Construction of an Ecological Security Pattern for the National Park of Hainan Tropical Rainforest on the Basis of the Importance of the Function and Sensitivity of Its Ecosystem Services

Abstract

The National Park of Hainan Tropical Rainforest (NPHTR) plays the crucial role of an ecological barrier for Hainan Island. It also serves as a valuable repository of tropical biological heritage for the world. The scientific establishment of an ecological security pattern (ESP) for the NPHTR is therefore essential for ensuring the robustness and stability of this regional ecosystem, safeguarding tropical biodiversity, and promoting sustainable development. This study integrates ecosystem service functions (ESFs) and ecological sensitivity to identify ecological sources (ESs) so that regional integrity is promoted and ecological security is ensured. Ecological corridors (ECs) are established on the basis of a minimum cumulative resistance (MCR) model and circuit theory. We integrate ESs, nodes, and corridors to construct the ESP of the NPHTR. The areas of extreme importance and sensitivity in the NPHTR account for 25.17% and 25.47% of its overall area, respectively, and are predominantly situated in the higher elevations of its eastern and central regions. Further, the ESs are mainly distributed in its western region, specifically Ba Wangling (BWL), Jian Fengling (JFL), and Ying Geling (YGL), covering an area of 1624.67 km² (38.06% of the total area) of the NPHTR. After correction, 47 ECs with a distance of 870.9 km have been established, with BWL and YGL serving as the core areas. The ecological pinch- and barrier points in the NPHTR cover areas of 11.49 km² and 16.35 km², respectively, primarily consisting of man-made landscapes such as farmlands and buildings. These areas are significantly disturbed by human activities. The NPHTR has an ESP of “one screen, one district, three belts, multiple points, and multiple corridors”. BWL and YGL, which contain numerous sources and corridors, are crucial ecological functional zones. To ensure the horizontal connectivity of rare wildlife, such as of Nomascus hainanus and Cervus eldii hainanus, the NPHTR should establish buffer or development zones. These findings offer valuable insights for the development and planning of ecological civilization on Hainan Island, as well as for the establishment and management of the ESP of other national parks and nature reserves.

1. Introduction

The Parties to the Convention on Biological Diversity have adopted the Kunming-Montreal Global Biodiversity Framework, which highlights the imperative of restoring a minimum of 30% of ecologically representative areas, protected nature systems, and other effective region-based conservation measures from a global perspective [1]. Achieving this objective requires a globally unified and rational spatial planning approach [2]. However, the rapid growth of the global economy and the extensive progress of urbanization have hindered this objective in numerous ways, which are indirectly manifested through the degradation of ecological service functions and values [3], landscape fragmentation [4], biodiversity loss [5], and the disruption of the ecological pattern [6]. Currently, China is in a crucial stage that warrants the construction of an ecological civilization. It is essential to effectively integrate aspects of the spatial structure optimization of its territorial elements with ecosystem service functions (ESFs) and comprehensively understand the relationships between ecosystem security, health, and services [7]. The urgent resolution of issues concerning the harmonization of protection and development, the enhancement of internal stability within regional ecosystems, and the preservation and assurance of their ecological security is imperative [8].

The establishment of an ecological security pattern (ESP) is critical for promoting high-quality development and ensuring human well-being. The current discourse on ESP has gained significant traction [9,10]. The main field of research has transitioned from biodiversity conservation to the assessment of the equilibrium between human society and ESFs [11]. The research scale has also shifted from macro-level assessments of the impact of national ecological protection and construction engineering technologies to regional-level assessments of ecosystem integrity [12]. Additionally, the direction of research has transitioned from qualitative landscape planning and analysis to the theoretical analysis of spatio-temporal patterns that are based on physics and RS, employing various methods, models, and indicators [13]. This research now primarily focuses on arid and semi-arid regions [14], mountainous areas [15], karst areas [16], and other complex topographical locations and sensitive but vulnerable areas. It also encompasses various spatial scales such as counties [17], provinces [18], river basins [19], and urban agglomerations [20]. In recent years, Chinese scholars have developed an ESP research paradigm that is based on the following steps, the “identification of ecological sources (ESs)—establishment of resistance surfaces—extraction of ecological corridors (ECs)” [21,22]. The methods for extracting ECs include techniques based on the importance of ecological conservation and landscape connectivity [23], the minimal cumulative resistance (MCR) model [24], and circuit theory [25]. Numerous studies demonstrate that integrating the MCR model and circuit theory offers significant advantages in extracting ECs and exploring regional ESPs [21,26]. However, research on the national parks with ecological safety barriers and their abundant biodiversity, intricate ecosystems, and crucial species habitat resources remain inadequate. Most research conducted on national parks primarily focuses on assessing ESFs [27,28], fostering active community participation [29], and promoting ecotourism [30]. The study of the construction of ESP and EC with comprehensive physical or geographical patterns in national parks often faces the challenge of overcoming existing administrative units, which makes reference cases relatively rare. Sensitivity assessments, EC construction, and the exploration of ESFs in collaboration with the National Park of Hainan Tropical Rainforest (NPHTR) are insufficient. The collaborative mechanism for constructing and optimizing ESPs in national parks, which is specifically aimed at protecting endangered wildlife and enhancing ESFs, requires further clarification.

The NPHTR is among the first five national parks officially established in China in October 2021 [31]. The park boasts the country’s most densely packed and impeccably conserved island rainforest [32]. The region possesses one of the most biologically diverse ecosystems on earth and serves as a habitat for over half of the world’s plant and animal species. The NPHTR holds significant national and global importance as an ecological security barrier for Hainan Island. The original tropical rainforest underwent significant reduction during the middle-to-late 20th century due to extensive industrial logging and human activities such as slash-and-burn destruction. By 1979, the forest area of the region was reduced to less than 1400 km², with only a 10.42% coverage rate of natural forest. The abrupt reduction in the regional ecological space posed a severe threat to the habitat of wildlife and vegetation, leading to fragmentation of the regional landscape. Although the NPHTR optimizes the integration of former nature reserves, forest parks, geoparks, and some natural forests with public welfare forests, it is also complex, heterogeneous, and possesses a discontinuous landscape. Moreover, the following challenges remain under-explored: (1) Its regionalized mode of management hinders the integration and connectivity of the NPHTR, while its fragmented and isolated distribution pattern exacerbates regional fragmentation and isolation. This hampers the comprehensive protection of tropical rainforest species and ecosystems. (2) The existing infrastructure, including roads, villages, settlements, and other man-made features in the NPHTR, exacerbate both the fragmentation of the regional landscape and different biological habitat segments. (3) Influenced by the establishment of the Hainan pilot free trade zone and its designation as an international tourism island, the Hainan provincial government has increased its investment in tourism development and road infrastructure construction [33]. The construction of the road around the NPHTR is particularly noteworthy. The improvement of the road network and the promotion of tourism have both positive and negative impacts. On the one hand, they enhance the connectivity of the region and facilitate economic growth. On the other hand, they contribute to habitat loss and degradation in the region, impeding species’ migration routes. Considering that the NPHTR has ecological, economic, and social aspects, this study starts from the perspective of regional ecological security and focuses on the establishment of integrated regional ecological services and ecological sensitivity assessments. It aims to explore the ESs, nodes, and corridors of the NPHTR, construct the ESP of the NPHTR, and identify key ecological functional areas in order to coordinate the development of the natural ecosystem, with the NPHTR as the main body and the optimal pattern for social demand.

In the above context, the previous research on the ESP in the NPHTR is inadequate. In terms of ensuring the ecological security of the region, this study identifies ESs by utilizing an evaluation that integrates the functions of ecosystem services and considers their ecological sensitivity. The ECs are constructed using the MCR model, and the ESP of the NPHTR is explored through the integration of ESs, nodes, and ECs to identify the optimal habitat and ECs for wildlife within the NPHTR. The specific objectives are as follows: (1) To couple ESFs and ecological sensitivity to construct the ESP of the NPHTR. (2) To utilize circuit theory and the MCR model to identify the essential ecological priority areas that require protection and ESs and optimize migration corridors for the NPHTR. (3) To determine the optimal living space and ECs between different management zones. (4) To collaboratively advance the spatial optimization of endangered wildlife habitats, ecological safeguarding, and the regional economy in the NPHTR.

2. Materials and Methods

2.1. Study Area

The NPHTR (18°33′16″–19°14′16″, 108°44′32″–110°04′43″) is situated within the dome mountain area in the central Hainan Island (Figure 1). The NPHTR is the most important and largest protected area in Hainan Island. From east to west, the NPHTR crosses nine cities and counties: Wanning, Lingshui, Baoting, Qiongzhong, Wuzhishan, Ledong, Baisha, Changjiang, and Dongfang. The NPHTR contains Wu Zhishan (WZS), Ying Geling (YGL), Jian Fengling (JFL), Ba Wangling (BWL), Diao Luoshan (DLS), Mao Rui (MR), and Li Mushan (LMS), as well as other natural mountain ranges with a concentrated tropical rainforest distribution, which serve as its main body. These primary mountains are divided into seven national park management bureaus, encompassing a total area of approximately 4269 km². The NPHTR features a tropical marine monsoon climate. Its soil is mainly latosols. The annual temperature ranges from 22.5 °C to 26.0 °C, exhibiting distinct dry and wet seasons. Rainfall is primarily concentrated between July and October, with an average annual precipitation of 1700 to 2700 mm [34].

The NPHTR has a permanent population of approximately 23,000 individuals, with the Li and Miao (ethnic minority) indigenous communities constituting 99% of the total. These communities are distributed across 129 natural villages, and their primary means of livelihood revolve around traditional agriculture, farming, and animal husbandry. The NPHTR encompasses diverse habitats, including tropical lowland rainforests, mountain rainforests, and cloud forests [35]. The forest coverage rate of the national park is 96%, while around 80% of the NPHTR is covered by natural vegetation. The NPHTR supports rich biodiversity, including 4367 species of higher plants and 651 species of wild vertebrates. It is home to 14 species classified as national first-class protected wild animals and an additional 120 species classified as national second-class protected wild animals. The NPHTR is a home to approximately 20% of the nation’s amphibian species, 33% of its reptiles, 38.6% of its avifauna, and 20% of its mammals. The NPHTR is an invaluable repository of tropical biodiversity and genetic resources, serving as the sole habitat of the critically endangered Nomascus hainanus.

2.2. Data Sources

Table 1. Basic information and sources of data.

Variable	Type of Data	Spatial Resolution (m)	Year	Source
Distance variables	Distance to road	30	2020
	Distance to settlement	30	2020	National Catalogue Service For Geographic Information (http://www.webmap.cn (accessed on 10 May 2024))
	Distance to water	30	2020
	Distance to railway	30	2020
LULCC data	Land use and land cover change (LULCC)	30	2020	Resource and Environment Science and Data Center of the Chinese Academy of Sciences
				(http://www.resdc.cn (accessed on 1 May 2024))”, “https://www.jl1mall.com/ (accessed on 1 May 2024))
Climate and environmental data	Soil data	30	-	World Soil Database (https://www.fao.org/home/zh/ (accessed on 12 May 2024))
	Digital elevation model (DEM)	30	2020	Geospatial Data Cloud (http://www.gscloud.cn/ (accessed on 3 May 2024))
	Normalized difference vegetation index (NDVI)	30	2020
	Net primary productivity (NPP)	30	2020	Derived from the results of remote sensing investigations and the assessment of national ecological status
	Precipitation	30	2020
	Temperature	30	2020
	Dryness index	30	2020	National Meteorological Information Center (https://data.cma.cn/ (accessed on 4 May 2024))
	Days of sand wind	30	2020

and Figure 2 record the data sources and spatial characteristics used in this study. The socio-economic data were sourced from the Hainan Statistical Yearbook (https://www.hainan.gov.cn/hainan/tjnj/list3.shtml (accessed on 12 May 2024)). The distribution data on the main wildlife in the park are from the NPHTR administration (http://www.hntrnp.com/ (accessed on 20 May 2024)).

2.3. Methods

With reference to relevant studies [36,37], taking into account the current situation in the NPHTR, and in accordance with the general plan for establishing the national park system and a general plan for the NPHTR (2023–2030), the indicators of water conservation (WC), soil conservation (SC), biodiversity conservation (BC), and wind prevention and sand fixation (WPSF) have been selected to assess the functional importance of the NPHTR. The ecological sensitivity of the NPHTR was evaluated by selecting the sensitivity of soil erosion (SSE) and the sensitivity of land desertification (SLD), which are indicative of sensitive ecological conditions. The research framework is shown in Figure 3:

2.3.1. Evaluation of the Importance of ESFs

(1)

WC

WC plays a pivotal role in regulating ecosystem water services, encompassing various functions such as surface runoff regulation, influencing nutrient cycling, and the augmentation of available water resources [38,39]. The assessment of the regional WC function in this study was performed by employing a water balance equation [40]. The calculation was performed using the formula provided below:

$$WC={\displaystyle \sum _{i=1}^j\left(P_i-R_i-E{T}_i\right)}\times A_i\times {10}^3$$

(1)

$$R_i=P_i\times a_i$$

(2)

where WC is water conservation (WC); P_i is the rainfall; R_i is the surface runoff; ET_i is the evapotranspiration; A_i is the area; and a_i is the surface runoff coefficient. The above indices are all in the class i ecosystem.

(2)

SC

SC not only effectively reduces soil erosion and protects and restores vegetation [41], but it also provides a habitat for wildlife, maintains biodiversity, and ensures regional ecological balance. In this study, the revised universal soil loss equation (RUSLE) was employed to calculate SC, using the following formula:

$$SC=R_n\times K\times LS\times (1-C)\times P$$

(3)

where SC is soil conservation (SC); R_n is the rainfall erosivity factor of the year ‘n’; K is the soil erodibility factor; LS is the slope length and slope factor; C is the vegetation coverage factor; and P stands for soil and water conservation measures. We refer to the calculation method of Yu et al. for the C, P, and LS factors [42].

The calculation methods used for R_n and K are shown in Formulas (4), (5), and (6), respectively:

$$R_n={\displaystyle \sum _{\mathrm{i}=1}^{12}1.735\times {10}^{\left(1.5\times \log \left(p_i^2/p\right)-0.919\right)}}$$

(4)

$$K=(-0.01383+0.15175K_{EPIC})\times 0.1317$$

(5)

$$\begin{array}{c}\hfill K_{EPIC}=\{0.2+0.3\exp [-0.0256m_s(1-m_{silt}/100)]\}\hfill \\ \times {[m_{silt}/(m_c+m_{silt})]}^{0.3}\times \{1-0.25orgC/[orgC+\exp (3.72-2.95orgC)]\}\\ \times \{1-0.7(1-m_s/100)/\{(1-m_s/100)+\exp [-5.51+22.9(1-m_s/100)]\left\}\right\}\hfill \end{array}$$

(6)

where P_i is the total rainfall of the month ‘i’; P is the total annual rainfall; and m_s, m_silt, m_c, and orgC represent the mass percentage of sand, silt, clay, and organic carbon, respectively.

(3)

BC

The protection of biodiversity serves as the foundation for achieving steady regional progression by capturing the status, dynamics, and vulnerabilities of regional ecosystems. Thus, it plays a pivotal role in shaping conservation strategies [43]. The NPP method was employed to assess the significance of BC functions. The evaluation model adapted is as follows:

$$BC=NPP\times F_{pre}\times F_{tem}\times (1-F_{alt})$$

(7)

where BC is biodiversity conservation (BC); NPP is the average classification index of the annual net primary productivity of vegetation and F_pre, F_tem, and F_alt are the annual average precipitation classification index, annual average temperature classification index, and altitude factor classification index, respectively.

(4)

WPSF

WPSF is a crucial service provided by regional ecosystems which encompasses the inhibitory and stabilizing effects of vegetation on wind and sand and the preservation of soil’s natural functions [44]. Its evaluation was conducted with the help of the following equation:

$$WPSF=NPP\times K\times F_q\times D$$

(8)

where WPSF is wind prevention and sand fixation (WPSF); F_q is the annual erosivity of the climate and D is the surface roughness factor.

(5)

Construction of a Model for the Assessment of ESFs

The assessment of the importance of integrated ESFs was obtained by superimposing WC, SC, BC, and WPSF. This was evaluated as follows:

$$ESI=Max\left\{WC,SC,BC,WPSF\right\}$$

(9)

where ESI is an evaluation level of the importance of an ecosystem’s integrated service function. The natural breakpoint method was employed in this study to categorize these into three levels: general importance, medium importance, and extreme importance.

2.3.2. Assessment of Ecosystem Sensitivity

(1)

SSE

This assessment was conducted by utilizing the modified soil and water loss equation, which is as follows:

$$SSE=\sqrt[4]{R_n\times K\times LS\times B}$$

(10)

where SSE is the sensitivity of soil erosion (SSE); LS is the slope length and slope factor and B is the fractional vegetation cover (FVC) classification index.

(2)

SLD

Based on the topography, geomorphology, and climate characteristics of the NPHTR, the SLD was calculated as follows:

$$SLD=\sqrt[4]{D\times G\times B}$$

(11)

where SLD is the sensitivity of land desertification (SLD); D is the classification index of the ecosystem type and G is the classification index of the slope factor.

(3)

Construction of Ecosystem Sensitivity Assessment Model

The comprehensive ecosystem sensitivity of the NPHTR was obtained by superimposing the SSE and SLD assessments as follows:

$$ES=M\mathrm{ax}\left(SSE,SLD\right)$$

(12)

where ES is the comprehensive sensitivity level of the ecosystem. The sensitivity of the NPHTR is categorized into three levels—generally, moderately, and extremely sensitive— using the natural breakpoint method.

2.3.3. Assessment of the Importance of Ecological Protection

The assessment of the importance of ecological protection is derived by overlaying the evaluations of the ESFs’ importance and the ecosystem’s sensitivity. The natural breakpoint method is then utilized to classify the protected areas into three categories: general, important, and extremely important protected areas.

2.3.4. Construction of ESP

(1)

The Identification of ESs and Nodes

ESs are crucial patches that play a pivotal role in providing essential ESFs and facilitating ecological processes while serving as vital habitats for wildlife [45]. The effective identification of these ESs is achieved through a comprehensive analysis of ecosystem services that incorporates both qualitative and quantitative approaches. Similarly, ecological nodes generally refer to the critical ESFs within ESs. Enhancing the protection of ecological nodes contributes to improving ecosystem connectivity. In this study, an initial rigorous screening process was conducted to identify extremely important protected areas, followed by the consolidation and integration of important ecological regions within the NPHTR to delineate the ESs. Finally, we chose the center point of the ESs as the ecological nodes [46].

(2)

MCR Model

The MCR model is capable of calculating the paths that exhibit the minimum value of cumulative resistance between the ESs, thereby facilitating the identification of potential ECs [47]. In this study, the construction of an ecological resistance surface was achieved through a quantitative approach and used to identify and extract ESs. The following formula was used for the calculations:

$$MCR=f_{\min }\times {\displaystyle \sum _{b=1}^t{\displaystyle \sum _{a=1}^q\left(D_{ab}\times R_a\right)}}$$

(13)

where MCR represents the minimum cumulative resistance, D_ab represents the spatial distance of species from source b to source a, R_a represents the resistance of source a to species movement, and f_min represents the positive correlation between the minimum cumulative resistance and ecological processes.

(3)

Construction of Ecological Resistance Surface

An ecological resistance surface is a fundamental component in the construction of an ESP. It accurately depicts the spatial distribution of ecological flow resistance among different ecological functional areas [9]. It is believed that the higher the resistance coefficient, the greater the resistance will be, and, consequently, the higher the cost of ecological land expansion. Based on the unique physical geographical pattern of the study area, and by considering factors such as the surrounding natural environment, human disturbances, and barriers posed by ground objects, a resistance surface index system was established, along with seven secondary indices (

Table 2. Ecological resistance assessment indicators.

Resistance Evaluation Index (Weight)	Index Classification	Landscape Resistance
Slope (0.146)	<8°	1
	8°–15°	3
	15°–25°	5
	25°–35°	7
	>35°	9
LULCC (0.39)	Forestland, water body	1
	Grassland	3
	Cropland	5
	Unused land	7
	Built-up land	9
Distance to road (0.082)	>5 km	1
	2–5 km	3
	1–2 km	5
	0.5–1 km	7
	<0.5 km	9
Distance to water (0.091)	<0.5 km	1
	0.5–1 km	3
	1–2 km	5
	2–5 km	7
	>5 km	9
Distance to railway (0.088)	>10 km	1
	5–10 km	3
	2–5 km	5
	1–2 km	7
	<1 km	9
Distance to settlement (0.135)	>2 km	1
	1–2 km	3
	0.5–1 km	5
	0.25–0.5 km	7
	<0.25 km	9
Terrain niche (0.068)	Using the range method for standardization, the higher the value, the lower the resistance

). These secondary indices include LULCC, slope, terrain niche, and the distances to roads, water, railways, and settlements. The resistance surface index was assigned by using ArcGIS, and the weight of each index was determined through a combination of the analytic hierarchy process and the entropy weight method [48]. The resistance surface was obtained by applying these weights and superimposing the assessment indicators accordingly. The outcomes of the evaluation of the resistance surface were categorized into low-, medium-, and high-resistance categories.

(4)

EC Extraction and Node Identification Based on Circuit Theory

ECs serve as a crucial conduit linking diverse ESs and function as a pivotal ecological landscape for the circulation of materials, flow of energy, and transmission of information within the ecosystem [49]. Ecological pinch points are areas characterized by high cumulative current values and serve as the primary pathways for simulated species migration, thus warranting prioritized ecological protection [2]. Ecological barrier points refer to regions that impede species movement between areas, and restoration efforts in these areas can enhance the connectivity of ESs [50]. In circuit theory, the ecological resistance value is considered to be the circuit resistance value, while the ecological flow represents random walking currents, enabling a more accurate reflection of species’ random walking characteristics [21]. The Linkage Mapper toolbox and Circuitscape plug-in were utilized based on the ESs and resistance surfaces established to accomplish the identification of potential ECs, as well as the extraction of ecological pinch points and barrier points within the study area [51]. Additionally, the importance of distinguishing ECs on the basis of a gravity model was emphasized [52].

3. Results

3.1. Analysis of ESFs Importance

The extreme importance of the NPHTR is attributed to its SC and BC, which have respective area proportions of 31.84% and 26.65% (Figure 4). Following these is its WC, while its WPSF is the least important, with a general importance of 78.19%. WC is predominantly characterized by general and medium importance, accounting for 50.20% and 34.36% of the NPHTR, respectively (Figure 4e). It is primarily distributed in JFL, BWL, and YGL, with a pronounced concentration in the eastern areas (Figure 4a). BC is characterized by a dominant presence of areas of general and medium importance, spanning extensive areas of 1731.37 km² and 1400.13 km², respectively. However, the spatial distribution of BC exhibits a distinct contrast to that of WC, showing a low distribution pattern in the east and a high distribution in the west (Figure 4b). SC is predominantly classified as of medium and extreme importance, as it constitutes 36.58% and 31.84% of the NPHTR, respectively, and is distributed throughout most regions within the NPHTR (Figure 4d). The dominant feature of WPSF is the prevalence of areas of general importance, which account for 78.18% of the total study area (Figure 4c). With the exception of JFL, BWL, and YGL, there are some continuous areas of medium importance, while all the other regions are characterized as being of general importance. This is because the rainforest is extremely resistant to wind erosion, so there is no significant difference in WPSF throughout the national park.

The integrated ESFs in the NPHTR are predominantly classified as being of general and medium importance, constituting 38.86% and 35.96% of the NPHTR, respectively (Figure 5). These areas are primarily situated in the northwest and southwest of the NPHTR, where the terrain is flat and grasslands and shrublands are predominant. Additionally, a certain amount of artificial vegetation, such as eucalyptus, rubber, and betel nut, is distributed in these areas. This artificial vegetation has a low coverage rate, simple structure, and is easily affected by human interference, which result in poor WC and SC capabilities. In the future, these areas will require manual intervention and effective revegetation to prevent soil erosion. The area of extreme importance measures 1075.79 km² in total and is primarily distributed across parts of DLS, YGL, LMS, MR, and BWL. These areas are characterized by higher altitudes, reduced human interference, and the preservation of a large amount of their original tropical forests. Local primary forests not only provide crucial ecological services but also ensure regional ecological security.

3.2. Ecosystem Sensitivity Analysis

The extremely sensitive areas in terms of the SSE in the NPHTR span 600.09 km², representing 14.06% of the overall NPHTR. They are primarily situated in tropical lowland rainforests with low-relief terrain in DLS, WZS, YGL, and MR (Figure 6). Their regional soil erosion is evident due to consistently high precipitation and the characteristic mountainous and hilly red soil in this area. The generally sensitive and moderately sensitive areas of the SLD account for 75.98% of the total of the NPHTR. Their distribution features are higher in the western and lower in the eastern parts, particularly in the sensitive areas of BWL, JFL, and YGL (Figure 6b). This indicates that the disparity in rainfall between the western and eastern sides of the NPHTR significantly impacts ecosystem sensitivity.

As shown in Figure 7, the NPHTR exhibits predominantly generally and moderately sensitivities, encompassing an extensive area measuring approximately 3181.5 km² (74.5% of the total area of the NPHTR). These areas are mainly distributed in lower-altitude regions such as DLS, LMS, and WZS (Figure 7a). They contain a certain amount of agricultural land and tropical cash crops. Driven by economic benefits, the scope and intensity of human activities increase in these areas, which lead to a decrease in regional ecological sensitivity. The areas of extremely sensitivity, namely YGL, BWL, JFL, and MR, are primarily concentrated in the core regions of the NPHTR due to their intricate topography and exceptionally high sensitivity. These areas are mainly distributed within tropical mountain rainforests, cloud forests, and other primary forests. These habitats are rich in forest resources, characterized by a noticeable altitude gradient and complex forest structure. Moreover, they serve as the natural habitat for rare wildlife species such as Nomascus hainanus, Manis pentadactyla, and Cervus eldii hainanus. The destruction of these areas would have a significant ecological impact, thus necessitating their protection.

3.3. Evaluation and Analysis of Importance of Ecological Protection

As shown in Figure 8b, the extremely important protected area of the NPHTR is 1287.36 km², constituting approximately 30.1% of the overall national park. It is mainly concentrated in JFL, YGL, BWL, MR, and WZS. The important protected areas are primarily distributed in the southwest and northwest, encompassing an approximate area of 2022.26 km². Benefiting from the stringent protection provided by the NPHTR management bureaus, these areas experience relatively minimal human disturbance. A significant amount of undisturbed vertical zoned primary forest vegetation has been conserved in the region, indicating its high level of ecological importance. The general protected areas are primarily distributed in the eastern area of BWL, the eastern area of WZS, and the southern area of LMS (Figure 8a). Although these areas are within the important ecological red-line area, they retain a certain number of ethnic minorities and traditional farming industries, as well as scattered built-up land [28]. Additionally, the Hainan provincial government has implemented a special plan for ecological tourism in the Hainan tropical rainforest national park (https://www.hntrnp.com/ (accessed on 20 May 2024)). Furthermore, the ‘Hainan Island tourism highway’ passes through this area. The development of local ecotourism has resulted in an increase in human disturbance, leading to the fragmentation of the regional landscape and the accelerated degradation of the native environment. Therefore, balancing economic development with regional ecological protection is a crucial challenge for the future.

3.4. Identification of ESs, Ecological Nodes, and Resistance Surfaces

Based on the evaluation results of the importance of ecological protection, 26 ESs were identified in this study (Figure 9). The area requiring protection covers 1624.67 km², representing 38.06% of the NPHTR. The ESs of BWL, YGL, JFL, and MR are primarily situated within this area, which is characterized by an intricate topography predominantly consisting of mountains and valleys. These areas exhibit characteristics commonly found in primary tropical rainforests, including typical mountain vegetation, minimal human disturbance, abundant species diversity, and a complex forest structure. The degree of regional landscape fragmentation is low. Further, the habitat’s quality is high and it is home to many rare wild animals and plants found in the NPHTR [53].

The proportions of low-, medium-, and high-resistance areas in the NPHTR are 34.11%, 45.09%, and 20.8%, respectively (Figure 10b). The spatial distribution of the areas’ features displays a pattern of high resistance around the periphery and low resistance in the central regions. The low-resistance zones, which are high-level ecological security areas, are distributed in large patches within YGL, LMS, WZH, MR, and other management branches with high ESFs (Figure 10a). The forest landscape is the dominant feature in these regions, and it exhibits a high level of ecosystem stability. Conversely, the areas of medium resistance are distributed in a buffering manner around the low-resistance zones and are scattered throughout the NPHTR as small patches. Lastly, the high-resistance areas are located at the periphery of the NPHTR, close to human habitation, and are subjected to significant human disturbances.

3.5. Construction of Ecological Security Patterns

With the support of ArcGIS, the identification of ECs in the NPHTR was conducted through the quantitative analysis of ESs, ecological nodes, and resistance surface data. (Figure 11). Considering the regional characteristics and physical geography of the NPHTR, 67 original ECs were merged and the redundant ones were removed. The resulting modified ECs were evenly distributed across the study area, effectively enhancing connectivity between ESs and ensuring the transfer of ecological factors. After modification, a total of 47 ECs, spanning 870.9 km, were retained (

Table 3. Changes in length of ECs of NPHTR before and after optimization (km).

	Length of Original	Length of the Revised	Change in Length
	ECs	ECs
BWL	226.45	148.18	78.28
DLS	249.01	57.33	191.68
JFL	201.01	108.96	92.04
LMS	120.86	81.56	39.29
MR	235.58	98.34	137.24
WZS	368.88	113.62	255.26
YGL	414.58	262.91	151.67

). Among them are 15 key ECs spanning 257.02 km, which constitute approximately 29.51% of the overall length of the ECs. These ECs are primarily distributed in BWL and YGL and they predominantly feature pristine natural forest landscapes that provide suitable habitats for species activities and effectively facilitate strategies such as biodiversity conservation and species migration (Figure 12). Due to factors such as ecological sources’ distribution and location in the NPHTR, the material exchange within these regions takes place over a shorter distance, which facilitates a relatively easy transfer of energy. In contrast, the ECs in DLS, MR, WZS, and YGL experienced a significant reduction in both length and numbers. Consequently, the interconnectivity and the energy flow among the ESs within these regions have become relatively limited. Ecological pinch points play a crucial role in facilitating the virtuous cycle of the ecological network and ensuring the sustainability of the ESP [54]. The total area of the ecological pinch points in the NPHTR is 11.49 km², and all are concentrated at the junction of YGL, WZS, and MR (Figure 12). Because these ecological pinch points are close to areas of human activity, human interference causes these potential ECs to be squeezed into narrow spaces, which hinder the migration of species. The total area of the ecological barrier points amounts to 16.35 km², and these are primarily distributed along the border between YGL and BWL in the northern region of the NPHTR. These predominantly encompass low-altitude areas that consist of farmland and human settlements. In general, the areas that are ecological pinch points and barrier points are predominantly artificial landscapes, which are highly disturbed by human activities and have low vegetation coverage [55]. This diminishes the overall quality of regional habitats and poses challenges for ensuring unimpeded flow in the ECs. Therefore, it is necessary to ensure the goals of ESP construction are met by implementing ecological restoration and reducing human activities in the future.

The NPHTR generally presents an ESP of “one screen, one district, three belts, multiple points, and multiple corridors” (Figure 12). The ‘one screen’ is an ecological barrier consisting of JFL, BWL, YGL, LMS, and DLS. This region not only serves as the ecological stronghold of Hainan Island, but it also boasts the most abundant forest resources. These lush rainforests serve as vital water reservoirs and play a crucial role in mitigating wind and flood hazards. Additionally, they act as a buffer against the threats posed by external social development and human activities, thus forming an ecological security barrier for the island. Next, the ‘one district’ in the ESP refers to the main ecological functional area that is formed by BWL and YGL. It holds significant value in protecting the terrestrial forest ecosystem of Hainan. YGL serves as the primary source of the Nandu and the Changhua rivers, which are the largest and the second largest rivers, respectively, on this island [56]. Similarly, BWL boasts the most well-preserved tropical rainforest in China and is exceptionally rich in wildlife resources, including Nomascus hainanus and Cervus eldii hainanus [57].

Next, ‘the three belts’ in the ESP refer to the ECs designed to enhance connectivity between NPHTR habitats and the areas containing rare and endangered animals and plants. These include the JFL-BWL-YGL, the MR-WZS-YGL, and the DLS-WZS-BWL belts. These corridors aim to optimize animal- and plant-dispersal activities while increasing habitat availability. In particular, the BWL and YGL corridors can effectively connect fragmented gibbon habitats and expand their range. By mitigating various threats to the survival and reproduction of the Nomascus hainanus population, this measure enables the species to gradually overcome its risk of extinction to a certain extent and continue its development and growth. Finally, the ‘multiple corridors and points’ refer to the main body of ecological land that contains all the sources of the different management branches of the NPHTR and serves as a key link in its ecological conservation.

4. Discussion

4.1. Necessity of Building ECs in NPHTR

The construction methods of an ESP mainly focus on the qualitative determination and quantitative analysis of ESs [58,59]. Their quantitative identification at the regional scale is mainly based on the use of ecosystem services, and an ecological sensitivity analysis is mainly used at the urban scale. Luo et al. conducted a simulation of the spatial characteristics of the ESP in the Weihe River Basin in 2040 under various scenarios, with the aim of determining an optimal model for sustainable development that balances biodiversity and economic growth in the region [50]. Meanwhile, Zhang et al. constructed an ESP for the Chengdu–Chongqing economic circle to address regional development and ecological protection conflicts [60]. Moreover, as the cornerstone of China’s natural protected area system, national parks are established with the primary objective of preserving the authenticity and integrity of natural ecosystems and maintaining biodiversity and ecological security barriers [61]. The constructed ECs of the NPHTR serve a dual purpose: firstly, they mitigate the habitat fragmentation caused by urban areas, rural settlements, roads, and various authorities in the region [62]. Secondly, they facilitate species diffusion and migration through the establishment of a corridor system while enhancing connectivity between islands within the protected area [63]. Thus, the multifunctional service capacity of tropical rainforests can be improved [64,65]. The distribution of nationally important protected animals in the NPHTR, as depicted in Figure 13, is primarily concentrated in BWL and JFL, aligning remarkably with our identified ecological functional areas. Notably, within BWL, the exceptional habitat quality and dense ECs provided by the ESP have expanded the potential migration routes for Nomascus hainanus and Cervus eldii hainanus. According to survey data, there has been a significant increase in the Nomascus hainanus population from approximately 33 individuals across five groups in 2020 to nearly 42 individuals distributed among seven groups by 2024 (https://www.hntrnp.com/ (accessed on 20 May 2024)).

Moreover, through a comparative analysis of the master plan for the NPHTR (2023–2030), this study revealed a high degree of alignment between the designated corridors and crucial ecological functional areas, with proposed establishment and restoration zones for bio-ECs. Notably, these ESs traverse the BWL-YGL and DLS-WZL regions. Therefore, based on the research paradigm of “ESs-ecological resistance surface—ECs,” we constructed a resistance model of “human activity—ecosystem—ecological guarantee.” Additionally, we integrated the evaluation models of ecological environment factors and ecological sensitivity to establish a regional NPHTR ecological environment factor system. Consequently, a multi-scale ESP was developed at the national park level. This approach facilitates the comprehensive identification of key ecological domains such as ESs, environmental factors, nodes, and important corridors. Furthermore, it aids in determining priority areas for protection (ecological pinpoints) and restoration (ecological obstacles), providing crucial foundations and optimal pathways for the migration, reproduction, and population recovery of rare and endangered species. Finally, under the background of promoting the construction of an ecological civilization, the ESP of the NPHTR, in this study, was constructed so that the objective of “one screen, one district, three belts, multiple points and multiple corridors” could be achieved. It shall provide a crucial foundation for the connectivity of ECs and will assist in regional ecological restoration within the NPHTR master plan (2023–2030).

4.2. Optimal Selection of Importance and Sensitivity Ecosystem Service Functions

The importance and sensitivity of ecosystems refer to their ability to adapt to external interferences and regenerate themselves. Assessing the importance and sensitivity of ecosystem services is a crucial prerequisite for promoting harmony between humans and nature, which is of great significance for maintaining environmental quality and ensuring sustainable social development [66]. Most studies typically focus on extracting policy-controlled areas such as nature reserves and ecological red lines, or ecologically superior regions like forest parks, as ESs [59]. However, the qualitative approach employed in these studies fails to comprehensively account for the inherent functions and sensitivity of these ecosystems. In this study, we quantitatively evaluated the differences in sensitivity and services of ecosystems in the study area by considering the geographical characteristics and key features of the ESs of the NPHTR. To avoid inaccuracies caused by relying on single indicators, we selected WC, SC, BC, WPSF, SSE, and SLD as evaluation indicators of the ecosystems’ importance and sensitivity. Additionally, our research findings demonstrate a high consistency between the distribution of ESs and the spatial distribution of the NPHTR’s core area. This further supports the notion that identifying ESs relies on selecting appropriate importance and sensitivity indicators.

Furthermore, the NPHTR is characterized by a forest landscape that holds an unequivocal dominance, boasting a forest coverage rate of 95.86% and a natural forest area proportion of 76.56%. This is a prototypical island-type tropical rainforest, preserving the authenticity of tropical natural habitats while exhibiting remarkable ecosystem resilience, stability, and landscape connectivity. Meanwhile, the NPHTR serves as the ecological pinnacle of Hainan Island and is the origin of major rivers including the Nandu, Changhua, and Wanquan Rivers. It plays a crucial role in safeguarding water and soil conservation, carbon sequestration and oxygen release, and preventing high winds and floods on the island. Consequently, when constructing the NPHTR’s ESP, greater emphasis is placed on selecting regional ecosystem function indices that not only enhance the rationality and precision of ecological source identification but also effectively reflect the objective of the construction of biodiversity protection and the maintenance of ecological system integrity within the NPHTR. Furthermore, this study integrates topography, forest landscapes, and other natural factors, along with human disturbance factors, to construct an ecological resistance surface, thereby providing a more precise depiction of the variations in the NPHTR and the influence of external factors.

Finally, based on statistics provided by the NPHTR administration, artificial forests cover an area of 824 km² within the national park, representing 19.3% of the park’s total area. These forests primarily consist of economically valuable tree species such as rubber and betel nut, as well as timber tree species including eucalyptus, casuarina, and Caribbean pine. Notably, these artificial forests are predominantly located in areas inhabited by the Hainan Li and Miao ethnic minorities. China has adopted a government-led multi-stakeholder governance model for its national parks, while the exploration of a multi-stakeholder collaborative governance model is still ongoing. The NPHTR has adopted a two-tier management system consisting of the Forestry Bureau of Hainan Province and the National Park Bureau. However, there appears to be limited enthusiasm and participation among local residents in the development of the national park. Despite the implementation of ecological relocation within the NPHTR, certain local areas, particularly in the middle and low mountainous and hilly regions of the northwest NPHTR, still witness unreasonable human activities leading to an increase in economic forests and timber forests. This has resulted in a simplification of the forest community structure, species composition, and the localized fragmentation of rainforests. Moreover, the study area experiences high-intensity rainfall events that enhance soil erosion sensitivity, thereby exacerbating the fragility of the ecological environment and giving rise to significant internal conflicts within the ecosystem. Therefore, selecting appropriate indicators that accurately reflect regional advantages’ importance and sensitivity can provide more precise observations regarding ecological environmental issues in the NPHTR.

4.3. Limitations and Prospects

There are certain limitations to this study. During the selection of the indicators of human interference, we have only considered the impact of varying levels of road distance on the NPHTR [67]. We did not account for the influence of the road network density on regional landscape fragmentation or the degree of interference from regional populations and economic pressure on the NPHTR. These aspects should be strengthened in follow-up research. Additionally, this study did not conduct a thorough investigation of actual ESs. According to the NPHTR’s geographical regionalization, the vertical ECs constructed in this research are highly connected. However, the horizontal ECs are limited to the BWL-YGL-WZS-LMS areas. Notably, the absence of horizontal corridors, such as those in JFL, MR, and DLS, could prevent the formation of networked ECs, thereby adversely impacting regional ecosystem connectivity. Furthermore, the corridor width was not determined in this study, which undoubtedly increases the cost of species migration in these areas [68]. The integrity of the NPHTR’s ESFs and the preservation of its biological diversity need to be considered in future research. Nevertheless, the peripheral buffer zone and the development area of the NPHTR should be rationally planned. Establishing and delineating transregional ECs to facilitate coordination among the seven national park services and the conservation system is essential. This approach aims to achieve horizontal corridor connectivity and the closed-loop connectivity of ECs of the NPHTR. Additionally, nine cities and counties around the NPHTR should be connected on the basis of potential corridors with strong ribbon connectivity. This will help realize the systematic integration of the NPHTR’s ECs, thereby improving the overall ecological security level of the region.

5. Conclusions

Based on the actual ecological and environmental characteristics of the NPHTR, its ESs were identified. These relied on ESFs and ecological sensitivity. Then, the ESP of the NPHTR was constructed on the basis of the coupling of important landscape elements with the ESs, nodes, and ECs. The results of the ESF analysis of the NPHTR indicated that the region exhibits higher values in terms of WC, WPSF, and BC. Due to variations in spatial rainfall patterns, there is a complete contrast in the distribution patterns of areas with extremely high values for the SSE and SLD between the eastern and the western sides. The areas of high ecological importance and sensitivity were primarily distributed in BWL, YGL, LMS, and MR. These areas encompass the tropical montane rainforest and alpine cloud forest that are found at higher elevations, while areas of lower value are situated within the tropical lowland rainforest found at lower elevations. The ESs within the NPHTR span a total of 1624.67 km², which is predominantly situated in the eastern areas of the NPHTR. Following our revision, a consensus was reached on the identification of 47 ECs that span a total length of 870.9 km. Ecological pinch points cover an area of 11.49 km², while ecological barrier points encompass an area of 16.35 km². These points are predominantly situated along the borders of national parks and face significant pressure from human settlements and agricultural land. The NPHTR as a whole presents an ESP of “one screen, one district, three belts, multiple points and multiple corridors”. The ECs effectively connect all major ecological functional areas. Planners and governments should enhance the ecological protection of local areas, minimize external human interference, and strategically plan the outer buffer and development zones of the NPHTR.