational and water–green corridors in the southwest, of which the number of ecological corridors is the highest in the southeast, mainly due to the fact that some of the smaller source areas play the role of stepping stones to reduce the cost of migratory distances while enhancing the connectivity between the source areas, while a small number of stormwater corridors and ecological corridors are distributed in the north due to the topography and transportation. When comparing the nodes’ spatial distribution, it can be seen that the ecological nodes are primarily found in the east and south. This is likely due to the city’s highly developed south and developing east. The eighteen streams on Cangshan Mountain are home to the majority of wetland nodes, which can help with runoff rate reduction, flood storage, and water quality enhancement. The overall spatial distribution of the landscape nodes is fairly balanced, with multi-functional, multi-type recreational nodes like temples, commercial districts, and science and technology parks on the west side, and parks and green spaces such as Shangguan Flower Park, Erhai Forest Park, Erhai Jade Cabbage Wetland, Erhai Ancient City, and Xiubeishan Forest Park as participatory ecological spaces.

3.3.4. Multi-Functional Composite Ecological Network Pattern Optimization Strategy

The coverage of the ecological network in Dali City is centered on ecological sources, but the eastern part of the city lacks large habitat patches and has poor connectivity with the north and south, so it is necessary to focus on increasing the construction of ecological sources in the eastern part of the city to enhance the protection of regional ecological sources. Some of the stormwater corridors and biological corridors are side-by-side or intersect to enhance the vegetation richness of the composite corridors and strengthen the connection between the corridors, which can improve the potential utilization rate of the corridors. Recreational corridors and ecological corridors intersect to form ecological breakpoints in municipal and main urban areas, respectively. There are breakpoints 16, 15, mainly distributed on the national highways G214, G320, and S221; and ecological corridors at the intersection of the road can be restored through the installation of above-ground flyovers and underground tunnels. Other actions include reducing the interference of town development and construction on all types of corridors, strengthening the spatial coordination of composite sources and corridors, and promoting the optimal construction of a multi-level, multi-objective red, green, and blue ecological network.

Dali City has a great variety of wild animals and plants, and rich vegetation types, and has built various kinds of nature reserves, wetland parks, and geological parks. In recent years, Dali City has carried out the project of returning farmland to forests, closing the mountains to cultivate forests, and continuously strengthening ecological environment construction while protecting and making good use of the existing forests. Unique natural landscape conditions and rich natural resources have created favorable conditions for ecological construction in Dali City. The cost–benefit analysis of ecological network construction and its subsequent development is a key link in assessing its value for environmental protection and sustainable development. As a result, the risk of ecosystem degradation is effectively reduced by promoting biodiversity conservation and restoration, further reducing the costs of governance in the future. Secondly, it improves the efficiency of resource use, such as water conservation, soil conservation, and other ecological services, and indirectly reduces the cost of resource acquisition and utilization. Finally, the construction of ecological networks promotes the development of green industries such as eco-tourism, injects vitality into the local economy, and achieves a win–win situation for both economic and ecological benefits.

4. Discussion

Building a multi-objective composite ecological network is crucial to the development and integration of recreational, water–green, and regional habitat networks. Using MSPA, circuit theory, and the landscape connectivity approach, a multi-level and multi-objective composite ecological network was built in Dali City for this study. The two most critical steps in the process of building ecological networks are the extraction of ecological sources and the identification of ecological corridors. Nowadays, ecological sources are identified by landscape connectivity [37], ecological sensitivity [7], and ecosystem service importance [53]. In this study, MSPA and landscape connectivity were used to identify ecological sources [54]. The construction of resistance surfaces is a prerequisite for the identification of ecological corridors [39,55] and in the past, the construction of resistance surfaces usually used topography, surface cover type, and road network factors as the leading indicators to classify and assign values to them. However, these methods ignore the high density of socio-economic activities brought about by urbanization [41].

Circuit theory and the MCR model are used to build ecological corridors. The MCR model can better depict the trend of ecological corridors [22], but it cannot pinpoint important nodes, such as “pinch points” and “obstacle points,” inside the corridors. In this study, circuit theory was used to construct the ecological corridors between the Dali municipal area and the main urban area [23]. The key ecological corridors were extracted using a gravity model. According to the results, which are in line with those of Li et al. [20] and Xu et al. [21], ecological sources are primarily distributed in Cangshan Mountain, Shangguan Town, and Fengyi Town. These are primarily core areas with widely dispersed woodland landscapes. The Cangshan region is home to the majority of forests, which are dispersed due to a variety of causes, including topography, climate, and human activity. As a result, there are few ecological sources found and there is a lot of edge fragmentation. The study region is centered on significant ecological corridors, which are areas of ecological connection and are crucial for the optimization and restoration of the entire landscape. The study identified 22 and 25 barrier points, respectively, in the municipal and main urban areas. These points were primarily spread inside the built-up area range and road network, indicating their location at the periphery of ecological sources or corridors. These findings are in line with prior research [56]. Furthermore, by examining the ecological corridor’s width, we discovered that, within the 30 to 150 m range, there was minimal variation in the landscape structure. This was associated with the type of landscape, which was primarily forested and featured a sizable forested area with strong landscape connectivity. This was in line with earlier research [57]. Narrow ecological corridors are not good for species survival because they have a negative “edge effect” on species’ movement and distribution. This is because, as a corridor’s width declines, so does landscape connectivity. Thus, covering as many different types of environmental gradients as possible will improve environmental variability and ensure biodiversity.

At present, the majority of research works have constructed networks on a single scale, emphasizing the region’s general average value at large scales while disregarding the variations at smaller ones [38]. The linkages between various ecological network levels and various administrative levels are also not widely studied [31]. Nor is the geographical variation in ecological landscape patterns at various scales taken into account [58]. It is important to take into account changes at various scales when building ecological networks and developing regional plans because of the scale effect, which makes ecological processes at different scales independent . The multi-scale and multi-level requirements of ecological restoration work in land space are combined in this study, and scale nesting is incorporated into the ecological network construction link in the study area, serving as an external support role and improving the effectiveness of regional ecological protection [59]. This not only builds the overall ecological security pattern at the mesoscale, but also takes into account Dali City’s small-scale land use characteristics and landscape connections. Additionally, this study built a habitat network, a water–green network, and a recreation network in Dali City from the perspectives of biological, hydrological, and humanistic processes. In contrast, an ecological network driven by a single goal that solely considers recreational or ecological conservation would be different [24,60,61]. It was shown that applying this method helps to increase the connectivity of landscape patches, which is advantageous for preserving regional landscape patterns and protecting biodiversity.

In the process of constructing the composite ecological network system, this study comprehensively applied the quantitative analysis method. This improves the traditional method based on empirical judgment and qualitative analysis. This study can provide practical references for urban and rural ecological construction and is of great significance for the sustainable development of fragile ecosystems. Although this study provides new insights into the construction of a composite ecological network, the following research deficiencies still exist: Firstly, because the natural environment is an organic whole, this study only utilizes the county scale and the urban scale, and future research can consider the relationship between larger scales. Secondly, the study concentrates on the tension between the increase in construction land and the deterioration of the terrestrial biological environment, primarily because of the study area’s relatively intact water network. Although certain significant water networks are taken into account, the significance of the water environment and the preservation of water systems need to be enhanced. Third, the ecological corridors created in this study are only useful for conserving biodiversity in the area; the requirements of certain species for corridor width have not been sufficiently taken into account. Future research should categorize ecological networks based on the requirements of various species. Ultimately, because of the data’s limitations, the impact of variables like residents’ travel patterns and spatial distribution on the recreation corridor was not fully taken into account. However, future research can optimize the suitability evaluation system by combining big data from multiple sources, including population travel patterns, settlement distribution, and holiday travel patterns.

5. Conclusions

Building a composite ecological network is an effective way to mitigate habitat fragmentation and biodiversity loss. This study used Landsat remote sensing data and forest resources type II survey data in Dali City for 2016, adopting source–sink theory, circuit theory, appropriateness analysis, and hydrological analysis in order to achieve ecologically sustainable development. The study was based on the principles of ecological protection and multi-functional coupling and coordination. The study constructed a composite ecological network of the municipal area and main urban area under the spatial perspective of red, green, and blue in Dali City. The conclusions are as follows:

(1)

In the “source-corridor-key node” ecological network, 3.65 km² of overlapping ecological sources was identified, mainly concentrated in the main urban area of the study area. There are 11 overlapping ecological corridors, 3 stormwater corridors, and 17 recreational corridors. The overlapping areas are the key to achieving overall ecological sustainability and biodiversity in the region and should be protected as a priority.

(2)

Two overlapping pinch points, three obstacle points, four wetland nodes, and seven open space nodes were identified between the municipal and the main urban area. The ecological pinch points and barrier points are areas where species migration and movement between source areas are strongly impeded, and ecological problems can be addressed through the nested integration of multi-scale spatial structures.

(3)

The triple-function corridors are mainly distributed in the central part of the study area and around Erhai Lake, with stormwater corridors and recreational corridors dominating in the west and ecological corridors mostly distributed in Fengyi Town and Haidong Town in the southeast. Therefore, in the future, the protection of Shuanglang Town and Xiaguan Town should be increased to build ecological sources and ecological corridors with stronger connectivity. At the same time, the protection of ecologically fragile areas in the west and south should be increased to reduce ecological damage and prohibit indiscriminate logging and deforestation to maintain ecological functions.

(4)

This study emphasizes the importance of a composite ecological network for ecological sustainability, which can be applied to different levels of territorial space restoration. In terms of constructing low corridors, corridors can be constructed through measures such as restricting land development and implementing vegetation restoration. In addition, according to the actual situation, additional protective measures can be installed in the watersheds where the population is concentrated, at the policy level, in order to strictly control the pollution of the watersheds caused by agricultural production and the sewage discharge from domestic life. This study can provide a reference for the composite ecological network of similar cities.