Comprehensive evaluation of environmental dimension reduction of multi-type islands: a sustainable development perspective

In recent years, the sustainable development of islands has attracted increasing attention from countries all over the world. An important prerequisite for promoting sustainable development is to understand the foundation and sustainable development potential of islands. Constructing index systems and models is an important means of evaluating the sustainability of islands. This study used factor analysis (FA) to construct an indicator system and set weights. Thirty-eight indicators were set from both natural and social directions to evaluate the sustainable development of seven typical islands in China. The FA removed the 10 indicators that were too relevant, and the 28 effective indicators were reduced into 9 main factors for evaluation. The results showed that the evaluation results are in line with the actual development of the island, which verifies the applicability of the model to different types of islands. The study also found that the changing trends of island social sustainability, tourism sustainability, ecological sustainability, resource sustainability, and economic sustainability are consistent. The value of fully balanced islands is higher than that of unbalanced or undeveloped islands. Among the seven islands, social islands have the highest total value, and ecological islands have the lowest total value.


Introduction
Since the twentieth century, oceans and islands have gradually become the focus of national and social attention Zhang et al. 2020b;Zheng et al. 2020b). The importance of island development, management, and ecological protection is increasing daily (Liu et al. 2018), and an island view of coordinated development, green development, and sustainable development has gradually formed. In response to the island's complex geography, resource endowments, and various conflicts arising from the development process (Douglas 2006), the island's development situation tends to shrink. Affected by the long-term concept of valuing land and ignoring sea, as well as unfavorable factors such as being far away from the mainland, inconvenient transportation, and hard life, the development of China's islands is in a lagging, disorderly, and extensive stage. In the process of island development, the following problems are present: the development order is chaotic, the construction level lags behind, the development level is low, and the resources and environment are seriously damaged. Since 2018, China has proposed the construction of ecological islands and reefs. To prevent ecological damage to islands, most island projects have been suspended. However, currently, it seems that how to balance the relationship between development and protection still has not been effectively resolved. The lack of accurate positioning, scientific evaluation, and comprehensive planning of the islands is very important reason.
Western maritime powers and international small island countries have done much scientific research on the scientific development and utilization of islands (Rigg and Richardson 1934;Tokusige 1939), such as island management (Kim 2020), island ecology (Petridis et al. 2017), and nonresident island development and utilization (Hwang and Ko 2018). Many countries with islands are more likely to adopt centralized and specialized agency management systems for island Responsible editor: Marcus Schulz * Shaoyang Chen nmdiscsy@126.com 1 management. The centralized formulation of marine island master plans through a unified organization makes it easier to achieve high resource utilization and ecological environment protection (Shi et al. 2015). International research on islands mostly focuses on ecological and environmental protection. The restoration of the island ecological environment is exemplified by New Zealand. As a small island country, New Zealand attaches great importance to ecological restoration research and has made a series of achievements in island ecological restoration (Towns and Ballantine 1993). Although China's efforts to protect the island ecological environment started relatively late compared to foreign countries, with China's emphasis on islands in recent years, the protection and management of islands has increased. Some achievements have been made in island development (Shen 1995), island protection (Zhang et al. 2020a), and ecosystem evaluation (Chen and Dong 2019). Compared with other countries with more successful island development (e.g., Australia, Thailand), the overall development and utilization level of uninhabited islands in China is low, and both efficiency and benefit are not commensurate with the relatively superior natural endowment status. The concept of sustainable development can be traced back to the 1970s which then quickly penetrated all areas of social development. In 2004, some experts and scholars (Li and Wang 2004) made the following definitions for the sustainable development of the island: adapt measures to island conditions, plan rationally, rely on scientific and technological progress, strengthen legal management, and rationally and effectively develop and utilize the island's ecological environment without reducing its carrying capacity so that it not only meets the needs of the present generation but also does not pose a hazard to the needs of future generations. Since then, the sustainable development of the island has become the primary topic in the study of the island environment. The development of islands is susceptible to human activities and environmental changes (Brauko et al. 2020). From the perspective of politics (Liao and Liu 2019), the economy (Baldacchino 2006;Liu et al. 2020;), resources (del Rio-Rama et al. 2020Zheng et al. 2020a), and the environment (Liu et al. 2017), disorderly islands' human activities and economic development models are not conducive to the sustainable development of islands (Zhao et al. 2016). The vulnerability of islands compared to land emphasizes the importance of island management and planning. Establishing the concept of sustainable development with strategic significance and a fair and just support system to provide a solid foundation and guarantee for the sustainable development of the island (Wang et al. 2006). Domestic and international research on the application of the concept of island sustainable development involves ecology , tourism (Moreno 2005), fisheries (Karcher et al. 2020), land resources (Zhang and Xiao 2020), etc.
Its emphasis is on balancing the natural ecology and social development of the island. For small island states in particular, sustainable development research is more relevant. Small island states pay more attention to how to achieve the sustainable development of islands (Tilley et al. 2019), and the study of protecting the island's ecological environment (Hafezi et al. 2020) and socioeconomic development (Mauthoor 2017) is more in-depth.
Currently, a series of evaluation applications for sustainable development have been carried out in different industries, such as business management , the automobile sales industry (Zhou et al. 2019), and water resource utilization (Dai et al. 2019). Huang et al. (1998) discussed the sustainability of urban eco-economics and selected 80 eco-economic indicators to evaluate the sustainability of Taipei City. Rajak et al. (2016) used fuzzy logic methods to evaluate the sustainability of urban transportation systems. Tang et al. (2019) selected 39 indicators from the three directions of economic, social, and ecological development to construct a city sustainability evaluation indicator system. Then, the entropy method was used to assign weights, and the gray correlation method was used for evaluation research. Che et al. (2021) selected 19 evaluation indicators from the four directions of environmental sustainability, economic development sustainability, social well-being sustainability, and technological innovation sustainability to construct a comprehensive regional coordination evaluation model. Sustainability evaluations were carried out in 31 provinces and regions in China. It can be seen from this that the evaluation research of sustainable development has developed in-depth from a single direction to a multidirection (Yang and Ding 2018). Generally, sustainable development is a composite ecosystem composed of three directions: nature, economy, and society (Singh et al. 2009;Tanguay et al. 2010;Peng and Deng 2020). However, the definition of sustainability shows uncertainty (Burgass et al. 2017). There are also many differences in the determination of the directions of the indicator system. It is not only related to the development of various directions, such as the environment, economy, society, and technology, but also related to the mutual relationship and interaction between the various directions (Che et al. 2021). Therefore, researchers determine different sustainability directions according to their needs generally (Shaker and Sirodoev 2016).
Since the establishment of the evaluation index system for the sustainable development of islands in 2004, many scholars have successively evaluated the sustainable development of islands (Ke et al. 2013;Ke et al. 2014;Gao et al. 2019;Long et al. 2020;Nesticò and Maselli 2020;Xu et al. 2020). The assessment of foreign islands is mainly centered on risk assessment, which explores the impact of disturbance on the island environment itself and human society, involving anthropogenic factors such as fisheries fishing (Gilman et al. 2014) and natural factors such as hurricane crossing (Sealey et al. 2020). Various models and frameworks have been developed to assess the sustainability of islands, such as ecological footprint (Fang et al. 2018;Dai et al. 2019) and data envelopment analysis (DEA) evaluation models (Wu et al. 2009). The ecological footprint is used as a measure of sustainability, but it has obvious shortcomings (Fiala 2008). The ecological footprint proposed by Rees focuses on ecological sustainability, ignoring the sustainability of the socioeconomic system (Rees 1996). DEA has drawbacks in evaluating efficiency and determining weights (Charnes et al. 1979;Wu et al. 2009). Extensive research has been conducted on the sustainability evaluation method based on the indicator system, which follows five conventional analysis stages: indicator selection, data processing, normalization, weighting, and aggregation, relying on a set of methods to ensure the scientific objectivity of the analysis (Miller et al. 2017). The total value score is calculated by the linear weighted sum of multiple indicator scores (Opon and Henry 2020). The choice of methodology involves much subjectivity, and the most commonly used methods include the analytic hierarchy process (Zhu and Wang 2017) and entropy method. The analytic hierarchy process (AHP) is affected by subjective factors (Tang et al. 2019), and expert opinions are highly subjective, resulting in weight predictions that contradict the actual situation. The entropy weight method does not consider the influence relationship between the index and the index, and the weight distribution is easily polarized. Currently, there is no unified and universally applicable island evaluation model at home or abroad (You et al. 2015;Karampela et al. 2017). Many indicator sets only reflect a limited range, and no indicator set is sufficiently comprehensive to characterize sustainability. The consistency and correlation of data also lead to uncertainty in the indicators (Opon and Henry 2020).
Drawing on the island research experience of major international marine countries and small coastal island countries, combined with the results of the evaluation of the sustainable development of domestic islands, this study used the network information collection method and statistical analysis method to study and analyze the representative data of the sustainable development of the islands and constructs the evaluation system of the FA method. FA is a multivariate statistical method used to reduce a large number of variables to fewer potential dimensions. It is also used to observe the relationship between data and test whether the assumed relationship or potential dimension in the data can be confirmed (Watson and Thompson 2006). The core of FA is correlation analysis. Usually, it uses covariance to measure the relationship between two variables. The key aspect is that the researcher decides how many factors to keep (Dinno 2009). FA was first applied in the field of psychology (Russell 2002) and is now widely used in various disciplines, such as earth sciences and oceanography (Bopp and Biggs 1981;Dimitriadou et al. 2019;Zheng et al. 2020c). This study used FA to reduce the dimensionality of indicators. First, FA eliminated useless indicators. Then, a large number of potentially relevant indicators were transformed into several unrelated main factors through linear combination, and fewer main factors were used to reflect the sustainable information of the island. The factor weight was obtained by calculating the variance contribution rate of the main factor in the dimensionality reduction process. FA is used to determine the weights in the evaluation of sustainable development, which can not only complete the sustainability evaluation of an island but also complete the comparison of development levels between different islands (Fu and Ma 2016;Yang et al. 2020). The study uniformly evaluated seven islands to test the feasibility of the evaluation method. Using different kinds of islands as samples for unified evaluation not only reduces the amount of necessary calculation for the individual evaluation of each island but also helps analyze the development of different types of islands. This approach is conducive to the state's hierarchical and classified management of islands and the macrocontrol of the development of different types of islands. The evaluation provides a theoretical basis for the sustainable development of the islands and a decision-making basis for the formulation of island development measures.

The study area
The results of the national survey of island names in sea areas show that there are more than 11,000 islands in China (Pan et al. 2018), including 12 major island counties (Zhao and Zheng 2017). The total area of islands accounts for approximately 0.8% of China's land area. According to whether the island has a household registration, it is divided into resident islands and nonresident islands. The evaluation model requires that the evaluation objects have universal applicability, so the study selected seven representative islands in terms of residents' life, tourism, natural environment, and ecology. The selected islands and reasons for selection are given in Table 1. The geographic locations of the seven selected islands are shown in Fig. 1.

Data acquisition
Island surveys are the basis for ecological environmental quality assessments of islands. Data acquisition methods include statistical yearbooks, literature inquiries, field surveys and measurements, satellite remote sensing, and camera equipment monitoring. The data in this experiment came from various regional government portals and district and county statistical yearbooks, which were authoritative and representative. The study collected the data of China's seven islands in 2019. The data acquisition URL is given in Table 2.

Method
The evaluation system is divided into the following four steps: the construction of the index system, the standardization of data, the determination of weight, and the comprehensive processing of index factors. The framework of the model is shown in Fig. 2.

Construction of indicator system
The index system goes through two steps: theoretical screening and dimensionality reduction screening. First, indicators were selected theoretically through the literature. Second, according to the SPSS FA, correlation screening and dimensionality reduction experiments were carried out on the indicators. Reflecting island sustainability information with fewer irrelevant main factors weakens the problem of inaccurate evaluation caused by repeated calculation of related indicators. In the theoretical screening stage, referring to the high-frequency indicators in the field of sustainable development evaluation in databases such as Science Direct, Web of Science, and China National Knowledge Infrastructure (CNKI), based on regional sustainable development journal literature (Fu and Ma 2016;Tang et al. 2019;Che et al. 2021), this study established the natural environment and social environment as the first-level indicators. It is necessary to evaluate the sustainability of the island from a comprehensive and multiangle perspective. Under the first-level indicators, secondary indicators were set up in terms of culture, economy, social development, resources, ecology, and environment. As a geographical unit different from the land, the island's sustainability has its own characteristics.
Then, referring to the characteristics of the island area, adjusted the indicator settings in the island sustainable development evaluation model (Ke et al. 2013;Gao et al. 2019;Nesticò and Maselli 2020), such as increasing indicators of cultural tourism, fishery economy, and tourism economy. Simultaneously, islands of different types and geographic locations will lead to differences in evaluation objectives. In the actual evaluation process, indicators were selected based on the functions of different islands. For the indicator division of uninhabited islands, social factors were negligible, focusing on their natural ecological environment. For tourist islands, the factors of island tourism were appropriately increased. For islands with special resources, emphasis was placed on the evaluation of their resource protection. A total of 38 indicators were covered to build an evaluation system for island environmental value elements. The evaluation index system is shown in Fig. 3.

Data standardization
The different formats of the data in the evaluation system result in the incomparability of different indicators, so the selected indicator data should be normalized. The SAVEE method (Chen 2011) provides standardized equations to realize the normalization of index data. This study improved the SAVEE standardized equations to achieve the normalization of quantitative data. The study used qualitative adjustment of the k value in the standardization formula to standardize different types of island index data to the same dimension to achieve the same evaluation standard. For quantitative data, within the limit distance X, the research object value b was standardized according to the equation in Table 3.
According to the impact of a single indicator on the total value of the island's environment, the index was divided into the positive correlation index, negative correlation index, and normal distribution index. The standardized formulas are showed as follows: The area is relatively small, the permanent population is small, and the island has a unique landscape. The island's economic development is dominated by tourism. Miaodao Islands (D4) Nanji Island (D5) Ecology Shedao Island (D6) The only island in the world where only vipers live. There is a national protection zone for vipers on the island to protect snake resources. Shanhu Island (D7) It is an island composed of coral reefs. The island is rich in coral and guano resources. It is one of the islands with the richest phosphate rock.
Positive correlation index: In the formula, y is the standardized value, b is the unstandardized index value, X is the limit distance value, and k={1, 3, 5, 7, 9} (k takes the value according to the degree of discretization of the index data; the greater the degree of discretization, the greater the value of k.) (1) y = − e k×(− ×(b+ )÷X)

Negative correlation index:
In the formula, y is the standardized value, b is the unstandardized index value, and X is the limit distance value.
Normal distribution index: (2) y = e − ×(b+ )÷X Fig. 1 Geographical location of the target island 20951 Environmental Science and Pollution Research (2022) 29:20947-20962 In the formula, y is the standardized value, b is the unstandardized index value, µ is the mean, and σ is the standard deviation.
The quantification of the qualitative index of the island environmental index system adopts the expert scoring method. The expert scoring method is a method of quantifying qualitative descriptions. The purpose is to count, analyze, and integrate the opinions of all participating experts and finally reach a consensus (Yang et al. 2019). Considering the impact of the indicators on the overall sustainable development of the islands and the comparative differences of the indicators between different islands, this study quantified the qualitative indicators based on the 0.1-0.9 scoring standard. The scoring standards are given in Table 4. When qualitative indicators were quantified, a number of experts in related fields score secondary qualitative indicators. The value of (3) y = e − 1 2 × − 2

Determination of weight
The contribution of indicators to sustainability is not the same (Mikulic et al. 2015), so indicator weighting is a necessary step. Determining the weight of the evaluation index is difficult in the construction of the evaluation system. There are many methods to determine the weight. Each method has its advantages and disadvantages (Ni 2002). Subjective weighting methods are highly subjective, and the data cannot be true and reliable. The objective weighting method uses rigorous mathematical algorithms and requires accurate data. A total of 38 indicators were selected in the study. Among them, some indicators may be relevant, making the indicator system and weights inaccurate. Therefore, to scientifically determine the weight of the index and accurately summarize the situation of the island, FA was adopted to determine the weight. This study used SPSS software to perform FA experiments to reduce the dimensionality of indicators and obtain weights. The main factor obtained through FA was the linear combination of variables. The weight was obtained based on the variance contribution rate of the main factors. FA requires that the number of indicators be less than the number of samples. However, this study contains 38 indicators and 7 samples in total. Therefore, this study considered both the principles and data, divided the indicators into different element layers and modules, and performed FA on each module. First, the indicators were qualitatively divided into five element layers, including the social element layer, tourism element layer, ecological element layer, resource element layer, and economic element layer. Using the average grouping method to weight the elements layers, the weight of a single elements layer was 20%. Then, factor dimensionality reduction analysis was performed on the selected indicators of each element layer. In the experiment, 10 invalid indicators were removed, and the dimensionality reduction operation was analyzed for the remaining 28 indicators. Through experiments, it was found that dividing the five element layers into 6 modules can simultaneously meet the KMO and Bartlett test conditions (KMO>0.5 & Sig<0.05). Then, FA dimensionality reduction was performed on the 6 modules. After continuous index adjustment, the FA divided the 28 indices into 9 main factors (Fig. 4).
FA can reduce the dimension of the index and transform the general index into several groups of unrelated comprehensive factors through linear combination. The main factor is the linear combination of the index, and the score of the main factor can be obtained according to the score coefficient matrix.
The main factor weight formula of FA is as follows: In the formula, ω i is the weight of the index, and e i is the contribution rate of the main factor.
The main factor score formula is as follows: In the formula, Y i is the score of the main factor, A ij is the score coefficient matrix, and y is the standardized value of the index.

Comprehensive processing of index factors
The index weights are integrated to analyze the overall environmental conditions of the island, and the weighting formula is shown as follows: Table 3 Quantitative data standardization equation There is a negative correlation between AQI and island value.
Educational institutions (300), total tourism income (500,000 yuan), total reception (800,000 people), per capita GDP (100,000 yuan), rural per capita GDP (100,000 yuan) The index is positively correlated with the island value, and the extreme value is selected according to the actual situation and expert experience.
Fitness venues (50), minimum living allowance (1500 yuan), total agricultural output value (10,000,000 yuan), total industrial output value (10,000,000 yuan), total retail sales of consumer goods (10,000,000 yuan), total fishery output value (1,500,000 yuan) The index is positively correlated with the island value, and the extreme value is selected according to the actual situation and expert experience.
Average annual precipitation (1200 mm), temperature (16℃), population per unit area (4 people/hm2) The index and the quantity of value have a normal distribution, µ is the mean, and σ is the standard deviation according to the actual situation. In the formula, ω i is the weight and Y i is the standardized value of the ith main factor.

Index dimensionality reduction and weight gain
The 38 secondary indicators were divided into 20 qualitative indicators and 18 quantitative indicators. The quantitative indicators included 13 positive correlation indicators, 1 negative correlation indicator, and 3 normal distribution indicators. Quantitative data were standardized by the SAVEE standard equation, and qualitative data were directly converted into a percentile form. Then, the qualitative and quantitative data were standardized. The standardized index value was transformed into a percentile form. The standardization results are shown in Table 5.
Through FA experiments, 28 indicators were reduced into 9 main factors. The weights and scores of the main factors are calculated by using Formulas (4) and (5). The weights (6) S = ∑ i × Y i and score calculation formulas of the main factors are given in Table 6.

FA condition test
FA requires KMO and Bartlett test conditions (KMO>0.5&Sig<0.05). The research carried out KMO and Bartlett tests on 6 modules. As shown in Fig. 5, the KMO test results of the 6 modules were all greater than 0.5, and the Bartlett sphere test results were all less than 0.05. The results meet the conditions of the FA variable test and prove that the adjusted index can be used for dimensional reduction experiments through FA.

Island factor score
The total scores of social factors, tourism factors, ecological factors, resource factors, and economic factors are calculated using Formula (6). The score results are shown in Table 7. Figure 6 shows that the individual factor scores of social islands, tourist islands, and ecological islands all have a downward trend. Compared with other factors, the downward trend of the resource factor is more moderate. The scores of social, tourism, ecological, resource, and The scores of the five factors of social islands are higher than those of tourist islands and ecological islands,  Index  Island  D1  D2  D3  D4  D5  D6  D7   b1  100  100  14  28  5  1  1  b2  60  100  100  100  100  100  81  b3  90  30  30  30  30  30  50  b4  100  97  96  16  66  21  83  b5  100  100  51  88  100  61  25  b6  80  80  90  90  80  50  90  b7  78  81  72  84  59  79  92  b8  80  90  70  90  70  70  70  b9  90  90  90  90  80  90  90  b10  90  80  -----b11  80  90  -----b12  90  70  80  90  80  90  60  b13  70  90  90  90  90  30  -b14  60  60  90  80  50  30  50  b15  50  50  90  70  30  30  30  b16  70  60  90  90  90  50  80  b17  70  90  50  80  90  --b18  50  50  90  90  90  80  30  b19  90  90  70  60  70  30  50  b20  ---90  -90  90  b21  80  58  62  46  88  --b22  83  98  -----b23  84  75  -----b24  90  74  -----b25  90  90  70  80  70  50  50  b26  90  80  -----b27  90  80  -----b28  90  90  70  60  70  10  30  b29  90  90  90  80  80  50  60  b30  13  4  -----b31  73  94  -----b32  34  78  -----b33  22  31  -----b34  21  100  3  7  7  --b35  77  100  44  97  19  --b36  99  100  43  89  12  --b37  86  90  89  100  92  --b38  75  79 ----- indicating that social islands have the highest level of development and utilization. The social, ecological, resource, economic, and tourism factors are all at a relatively high level, which implies that the development of social islands is balanced. Ecological islands are mostly uninhabited islands. Although ecological islands are rich in ecological resources, they are ecologically fragile, and the islands are almost in an undeveloped state. Tourist islands are rich in natural landscapes, mainly for the development of tourism. The scores on ecological factors of ecological islands are significantly higher than the values of other factors. Shanhu Island has valuable resources such as coral reefs, and Shedao Island has unique snake resources. Therefore, both islands have high ecological protection value. The evaluation result is consistent with the actual situation of the islands. The social factor score reflects the quality of life, education, and social security of the entire island to a large extent. The tourism factor score is set according to the geographical characteristics of the island and reflects the sustainability of the island's tourism industry. The ecological factor score presents the island's ecological level and the island's ecological protection and is also an important factor for the sustainability of the island. The resource factor is similar to the ecological factor and is an important factor for the island's sustainability, reflecting the utilization of the island's resources. The factor score intuitively reflects the level of economic development of the island. Analyzing the interrelationship between the island main factors shows that the changing trends of island social sustainability, tourism sustainability, ecological sustainability, resource sustainability, and economic sustainability are consistent (Liu et al. 2021). This consistency shows that the sustainability of a single element of the island has the potential to drive the sustainability of other elements. This potentiality is of great significance to the development of islands based on their advantages in island planning. For example, the establishment of ecological landscapes on tourist islands promotes the development of island tourism, thereby promoting the overall economic development of the islands and driving the sustainability of the entire island society.
Analyzing the results of the island factor score in Fig. 6, the score of a single factor shows a downward trend,  indicating that there may be a certain correlation between the factors. Figure 6 shows that all factors show the same changing trend. The main reason may be that in the process of island development, society, tourism, ecology, resources, and economy will restrict and influence each other. Compared with other factors, the change trend of the resource factor is more gradual. After analysis, this may be because resource factors are less affected by human activities than social factors, ecological factors, tourism factors, and economic factors. Social, tourism, economic, and ecological factors are formed by human participation in the construction of islands, which are more affected by human activities and have more drastic changes. This observation shows that the value of an island is not only affected by the characteristics of the island itself but also restricted by human development and construction activities.

Island total value score
Formula (6) is used to obtain the total score of the island. The total score of the island is given in Table 8. Table 8 shows that the Zhoushan Islands have the highest score of 87.625, and Snake Island has the lowest score of 20.587. Social islands such as Chongming Island and Zhoushan Islands have the highest scores, exceeding 70; tourist islands have a medium score, approximately 40; ecological islands have the lowest score, approximately 20.
The comprehensive evaluation results of different types of islands show that the value of balanced development islands is higher than that of unbalanced development islands or undeveloped islands (Zhou et al. 2015). The comprehensive value of social islands is higher than that of tourist islands, while the comprehensive value of ecological islands is the lowest. The total value of Chongming Island and Zhoushan Islands is the highest compared to other islands. The results indicate that the development of Chongming Island and Zhoushan Islands is in good condition, the facilities on the islands are well constructed, and the islands have the strongest development level. This phenomenon is due to the large area of Chongming Island and Zhoushan Islands, the large population base, and the high level of social development that drives the level of island development and utilization. Their social development and the island's natural level are relatively balanced. Weizhou Island, Miaodao Islands, and Nanji Island are second in value. This is mainly due to the relatively small area of the island, the small inhabitant population and the uneven development of the island. The islands have a focus on development according to their own characteristics. There are some residents  living on Weizhou Island, Miaodao Islands, and Nanji Island. The islands have rich and unique natural landscapes. Good natural scenery and ecology has attracted many tourists. The residents of the islands mainly make a living from fishing and tourism. Therefore, these three islands are defined as tourist islands. Due to their geographical location and geographical area, these islands are not suitable for many residents. The degree of island development and construction is lower than that of Chongming Island and Zhoushan Islands. The islands mainly rely on the development of tourism, and their value will be relatively low. Shedao Island and Shanhu Island have the smallest value and the weakest level of development. They are in the undeveloped or weakly developed stage. The two islands are uninhabited islands and are almost undeveloped. The islands are very small and lack freshwater resources, which is not conducive to the lives of residents. Shedao Island is the only island in the world that survives a single species of blackbrowed vipers. The toxins of vipers have scientific research value. China has established nature reserves to protect snake resources and the survival and reproduction of vipers. Shanhu Island is an island composed of coral reefs. The island is rich in corals and guano resources. It is one of the islands with abundant phosphate rock. Shedao Island and Shanhu Island are islands that cherish ecological resources. Although the degree of development and construction of the islands is very weak, the ecological value of resources cannot be ignored. The study of the overall value of islands requires a comprehensive and multiangle evaluation and monitoring of the sustainable development level of island society, tourism, ecology, resources, and economy. In this study, these factors and the characteristics of the island were comprehensively considered, formulate an island development plan, and promote the sustainable development of the island. Currently, the contradiction between island development and the environment is more prominent. For islands with comprehensive and balanced development, such as social islands, attention is given to the relationship between balanced development and the environment. As the development of the island continues, the island's environmental protection plan must be carried out. For islands with unbalanced development, such as tourist islands, while developing the island's advantageous industries, attention should be given to the island's social sustainability and ecological sustainability to move toward balanced development. The unified evaluation of different types of islands is of great significance to the hierarchical and classified management of islands.

Conclusion
The changing trends of island social sustainability, tourism sustainability, ecological sustainability, resource sustainability, and economic sustainability are consistent, indicating that the sustainability of a single element of the island has the potential to drive the sustainability of other elements. This potentiality is of great significance to the development of islands based on their advantages in island planning. The value of fully balanced islands is higher than that of unbalanced or undeveloped islands. The sustainability of islands requires comprehensive and balanced development, especially the relationship between balanced development and ecology. The unified evaluation of different types of islands can provide services for the hierarchical and classified management of islands. After the island is evaluated, it can be classified by level and category, and then the manager can develop and utilize the island rationally according to the island's own characteristics.
This study used FA to process indicators. The theoretically selected island sustainability indicators may have relevance, resulting in inaccurate evaluation of island sustainability. This method considers the correlation between indicators and reduces the dimensions of multiple indicators. FA turns indicator variables into low-dimensional uncorrelated main factors. The main factor is a linear combination of indicator variables. The FA used a small number of irrelevant main factors to reflect the island's sustainability information. The FA method can not only complete the sustainability evaluation of an island but also complete the comparison of development levels between different islands. This research also proposed different standardized equations through the analysis of different types of island indicators. For indicators of different types of islands, appropriate standardization weights were set to make data standardization fair. The limitation of this study is that some indicators cannot obtain quantitative data, only qualitative descriptions. This study used the expert scoring method to take the average of many expert opinions to make the quantification of qualitative indicators as less subjective as possible. To date, FA has been successfully applied to inland urban ecosystems by previous experiments Fu and Ma 2016). In this study, FA was successfully applied to island ecosystems, and its applicability in open coastal ecosystems needs to be verified. Data availability In this study, the environmental data of the seven islands mainly come from two kinds of websites (Table 2). One is the official website of local government, including the municipal government official website and the district government official website, and the other is the local tourism website. Among them, most of the indicator data come from the statistical yearbook on the government website. The datasets of both websites are open to public readers.

Declarations
Ethics approval and consent to participate Not applicable.
Consent for publication All authors are responsible for the article and agree to publish.

Competing interests
The authors declare no competing interests.