Ecological Security Assessment Based on the “Importance–Sensitivity–Connectivity” Index and Pattern Construction: A Case Study of Xiliu Ditch in the Yellow River Basin, China

Abstract

Resource, environmental, and ecological issues have become major constraints to the development of many regions. The Yellow River Basin is an important barrier for maintaining ecological security in northern China, but it has been impacted by problems such as severe soil erosion and declining biodiversity. The rational construction of ecological security patterns is important to enhance ecosystem functions and maintain regional ecological security. In this study, a comprehensive ecological security assessment system was constructed by selecting ecosystem service importance, ecological sensitivity, and landscape connectivity to assess the ecological security of Xiliu Ditch, an ecologically fragile region of the Inner Mongolia section of the Yellow River Basin in China. The assessment results showed significant spatial heterogeneity, with medium- and low-security value areas dominating, while high-security value areas accounted for only 18.7% of the study area. Seventeen ecological sources were identified from the high-security areas, which were mainly composed of grassland, woodland, and water bodies, most of which are distributed in the southern part of the study area. Twenty ecological corridors were selected by the minimum cumulative resistance model and gravity model and classified into 15 construction corridors and 5 potential corridors. Forty-six ecological nodes were defined, including twenty strategic points, nine potential strategic points, and seventeen break points. On this basis, we constructed an ecological security pattern of “two belts, three cores, six zones, multiple corridors and multiple nodes” and proposed corresponding ecological governance measures. This study explores the ecological security pattern at the small watershed scale, which helps to realize the fine management of the Xiliu Ditch basin and, on this basis, can provide scientific support for the ecological protection and sustainable development of the Yellow River basin. In addition, the ecological security assessment system proposed in this study can provide new ideas for the construction of ecological security patterns in similar ecologically fragile areas around the globe.

1. Introduction

The loss of natural ecological space has led to changes in ecosystem structures and functions and has created a series of environmental problems, such as biodiversity declines, soil erosion, and forest area reduction [1,2,3]. Therefore, guaranteeing the stable operation of ecosystems to promote regional sustainable development has become an imperative task for global ecological governance [4]. Ecological security usually refers to the ability of an ecosystem to provide ecosystem services for plant and animal survival, human activities, and socioeconomic development [5]. Building a reasonable ecological security pattern plays an important role in maintaining ecosystem integrity and promoting regional high-quality development [6].

Since its introduction in the 1990s, the theory of ecological security patterns has gradually become a research hot spot in global ecology, environmental science, and urban planning [7]. Research methods have evolved from initial qualitative planning to a combination of multiple tools, such as spatial data computation [8], static pattern optimization [9,10,11], and dynamic trend simulation [12,13]. The research framework of “ecological source identification–resistance surface creation–ecological corridor extraction” has been widely used by global scholars in the construction of ecological security patterns [14,15]. First, ecological sources refer to ecological patches that are important to ecosystem integrity and are the basis of ecological security patterns [16]. The identification methods are mainly divided into two categories. One category selects patches with better habitat quality directly, such as parks, nature reserves, waters, and woodlands, as ecological sources, using methods such as morphological spatial pattern analysis (MSPA) [17,18,19], landscape connectivity analysis [20,21], and the integrated valuation of ecosystem services and trade-offs (InVEST) model [22,23,24]. Another category considers the spatial characteristics of the study area for multi-indicator ecological assessments, such as ecosystem service importance assessments [11,25], ecological sensitivity assessment [26,27], and environmental suitability assessments [28,29]. Such methods pay more attention to the functional attributes of ecosystems, as this approach is conducive to accurately extracting areas with potential environmental problems and providing an effective scientific basis for the delineation of ecological functional areas [30]. Second, the flow of ecological elements between sources must overcome resistance, and most studies integrate natural and anthropogenic disturbance factors to construct resistance surfaces that meet the current situation of the study area [31]. Third, the minimum cumulative resistance (MCR) model [32,33,34] and the least-cost path (LCP) model [35,36,37] are common methods used for extracting ecological corridors. By determining the path of least resistance between patches as the corridor, the MCR model can well simulate the obstruction effect of landscapes on the spatial movement process [2,3]. The LCP model reflects the cost consumed by an organism when moving from a source to a destination and is widely used to simulate and plan biological movement routes [38,39]. However, these models cannot compare the importance of different corridors. Therefore, current studies often quantitatively analyze the priority of ecological corridors by combining circuit theory [19,40], gravity model [41,42], and other methods, and these combinations can be used to screen out important corridors that need priority ecological restoration [43].

Although there have been many studies on the construction of ecological security patterns, research directions have focused mostly on prefecture-level cities or urban agglomerations with rapid urbanization. They have focused less on areas with fragile ecological environments and have not yet formed a standardized model for the comprehensive assessment of ecological security [44]. Related studies commonly use mathematical models such as integrated index method [25], hierarchical analysis, fuzzy comprehensive evaluation, pressure–state–response (PSR) model [45,46], driving force–pressure–state–impact–response (DPSIR) model [47], Technique for Order Preference by Similarity to an Ideal Solution (TOPSIS) model [48], and ecological footprint model [49] for ecological security evaluation, which can better reflect the coupled benefits of socioeconomic development and ecological protection [50]. Among them, the integrated index method can identify the driving factors of ecological security changes relatively scientifically [51]. Most studies have constructed ecological security assessment systems from a single perspective of ecosystem service functional importance or ecological sensitivity assessment [52,53,54]. Several scholars have studied carbon sequestration capacity, soil and water conservation services, and recreational services (RCS) [44,54,55], and some scholars have applied them to alpine areas [56], watersheds [30], arid and semiarid zones [57], etc. All these studies have promoted the development of theories and methods of ecological security assessment. However, due to scholars’ different understandings of ecological security assessments, the selected indicators are inconsistent [58], which may make it difficult for the identified key restoration areas to meet the development requirements under the multiobjective orientation of ecology and economy [59]. Therefore, it is important to select more comprehensive indicators to construct a holistic ecological security assessment system to improve the carrying capacity of the regional ecological environment [57,60], which is also the current research development trend.

Optimizing the national spatial development pattern and enhancing regional security are important tasks in the construction of China’s ecological civilization [61,62]. The Yellow River Basin, as the most severe area of soil erosion in China, is prone to natural disasters and has long been the research focus of sustainable ecological development in China [7]. The Chinese government has promulgated the “Yellow River Basin Ecological Environmental Protection Plan”, “Outline of the Yellow River Basin Ecological Protection and High-Quality Development Plan”, and other documents to guide the ecological protection of the Yellow River Basin, clearly choosing to focus on the ecological protection of small basins and to fully form the ecological security pattern of the Yellow River Basin by the middle of this century. Zhang et al. constructed an ecological security pattern for the Yellow River Basin, and the results show that ecological sources and corridors are distributed mainly in the eastern part of the Yellow River Basin; however, there are almost no large sources distributed near the upper Inner Mongolia section, in which soil erosion is severe [58]. The sediment carried within the river mainly comes from the Ten Kongdui Basin; thus, flood control and sand management are the top priorities in the ecological restoration strategy of the area. The Ten Kongdui Basin is composed of ten similar small basins. This paper chose the Xiliu Ditch as the research object to study ecological security. This river gully is a primary tributary of the Yellow River, and it is a river gully with serious ecological problems affected by natural factors and human activities. The local government has carried out a feasibility study on the comprehensive treatment project, and the current governance degree of the whole Ten Kongdui Basin is only 32.79%. Since the 1980s, local projects such as “pilot small watersheds in the middle reaches of the Yellow River”, “backbone dam projects”, and “sea buckthorn planting demonstration area” have been carried out. Five small watersheds have been comprehensively managed, and four Yellow River inlets have been upgraded. However, due to insufficient technological support, the transformation rate of new technologies and achievements is not high, and the ecological environment of the region has been in a state of local governance and overall deterioration, so the technology of regional systematic governance needs innovation. Most of the current research areas on the upper reaches of the Yellow River focus on the spatial scale of provinces, cities, and counties, and the main stream of the Yellow River is often the focus of research [3]; in contrast, relatively few studies have been conducted on ecological security patterns at the small basin scale. Exploring the construction of ecological security patterns at the small basin scale is conducive to improving the fine management of the regional ecological environment and filling the research gap in relation to the construction of ecological security patterns in this region.

In this context, this study first selected three indicators of ecosystem service importance, ecological sensitivity, and landscape connectivity to construct an ecological security assessment system; identified patches with high ecological security value as ecological sources; combined them with the MCR model, gravity model, and hydrological analysis to screen important corridors and nodes; and then superimposed the extracted ecological sources, corridors, and nodes to form the ecological network of the Xiliu Ditch Basin. Based on this, we divided the functional zones, determined the ecological planning structure, and finally constructed the ecological security pattern of Xiliu Ditch. We proposed ecological restoration strategies corresponding to the distribution of ecological security patterns and fine management practices of small watersheds to improve the ecological environment of Xiliu Ditch to promote the protection, governance, and high-quality development of the Yellow River basin with respect to area. In addition, the study aimed to establish a comprehensive ecological security assessment system based on the “Importance–Sensitivity–Connectivity” index, which can provide new ideas for identifying ecological sources for other ecologically fragile areas around the globe.

2. Study Area and Materials

The Xiliu Ditch Basin (109°31′ E~109°53′ E, 39°51′ N~40°30′ N), with an area of 1521 km², is located in Ordos city, Inner Mongolia Autonomous Region, China, and it is a primary tributary of the upper section of the Yellow River (Figure 1). The Xiliu Ditch Basin belongs to a temperate continental climate. It is arid and rainless throughout the year. Rainfall is concentrated in summer, with an annual rainfall of 200–400 mm. The flood and dry periods are obvious; flash floods, droughts, and other natural disasters are frequent; and the ecological environment is fragile. As the representative river with the largest watershed area in the famous Ten Kongdui area, the Xiliu Ditch has a complex geomorphology. The upstream area is the Loess Grassland Area with narrow riverbeds, short ditches, and rapid flows, leading to serious water and soil erosion problems. Some enterprises rely on mine resources for mining, which results in serious industrial pollution and destruction of mining ecosystems. The Kabuki Desert runs east–west through the midstream of the Xiliu Ditch and contributes coarse sand to the Inner Mongolia section of the Yellow River. Its eastern land use type is woodland, while the western part is dominated by construction land and unused land. Water and sediment problems are of great concern. Downstream is the Yellow River Plain Area, which has 65% of the region’s population and 70% of its arable land. Agricultural and grazing activities are more frequent, and there are problems such as indiscriminate discharge of domestic sewage from farmers and herdsmen, serious agricultural surface pollution, degradation of grassland due to overgrazing, etc. Meanwhile, the siltation of sediment from the middle and upper reaches causes blockage of the Yellow River mouth. Due to the influences of both natural and human activities and the lack of ecological governance in the late period, the ecological problems In the Xiliu Ditch Basin have seriously threatened the sustainable development of the region and the ecological security of the Yellow River; thus, ecological restoration needs to be carried out in this area.

The data used in this study include land use data, climate and environmental data, and socioeconomic data. Among them, the land use data were generated based on the interpretation of the Landsat series satellite images. According to the classification scheme developed by the Chinese Academy of Sciences, land use was divided into 6 primary land categories, which include arable land, woodland, grassland, water, construction land, and unused land, and 18 secondary categories, with a comprehensive evaluation accuracy of 91%. With reference to the Guidelines for Assessment of Resource and Environment Carrying Capacity and Suitability of Territorial Spatial Development (Trial) issued by the Ministry of Natural Resources of China in January 2020, this study collected relevant climate and environmental data for ecological security assessment. The socioeconomic data affected the determination of ecological corridors and nodes in the later stage. All data were unified in the World Geodetic System 1984 Coordinate System with a spatial resolution of 30 m × 30 m (

Table 1. Detailed description of data.

Data	Subdata	Year	Spatial Resolution	Sources
Land use data	Land use	2022	30 m	https://earthexplorer.usgs.gov/
Climate and environmental data	DEM	2022	30 m	http://www.gscloud.cn/
	Average annual precipitation, average annual temperature	2021	1 km	http://www.worldclim.org/
	Normalized difference vegetation index (NDVI)	2021	250 m	https://lpdaac.usgs.gov/
	Net primary productivity (NPP)	2020	500 m	https://www.nasa.gov/
	Soil Information	-	1 km	http://westdc.westgis.ac.cn/
	Meteorological data	2021	Station	http://data.cma.cn/
Socioeconomic data	Roads, water systems	2022	Shapefile	https://www.openstreetmap.org/

).

3. Methodology

This study included four main steps, as shown in Figure 2. First, the ecological security of the Xiliu Ditch was evaluated according to the ecosystem services importance, ecological sensitivity, and landscape connectivity. Second, according to the evaluation results, the patches with high ecological security values were identified as ecological sources. Then, the MCR model was used to calculate the potential ecological corridors among all sources, and the gravity model was combined to screen and classify them. The ecological nodes were determined by intersecting the extracted important corridors with the hydrological analysis results. Finally, we superimposed the ecological sources, corridors, and nodes to construct the ecological security pattern of the Xiliu Ditch and proposed corresponding ecological restoration and planning strategies.

3.1. Identify Ecological Sources through Ecological Security Assessment

Ecological security assessment is a comprehensive assessment of the quality of various service functions provided by an ecosystem and its self-regulating ability to cope with external disturbances. It is often used to detect potential ecological problems, and its evaluation results can be an important prerequisite for identifying high-quality sources [57]. In this study, the ecological assessment system of the Xiliu Ditch was constructed based on the ecosystem service importance, ecological sensitivity, and landscape connectivity, and the corresponding weight was assigned to each index layer factor by the Delphi method (

Table 2. Ecological security assessment indicators.

Target Layer	Rule Layer	Weight	Index Layer	Weight
Ecological security value	Ecosystem service importance	0.49	Wind and sand control importance	0.16
			Water conservation importance	0.10
			Soil and water conservation importance	0.15
			Habitat quality importance	0.08
	Ecological sensitivity	0.31	Land desertification sensitivity	0.09
			Soil erosion sensitivity	0.17
			Soil rocky desertification sensitivity	0.05
	Landscape connectivity	0.20	Landscape connectivity	0.20

), relying on expert experience to make decisions quickly and effectively [63]. The spatial pattern of ecological security assessment was obtained by superimposing the assessment results of each factor by the “raster calculator” function in ArcGIS, and the areas were divided into high-security value areas, medium-security value areas, and low-security value areas. On this basis, patches with high ecological security values were considered ecological sources.

3.1.1. Ecosystem Service Importance Assessment

Ecosystem service function is an intermediate factor linking ecosystems and human well-being and can balance the contradiction between socioeconomic development and ecological protection [16]. As a primary tributary of the Yellow River, the Xiliu Ditch Basin is affected by flash floods, broken topography, and other factors, resulting in sediment accumulation and poor vegetation. Therefore, four types of assessment factors, including wind and sand control, water conservation, soil and water conservation, and habitat quality, were selected to assess the ecosystem service importance. Considering the availability of data and with reference to the document “Guidelines for the Delineation of Ecological Protection Red Line” (Trial) (Ecology [2017] 48 of the Environmental Affairs Office), three indicators of wind and sand control importance, water conservation importance, and soil and water conservation importance were calculated using the NPP quantitative assessment index method, while habitat quality importance was calculated using the model method. According to the weights of these four factors, spatial superposition analysis was implemented, and the results were divided into three levels: extremely important, very important, and general important.

(1)

Wind and Sand Control Importance

Wind and sand control importance is often used to assess the ability of ecosystems to prevent land desertification and protect agricultural land from wind–sand damage [64]. The Kabuki Desert runs east–west through the middle reaches of the Xiliu Ditch, so the water–sand problem in the study area is of great concern. The selection of this assessment indicator helps to identify the current and future priority areas that assume the function of wind and sand control. The calculation formula is as follows:

$$S_{ws}=NP{P}_{mean}\times K\times F_q\times D$$

(1)

$$F_q=\frac{1}{100}{\displaystyle \sum }_{l=1}^{12}{u}^3\left\{\frac{ET{P}_i-P_i}{ET{P}_i}\right\}\times d$$

(2)

$$ET{P}_i=0.19{\left(20+T_i\right)}^2\times \left(1-r_i\right)$$

(3)

$$u_2=u_1{\left(z_2/z_1\right)}^{1/7}$$

(4)

$$D=1/\cos \theta$$

(5)

where $S_{ws}$ is the wind and sand control service capability index; $NP{P}_{mean}$ is the net primary productivity; K is the soil erodibility factor; $F_q$ is the annual mean climatic erosivity; u is the monthly average wind speed at the height of 2 m; $u_1$ and $u_2$ represent the wind speed at heights of $z_1$ and $z_2$; $ET{P}_i$ is the monthly potential evaporation (mm); $P_i$ is the monthly precipitation (mm); d is the number of days in the month; $T_i$ is the monthly average temperature; $r_i$ is the monthly average humidity; D is the surface roughness factor; and θ is the slope (radian).

(2)

Water Conservation Importance

The process by which ecosystems intercept precipitation, suppress evaporation, regulate runoff, and purify water is called water conservation [65]. This study used the ecosystem water conservation service capacity index as an assessment indicator, calculated as follows:

$$WR=NP{P}_{mean}\times F_{sic}\times F_{pre}\times \left(1-F_{slo}\right)$$

(6)

where WR is the ecosystem water holding capacity index; $NP{P}_{mean}$ is the net primary productivity of vegetation; $F_{sic}$ is the soil infiltration factor; $F_{pre}$ is the average annual precipitation factor; and $F_{slo}$ is the slope factor.

(3)

Soil and Water Conservation Importance

Soil and water conservation aims to reduce soil erosion through the interception and absorption of rainfall by vegetation and its roots, and its importance is related to slope, vegetation coverage, and land use type factors [66]. The calculation formula is as follows:

$$SC=NP{P}_{mean}\times \left(1-K\right)\times \left(1-F_{slo}\right)$$

(7)

where SC is the ecosystem soil and water conservation service capacity index.

(4)

Habitat Quality Importance

Habitat quality reflects the ability of an ecosystem to provide suitable living conditions for individual species or populations and is often assessed by the Habitat Quality module of the InVEST model [67]. Considering the impact of human activity intensity on the habitat, we took the interpreted secondary land use classification of the study area as the original data and selected arable land, rural settlements, other construction land, and sandy land easily affected by human activities from it as the habitat threat sources. The specific parameters are shown in

Table 3. Sensitivity of land use types to habitat threat sources.

Land Use Category	Land Use Type	Habitat Suitability	Arable Land	Rural Settlement	Other Construction Land	Sandy Land
12	Arable land	0.3	0.3	0.6	0.4	0.6
21	Woodland	1	0.7	0.8	0.65	0.65
22	Shrub woodland	1	0.6	0.65	0.6	0.7
23	Open woodland	0.8	0.6	0.6	0.5	0.7
24	Other woodland	1	0.8	0.85	0.7	0.65
31	High-cover grassland	0.8	0.5	0.55	0.35	0.8
32	Mid-cover grassland	0.7	0.55	0.6	0.4	0.8
33	Low-cover grassland	0.6	0.5	0.5	0.3	0.6
4	Water body	0.9	0.65	0.65	0.6	0.65
52	Rural settlement	0	0	0	0	0
53	Other construction land	0	0	0	0	0
61	Sandy land	0.1	0.1	0.3	0.5	0.1
63	Saline land	0.1	0.1	0.3	0.4	0.6
64	Marshland	0.5	0.4	0.4	0.3	0.3
65	Bare land	0	0	0	0	0

. The higher the intensity of human activities is, the lower the habitat quality and biodiversity [30]. The calculation equation is as follows:

$$Q_{xj}=H_j\left[1-\left(\frac{D_{xj}^z}{D_{xj}^z+k^z}\right)\right]$$

(8)

where $Q_{xj}$ is the habitat quality of raster x in land use type j; $H_j$ is the habitat suitability of land use type j; $D_{xj}^z$ is the total habitat threat level of raster x in land use type j; k is the half-saturation constant, which is often regarded as 0.5; and z is the normalization constant (z = 2.5).

3.1.2. Ecological Sensitivity Assessment

Ecosystem sensitivity quantitatively reflects the self-regulation ability of ecosystems to cope with human disturbance and natural environmental change [68]. A decrease in ecosystem sensitivity is not conducive to the stability of ecological security patterns. Therefore, in this study, taking into account the actual situation of Xiliu Ditch, and referring to the “Guideline for the Assessment of the Carrying Capacity of Resources and Environment and the Suitability of Territorial Space Development” (trial) issued by the Ministry of Natural Resources in January 2020, three indicators of land desertification sensitivity, soil erosion sensitivity, and soil rocky desertification sensitivity were selected to evaluate the ecological sensitivity (

Table 4. Ecological sensitivity assessment indicators.

Indicator	Definition	Formula	Specific Parameters and Description
Land desertification sensitivity	Land desertification sensitivity refers to the gradual decline in soil productivity caused by human activities [64].	$D_i=\sqrt[4]{I_i\times W_i\times N_i\times C_i}$ (9)	$D_i$$\mathrm{is} \mathrm{the} \mathrm{land} \mathrm{desertification} \mathrm{sensitivity} \mathrm{index}, I_i$$\mathrm{is} \mathrm{the} \mathrm{dryness} \mathrm{index}, W_i$$\mathrm{is} \mathrm{the} \mathrm{number} \mathrm{of} \mathrm{days} \mathrm{with} \mathrm{sand} \mathrm{and} \mathrm{wind}, N_i$$\mathrm{is} \mathrm{the} \mathrm{soil} \mathrm{texture} \mathrm{type}, \mathrm{and} C_i$ is the vegetation coverage.
Soil erosion sensitivity	Soil erosion is mainly influenced by topography, precipitation, soil properties, and vegetation, resulting in simultaneous losses of water and soil [65].	$S{S}_i=\sqrt[4]{P{R}_i\times K_i\times L{S}_i\times C_i}$ (10)	$S{S}_i$$\mathrm{is} \mathrm{the} \mathrm{soil} \mathrm{erosion} \mathrm{sensitivity} \mathrm{index}, P{R}_i$$\mathrm{is} \mathrm{the} \mathrm{rainfall} \mathrm{erosion} \mathrm{force}, K_i$$\mathrm{is} \mathrm{the} \mathrm{soil} \mathrm{erodibility} \mathrm{factor}, L{S}_i$$\mathrm{is} \mathrm{the} \mathrm{topographic} \mathrm{relief} \mathrm{factor}, \mathrm{and} C_i$ is the vegetation cover factor.
Soil rocky desertification sensitivity	Soil rocky desertification refers to the loss of surface soil and land agricultural value due to soil erosion and other problems [65].	$S_i=\sqrt[3]{D_i\times P_i\times C_i}$ (11)	$S_i$$\mathrm{is} \mathrm{the} \mathrm{regional} \mathrm{rock} \mathrm{desertification} \mathrm{sensitivity} \mathrm{index}, D_i$$\mathrm{is} \mathrm{the} \mathrm{land} \mathrm{use} \mathrm{type} \mathrm{classification}, P_i$$\mathrm{is} \mathrm{the} \mathrm{slope} \mathrm{of} \mathrm{the} \mathrm{terrain}, \mathrm{and} C_i$ is the vegetation cover factor.

), and the assessment results were reclassified into three levels by the natural break point method, and finally, the study area was classified into extremely sensitive areas, very sensitive areas, and general sensitive areas by the weighted superposition of the “raster calculator”.

3.1.3. Landscape Connectivity Assessment Based on MSPA

MSPA relies on the principle of morphology to identify important ecological patches in land use types in the study area through the processes of measurement, recognition, and segmentation [69]. Landscape connectivity can quantify the degree of circulation of ecological elements among patches, and stronger landscape connectivity is more conducive to maintaining the stability of ecosystems [70]. The key indicators commonly used to assess connectivity are the integral index of connectivity (IIC), probability of connectivity (PC), and patch importance index (dPC) [63]. The calculation formulas are as follows:

$$IIC=\frac{-b{\displaystyle {\sum }_{i=1}^n}{\displaystyle {\sum }_{j=1}^n}\frac{a_i{a}_j}{1+n{l}_{ij}}}{A_l^2}$$

(12)

$$PC=\frac{{\displaystyle {\sum }_{i=1}^n}{\displaystyle {\sum }_{j=1}^n}{a}_i{a}_j{P}_{ij}^{*}}{A_l^2}$$

(13)

$$dPC=\frac{PC-P{C}_{remove}}{PC}$$

(14)

where n denotes the total number of patches in the region; $a_i$ is the area of patch i; $a_j$ is the area of patch j; $P_{ij}^{*}$ is the maximum of the product of all path probabilities between patch i and patch j; A is the total area of the study area landscape; and $P{C}_{remove}$ denotes the possible connectivity index of the landscape after removing an ecological element.

In this study, first, woodland, grassland, and water bodies were selected as foreground data, and the remaining land use types were employed as background data. The foreground connection was set to 4. An edge width of 60 m was selected in the analysis process. The conversion process was set to “open”, and the text option was set to “open”. MSPA was performed using the Guidos tool to obtain a total of seven landscape types, core, bridge, edge, loop, peroration, branch, and islet, and the core was extracted for later landscape connectivity assessment. Then, the interpatch connectivity distance was set to 2 km, and the connectivity probability was set to 0.5 based on Conefor 2.6 to calculate the connectivity level among patches. Finally, dPC values of 0.8 and 0.2 were selected as the critical thresholds to classify the three levels of high, medium, and low connectivity.

3.2. Screening Ecological Corridors

3.2.1. Constructing Resistance Surface

The resistance surface is a collection of the resistance that must be overcome during the migration of ecological elements between different sources [32]. With reference to relevant studies [3,28], the four factors of land use type, elevation, slope, and distance from rivers were selected to construct a comprehensive resistance index system (

Table 5. Resistance indicator system.

Resistance Indicator	Resistance Value					Weight
	1	3	5	7	9
Land use type	Woodland, Grassland	Water body	Arable land	Unused land	Construction land	0.62
Elevation (m)	<1100	1100–1200	1200–1300	1300–1400	>1400	0.19
Slope (°)	<2	2–4	4–6	6–10	>10	0.12
Distance from rivers (m)	<1000	1000–200	2000–3000	3000–5000	>5000	0.07

). The land use type determines the suitability of the area for species survival. The higher the elevation and slope values, the more difficult it is for species to migrate. As a primary tributary of the Yellow River, the Xiliu Ditch provides ecological functions, such as purifying harmful substances and improving the environment. The closer the distance to the river, the better it is for the expansion of the ecological source.

3.2.2. Calculating Potential Corridors

The MCR model can calculate the minimum cost distance of each landscape element from the original source to the target source to construct the potential ecological corridor [3]. The calculation is as follows:

$$MCR={\displaystyle \int }min{\displaystyle \sum }_{j=n}^{i=m}\left(D_{ij}\times R_i\right)$$

(15)

where $D_{ij}$ denotes the spatial distance from ecological source i to j; $R_i$ is the resistance coefficient of ecological source i to species movement; and m and n denote the number of ecological source i and ecological source j, respectively.

3.2.3. Extracting Important Corridors

Screening out important corridors can provide a reference for determining the priority of ecological restoration measures [4]. In this study, the interaction matrix between each ecological corridor was generated using the gravity model, and the corridors with an interaction strength >1 were selected as important ecological corridors, which will be prioritized for future ecological construction. Corridors with intensities in the interval of [0.5,1] were classified as potentially important corridors, and the rest were classified as general corridors. The equation of the gravity model is as follows:

$$G_{ab}=\frac{N_a-N_b}{D_{ab}^2}=\frac{L_{max}^{2}ln\left(S_a\right)ln\left(S_b\right)}{L_{ab}^2{P}_a{P}_b}$$

(16)

where G denotes the magnitude of the gravitational force; N is the weight value; D is the standardized value of the potential corridor resistance between patches; P represents the patch resistance value; S is the patch area; L represents the cumulative resistance value; and $L_{max}$ is the maximum value of the cumulative resistance of the regional corridors.

4. Results

4.1. Spatial Patterns of Ecological Safety Assessment

The spatial patterns of the four types of ecosystem service importance and the integrated results are shown in Figure 3, and they exhibited obvious spatial heterogeneity. The extremely important area of wind and sand control in the Xiliu Ditch Basin was 910.35 km², mainly distributed in the northern Yellow River Plain Area and the southern Loess Grassland Area, while the central Kabuki Desert had poor wind and sand control ability. The southern part of the study area and the eastern side of the Kabuki Desert were identified as extremely important areas for water conservation; this region had an area of 532 km², accounting for 35% of the study area. Soil and water conservation services showed the spatial distribution characteristics of being “high in the north and south and low in the middle”, mostly for general important areas and very important areas, and the extremely important areas were more fragmented. The habitat quality of the entire Xiliu Ditch watershed was poor, and the extremely important areas were distributed mainly near water bodies and woodlands on the eastern side of the Kabuki Desert, covering an area of 341 km², with relatively rich vegetation and strong water storage capacity. Overall, the extremely important areas of integrated ecosystem services covered 459 km², accounting for 30.2% of the study area; these areas were distributed mainly in the southern and part of the northern woodland. The general important areas were distributed mainly on the north and west sides of the central Kabuki Desert, with an area of 441 km². Specifically, the northern Yellow River Plain area had relatively poor ecosystem service capacity because of the large areas of arable land and unused land, which are vulnerable to human factors.

The spatial pattern of ecological sensitivity is shown in Figure 4. There were obvious differences in spatial distribution between different types of ecological sensitivity. The difference in the spatial distribution of the same type of ecological sensitivity in the north–south direction was greater than that in the east–west direction. The land desertification sensitivity was high in the north and low in the south. The extremely sensitive area is 179 km², accounting for 11.8% of the study area. The soil nature of these areas was sandy loam, and factors such as high wind erosion and agricultural production had a strong destructive effect, making it easy for desert landscapes to form. Soil erosion sensitivity is characterized by a high southern and low northern distribution. The extremely sensitive area of soil erosion had an area of 742 km², of which grassland had the largest area, accounting for 31.6%. It was mostly distributed around the ridge upstream of the Xiliu Ditch, as this area is often subject to natural erosion, such as wind and water forces, due to the undulating terrain; thus, this region requires focused attention. The overall rainfall in the study area is low, so the risk of soil rocky desertification is reduced. The southern lithology is mainly subpure carbonate rock, which is exposed to bedrock because it is in the mountainous area, showing a certain sensitivity to soil rocky desertification. From a comprehensive perspective, the final ecological sensitivity of the Xiliu Ditch Basin varied significantly in space, showing the characteristics of patchy distribution in very sensitive areas, general sensitive areas, and spotty distribution in extremely sensitive areas. The extremely sensitive area is 54 km², accounting for only 3.6% of the study area, and it is distributed in the northern plain. The land use types were mainly grassland, arable land, and unused land, accounting for 39.8%, 23.3%, and 16% of the extremely sensitive areas, respectively. These areas are sensitive to external disturbance and require priority protection. The general sensitive areas were located mainly in the central and southern parts of the city, with an area of 649 km²; these areas are more capable of coping with external environmental disturbances and can be moderately developed.

The landscape connectivity classification generated based on the MSPA model is shown in Figure 5. The core areas, which can be used as ecological sources, accounted for 62.14% of the total area and were concentrated in the southern part of the study area. The land use types were composed mainly of woodland, grassland, and water. Bridges, as structural corridors connecting core areas, had great impacts on landscape connectivity, accounting for approximately 0.05% of the total area. Landscape connectivity in the study area was dominated by high connectivity, with an area of 846 km² (55.6%), followed by medium connectivity (18.6%) and low connectivity (25.8%). The highly connected area within the core accounted for approximately 89.5%. The patches therein were more connected and provided a good basis for the selection of ecological sources at a later stage.

4.2. Distribution of Ecological Sources

The final distribution of ecological security in the Xiliu Ditch Basin was derived by overlaying the results of the assessment of ecosystem service importance, ecological sensitivity, and landscape connectivity (Figure 6a). The overall ecological security situation was poor, and medium- and low-security areas were widely distributed. The area of the high-security value areas was 285.5 km², accounting for 18.7% of the study area, and most of the area was distributed in the Loess Grassland Area. High-security value areas were selected as the resources of ecological sources. Smaller landscape patches provide limited ecological benefits and are not conducive to material and energy flows; therefore, patches with an area smaller than 1.2 km² were not selected. Finally, seventeen ecological sources were extracted (Figure 6b). The remaining areas were regarded as potential ecological sources, which can be developed into ecological sources in the future by expanding their ecological benefits through the construction of forest belts, water conservation, and other ecological measures. The results show that the ecological source area of the Xiliu Ditch Basin was 196.05 km², accounting for 12.9% of the total area. The main land use type of the sources was grassland, which accounted for 98% of the total area of ecological sources, and the remaining land was woodland, water body, and arable land. Compared with large areas of grassland, the study area had a high degree of fragmentation of woodland, water body, and arable land, so there were few parts that could be classified as ecological sources. The ecological environment of the constructed and unused land is so poor that these areas do not have the potential to become ecological sources. Most of the ecological sources were distributed in the Loess Grassland Area in the southern part of the study area with high topography. Although there are woodlands distributed in the central Kabuki Desert region, there are no eligible ecological sources distributed due to the poor abilities of wind and sand control and water conservation; land desertification is serious, and soil erosion can easily occur, which poses a greater threat to the ecological security of the downstream. The northern Yellow River Plain Area has relatively frequent human activities, so the ecological sources were relatively smaller and more fragmented in terms of their distribution. The largest ecological source was the Bashtu Gully Source, which had an area of 63.04 km², accounting for 32.2% of the total ecological source area. This was followed by the Azi Gully Source and the Laochang Bay Source, with areas of 39.1 km² and 33.57 km², respectively. These areas are the most important ecological sources in the Xiliu Ditch Basin, and they are distributed near water bodies, with good soil and water conservation ability, and should be prioritized for protection in the future.

4.3. Identification of Ecological Corridors and Nodes

The integrated resistance surface of the Xiliu Ditch Basin was obtained by the weighted superposition of four resistance factors (Figure 7a). The ecological resistance values in the study area ranged from 1 to 8.7, with a mean value of 3.4. From the perspective of spatial distribution, the west side of the central Kabuki Desert was higher, and its two types of land use, unused land and construction land, subject the area to more anthropogenic interference, which is not conducive to the migration and spread of species. Due to the large area of grassland in the south, the resistance value was relatively low, which is conducive to the operation of ecological flow. This is also the main distribution area of ecological sources. In the north, the resistance values varied significantly due to the high degree of landscape fragmentation. In general, the high resistance value areas were mostly distributed in the low-security value areas in the ecological security assessment. This scenario means there are strong limitations on energy transfer and species migration in the ecosystems in the study area.

Based on this, the MCR model was used to construct ecological corridors connecting the 17 ecological sources (Figure 7b). The gravity model was used to remove the duplicated and redundant corridors, resulting in 20 ecological corridors in the Xiliu Ditch Basin with a total length of 480.3 km, which showed a network distribution pattern and were uniformly distributed in the northern Yellow River Plain Area and the southern Loess Grassland Area. Furthermore, only two corridors along the river crossed the central Kabuki Desert. The interaction matrix was used to identify 15 construction corridors with a total length of 375.5 km, and 5 potential ecological corridors with a total length of 104.8 km were identified, which formed the basic framework of the ecological network in the study area. All potential ecological corridors were distributed in the southern part of the study area, and the interaction force of this type of corridor was relatively low; however, because these corridors connect ecological sources with large areas and high biodiversity, they have the potential to improve energy transfer efficiency in the future. In addition, 46 ecological nodes were identified, including 20 strategic points, 9 potential strategic points, and 17 break points; most were located within a 1 km radiation range of ecological sources. These ecological nodes are important components of the ecological network and serve as “stepping stones” for organisms to move through the corridor, providing them with energy for transient movements.

4.4. Construction of Ecological Security Pattern

Based on the identified ecological sources, corridors, and nodes, combined with the current land use situation, ecological security assessment distribution, and other attributes, the ecological security pattern of “two belts, three cores, six zones, multiple corridors and multiple nodes” was proposed (Figure 8), providing a reference for the ecological environment governance of the Xiliu Ditch Basin. The two belts included the Northern Wangmao–Bashtu Gully Ecological Construction Belt and the Laochang Bay–Bashtu Gully Ecological Replenishment Belt and had a relatively concentrated corridor distribution. The three cores included the Bashtu Gully Ecological Core, Laochang Bay Ecological Core, and Northern Wangmao Ecological Core, which radiate outward and connect multiple ecological sources. The six zones included the plain ecological barrier zone, arable land improvement zone, sand control zone, erosion ditch sand-fixing zone, mining pollution prevention zone, and highland soil and water conservation zone. The constructed ecological corridors and nodes ensure the continuity of the energy cycle and species migration and ensure the stability of regional ecological security.

5. Discussion

5.1. Rationality of Ecological Security Assessment

Building an ecological security pattern is considered an effective way to maintain the stability of regional ecosystems. Conducting ecological safety assessment is one of the key steps and can help decision makers accurately identify ecological patches that need priority protection and key treatment, thus providing guidance for subsequent ecological restoration measures, such as protected area delineation and corridor construction [2]. Relevant studies have been conducted to construct an overall framework for ecological security assessment from an integrated landscape perspective. Li et al. conducted a comprehensive assessment of ecological security in the Nujiang River region based on ecosystem service importance, ecological sensitivity, and landscape connectivity; the authors provided suggestions for ecological restoration in alpine valley areas [44]. Wu et al. identified ecological sources based on ecosystem services, landscape connectivity, and nature reserves to construct an ecological security pattern to promote ecological conservation in the Black River Basin [71]. These studies fully integrated the synergistic effects of ecosystem services to the extent that the ecological security patterns that were constructed were more scientific [30]. Xiliu Ditch is in the Yellow River basin, which is the world’s fifth longest river, and is the main birthplace of Chinese civilization. Ecological protection and restoration are the main goals of its development. The “Guideline for the Assessment of the Carrying capacity of resources and environment and the suitability of territorial Space Development” (trial) issued by the Ministry of Natural Resources in January 2020 clearly stipulates that ecological protection functions are evaluated through two indicators: ecosystem services importance and ecological sensitivity. The five factors of biodiversity maintenance, water conservation, soil conservation, wind and sand control, and coastal protection are included in the ecosystem service importance index. Soil erosion, soil rocky desertification, land desertification, and coastal erosion sensitivity are included in the ecological sensitivity index. Considering that the Xiliu Ditch Basin is located inland in China, the factors related to the coast were not included in the evaluation system. In addition, due to the availability of data, such as species distribution, we chose the habitat quality factor to replace the biodiversity maintenance factor by referring to Li et al.’s study [2]. Due to the fragmented surface morphology and undulating topography of Xiliu Ditch, the ecological corridor is prone to fracture. Therefore, we introduced landscape connectivity as an assessment index, combined with the MSPA model. Many studies have directly used the results of MSPA and landscape connectivity as the basis for screening ecological sources [4,21], focusing too much on intersource connectivity and neglecting other influential factors. In summary, we created a comprehensive ecological security assessment system by considering three factors: ecosystem service importance, ecological sensitivity, and landscape connectivity. Among them, ecosystem services mainly provide the basis for ecological compensation [59]; ecological sensitivity is often used to guide ecological environmental protection and scientific use of land [65]; and landscape connectivity focuses on protecting ecosystem functions [72]. Each of the three factors plays an important role in improving the ecological benefits of regional ecosystems, reducing development risks, and enhancing network connectivity. The ecological security assessment system we constructed follows the logical process of problem orientation to theory guidance to method selection, achieving interdisciplinary and multisource data integration. Therefore, the system can fully reflect the ecosystem security status of the study area and provide new ideas for the identification of ecological sources in other ecologically fragile areas around the globe.

5.2. Suggestions for Optimizing the Ecological Security Pattern

Optimizing the ecological security pattern can effectively improve the connectivity of the regional landscape and promote the sustainable development of the regional ecology. According to the spatial distribution of ecosystem service importance and ecological sensitivity evaluation, the distribution of extremely important areas of wind and sand control and water conservation is wider than that of soil conservation and habitat quality, and the area of extremely sensitive areas of land desertification and soil erosion is much larger than that of soil rocky desertification. The Kabuki Desert in the center has a great impact on ecological security, resulting in a particularly difficult task of wind and sand control and water conservation in both the upstream and downstream parts. Water is the core element of the regional ecosystem, and water conservation service is an important ecological guarantee to support the socioeconomic development of the Xiliu Ditch Basin and even the Yellow River Basin [73], especially downstream; due to the flat terrain, water bodies carry most of the production and living needs of local farmers and herdsmen. Therefore, the water and sand problem is the key factor affecting the ecological security of Xiliu Ditch. This study proposes the following restoration strategies for the constructed ecological security pattern of the Xiliu Ditch with a view to alleviating local water and sand problems such as soil erosion and sedimentation.

(1) Two Belts: The Northern Wangmao–Bashtu Gully Ecological Construction Belt connects the northern and southern parts of the study area, and the middle section crosses the Kabuki Desert along the water body of the Xiliu Ditch. Ecological construction should focus on the prevention and control of desertification in this section. The Laochang Bay–Bashtu Gully Ecological Replenishment Belt connects the two largest ecological sources in the study area from east to west. Rivers and mountains hinder the construction of corridors between these two sources, and it is necessary to promote the extension of green space to the east and maintain the ecological balance between the east and west by strengthening the vegetation around the ecological replenishment belt.

(2) Three Cores: In response to the trend of existing landscape fragmentation in the study area, it is necessary to strengthen the protection of existing sources, establish buffers to protect the core sources, and expand the radiation range of the ecological core.

(3) Six Zones: The arable land improvement zone is low and flat, and the area of distributed sources is small; these areas should be further reclaimed and restored to become green. This region should also strengthen the construction of forest networks around farmland and transform low- and medium-yield fields into circular farms, which encompass the local characteristics of ecological agriculture. The plain ecological barrier zone is distributed on the west side of the downstream of Xiliu Ditch, and the river is influenced by the middle and upper reaches and carries a large amount of sediment. Improving the forest planting structure in this area could reduce the sediment transport from the Xiliu Ditch to the main stream of the Yellow River, which would improve the ecological restoration of the beach at the mouth of the Yellow River. The Kabuki Desert is divided into the sand control zone; thus, biological locking edge forest belts should be constructed on the north and south sides of the desert in a pattern of “south encircling, north blocking and middle cutting” to prevent the desert from encroaching further south, expanding north, and moving east. In the erosion ditch sand-fixing zone, the main objective is to prevent soil erosion and control land desertification. The construction of fish scale pits and sediment retention dam systems would be effective. The mining pollution prevention zone should continue to implement mine geological environment remediation. Priority ecological treatment should be given to the sources within the highland soil and water conservation zone to regreen grasslands and enhance the water-capture capacity of the region.

(4) Multiple Corridors and Nodes: Through the construction of ecological corridors, an interconnected spatial network is formed in the study area. The corridors within the central Kabuki Desert should focus on protection by mechanically laying straw checkerboard barriers around the corridors to form buffers. The construction process of the rest of the corridors should strengthen desertification control and improve vegetation cover. Building forest strips or grasslands around ecological nodes on the corridor for soil and water conservation makes them a “pedal” to promote biological flow.

5.3. Limitations of the Study and Future Directions

There are some limitations of this study. First, this study used the MCR model and gravity model to identify and screen ecological corridors, and although it can better reflect the connectivity of the ecological network, it cannot determine the width of the corridor, which directly affects the ecological function. In the future, circuit theory or the ant colony algorithm will be used to simulate the direction and width of the ecological corridor to explore the optimal corridor width. Second, we did not consider the spatial and temporal variations in land use types when constructing the ecological security pattern. Regional land use variations can significantly alter ecosystem patterns, leading to changes in the supply of ecosystem services [74]. Related studies are gradually combining land use change simulation with ecological security pattern construction [75,76]. In the future, we will consider introducing the PLUS model to simulate land use under different development scenarios and adjust ecological networks predictably with this new idea of dynamic conservation. Another limitation that exists is that our study did not consider interregional impacts when constructing the ecological security pattern, and future studies need to consider ecological linkages and remote coupling of ecological security patterns across different regions.

6. Conclusions

This study constructs a comprehensive ecological security assessment system by combining ecosystem service importance, ecological sensitivity, and landscape connectivity. Based on this system, the ecological security of the Xiliu Ditch basin is assessed; the areas with high ecological security values are identified as ecological sources; ecological corridors and nodes are extracted using the MCR model and gravity model; and finally, the ecological security pattern of the Xiliu Ditch is constructed by superimposing them. The results show the following:

(1) The overall ecological security condition is poor, with medium- and low-security value areas dominating. High-security value areas cover 285.5 km², accounting for only 18.7% of the study area, and they are mainly in the southern part of the study area.

(2) Based on the ecological security assessment, seventeen ecological sources are identified. In total, 20 ecological corridors with a total length of 480.3 km are extracted and divided into 15 construction corridors with a total length of 375.5 km and 5 potential ecological corridors with a total length of 104.8 km. Forty-six ecological nodes are identified, including twenty strategic points, nine potential strategic points, and seventeen break points.

(3) The ecological security pattern of “two belts, three cores, six zones, multiple corridors and multiple nodes” is determined. It provides guidance for ecological restoration in this region and in the Inner Mongolia section of the Yellow River Basin. In addition, the ecological security assessment system constructed in this study provides new ideas for the study of ecological security in similar ecologically fragile areas around the globe.