1. Introduction
When it comes to ensuring sustainable regional development, ecological safety is the starting point and a necessary pre-condition. The loss of habitat, fragmentation, and other threats to biodiversity have resulted in a steady decline in both the number of animals and the quality of the regional ecological environment [
1,
2,
3]. On the other hand, ecosystems are fragile and sensitive, so the intensification of human activities will inevitably cause damage to the ecological security pattern (ESP) [
1,
4]. The rise of urbanization, in particular, has a negative impact on the ESP, increasing the level of unpredictability that regional eco-systems must deal with [
5]. For this reason, more and more countries are realizing how important the ESP is and incorporating it into their national security systems [
6,
7]. Researching how to ensure regional ESP is also a key part of regional sustainable development [
5,
8].
The effective identification of the ESP is an important method for the rational planning of ecological regions [
9]. First, the ESP typically entails building ecological sources, resistant surfaces, ecological corridors, and ecological pinch points [
10]. As a result, by employing the “point–line–plane” approach, the ESP can make it easier for creatures to move between habitats while maintaining the integrity of the ecosystem structure. This can protect biodiversity against the long-term effects of habitat fragmentation [
11]. Among these are ecological sources, which are the patches that play an important role in biodiversity conservation. In the past, in the process of source selection, scholars have generally used qualitative ecosystem types [
12], quantitative habitat importance [
13], ecological sensitivity [
14], and ecosystem services [
3] to unify the sources in the ESP. The preceding considerations are adequate for determining the commonality of sources but are insufficient for studying their characteristics. For example, the ecological corridors between recreation and nature reserve sources may not be suitable for either species migration or human recreation. Morphological spatial pattern analysis (MSPA), which focuses on structural connections [
15], makes it easier to choose ecological sources in a scientific way. It also helps to define how species move through the landscape in an objective manner [
16].
Currently, various research theories and methods have been developed for the construction of the ESP, including a graphical approach [
17], circuit theory [
18], and the Minimum Cumulative Model (MCR) [
19]; the latter of which is based on the “source–sink” theory, and has become a mainstream assessment method due to its flexible additivity [
5]. By computing resistance surfaces and subsequently extracting important ecological nodes and corridors, it has been widely utilized to pinpoint ecological sources and mimic ecosystem processes [
20]. However, although this method can characterize the orientation of ecological corridors, it usually ignores the stochastic wandering behavior of species and fails to specify the specific extent (corridor width) and key nodes (pinch points) of corridors. As a result, in this work, we use circuit theory to forecast the motion patterns in complex landscapes, addressing a major limitation of the MCR model previously used to identify key landscape patches [
21]. In summary, this paper combines the MSPA-Circuit theory to analyze regional ESP because the MSPA method places an emphasis on the system’s internal and structural connectivity, whereas circuit theory is employed to effectively make up for the MSPA method’s lack of functional connectivity and to locate crucial pathways in ecological systems [
22,
23]. By combining the two methods, conservation strategies for the ESP that are more scientifically sound can be developed.
It is noteworthy that most studies on the ESP have focused on the identification of ecological sources and the identification of research paradigms for ecological corridor construction but have typically neglected to effectively evaluate the ESP and provide subsequent optimization measures [
5,
8]. In the earlier days, the ESP construction was aimed at biodiversity conservation; however, with the development of an ecosystem service assessment and the recognition of the importance of ecological security [
3,
24], it was found that the ESP would then have an impact on local socio-economics [
25,
26], and the subsequent attention was gradually focused on the spatial structure of ecosystems [
27], ecological functions and processes [
28], coupled ecosystem services [
29], and other related components [
5]. Therefore, this study enhances the analysis process of evaluation and optimization for key elements that make up the ESP, such as patches, corridors, and nodes, in order to achieve the important goal of identifying effective management of the ESP at a regional scale.
At the same time, contemporary researchers have concentrated on the administrative boundaries set by prefecture-level cities [
30], provinces [
20], and urban agglomerations [
31], as these boundaries are clear and relevant data are easily accessible, which does add some convenience to the study. However, the spatial development of a biological species can hardly be limited by artificial administrative boundaries. Therefore, in this paper, we attempt to propose a set of ecological safety planning schemes. For the region to grow in a healthy and sustainable way, administrative borders need to be broken down and a set of cross-regional plans for ecological security need to be made. The ecological region around Taihu Lake in China was selected as a typical and representative case site. The Yangtze River’s ecological regulator, China’s Taihu Lake Ecological Region, includes many administrative regions of Jiangsu and Zhejiang, has a shoreline of about 400 km, and is rich in ecological resources. Therefore, we need to build the ESP across regions as soon as possible to protect ecosystems and keep them working.
We employed the MSPA–Circuit theory to investigate the ESP of the ecological region around the Taihu Lake Basin, considering the following three major objectives: (1) to creatively propose a framework for the “construct–evaluate–optimize” research method for the ESP; (2) to explore the ESP characteristics under ecologically dominant functions; and (3) to propose a planning scheme for ecological safety protection and restoration for ecological regions. These goals are intended to provide strong theoretical support for cross-regional ecological security management.
5. Results
5.1. Construction of the ESP
The results of MSPA-based ecological source identification in the ecological region around Taihu Lake are shown in
Table 4 and
Figure 4. The study area contained 27,186 regional core landscapes with a total area of 9145.86 km
2, accounting for 85.23% of the total area of the extracted prospects. The marginal area adjacent to the research area’s core landscape was 1071.49 km
2, which made up 9.99% of the entire foreground area. This indicates that the study area’s prospects had a great marginal effect. The connecting branch line and pore area accounted for 1.77% and 1.53% of the total potential area, respectively. The bridge region could not adequately facilitate the ecological flow of energy and matter, as it made up only 0.86% of the total foreground area. Furthermore, only 0.38% of the potential space was taken up by small islets, and only 0.24% of the total foreground area was found to support the spread of species through the ecological patches.
The core region comprised a sizable habitat patch, which could support a variety of creatures and contribute to biodiversity preservation. The first 20 core areas were chosen as viable ecological sources for the habitats of specific species, based on the size of the source area [
7]. Taihu Lake, GE Lake, Yangcheng Lake, Chang-dang Lake, Chenghu Lake, Dianshan Lake, Huangpu River, and Xiazhu Lake were some of the source locations. The biological source areas in the forest and grasslands were generally separated by the rugged and hilly area southwest of the Taihu Lake tourist region.
Based on the height, slope, landscape type, vegetation covering, and distance from a river, a naturally integrated resistance surface for biodiversity was found to exist in the Taihu Lake basin (
Figure 5A). Using information from night-time lighting, the drag surface of the Taihu Lake River basin was adjusted (
Figure 5B). Shanghai, Suzhou, Wuxi, and Changzhou in Jiangsu Province, as well as Hangzhou and Jiaxing in Zhejiang Province, were found to be high-value areas. Although the Taihu Lake region was the most significant ecological supply for the Taihu Lake River basin, it also exhibited a high degree of ecological resistance. For this reason, we should focus on the areas around Taihu Lake and build an ecological network for the protection of biodiversity.
Strips of biological land, known as corridors, should be made available to various species in order to connect ecosystems, avoid species isolation, maintain minimum numbers, and safeguard biodiversity. An ecological protection network is made up of ecological sources and corridors. In this line, 37 natural corridors with a combined length of 738.43 km were discovered (
Figure 6). The corridors connected the habitats where species may thrive and could support the movement of organisms between the habitats. The absolute distance between sources in the study region is shown by red lines, which depict the Euclidean distance of the links between sources. The geometric center of gravity of each source served as the connecting element.
5.2. Evaluation of the ESP
Based on the global connectivity index, we calculated the significance of the core ecological sources suitable as various species habitats for the top 20 ecological sources. These 20 eco-source locations were also given site names, as listed in
Table 5, making the results more useful and usable.
From the perspective of an ecological source of water, the lake body of Taihu Lake is the most important ecological source, providing an important channel for migratory birds. In the Suzhou area of Eastern Taihu Lake, the Yangcheng Lake Importance Score was 0.19, while the Chenghu index was 0.02. In contrast, the Huangpu River, which is primarily found in Shanghai, had a lower significance score (0.01), due to its limited width and great distance from other sources.
From the viewpoint of the ecological source of forest land, it was seen as an important living space for species, due to the large area of forest cover in the hills of western Zhejiang. The relevance index of the 02 forest source region, in comparison to the 01 Taihu Lake and mountains, was lower. According to the results, the relevance index of the source area and its area were not significantly positively correlated. For instance, Huzhou’s pictorial location No. 19 was just 30.83 km2 in size, yet its importance index reached 30.10. The above results indicate that we should pay more attention to how source regions are linked, and that the size of the ecological source is a factor affecting the survival of species.
In order to effectively guide the development and regulation of ecological networks in ecological regions, it is helpful to have a clear understanding of hot-spot locations and the directional aspects of ecological resistance impacting biodiversity. The entire Taihu Lake basin was partitioned into a 1 km × 1 km grid using ArcGIS, and this grid was utilized to create spatial statistics on the ecological resistance surface. The average grid resistance value is shown spatially, relative to the grid.
As shown in
Figure 7A, the spatial heterogeneity of habitat quality in the ecoregion around Taihu Lake was significant. Significant low-value areas included the lake body of Taihu Lake and the forested mountainous area in western Zhejiang Province. These areas are to the southwest of Taihu Lake. The ecological resistance hot-spot areas show a significant point–surface pattern distribution, where the centers of the clusters are mostly densely populated economic centers. The Wuxi-Suzhou-Shanghai line, on the other hand, displayed a continuous surface distribution, emphasizing the importance of cold- and hot-spots in space.
The standard deviation ellipse of the ecological resistance surface generally exhibited a northwest–southeast direction, as shown in
Figure 7B, based on the study results of the ecological resistance direction for the ecological region around Taihu Lake. The azimuth angle was 104.13°, the long axis of the standard deviation ellipse was 66,611.03 km, and the short axis was 82,764.23 km. The center of gravity was situated east of the body of water in Taihu Lake. According to the continuous plane direction of the ecological resistance hot-spot, the general direction was to the east and north of Taihu Lake. The lake body of Taihu Lake and the forest mountain ecological source in Western Zhejiang were noteworthy, when combined with the findings of the ecological source study. The low-value biological source areas, on the other hand, were mostly located to the east of Taihu Lake, affected by both natural and man-made factors.
We established a computation using the ecological activity paths and various distances between different ecological sources, in order to fairly estimate the degree of interconnectedness between the ecological sources due to the significant gravitational differences between them (
Table 6). Specifically, we covered the following five areas: (1) In the measuring space, the Euclidean distance represented the “normal” (i.e., straight line) separation between two ecological sources; (2) the cost-weighted distance can be thought of as an extension of the Euclidean distance, which assigns the ecological resistance cost factor to the distance passing through an ecological source; (3) the least expensive way between the two ecological sources, as measured by the unweighted length of the minimum cost path and the generated path, might be determined to be the best option in terms of the expense of constructing the ecological corridor; (4) the cost-weighted distance to Euclidean distance ratio (or Cwd/Euc) between ecological sources indicate that it was challenging for the straight-line distance between ecological sources to accurately reflect the actual value under diverse resistance situations; and (5) the ratio of the cost-weighted distance to the unweighted length of the minimum cost path (or Cwd/LCP) indicate how the channel connection between source locations affected how far the ecological path moved.
5.3. Optimization of the ESP
Ecological nodes are the springboards and turning points for species in the ecological region, which are generally located at the weakest point, in terms of corridor function. They are primarily divided into key ecological protection nodes and key ecological restoration nodes, with the former being critical to ensuring that the ecological corridor effectively plays ecological functions and can guarantee the smoothness of the ecological corridor, while the latter is the area that hinders the confluence of the ecological corridor. In particular, consideration of the latter area, which hinders the connection between ecological sources, can allow us to greatly improve the landscape connectivity and ecological stability of the ecological region around Taihu Lake. For this reason, we measured and extracted 36 key ecological protection nodes and 24 key ecological restoration nodes.
Furthermore, as most prior studies have only conducted quantitative analysis, we synthesized relevant planning policies issued by government units to improve the viability of ecological security strategies around the Taihu Lake basin, in light of emerging needs for national and regional strategic development. Basin plans were included, such as the Comprehensive Plan of the Taihu Lake Basin, the Comprehensive Plan of Flood Control for the Taihu Lake Basin, the Comprehensive Plan of Water Resources for the Taihu Lake Basin, and the Overall Plan for Comprehensive Management of the Water Environment around the Taihu Lake Basin. We further determined the spatial characteristics of the basin’s ecological network and joined all the basin’s parts to align the water resources to the national strategy and proposed a regulation strategy based on the ESP at the district and county scales for the Taihu Lake basin. Finally, as shown in
Figure 8, we proposed the following “four zones and one belt” ecological optimization pattern: an ecological protection and restoration area in the west of Taihu Lake, an ecological restoration area around Taihu Lake, a Yangcheng-Huangpu River ecological key control area, an important ecological conservation area in Western Zhejiang, and an ecological corridor construction region to the southeast of Taihu Lake.