1. Introduction
With the rapid development of urbanization, there are more and more human settlement construction activities such as urban land expansion, traffic and municipal corridor construction, agricultural reclamation and planting, animal husbandry, and industrial mining. Moreover, urban and rural ecological space is in a passive adaptation state, with erosion, movement, and succession occurring constantly. Furthermore, the shrinking of ecological patches and the rupture of corridors also result in the obstruction of ecological processes, the decline of ecosystem services, and the loss of biodiversity [
1,
2]. The concept of ecological infrastructure (EI) was formally proposed in the Man and Biosphere Program in 1984. Originating from a biodiversity conservation study, EI contains similar concepts, including the concepts of ecological network, green infrastructure network, and biodiversity conservation network [
3]. EI focuses on building spatial patterns, thereby guaranteeing ecological processes, including two key elements: ecological sources and ecological corridors. An ecological source is the source and sink of an ecological process and serves as the habitat for organisms, while an ecological corridor is the flow path of the ecological process and serves as the channel for organisms to spread and migrate among patches in different habitats [
4]. As indicated by the study, it was more conducive to the exertion of ecosystem service functions and responses to climate change to connect the discrete and isolated ecological sources functionally and maintain the integrity of the regional landscape compared to adopting an incremental approach focusing on isolated ecological patches and the expansion of natural habitat areas [
5]. EI networks can maintain biodiversity, ecological processes, and the sustainability of ecological functions, thus promoting ecosystems to have stronger anti-interference abilities and resiliencies [
6,
7]. Moreover, the ecosystem services provided by an EI network can optimize the landscape, enrich recreational activities, and enhance aesthetic value, which is of great significance for improving the quality of human settlements [
8].
In terms of EI network construction methods, scholars have conducted numerous studies on EI networks in many fields such as island ecology [
9], landscape ecology [
10,
11], land use and planning [
12], and ecosystem services [
13]. Currently, by following the principle of landscape ecology, the path of “ecological source identification-corridor extraction-network optimization” is a common practice used in constructing regional EI networks [
14,
15]. Broadly speaking, ecological sources include the habitats of animals and plants as well as patches that provide important ecosystem services and have high ecological benefits. Moreover, the theoretical basis for ecological source selection is the “source-sink” theory in which a source area is directly proportional to the diversity of habitats. Furthermore, forests, grasslands, wetlands, and water bodies meeting a certain area threshold can all be selected as screening criteria [
16] or can be identified by performing morphological spatial pattern analysis (MSPA) [
17] and ecological security pattern evaluation [
18]. Area identification of ecological sources do not consider the connectivity of ecological source areas, while the ecological security pattern assessment method requires a large amount of data and is more complex. MSPA has often been used because of its advantages of using small amounts of data and producing visualizations of analysis results. Because the number of identified ecological sources is often large, it has often been combined with area and connectivity analysis to screen ecological sources. Presently, there are two methods widely used for corridor extraction. One method is based on Forman’s “patch corridor” concept [
19], which uses the minimum resistance model to extract potential EI corridors [
20]. The other method involves introducing the connectivity model of circuit theory into ecological process simulation [
21] with the low-resistance corridor used as the extraction corridor. When there are many ecological sources, the number of corridor path results is also large, so it is necessary to classify the relative importance of these corridor paths to reduce construction cost and ensure implementation. Corridor classification is mostly based on the Green Infrastructure Assessment method, but the amount of data required is large and Weight values are subjective. In addition, quantitative analysis is often applied in network optimization, and a network structure index [
22] and landscape index [
23] are usually employed for optimization.
Since 1992 when the Convention on Biological Diversity was adopted by the United Nations Conference on Environment and Development, China, as a contracting party, began to devote itself to strengthening the construction of ecological nature reserve systems and repairing regional ecological environments degraded by rapid urbanization. According to the exploration of many Chinese scholars on ecological network construction methods in Fujian Province [
24], Taihu Lake Basin [
25], Nanjing City [
26], the Yangtze River Delta Integration Demonstration Zone [
27], and Dianchi Lake [
28], EI corridors possess a large span on the macroscale, playing a more flexible role in control and superior guidance. Moreover, county ecological space is the key level to undertake provincial ecological security patterns and implement ecological demonstration projects [
29]. In recent years, against the backdrop of giving priority to economic development, county space has often been faced with problems such as fragmentation of ecological space and the fracture of ecological corridors. How to build and screen important ecological corridors and propose feasible ecological network construction strategies in fragmented landscape patterns is crucial to county ecological restoration. Due to the redundancy of constructing ecological corridors by combining MSPA and MCR, implementation costs are too high and ecological corridors need to be screened. Considering the objectivity of screening, the availability of data, and the convenience of operation, this study introduces the gravity model to classify the importance of corridors on the basis of existing EI network planning methods and proposes to construct a quantitative method system for identifying ecological source areas based on “MSPA + connectivity evaluation”, extracting important ecological corridors based on “MCR + gravity model”, and optimizing corridors and identifying ecological strategic points based on “network structure evaluation”. Langzhong county in Sichuan province of China was taken as a case study to apply this method, and based on the spatial pattern of EI networks and the results of gravity model analysis, a progressive corridor construction scheme and a differentiated ecological zoning restoration strategy are proposed. We hope that this study can provide inspiration for the study of county ecological network construction methods and restoration strategies.
5. Conclusions
In view of ecological space optimization and control, there is a relatively clear pattern and framework in top-level design (including double evaluation of provincial land space and delineation of an ecological red line) and bottom-level implementation (involving land remediation measures and key ecological restoration projects). Nevertheless, identification of ecological restoration units, spatial layout of ecological facilities, and control methods in the middle level are still under discussion. By constructing the county EI network, better guidance can be provided to site selection, implementation, and construction of ecological restoration projects. To reduce subjective interference in ecological source selection and determine the optimal path and spatial position of ecological corridors, this paper considered Langzhong County, Sichuan Province as the study area, used MSPA and the MCR-Gravity Model in building the EI network of Langzhong County, and proposed strategies for ecological source protection, corridors, and stepping-stone construction as well as regional ecological pattern restoration in Langzhong County. This paper has the objectives of optimizing the ecological background of Langzhong County, protecting the biodiversity of the county, and enhancing the ecosystem services, which has direct significance to ecological security of the middle and upper reaches of the southwest river basin.
The study conclusions are as follows:
This study recommends plans to protect 50 ecological sources, mainly located in the northeast and southwest marginal areas of Langzhong County. The land-use types give priority to woodland supplemented by grassland and cultivated land. A total of 105 ecological corridors are proposed to be constructed, classified into three grades based on gravity values. The primary corridors are located in areas with dense ecological sources, the secondary corridors radiate outward around the primary corridors, and the third-level and fourth-level corridors mainly connect with long-distance patches to form an outer ring. Moreover, this study recommends plans to construct nine stepping-stone patches in long-distance corridors, nine stepping stones at the intersections of land ecological corridors, and six stepping stones at the intersections of land and water corridors (by planting wetlands and designing shoals and deep ponds for upgrading), thereby improving the success rate of species migration, protecting biodiversity, and enhancing the smoothness of material and energy flow. Additionally, this study recommends plans to repair 18 ecological breakpoints of EI corridors cut off by highways, railways, national highways, and provincial highways and restore connectivity by constructing underground passages, tunnels, and flyovers. Generally, it proposes protective countermeasures according to a regional ecological pattern with three vertical and two horizontal ecological belts, four ecological control districts, and six clusters of EI networks.
As for this study, there are still several aspects for further discussion as follows: (1) Different study scales have different influences on the results. To maintain small patches with good connectivity, this study adopts a study granularity of 30 × 30 m based on the related literature [
45], and future studies at different scales and granularities need to be conducted. (2) This study focuses on the spatial distribution of an EI network. Corridor width should be considered in future planning and design. A buffer zone with an appropriate width should be built according to corridor route selection. Additionally, the current ecological space can be fully utilized, thus improving construction efficiency. (3) The study on the analysis and evaluation of farmland quality and supply service capacity is still shallow. Hence, it is necessary to further explore the action mechanism and coping strategies of each ecological function of GI at the city level.