Ecological Network Construction in High-Density Water Network Areas Based on a Three-Dimensional Perspective: The Case of Foshan City

Abstract

The acceleration of urbanization has resulted in varying degrees of impact on the stability and health of high-density urban ecosystems. Building urban ecological networks is crucial for safeguarding biodiversity and sustaining ecosystem vitality. In this study, the city of Foshan was selected as the study area, which is a prime representative of a high-density water network city. Additionally, a morphological spatial pattern analysis was employed to identify the ecological source. We built an ecological resistance surface using geographic, natural, and behavioral elements, adjusting it based on the density of the water network and the building height. Following this, the circuit theoretical model was utilized to create an ecological network by identifying ecological corridors. There were three key findings. First, the ecological network consisted of 30 ecological source sites and 53 ecological corridors, and 103 ecological “pinch points” and 193 ecological barrier points were identified. Second, the ecological sources were predominantly situated in the southwestern and northern parts of Foshan City. Meanwhile, the suburbs of Foshan City contained the primary ecological barrier points, mainly stemming from new construction sites, while the key ecological “pinch points” were concentrated at river junctions. The third outcome was the recommendations to (a) boost the connectivity of the ecological network in the suburbs, (b) improve the connection of the water network in urban areas, and (c) focus on enhancing landscape connectivity. The objective was to develop approaches for optimizing urban ecological networks, leading to better connectivity and improved ecological network quality.

1. Introduction

Ecological degradation, loss of species habitats [1], increased landscape fragmentation, land use changes, and ecological light pollution have been caused by rapid urbanization, rural modernization, and intense land development and utilization in specific areas. Consequently, this series of events has affected species migration and reduced biodiversity [2,3]. To date, nearly one million species have been lost globally, with various groups of species facing a rising risk of extinction. Human development and the construction of grey infrastructure have led to the fragmentation of habitats, which is a key factor in this phenomenon. This has led to the risk of loss of connectivity of migratory corridors for plant and animal species, which is a vital avenue for biodiversity conservation [4]. Ecological networks are interconnected systems consisting of core habitat patches and the ecological corridors that link them [5]. They play a role by establishing, restoring, and maintaining ecological corridors, thereby conserving the fragmented biodiversity in the system [6]. Research has demonstrated that urban ecological networks effectively conserve and enhance urban biodiversity by offering uninterrupted pathways for animal migration [7].

From the 1990s onward, many academics have formulated ecological network models and techniques across different spatial scales, drawing from landscape ecology principles. Ecological source areas are identified and extracted through the identification of ecological patches. Resistance values are assigned to elements affecting ecological circulation to construct resistance surfaces, and the shortest paths traveled by biological flows between source areas are defined as ecological corridors. This process ultimately forms a research framework based on the “source–resistance surface–corridor” paradigm [8,9,10]. The establishment of ecological networks typically relies on two aspects: ecological sources and ecological corridors. Currently, two main strategies are deployed to identify ecological sources. The first is the land class method, which frequently selects nature reserves [11,12], urban green spaces [13], and natural land cover types as ecological resources [14]. The other is the comprehensive method, which includes network topology and ecosystem services [15], the inverse granularity method [16], the principal component analysis method [17], and the ecological network research method of landscape pattern analysis [18], among others. Nevertheless, it must be noted that the above research methods have several limitations, including the “ecological corridor” paradigm and the subjective nature of the assessment process of the land class method, which is less likely to consider the internal differences within the same land type. In contrast, the comprehensive method selects multiple indicators for case studies to reflect the diverse environmental conditions in different locations. For these reasons, this study adopted the morphological spatial pattern analysis (MSPA) method [19,20,21], which has been extensively used in recent years to identify ecological source sites. MSPA utilizes land use grid pixels in the study area for computation, identification, and segmentation to create detailed images. When juxtaposed with the two previously employed methods, MSPA can objectively determine the nature and structure of the landscape and quantitatively identify ecological source areas. Building ecological corridors necessitates the development of a comprehensive resistance surface [22]. Generally, it is constructed by weighting and correcting geospatial information like land use type and elevation [23] and forming an optimal channel for material and energy flows in the region guided by the minimum resistance value [24].

However, human activity significantly affects the stability of ecological networks in high-density developed areas, especially high-density water network areas and high-density intensive development cities with well-developed river and lake systems. Limited by the development of water network geomorphology, areas with high human activity are more concentrated than in other areas, leading to a more intense impact of the high-density urban form on ecological processes. Previous studies mostly constructed resistance surfaces based on two-dimensional planes, which are very friendly to plain areas and areas rich in natural ecological resources; however, there are insufficient considerations for high-density areas [19]. Therefore, this paper proposes a comprehensive resistance surface construction method based on a 3D “geography–nature–human” perspective to boost the accuracy and scientific validity of ecological resistance surface construction in high-density water network areas. An ecological corridor constructed using the minimum resistance value can clarify the optimal route for biological circulation and material flow, but it cannot determine the spatial extent and key nodes of ecological corridors. McRae and other scholars have substantiated the significance of this circuit theory model in compensating for this limitation and extracting ecological corridors [25,26,27,28]. By simulating the characteristics of random current travel and the magnitude of current intensity, circuit theory can predict the direction of biotic movement and identify the key points of ecological corridors. It can also effectively determine the multiple diffusion paths of species among ecological source sites. The circuit theory model employed in this research can effectively respond to the width of ecological corridors, and by conducting a centrality analysis, the priority of ecological sources and ecological corridors can also be obtained.

In recent years, the development strategy of high-density cities has also pivoted toward prioritizing quality over speed. The current difficulties and challenges facing these areas involve protecting and restoring the urban ecological environment and promoting the harmonious coexistence of socioeconomic and ecological systems. China is implementing comprehensive land management practices nationwide to promote high-quality urban development. Land spatial planning emphasizes the importance of ecological space, addressing issues like spatial disorder, ecological function degradation, and inefficient land utilization. It involves land organization, ecological protection and restoration, and historical and cultural protection. Ultimately, it is crucial to comprehend the changing relationships between humans and nature, land and industry, and culture and landscape as a whole. Based on this, this study selected Foshan City as the study area, which is known to be a typical high-density city with a developed river network and water system. We explored how biodiversity is influenced by geographic, natural, and human elements, examining the impact of high-density urban building heights on flying species and the harm of artificial lighting on nocturnal land animals. Then, we incorporated these factors into the creation of resistance surfaces to build a 3D resistance surface, which we comprehensively superimposed to establish the ecological network space across Foshan City. Overall, a quantitative framework for ecological network construction and optimization was generated. This study attempted to explore two questions. First, how can one develop a complete ecological network construction framework in water network areas heavily impacted by human activity? Second, what strategies can be implemented to enhance the efficiency of ecological flows in various urban landscape areas?

2. Materials and Methods

2.1. Study Area

Situated in the western area of the Guangdong–Hong Kong–Macao Greater Bay Area, the city of Foshan is a prime example of a densely populated city with advanced urbanization, industrialization, and modern agriculture. As of 2023, the city of Foshan has five municipal districts under its jurisdiction, with a total area of 3797.79 km² (Figure 1). The Xi Jiang and Bei Jiang Rivers, along with their tributaries, flow throughout Foshan City as part of the Pearl River system. Plains, low hills, and rivers account for 70.9%, 20.0%, and 9.1% of the city’s total area, respectively, which is typical of the deltaic river network region. Agricultural land, like arable and garden land, covers the plains, while natural forests dominate the hills. The region also features an extensive river network and water system. the city of Foshan is home to seven national key wildlife species, namely, small ling cats, tiger frogs, brown-winged jay-cuckoos, yellow-billed egrets, small jay-cuckoos, sparrowhawks, and snake eagles, all of which have been found and recorded. Equally important, this city is also a transit point and habitat for international migratory birds, with Huangji village in the town of Jiujiang, Nanhai District, being a key migratory stopover.

2.2. Data Source and Preparation

This research primarily relied on land use information and geospatial data (

Table 1. Source of research data.

Data Type	Data Sources
Administrative division data	Ministry of Natural Resources Standard Map Service System (http://bzdt.ch.mnr.gov.cn/, accessed on 3 June 2024)
Land use data	Data Center for Resource and Environmental Sciences, Chinese Academy of Sciences (http://www.resdc.cn, accessed on 5 June 2024)
DEM data	Geospatial Data Cloud (http://www.Gscloud.cn/, accessed on 5 June 2024)
Floor height data	BIGEMAP GIS Office platform

). The foundation for our alignment was the 2020 Land Use Data of Foshan City from the Resource and Environmental Science Data Center of the Chinese Academy of Sciences (http://www.resdc.cn, accessed on 5 June 2024). To facilitate data overlay and calculation, the raster data were uniformly resampled and converted to a 30 × 30 m raster. Subsequently, six categories were created to reclassify the land use data: cropland, forest land, grassland, water source, construction land, and unused land. We acquired the digital elevation model (DEM) from the geospatial data cloud (http://www.Gscloud.cn/, accessed on 5 June 2024). Meanwhile, the floor height data were retrieved using the BIGEMAP GIS Office platform, while the water network density, night lights, and roads were acquired from the Data Center for Resource and Environmental Sciences of the Chinese Academy of Sciences (http://www.resdc.cn, accessed on 5 June 2024).

2.3. Study Design and Setting

This study included four main steps (Figure 2), the first of which was to identify the ecological source areas and obtain the core and bridging areas as the ecological source area. The second step was the construction of a 3D resistance surface based on the influence of geographic–natural–human elements on ecological circulation. The third step was the construction of an ecological network, specifically using the circuit theory model to construct the ecological network of Foshan City through the identification of “pinch points”, obstacle points, corridor centrality, and priority analysis. The fourth step was zoning construction and management. Combined with the characteristics of Foshan’s water network, three ecological restoration zones were delineated, and a feasible strategy for ecological optimization is proposed to provide research value for the construction of ecological networks in water network cities.

2.4. Identification of Ecological Sources

Soille et al. introduced the morphological spatial pattern analysis (MSPA) method [29], which was utilized in this study. Foreground images were created by extracting forest land, grassland, and water surfaces from the current land use status of the study area, and the rest of the land use types were used as background images. The Guidos Toolbox identified seven MSPA landscape types: core areas, edge areas, traffic circles, spurs, isolated islands, gaps, and bridging areas. In this case, 3 pixels were selected for the edge width, and the others were set to default values. The morphological basis for ecological source identification was established by extracting the core and bridging areas, which contribute most to maintaining landscape connectivity [13]. The principle of habitat diversity in landscape ecology states that the area of patches is positively related to the diversity of habitats [30]. Based on the current conditions of the study area, patches larger than 2000 m² were selected as the ecological source areas.

2.5. Construction of an Ecological Resistance Surface

In this research, an ecological resistance surface was constructed by merging various geographic–natural–human multi-indicators. Considering that geographic elements are the substrate of ecological networks, natural elements reflect the urban characteristics of the water network, while human elements involve the limitation of human activity in cities. We considered the natural background land use type as a coercive factor, while other topographic terrains and human activity elements were viewed as corrective factors. Different geographical conditions and environmental factors impact biological migration to varying degrees. Statistics show that global nighttime artificial light has been increasing at a rate of almost 6% annually (ranging from 0% to 20%) over the last century [31] due to advancements in society and the development of urban and rural construction [32]. Artificial nighttime lighting disrupts the natural light–dark cycle and interrupts diurnal patterns, particularly in urban ecological networks. This disruption affects the normal reproduction [33], growth, and physiology of animals [34], as well as their nocturnal travel patterns [35,36], leading to changes in population size. The selection of resistance factors in this study included land use type, night lighting, building height, distance from roads, elevation, corrected surface undulation, and water network density. The density of the water network was analyzed in terms of point density, calculating the magnitude of each unit of surface based on the point elements that fall within the neighborhood around each 1000 × 1000 m unit, with higher magnitudes representing lower resistance values. The reason for increasing building height as a resistance factor is that high-density urban development not only intensifies land use but also makes much greater use of vertical space compared to other cities. This vertical expansion creates a certain level of resistance to bird migration. Higher building heights may result in higher resistance values. Values were allocated to the resistance factors, taking into account the variations in habitat suitability caused by landscape type and topographic features, as well as the principle of distance attenuation. We invited relevant departments of Foshan City and ecological experts to compare, score, and assign weights to each index (

Table 2. Three-dimensional resistance surface impact factors and their weighting assignments.

Resistance Factor		Grading Index	Resistance Value	Weight
Geographic elements	Land use type	Cropland	3	0.3426
		Unused land	4
		Construction land	5
	DEM/m	<150	1	0.2037
		150–300	2
		300–450	3
		450–600	4
		>600	5
	Slope	<8	1	0.1042
		8–15	2
		15–25	3
		25–45	4
		>45	5
Natural elements	Water network density	650–1109	1	0.0428
		400–650	2
		200–400	3
		65–200	4
		<65	5
	Night light data	6275–7938	1	0.0526
		7938–9140	2
		9140–11,024	3
		11,024–14,482	5
		14,482–38,442	5
	Building/m	3–9	1	0.1456
		9–33	2
		33–72	3
		72–132	4
		132–219	5
Human elements	National highway/m	>10,000	1	0.1085
		5000–10,000	2
		2000–5000	3
		1000–2000	4
		<1000	5
		>5000	1
	Provincial highway/m	2000–5000	2
		1000–2000	3
		500–1000	4
		<500	5
		>1000	1
	County highway/m	500–1000	2
		200–500	3
		100–200	4
		<100	5

).

2.6. Ecological Corridor Extraction and Ecological Network Construction

Connecting fragmented patches through ecological corridors is crucial for facilitating species migration and ensuring the continuity and cyclicity of ecological flow and processes in the region. Circuit theory applies physical principles like resistance and current in an ecological context. It utilizes the characteristic of random wandering of electric charge and considers ecological surfaces as conductive surfaces. Another key point is that this theory calculates the effective ecological resistance value based on resistive surfaces and can pinpoint multiple ecological flow paths with a certain width between ecological source points [37]. The study involved using the Linkage Mapper toolbox to simulate ecological corridors in Foshan City by applying circuit theory for developing ecological networks.

2.7. Ecological “Pinch Points” and Ecological Barrier Points

Ecological “pinch points” and barrier points are areas that significantly impact the connectivity of ecological networks and are crucial for corridor connectivity, making it necessary to prioritize them for protection and restoration [38]. This investigation applied the Linkage Mapper toolbox to detect ecological “pinch points” and barrier points using the Pinchpoint Mapper and Barrier Mapper (Linkage Mapper 3.0) tools.

2.8. Ecological Source and Corridor Importance Assessment

Circuitscape forecasts and simulates material and movement flow between ecological sources by employing circuit theory. The probability of species migration between ecological sources is positively correlated with the cumulative current values between nodes. Corridor centrality serves to underscore the importance of ecological corridors in ecological networks. A higher current value suggests stronger connectivity and significance of the ecological element in the ecological network. We performed a centrality analysis with the Centrality Mapper tool and sorted the obtained ecological corridors and source centrality into three levels.

3. Results

3.1. Ecological Land Identification and Ecological Source Extraction

This paper considered the ecological space of Foshan City as the foreground of the MSPA (Figure 3). Subsequently, the proportion and area of each landscape type were calculated accordingly (

Table 3. Characteristics of each MSPA class.

Landscape Type	Area (km2)	Proportion of Ecological Space (%)
Core	890.3	67.21%
Bridge	62.97	4.75%
Branch	62.37	4.71%
Loop	5.86	0.44
Islet	13.81	1.04%
Perforation	23.62	1.79%
Edge	265.8	20.06%
Total	1324.73	100%

). The core area, one of the seven landscape types, dominates in size, encompassing 890.30 km², equivalent to 67.2% of the foreground elements. It is predominantly situated in the southwestern and northern sections of the study area. Spurs, bridges, and loops are essential for constructing corridor components that facilitate the circulation between urban ecological flows. The isolated landscape category includes small islands, which make up 0.36% of the total. The bridging area connects different core areas and can be used for species to communicate across the core areas, with an area of 62.97 km². The foreground elements cover only 4.8% of the total area, a small fraction, and the core area patches lack close linkage and communication.

Ecological patches larger than 0.02 km² were selected as ecological source sites due to their completeness and significance, taking into account the study area’s size. Additionally, a total of 30 ecological source sites were identified in Foshan City, with a total area of 745.33 km², accounting for 78.19% of the total area of the core and bridging areas and 19.73% of the total area of the study area. Overall, in Foshan City, the distribution of ecological source sites is relatively concentrated, primarily in the southwest and north of the mountainous forest land and the adjacent small ecological green space, as well as in the waters of the central, western, and southern parts of the city, with fewer source sites in the remaining area. Notably, in the heart of the municipality, where urbanization is most pronounced, a notable scarcity of ecological resource land is evident. The remaining area exhibits a lower distribution of source lands, consisting mainly of broken and intertwined urban land and cropland, with poor circulation of ecological elements and a lack of source land conditions. The energy flow between the northern and southern source areas is largely connected by the Xi Jiang and Bei Jiang watercourse-type source areas. However, urban development is encroaching on watercourse-type source areas, causing fragmentation of green spaces, which disrupts the continuity needed for effective material energy circulation.

By conducting a centrality analysis on ecological source sites, we determined the comparative significance of each site. The centrality of each source site was also obtained through the Centrality Mapper tool and divided into three levels using the natural breakpoint method. Figure 4 illustrates the three primary source sites, six secondary source sites, and 21 tertiary source sites. The significance of primary source sites lies in their pivotal role in species diversity and material energy transfer, whereas secondary and tertiary source sites, though smaller in scale and energy output, are important habitats for species. Enhancing the ecological quality of source areas is essential for securing the urban ecological network.

3.2. Construction of an Geographic–Natural–Human Ecological Resistance Surface

The ecological resistance surface of Foshan City was obtained by superimposing the weights of geographic–natural–human resistance elements (Figure 5). Medium and high levels of resistance define the ecological resistance surface, with areas of 1395.07 km² and 1323.18 km², accounting for 36.79% and 34.84% of the study area, respectively. Influenced by the spatial pattern of Foshan City, the resistance surface presents a semi-circular structure (Figure 6). More specifically, the medium-resistance surface is primarily distributed in the periphery of the main urban area, and the high-resistance region is densely clustered within the primary urban area, while the rest is relatively dispersed. The focus of these regions is on development and construction. The ecological source area faces constraints on its expansion due to human activity, causing heightened resistance to ecological flow, particularly in densely populated residential and commercial regions. Moreover, the ecological low-resistance surface occupies the smallest proportion, accounting for 28.37% of the total area of the study area. The low resistance areas are mainly located in the northern part of the Gao Ming and Sanshui districts and the main waterways extended by the Beijiang and Xijiang rivers, mirroring the distribution pattern of ecological source areas. The topography of these areas is more complex, with sporadic high-resistance surfaces attributed to road construction and development activities like ecotourism.

3.3. Spatial Extent of Ecological Corridors

We utilized the Build Network and Map Linkage tools to locate ecological corridors and calculated the width of those corridors using a threshold of 2000 m. This study identified 53 potential ecological corridors totaling 239.27 km in length (Figure 7). The spatial distribution of ecological corridors in Foshan City varies significantly. The southwestern and northern parts of the city have a denser distribution of patches, which poses migration challenges from the central urban zone and results in very few corridors connecting to ecological source areas. Specifically, the Gaoming District in the southwestern part of Foshan City is characterized by woodland and grassland. However, the abundance of mountains often leads to ecological corridors between roads becoming blocked and fragmented by short corridors that are less than 100 m long. Although these corridors are short, they play a pivotal role in improving the connectivity of the urban ecological network. Due to high-density urbanization, public green spaces are dispersed throughout Chan Cheng District, a section of Nanhai District, and the town of Lecong in Shun de District. Small core areas in this region are highly valued as key connectors within the urban ecological network. Meanwhile, Sanshui District in the north and Shun de District in the south exhibit a pattern of large aggregations with small dispersion, with an even distribution of ecological corridors dominated by watercourses. The main components of these corridors include woodland, grassland, and watercourses, demonstrating their vital role in facilitating connectivity and dual circulation.

3.4. Prioritization of Ecological Corridors

The purpose of centrality analysis is to distinguish important corridor routes to strengthen their connectivity. Urban ecological networks rely heavily on sources with the highest centrality. To compare the relative restoration priorities among urban ecological corridors and to reduce conservation costs, a hierarchy of urban ecological corridors should be implemented. We adopted the natural breakpoint method to identify appropriate locations for constructing urban ecological corridors. Figure 8 displays four primary corridors, 23 secondary corridors, and 26 tertiary corridors. Primary corridors play an important role in species richness, migration, and dispersal. Secondary corridors complement primary corridors. Even though tertiary corridors currently have a low density, they are crucial for animal migration, highlighting the pressing requirement to restore ecological spaces and increase ecological circulation.

3.5. Identification of Ecological “Pinch Points”

Ecological “pinch points” refer to areas within the theoretically generated corridors of electric circuits that have higher current values and narrower current widths. These areas reflect their greater importance in ecological flows and the characteristics of fewer alternative paths. They play a crucial role in landscape connectivity and maintaining regional ecosystem security. Therefore, emphasis should be placed on key ecological protection and management in the “pinch point” area. In this paper, cost-weighted corridors of 1000 m and 10,000 m were selected respectively, and the “All-to-one” model was chosen to calculate current density and identify ecological “pinch points”. After comparison, it is deemed more reasonable to take 1000 m as the cost-weighted corridor (Figure 9). Key areas with patches greater than 1 ha were selected as ecological “pinch points”. Finally, 103 ecological “pinch points” were identified. In terms of spatial distribution, the ecological “pinch points” in Foshan City are mainly located in the northeastern part of Southwest Street in Sanshui District, the waters of Keitang at the intersection with Yundonghai Street, the waters of Keitang in Lubao Town, the green areas in the northwestern part of Mingcheng Town in Gaoming District, the intersection of Ronggui Street and Daliang Street in Shunde District, the intersections of Longjiang, Lechong, and Leliu Streets, and the intersections of Nanzhuang, Xiqiao, and Danzao Towns in Nanhai District. The land types of the ecological pinch points are mainly based on dike ponds, river channels, and green areas.

3.6. Identification of Ecological Barrier Points

The Barrier Mapper tool of Linkage Mapper can identify ecological barrier points. These areas are where the movement of organisms in habitat patches is impeded, and prioritizing their restoration can boost the connectivity of ecologically important spaces. For this investigation, we used 50, 100, 150, and 200 m as the radius for the iterative comparison, concluding that 150 m was the most suitable. We collected 193 ecological barrier points (Figure 10), with the largest area spanning 3.47 km². The findings revealed that the ecological barrier points are mostly distributed in the north and southeast of Foshan City. Concerning spatial distribution, ecological barrier points are distributed at the junctions of ecological sources and corridors, as well as in the middle of ecological corridors. The towns of Shishan, Lishui, and Leping in Foshan City face greater ecological barrier points (Figure 10). Although not the most degraded habitat quality sites in the study area, the ecological barrier points are positioned in the ecological corridors, making them crucial for restoration efforts to enhance ecological flow. The land use types in the ecological barrier points primarily include construction land, farmland, and transportation land (Figure 11). These barrier points are distributed as dots and lines, forming a relatively continuous pattern across the region. The impact of human activity on the ecological corridors is evident, leading to clear fragmentation among the ecological source areas.

3.7. Ecological Corridor Width and Repair Priority

Width has a significant impact on the ecological functioning of corridors. A corridor that is too narrow can be detrimental to sensitive species while reducing the corridor’s ability to filter pollutants and other functions. Among all possible corridor locations, priority restoration areas deserve the most attention (Figure 12). The width value of ecological corridors varies in different areas. When the width is small, the corridor width has little or no effect on the number of species. The width effect becomes obvious only after reaching a certain width threshold. Relevant studies have shown that this threshold is 7–12 m [37]. Foshan City’s corridors are predominantly narrow, with widths of approximately 12 m, primarily because of human activity and urban ecological barrier points. In the absence of prompt restoration efforts, the preservation of species diversity remains uncertain. The maximum width of the ecological corridors is 380 m, dominated by forested land and basal pond waters. The corridor construction of Dongping Waterway and Shunde Waterway holds the utmost significance, as it serves as the primary link connecting the north and south of Foshan City for species migration (Figure 13).

4. Discussion

Following the identification of ecologically significant areas in the previous section, the ecological security pattern was constructed, and the ecological source area was classified as an ecological protection area. Additionally, the ecological “pinch points” were labeled as ecological critical areas, while the ecological barrier points were deemed ecological improvement areas. Proposed strategies for upgrading and optimization are listed in

Table 4. Main locations and typical representatives of the three zoning districts.

Ecological Area	Specific Area	Typical Areas
Ecological protection area	Yunyong Forest Farm, Hui Long township	Suburban mountainous areas represented by Gaoming District
Ecological improvement area	Shishan, Lishui, and Leping townships	Suburban agricultural water network areas represented by Sanshui District
Ecologically critical area	Ronggui Street meets the river in Daliang Street	Urban water network areas represented by Shunde District

.

4.1. Carrying Out Zonal Restoration and Optimizing the Ecological Network

(1) Ecological protection areas

The evolution of the city has brought about a dramatic change in land utilization. Along with this, grassland and forest land have been gradually expanded by the influence of human infrastructure construction, but the disorganized construction and development processes have resulted in the fragmentation of the ecological space. Additionally, the road network spans multiple landscape types, each presenting different levels of impediment to the movement of organisms. Hence, these areas should prioritize safeguarding the ecological background, restoring the natural vegetation structure, strictly limiting encroachment and destruction of the surrounding ecological spaces, and maintaining the integrity and sustainability of ecological functions rather than focusing on recreational development.

(2) Ecological critical areas

Ecological “pinch point” areas with high circuit density are classified as ecological critical areas, where ecological materials and energy circulate and interact, promoting ecological function enhancement. It is vital to enhance artificial ecological restoration, elevate ecological quality, widen ecological corridors, and boost ecological functions to reduce current density and establish a more stable ecological network.

(3) Ecological improvement areas

The variety of construction land types with complex functions in urban settings, coupled with cropland in the suburbs, poses challenges like inadequate green infrastructure in high-density cities. Consequently, this creates connectivity obstacles in ecological corridors. Specifically, industrial plants, village roads and houses distributed in villages and towns, and other land uses can sever connections between ecological source areas. Thus, optimizing ecological improvement areas is pivotal for enhancing the overall connectivity of ecological networks.

4.2. Conducting Categorized Responses to Boost Ecological Functions

(1) Construction of animal-friendly linear transportation infrastructure

The fragmentation of ecological sources is chiefly due to linear transportation infrastructure. Moreover, the development of linear transportation systems can directly or indirectly affect the environment, facilitating safer and more efficient movement of people and resources. However, it can also lead to habitat fragmentation or loss, interfere with animal activities, and act as barriers or “filters” in animal migration and biological circulation. For instance, the Gaoming District is a predominantly mountainous woodland area rich in ecological resources. In this case, the growing population’s demand for infrastructure development, including roads and railroads, has negatively impacted ecological connectivity by fragmenting ecological sources, degrading habitats, and obstructing connectivity. Concomitantly, denser human gatherings and activities are taking place, leading to greater effects on ecological connectivity. As a result, the ecological corridors identified using Linkage Mapper analysis are described as multiple and short. To address this, constructing dedicated underpasses for animal migration can help ensure biological circulation between ecological sources. Equally important, regions where new development projects, such as linear infrastructure construction, are authorized should focus on promoting ecological connectivity. Developing less disruptive and more ecologically permeable linear transportation infrastructure for urban biodiversity, as well as preventing and limiting the fragmentation of source patches, are core components of preserving ecological connectivity. Road system planning should account for the biological connectivity of isolation zones on both sides of linear transportation routes and include animal connectivity pathways at points where ecological corridors are obstructed. Additionally, avoiding disruption of ecological sources and corridors helps maintain the integrity of biological connectivity. Diverse strategies can also be employed to mitigate the effects of highways and railroads on biodiversity and to achieve more effective avoidance, minimization, mitigation, and remediation measures, ultimately maximizing ecological connectivity.

(2) Utilizing the value of multiple effects of ecological corridors

If only some ecological sources were connected to develop a traditional ecological corridor, the ecological corridor would lack multi-effect value. Given the heterogeneous urban functions and landscape structure of urban–rural integration, it is essential to prioritize the dynamic ecological service functions of ecological corridors and leverage the ecological–social–economic benefits. For example, Sanshui District is located on the north side of Foshan City, on the periphery of the central city. In this case, the land use types mostly include industrial land, dike ponds, watersheds, and village land. As urban de-industrialization progresses and suburban industrialization grows, the transformation of suburban agricultural areas into industrial zones has exacerbated urban sprawl. Simultaneously, it has produced a series of problems like soil and water environmental pollution, hydro-ecological degradation, and declining ecological functions. As a result, the forthcoming strategies can be utilized to amplify the ecological functions. First, the gradual restoration of ecological barrier points should be implemented, and an ecological corridor network should be established. Next, the agricultural landscape should be preserved, and the minimum requirements for the three living spaces should be strictly adhered to. Sustainable progress in the agricultural sector should be fostered by leveraging the multi-dimensional characteristics of the dike pond’s land and water interactions and biodiversity. Following this, material land and water cycle and the double cycle within the watershed should be implemented, and the environmental impact should be reduced.

Subsequently, organic production methods should be adopted, based on which the productive, living, and ecological functions of agriculture should be tapped. By increasing economic gains and implementing actions like restoring water ecosystems and optimizing rural water landscapes, a mutually beneficial outcome for the economy and the environment can be obtained. Finally, an agricultural mosaic system should be established in ecological corridors, with a unified agroecological network for coordinated governance and restoration. Depending on their urbanization process and urban–rural pattern, regions can customize the number and width of ecological corridors they create. Simultaneously, a scientifically and reasonably designed agricultural spatial layout should be integrated with the construction of ecological corridors. In this way, we can obtain the perpetuation of land productivity and conserve arable land resources, all while safeguarding the basic survival rights of other organisms and promoting the coexistence of humans and organisms within these areas.

(3) Promote the multifunctional development of waterfront space

Recent economic advancements have resulted in a growth in the overall economic scale yet have also given rise to a range of issues. The water environment in the area has suffered damage to varying extents due to haphazard land development. Shunde District, as the sub-center of Foshan City, belongs to the typical high-density water network cityscape. Its ecological source is largely dominated by watersheds and dike ponds, and the ecological corridors are also dominated by river systems. High-intensity land use development has resulted in construction land and roads encroaching on both sides of the river system, diminishing ecological connectivity. Based on the above analysis of ecological “pinch points” and barrier points, the ecological “pinch points” in Shunde District are mostly distributed in the river watercourses or the intersections of the water network. Simultaneously, the ecological barrier points are chiefly caused by urban roads and railroads. River waterway management should be differentiated from the outdated and unsustainable single-element approach. Instead, it should involve a comprehensive analysis and management of the entire river system, including its structure, function, and ecology. Overall, this study suggests five measures:

(1)

Strict boundary control is necessary for the development of the construction land along both sides of the river channel, which is supported by four reasons. First, this approach ensures the continuation of the green space system on both sides of the river channel. Second, it executes green space restoration and stitching for the missing parts and then restores its ecological structure. Next, this method further consolidates the natural foundation of water areas, woodlands, and other natural resources. Finally, it opens up green public spaces to restore the original ecological continuation function.

(2)

The main approach to restoring the wetland beach area in water network cities involves dealing with agricultural surface source pollution, restoring the wetland base, and utilizing other technical means to maintain the corridor connectivity with other ecological source areas.

(3)

Waterfront space is often a high-vitality area of the city, and to address water pollution issues, a sewage interception system has been implemented along the river. It is advised to incorporate pipeline interception measures for sewage collection and the retrieval and purification of metal and chemical pollutants produced by industries that substantially degrade the river’s water quality. Additionally, the water ecosystem of the dike pond needs to be maximized to purify the mildly polluted sewage.

(4)

Migratory corridors for nocturnal animals need to be established in response to light pollution. High-rise buildings should be placed along the river at a specified distance to minimize the harm of light pollution to the natural space.

(5)

Animal-friendly infrastructure construction needs to be actively carried out in ecologically critical areas. Additionally, animal passages, such as ecological overpasses and culverts under bridges, should be constructed to support ecological movement.

In the context of ecological civilization construction, ecological restoration and urban rehabilitation are major issues faced by today’s government. Past urban development has inevitably caused destruction, so utilizing natural resources as potential for development is crucial. This paper provides certain conclusions on this issue. Specifically, zoning controls should be implemented in water network areas to enhance ecological connectivity. The research results provide theoretical guidance for the sustainable development and ecological design of water network cities such as Foshan City and can help establish the basic ecological spatial pattern.

5. Conclusions

In this study, Foshan City, a city with a high-density water network, was selected as the research area. We put forward an ecological network construction framework tailored to the features of high-density water network cities. In the framework, the ecological source was identified based on geographic elements, considering human and natural factors. Meanwhile, the ecological network was established by merging the development of a high-density water network city with the incorporation of resistance surfaces based on building floor heights and water network densities, along with geographic factors. It introduces a new research angle for the construction of ecological networks and ecological restoration in urban areas. The main conclusions are as follows:

(1)

Foshan City’s ecological sources are primarily situated in the western and northern areas, with a concentration of ecological corridors in the western part of the study area. It features a high degree of landscape connectivity, which is conducive to the protection of biodiversity.

(2)

According to the circuit theory, a total of 53 ecological corridors were identified, spanning 239.27 km in length. The distribution of these corridors was found to be uneven, predominantly in the northern and southwestern areas.

(3)

A total of 103 ecological “pinch points” and 193 ecological barrier points were identified using the circuit theory as ecological critical areas and ecologically improvement areas, respectively.

After identifying the ecological network in the high-density water network area of Foshan City, we recommended measures for protecting, restoring, and optimizing it in line with the city’s future development. This includes clarifying the ecological source areas that must be protected without compromise, the ecological improvement areas that are top priorities for solutions, and the ecological critical areas that are targeted for upgrading and restoration of their connectivity functions. Construction sites should steer clear of ecological source areas and ecological corridors to provide favorable conditions for species’ habitats and biological circulation. In ecological restoration, efforts should be intensified to boost ecological protection and network restoration to limit the adverse effects of large-scale production, construction, and other human activities. In contrast to prior research, this study specifically examined high-density water network areas, incorporating water network density and building height as resistance factors while underscoring the role of human activities in exacerbating ecosystem disturbances. It also determined the width of ecological corridors through cumulative current strength to provide a reference for constructing urban ecological networks. Nevertheless, this paper has two primary limitations. First, the source area identification process did not consider the impact of ecosystem services on the scope of the source area. Second, target species were not selected as indicator species for building height while determining resistance values. This may have affected the delineation of resistance values.