Spatial-Temporal Evolution Patterns and Obstacle Factors of Urban–Rural “Economy–Society–Ecology” Coordination in the Yangtze River Delta

Abstract

Background: In the context of sustainable development, urban–rural integration is not solely focused on high economic growth but has been expanded to a wider range of social and ecological fields. Objectives: To analyze the spatial and temporal evolution of the coupling coordination level of urban–rural economic, social and ecological integration subsystems in the Yangtze River Delta and to explore its main obstacles, in order to provide countermeasures to promote the sustainable development of urban and rural areas. Methods: Panel data of 16 cities in the core area of the Yangtze River Delta from 2005 to 2020 were selected, and the entropy method was used to calculate the comprehensive development level of each subsystem, based on which the coupling coordination degree was measured by the coupling coordination model, and the obstacle degree of each indicator was obtained by the obstacle degree model. Outcomes: The urban–rural integration level kept a steady rise, and the “economy–society–ecology” coordination level transformed from borderline imbalances to moderate coordination. The high-value areas presented an initial “Π” shape distribution that later turned into a “>” shape, while the backward areas were primarily located at both ends of the north and south. The key obstacle factors included per capita GDP, population urbanization rate, urban and rural basic pension insurance coverage, faculty–student ratio in urban and rural primary and middle schools, per capita park green land, road network density and fertilizer application per unit area of sown. Recommendations: The systematic coordination of urban–rural integration should be promoted according to local conditions, with emphasis on broadening urban–rural communication channels, promoting the equalization of urban–rural public services and establishing the urban–rural collaborative mechanism for environmental maintenance.

1. Introduction

The impact of urban–rural relations on the social landscape is significant, given its universal role as a modality facilitating linkages and mutual interactions between urban and rural domains. Driven by industrialization and urbanization, the demographic characteristics, spatial structures and development patterns of city and country in various regions have undergone huge transformations [1], which have driven changes in the dynamic movement and accumulation of production inputs [2], gradually leading to rural exodus [3], a low level of science and technology, and inadequate supply of basic public services [4]. Therefore, developed countries have taken the lead in focusing on urban–rural relations to rethink extensive modes of economic growth and urbanization [5], redefining the meaning of rural existence, and implementing beneficial measures such as agricultural modernization and the reallocation of excess rural labor [6,7]. Being the representative developing country globally, China also possesses an enduring system of urban–rural dual structure [8]. According to data from the National Bureau of Statistics of China, the ratio of per capita disposable income between urban and rural residents in 2021 was 2.50, a decrease of 0.38 compared to 2012. This indicates that the relative income gap between urban and rural residents continues to narrow under the promotion of policies related to new urbanization and rural revitalization. However, the speed advantage cannot conceal the significant gap between China and developed countries and the existing shortcomings of urban–rural integration development still exist. As pointed out by Zhao Junya [9] and Feng Yongtai [10], the gap between urban and rural areas in resource factors such as industrial endowments, living standards, and public services is still significant, leading to a weak foundation and insufficient endogenous driving forces for rural development. Under the wave of global sustainable development, the balance between human social development and ecological environment protection is increasingly valued [11], and thus the urban–rural integration is gradually exposing new problems. Wen Feng’an [12], Zhao Zhiqiang et al. [13] thought that the lack of attention to the field of urban–rural ecological integration in China has led to a series of sustainable development challenges such as the deficit of natural resources, the lack of green governance, and economic momentum conversion, so they called for the establishment of a coordinated urban–rural ecological environment system.

The Yangtze River Delta is a crucial area of economic development in China, with sophisticated urban and industrial evolution, and has representative and pioneering conditions for achieving urban–rural convergence. However, certain inadequacies and imbalances are still presented during the urban–rural interaction because of notable disparities in economic progress, social life, and ecological resources across regions. In particular, the neglect of the inner connection and coordination of urban–rural development has laid hidden dangers for the sustainable and robust advancement of the interdependent urban–rural dynamic, gradually revealing such problems and dilemmas as the significant regional development differences, the intensified urban siphon effect, and the mounting environmental regulatory demands [14,15,16]. Therefore, the study’s objective is to construct a reasonable analytical framework for urban–rural integration development in the Yangtze River Delta, focusing on the coordinated mode among subsystems in the internal structure of the urban–rural integration composite system. The main contributions and innovations are as follows: Firstly, from the theoretical analysis, this article combined the theory of urban–rural continuum with the theory of composite ecosystems to construct an indicator system and measurement method for urban–rural integration development that included three subsystems: economic integration, social integration, and ecological integration. This makes up for the shortcomings of neglecting the internal connections between urban and rural areas in previous research. Secondly, from the research perspective, the article took the lead in paying attention to the interaction relationship between the economic, social, and ecological subsystems within the urban–rural integration system. We made a breakthrough in the single research perspective of calculating the comprehensive score of the composite system and chose to use the coordination level of the three subsystems as the evaluation basis for the sustainable development of urban–rural integration, making up for the previous research’s neglect of the imbalance among subsystems. Thirdly, from the research object, we chose the YRD region to analyze its spatiotemporal evolution characteristics of urban–rural “economy–society–ecology” coordination based on panel data, filling the gap in urban–rural research at the municipal level. Finally, according to the results of the spatiotemporal evolution and obstacle factor analysis, we enriched the countermeasures and suggestions for promoting urban–rural coordinated development in the YRD region and proposed the future research direction.

The remaining research is structured as follows: theoretical and literature review are presented in Section 2, materials and methods are presented in Section 3, empirical findings and results are presented in Section 4, discussion and conclusions are found in Section 5, and Section 6 presents the study’s limitations and prospects.

2. Literature Revie

2.1. Theoretical Background

The study introduced the integration analysis perspective of the urban–rural continuum theory and the composite ecosystem theory as the basis for constructing an analytical framework for the urban–rural “economy–society–ecology” coordination. Among them, the urban–rural continuum theory believes that after an economy enters the stage of urban–rural integration, the interaction between urban and rural society increases, and economic and social characteristics continue to penetrate each other. Therefore, it advocates breaking down the segmented development system, paying attention to the differences within urban and rural areas, and emphasizing the connection and integration. It also attempts to understand the economies that appear in different levels of regions on the urban–rural continuum, political and social phenomena, and their causes [17]. For example, Von Braun [18] combined the urban–rural continuum theory with the regional network theory and found that reducing the cost of factor flow on the urban–rural continuum can enhance urban–rural spatial integration, stimulate its trade increase, and thus improve the level of urban–rural connectivity. Requena [19] and Thiede et al. [20] conducted surveys on residents’ happiness and income using the urban–rural continuum paradigm, and the results showed that relying solely on urbanization cannot solve all problems in urban–rural development. The composite ecosystem theory points out that human development issues are usually inseparable from economic development, social life, and natural environment. Three different subsystems exist and develop independently, but they also affect and constrain each other. They must be judged as a complex and composite system [21]. The theory is usually used to evaluate the comprehensive development status of a certain region or as a basis for regional functional design. For example, Cai Chaoyue et al. [22] evaluated the high-quality economic, social, and ecological development levels of 284 cities in China based on this theory. Ma Jia et al. [23] explored the path of creating ecological landscapes as rural ecological infrastructure by sorting out the interrelationships between rural social, economic, and natural systems. It can be seen that the composite ecosystem theory has played a certain role in the research of urban and rural development planning, but it is still less applied to the research of urban–rural integration and coordinated development issues.

Overall, urban–rural integration development under the urban–rural continuum theory and the composite ecosystem theory requires the integration and coordinated interaction of multi-level fields such as economy, society, and ecology between urban and rural areas, ultimately forming a composite development system of long-term harmonious coexistence and mutual promotion.

2.2. Literature Review

At present, the academic research on urban–rural integration development roughly follows two main lines. The first line is to study the connotation and logical mechanism of urban–rural integration from the perspective of the evolution of urban–rural relations. Marx and Engels’ theory of urban–rural relations believes that the progress of human society is accompanied by the evolution of urban–rural relations, and the transformation of urban–rural relations will go through a stage of urban–rural chaos, urban–rural separation, and urban–rural opposition, ultimately entering the stage of urban–rural integration and development. The urban–rural structure school also believes that the urban–rural development structure undergoes an evolutionary process from early-urban biased to urban–rural dependent and then to urban–rural networked connections, reflecting the multi-level and gradual integration development logic of population, space, economy, and value between urban and rural areas [24,25,26]. Another main line is to evaluate the development of urban–rural integration from the perspectives of management and economics. The urban–rural gap is a manifestation of insufficient urban–rural integration and development, and many scholars focus on analyzing the urban–rural gap from different fields such as healthcare, education, human resources, and social welfare [27,28,29,30]. In addition, some scholars have focused on measuring the level of urban–rural integration. One approach is to select an appropriate indicator system. It is common to calculate comprehensive scores from multiple dimensions such as economy, space, and life to quantify the level of development [31,32]. Some scholars have also taken a different approach. Guo Haihong et al. [33] broke away from the static perspective and reflected the dynamic evolution of urban–rural integration from three aspects: premise, motivation, and outcome; Lu Yangchun et al. [34] constructed an indicator system of “elements–structure–function”; Cao Xue et al. [35] focused their research on the flow of urban–rural factors and constructed a “people–land–industry” evaluation system. The second is to choose an appropriate measurement method. The measurement methods for the level of urban–rural integration development mainly include the entropy weight method [31,34], the principal component analysis method [35], and the analytic hierarchy process [36]. The most commonly used method is the entropy weight method, which, as a classic objective weighting method, can effectively reduce the impact of subjectivity on decision-making results.

2.3. Evaluation Summary

The above literature explored the evolution path of urban–rural relations from fragmentation to integration from a theoretical perspective and evaluated regional urban–rural integration from an empirical perspective, providing a certain reference and basis for this study, but there are still certain shortcomings. Firstly, existing quantitative research on urban–rural integration development mostly focuses on the national or provincial level, with few evaluating urban agglomerations, especially for the YRD region. Therefore, this article selects this representative frontier area of urban–rural development as the research object, filling the gap in urban–rural research at the level of urban agglomerations. It can provide experience and reference for the subsequent development of most regions in the country. Secondly, most studies consider urban and rural areas as independent regional units and construct indicator systems separately, or simply weight and stack the development levels of various systems to obtain the level of urban–rural integration development in a region, neglecting the interaction relationship between urban and rural areas and various subsystems within the urban–rural development system. Therefore, this article combined the urban–rural continuum theory with the composite ecosystems theory to improve the traditional segmented urban–rural development indicator system based on the theory of binary structure. From the perspective of urban–rural interaction, it constructed an urban–rural integration development indicator system that included three subsystems: economic integration, social integration, and ecological integration. With the help of the entropy weight method, the development level of each subsystem was determined. On this basis, drawing on the research method of Zhou Wenhui et al. [37] to evaluate the mutual influence and coordination degree of various elements within the system, a coupling coordination model was selected to calculate the level of urban–rural “economic–social–ecological” coordination as the evaluation basis for sustainable development of urban–rural integration, effectively avoiding the deviation of evaluation results caused by the hidden imbalance between subsystems. Finally, although some scholars have explored the influencing factors of urban–rural integration development, most of them are limited to single resource elements such as human resources, land, and public services [38,39,40], and most are concentrated at the economic and social levels, lacking a comprehensive analysis of the internal elements of the composite urban–rural development system. Therefore, we refer to the research methods of Qiu Shuang et al. [41] and Liu Jinfeng et al. [42] to introduce an obstacle degree model in order to better explore the internal key factors hindering the development of urban–rural “economy–society–ecology” coordination in the YRD region.

3. Materials and Methods

3.1. Research Region

The Yangtze River Delta (YRD) is a region in eastern China whose core areas include 16 cities in Jiangsu Province, Zhejiang Province, and Shanghai Municipality (Figure 1). It is known for its strategic location on the coast, fertile land, and thriving economy. However, due to the expansion of the construction scope and the vicissitude of the region caused by the flow of resources, the heterogeneity in the YRD region has become increasingly prominent, showing the characteristics and evolution patterns of integrated development between urban and rural areas at various stages.

3.2. Data Sources

This paper utilizes panel data from 16 cities located in the YRD region, covering the period between 2005 and 2020. The main data are obtained from various publications, including China City Yearbook, China Rural Yearbook, China Statistical Yearbook for Regional Economy, and official yearbooks of Chinese provinces and cities, as well as government websites or bulletins. Some of the missing data are calculated using the linear interpolation method and then supplemented.

3.3. Research Indicators

Urban–rural integration is not about eliminating differences between urban and rural areas but rather placing them on an equal footing so that they can blend and permeate each other, forming a unified entity with close connections, complementary functions, and shared benefits. The urban–rural integration is a unified development of interdependence, close cooperation, and reasonable structure between urban and rural areas at the economic, social, and ecological levels. Therefore, when selecting evaluation indicators, this article combined the urban–rural continuum theory with the composite ecosystems theory to establish an indicator system for the development level of urban–rural integration based on the principles of scientificity, systematicity, integrity, and accessibility. There are three subsystems: economic integration, social integration, and ecological integration, including a total of 24 indicators (

Table 1. Evaluation indicator system for urban–rural integration.

Subsystem	Primary Indicator	Secondary Indicator	Attribute	Weight
Urban–Rural Economic Integration	Economic state	A1 Per capita GDP/yuan (RMB)	+	0.124
	Industrial Development	A2 Proportion of non-primary industry output value/%	+	0.033
		A3 Proportion of rural non-primary industry employment/%	+	0.024
	Resident income	A4 Ratio between disposable earnings of urban and rural residents/%	−	0.029
		A5 Ratio between salaries of urban and rural residents/%	−	0.033
	Resident consumption	A7 Ratio between consumer spending of urban and rural residents/%	−	0.055
		A8 Ratio between Engel’s coefficient of urban and rural residents/%	+	0.035
Urban–Rural Social Integration	Urbanization	B1 Population urbanization rate/%	+	0.095
	Transportation and communication	B2 Ratio between transportation and telecommunication expenses of urban and rural residents/%	−	0.025
		B3 Road network density/%	+	0.055
	Culture and education	B4 Ratio between cultural and educational expenses and of urban and rural residents/%	−	0.029
		B5 Faculty–student ratio in urban and rural primary and middle schools/%	+	0.086
	Medical and healthcare	B6 Ratio between healthcare expenses of urban and rural residents/%	−	0.021
		B7 Licensed physician coverage of urban and rural residents/%	+	0.023
	Social Security	B8 Ratio between transfer incomes of urban and rural residents/%	−	0.030
		B9 Basic pension insurance coverage of urban and rural residents/%	+	0.093
Urban–Rural Ecological Integration	Ecological greening	C1 Greening coverage of built-up land/%	+	0.009
		C2 Park green land per capita/m2	+	0.084
	Energy saving and emission reduction	C3 Energy intensity of GDP/t/yuan (RMB)	−	0.029
		C4 Fertilizer application per unit area of sown land/t/hm2	−	0.046
	Environmental Governance	C5 Solid waste recycling rate/%	+	0.016
		C6 Domestic waste non-hazardous treatment rate/%	+	0.006
		C7 Centralized treatment rate of sewage/%	+	0.019

).

(1)

Economic integration subsystem

The integration of urban and rural economy is the material foundation for promoting common prosperity, including four primary indicators: urban–rural economic state, industrial development, resident income, and resident consumption. Among them, the growth of the urban–rural economy is a prerequisite for economic integration, generally manifested as the final result of production activities, and the linkage and upgrading of urban–rural industries are its key supports, closely related to the urban–rural economic structure and labor division [43]. Therefore, referring to the indicator system of Ma Zhifei [32] and Liu Peng [44], the per capita GDP was selected to measure the urban–rural economic state, and the proposal of non-primary industry output value and rural non primary industry employment were selected to reflect the level of urban–rural industrial development. In addition, the material foundation directly reflects the wealth level of urban and rural residents—that is, the income level of urban and rural residents—and then reflect their consumption willingness and ability. Specifically, this involves relying on effective industrial interaction between urban and rural areas to narrow the wealth gap of residents, achieve a mutually beneficial economic cycle, and ultimately move towards integration. Referring to the study by Shi Jiangang et al. [31], four indicators were selected to measure these: the ratio between disposable earnings, salaries, consumer spending, and Engel’s coefficient of urban and rural residents.

(2)

Social Integration Subsystem

Urban–rural social integration is the basic value orientation of urban–rural integration development, closely related to residents’ lives. It refers to overcoming the obstacles of regional division between urban and rural areas, improving the accessibility through the construction of transportation facilities, accelerating the circulation speed of population, materials, and other factors, thereby narrowing the allocation gap of various social resources and enabling urban and rural residents to enjoy public facilities and services equally and fully. This improves their social quality of life, including five primary indicators: urbanization, transportation and communication, culture and education, medical and healthcare, and social security. Among them, solving the problem of farmers entering cities is the first step in promoting urban–rural social integration and is a prerequisite for achieving diversified employment of rural population and promoting large-scale agricultural production. This needs to be measured by the population urbanization rate, which represents the proportion of urban population to the total population [45]. In terms of the evaluation of urban and rural facility construction and resource allocation, this article combines the research indicators of Guo Haihong [33], Wang Dachao [46], and Gao Jing [47] to divide it into two parts: the urban–rural service expenditure gap and the urban–rural equal supply. The former represents the willingness and ability of urban and rural residents on the demand side to enjoy public facilities and services, while the latter represents the accessibility and adequacy of various resources on the supply side. Therefore, at the demand level, we selected the ratio between transportation and telecommunications expenses, cultural and educational expenses, healthcare expenses, and transfer income of urban and rural residents as the secondary indicator. Among them, transfer income refers to various transfer payments made by the state, units, and social organizations to households and income transfers between households, including government retirement pensions, unemployment benefits, compensation, etc. Generally, local finance is relatively abundant, and the average transfer income of residents is relatively high. Therefore, the urban–rural transfer income gap is also one of the main reasons for the urban–rural social security gap. At the supply level, the ratio between road network density, the faculty–student ratio in urban and rural primary and middle schools, licensed physician coverage, and basic pension insurance coverage of urban and rural residents were selected as a secondary indicator. Among them, road transportation is the most common mode of transportation between urban and rural areas, and road network density represents the close degree of spatial correlation between urban and rural areas. Faculty–student ratio and licensed physician coverage reflect the service quality and efficiency of schools and hospitals, respectively, representing the supply level of education and medical resources in a region. Basic pension insurance refers to the system in which social insurance agencies pay pension and other benefits to workers in accordance with the law after they reach the retirement age set by the state or exit their employment due to other reasons. Its coverage rate to some extent reflects the level of social security and corresponding public service capabilities in urban and rural areas and is a guarantee of social harmony and stability in urban and rural areas.

(3)

Ecological integration subsystem

The integration of urban–rural ecology is the environmental foundation and resource guarantee for the integrated development of urban and rural areas, suggesting this may transform the urban–rural development model of pursuing economic growth at the excessive environmental loss by promoting ecological environment construction and adjusting production and lifestyle. While maintaining the fundamental green and sustainable development of the region, it needs to improve the living environment of urban and rural areas and fully and efficiently utilize resources, thereby enhancing the production and renewal capacity of the overall urban and rural environment. Therefore, the ecological integration subsystem specifically includes three primary indicators: ecological greening, energy saving and emission reduction, and environmental governance. Among them, the measurement method of ecological greening referred to the indicator system proposed by Xie Lei et al. [48], selecting two secondary indicators, namely green coverage of built-up land and park green land per capital, to reflect the basic living environment of urban and rural residents. Energy intensity of GDP and fertilizer application per unit area of soft land were used to measure the level of energy saving and emission reduction. The former represents the total amount of energy consumed in industrial production converted into standard coal, while the latter represents the total amount of fertilizer used in agricultural planting, reflecting the energy consumption and possible pollution emissions of major production activities in urban and rural areas. At the level of environmental governance, we referred to the indicator system proposed by Shi Jiangang et al. [31] and Zhang Haipeng et al. [49] to select three indicators: solid waste recycling rate, domestic waste non-hazardous treatment rate, and centralized treatment rate of sewage. These indicators comprehensively demonstrate the treatment and reuse level of the main pollutants generated in the production and living processes. Enhanced capabilities at this level can effectively reduce the bidirectional negative impact of various pollutants spreading and flowing through natural media such as the water and atmosphere between urban and rural areas [50], achieving a harmonious ecological environment.

3.4. Research Methods

3.4.1. Coupling Coordination Model

After standardizing the original data, the indicator weight w (Table 1) was obtained using the entropy method. Then the comprehensive score of urban–rural integration and its three subsystems could be calculated by using Equation (1).

$$F_{ij}=\sum _{j=1}^m{w}_j{X}_{ij}$$

(1)

$$C_n=\sqrt[3]{\frac{L_i\times M_i\times N_i}{{\left(L_i+M_i+N_i\right)}^3}}$$

(2)

$$D=\sqrt{C_n\times T}$$

(3)

$$T=a{L}_i+b{M}_i+c{N}_i$$

(4)

In the formulas, C_n is the coupling degree among subsystems, indicating the status to which they affect each other. T is the comprehensive level, in which the coefficients are taken as the mean value of 1/3. L_i, M_i, and N_i are the comprehensive scores of the three integration subsystems. D reflects the level of benign coordination as measured by the degree of coupling coordination [51], and its value is categorized as the following (

Table 2. State division criteria of coupling coordination degree D.

D Value	State	D Value	State
(0, 0.2]	Severe instability	(0.5, 0.6]	Primary coordination
(0.2, 0.3]	Moderate disequilibrium	(0.6, 0.8]	Moderate coordination
(0.3, 0.4]	Mild disequilibrium	(0.8, 0.9]	Good coordination
(0.4, 0.5]	Borderline imbalance	(0.9, 1]	Quality coordination

).

3.4.2. Barrier Degree Model

The model mainly measures the degree of factor influence, indicator deviation, and barrier effect. Among them, the factor influence reflects the contribution of each indicator to the rating of urban–rural integrated development; the indicator deviation reflects differences between indicators and evaluation goals; the barrier degree is the negative impact of the criteria layer and factor layer on urban–rural integration.

$$M_j=w_j\times R_{ik}$$

(5)

$$I_{ij}=1-X_{ij}$$

(6)

$$O_{ij}=\frac{M_j\times I_{ij}}{{\sum }_{j=1}^m{M}_j\times I_{ij}}\times 100\mathrm{\%}$$

(7)

$$O=\sum O_{ij}$$

(8)

In the formulas, M_j denotes the factor contribution; w_j represents the indicator weight; R_ik is weights’ sum of the secondary indicators; I_ij shows the indicator deviation degree; and X_ij is the standardized indicator. O_ij is the barrier degree of the jth indicator to the urban–rural coordinated advancement, and O is the barrier degree of the corresponding criteria layer.

4. Results

4.1. Analysis for the Development Level of Urban–Rural Integration

As shown in Figure 2, the development level of urban–rural integration steadily increased from 2005 to 2020, with an overall score of 86.28%, improving from 0.3673 to 0.6842. Among them, the social integration trend was the most similar to the comprehensive one, with a gradual improvement of 91.92%. At the beginning of the examination, the integration development of the ecological subsystem was leading, while the economic subsystem was the most backward. However, the latter’s development rate stayed ahead and caught up with the social and ecological integration subsystems after 2016 to become the subsystem with the highest overall score. The comprehensive development score of urban–rural ecological integration improved from 0.4781 to 0.6649, with an improvement rate of 39.07%, which was much lower than the improvement rate of the development of other integration subsystems, indicating that there may have been too much attention given to economic development, sacrificing ecological environment development. There still existed the phenomenon of uncoordinated development among the subsystems.

4.2. Analysis for the Coupling Coordination Level of Urban–Rural Integration

The coupling degree and coupling coordination degree of the economic, social, and ecological integration subsystem in the YRD region from 2005 to 2020 were taken to draw Figure 3. The coupling degree ranged between 0.64 and 0.67, with a stable level of grinding-in. The development state of coupling coordination included three types, from borderline imbalance to primary coordination and then to moderate coordination, respectively, with 2006 and 2012 as the time points. It indicated that, with the progress of society and the strengthening interaction among the three systems, the integration gradually approached the good relationship of complementary and coordinated development but still needed to be optimized with the development goal of high-quality coordination.

4.2.1. Time Series Evolution Characteristics

The coupling coordination degree of 2005, 2010, 2015, and 2020 were selected and plotted in Figure 4, where the horizontal line represented the relevant values, and the vertical axis represented the corresponding kernel density.

Firstly, according to the barycenter of the kernel density curve, it shifted to right year by year from 2005 to 2020, and the magnitude of the shift between 2005 and 2010 was larger than that between 2010 and 2020, indicating that the urban–rural coordination level increased rapidly in the early period, but then it fell. Secondly, the curve shape changed from “flat and wide” to “sharp and narrow”, which indicated that the coordinated development of each region “advances in tandem”. Therefore, cities distributed at both ends of the curve decreased. The right trailing edge shrank year by year, and the left trailing edge tended to lengthen significantly during 2015–2020, indicating that the number of cities on the left side gradually exceeded that on the right side, meaning that cities with a less-than-average coupling coordination level of urban–rural integration were more common. Finally, the height of the curve crest steadily climbed, implying that the regional differences in the coupling coordination level of urban–rural integration tended to narrow. In 2015, the major peak was on the left side, but it had an obvious “bimodal” shape, and the right side of the secondary peak was close to the main peak, meaning that there were high-quality integration cities emerging at that time, which made the integration performance of the YRD region show a bipolar pattern. In 2020, the multiplier-developing trend had been eased, but there was still a situation of “peaking” on the trailing edge of the left side, and the low-value cities area saw an uptick.

4.2.2. Spatial Pattern Evolution Characteristics

The spatial distribution maps were drawn for the years of 2005, 2010, 2015, and 2020 (Figure 5). The level of coupling coordination was classified into four hierarchical gradients from low to high, using the natural breakpoint method. Different states of coordination were represented by triangular, square, and circular icons, according to the classification criteria in

Table 3. Barrier degree of the main obstacle factors.

Year	A1	B1	B9	B5	C2	B3	C4
2005	12.88%	12.22%	12.16%	8.38%	6.86%	5.85%	5.72%
2006	13.23%	12.60%	12.57%	8.73%	6.71%	5.96%	5.52%
2007	12.35%	12.83%	12.42%	9.29%	6.05%	5.68%	5.87%
2008	12.59%	12.89%	12.08%	8.66%	5.79%	5.27%	6.23%
2009	13.57%	12.85%	11.68%	7.68%	5.93%	4.94%	5.68%
2010	13.23%	12.48%	11.52%	9.18%	6.37%	5.80%	5.52%
2011	13.01%	12.23%	11.08%	8.52%	6.40%	5.77%	4.25%
2012	13.31%	13.08%	10.74%	8.48%	6.41%	5.86%	4.69%
2013	13.21%	12.85%	10.40%	7.86%	7.28%	5.85%	4.57%
2014	13.55%	13.17%	9.19%	8.96%	6.91%	5.85%	4.02%
2015	13.67%	13.33%	9.76%	8.77%	8.33%	6.23%	4.01%
2016	13.25%	13.26%	10.56%	9.12%	8.37%	6.35%	3.98%
2017	13.68%	13.01%	11.25%	8.86%	10.94%	6.32%	3.77%
2018	13.71%	11.68%	11.40%	9.88%	12.32%	6.61%	3.86%
2019	10.92%	11.24%	11.66%	11.66%	7.36%	7.15%	4.32%
2020	11.17%	10.45%	12.12%	11.18%	7.10%	6.60%	5.20%
Mean value	12.96%	12.51%	11.29%	9.08%	7.45%	6.01%	4.82%

. In general, the urban–rural coordination in the YRD region exhibited a relatively dispersed distribution pattern. It may be because the development of each city had certain similarities, but the mutual connections and interactions between adjacent areas had not yet reached a stable and orderly state. In addition, obstacles such as local protectionism have also made it necessary for the breaking of urban barriers and the full realization of the spillover effects of each city’s advantages to take time to achieve.

From the relative level of coupling and coordination, the low-value areas to some extent locked the north and south ends. In the early stage, high-value areas were distributed roughly in the shape of “Π”. In 2005, fewer than half (43.75%) of the 16 core cities had achieved primary coordination. These cities were mostly situated in the urban agglomerations of Suzhou, Wuxi, Changzhou, Zhenjiang in the southern Jiangsu Province, and the provincial capital of Nanjing, as well as Hangzhou and Ningbo, while the rest were on the verge of imbalance. Among them, Shanghai, Nanjing, Suzhou, Wuxi, and Hangzhou relied on their geographical advantages in politics and economy and a relatively strong industrial foundation, and their core urban areas have always maintained the developed economy, with dense industrial and manufacturing industries. The expansion of their industries and functions has a strong radiation effect on the surrounding rural areas while also driving further development of people’s livelihood and ecology. Changzhou, Zhenjiang, and Ningbo’s economic foundations were slightly inferior to the above cities, but they had good infrastructure and supporting public services, focusing on ecological and livable environment construction, and easily formed a benign interaction among the economy, society, and ecology. In 2010, all cities in the research area reached the state of primary coordination, but the two cities of Taizhou and Shaoxing in Zhejiang Province remained at a lower level, very close to a state of imbalance. The high-value areas showed a “>”-shaped distribution during the latter period. In 2015, all cities in the research area reached a moderate level of coordination, and the overall spatial pattern did not change much. It is worth noting that Shanghai’s coupling coordination level dropped from the fourth level to the second level, indicating that although it has certain advantages in development foundation, its development rate is relatively slow compared to other cities. By 2020, Nanjing, Zhenjiang, and Wuxi still remained at the fourth level, while Shanghai, Suzhou, and Hangzhou belonged to the lower level. Zhoushan City was a special case, rising from the lowest level in 2005 to the highest level. It is located at the junction of the Yangtze River and the East China Sea coast, with the geographical characteristics of an archipelago city. It had long been dominated by fisheries and tourism and had been trying to fully leverage its ecological advantages to promote economic and social development while paying great attention to ecological environment protection. Therefore, although Zhoushan’s economic strength was relatively backward, its ecological fishery was developed, and the urban–rural boundary was not clear, it was indeed possible for it to get a small urban–rural disparity and a high level of system coordination.

For further analysis, the geographical data of each city was used as the X and Y values of the horizontal attribute, and the coupling coordination level was used as the Z value of the height attribute. Through fitting with a second-order trend surface function, the results are shown in Figure 6, where the green curve represents the spatial fitting line in east–west orientation and the blue curve represented the fitting line in north–south orientation.

The east–west fitting curve of the urban–rural coordination changed from a “V”-shaped curve to a positive “U”-shaped curve, indicating that the coordination gap between the middle and two ends of the YRD region gradually widened. The curve changed from being higher in the west than in the east to a flat shape at both ends, which may be due to the development of the two provincial capital cities of Nanjing and Hangzhou driving the development of surrounding cities during the study period. Although Shanghai had a higher level in the early stage, its development was relatively slow, and its advantages in coordinated development were not obvious. Later, with the urban and rural integration progress, the radiation effect of large cities also weakened, and the differences between the eastern and western regions gradually narrowed.

The north–south fitting curves were in an inverted “U” shape, with the north higher than the south, from 2005–2010. The gap between the two ends was alleviated in 2015 and then widened again in 2020, but the coordinated development level of the central cities remained higher than that of the northern and southern ends. Combining with Figure 4 to explore the reasons, it can be seen that low-value areas are mainly concentrated at the northern and southern ends, including Yangzhou, Taizhou, Nantong, Shaoxing, and Taizhou, with their coupling coordination level often stable in the first or second gradient. Cities with higher levels, such as Shanghai, Nanjing, Wuxi, and Changzhou, are mostly located in the central by north region, forming a spatial pattern of higher in the middle and lower at both ends.

4.3. Analysis for the Obstacle Factors of the Coupling Coordination Level of Urban–Rural Integration

On the basis of measuring the coupling coordination degree of urban–rural integration subsystems in the YRD region, in order to further explore the key constraints of the level of urban–rural “economy–society–ecology” coordination from 2005 to 2020, this study continued to use the barrier degree model to calculate the barrier degree of each indicator (A1–C7) in Table 1. Considering the number of secondary indicators, the top seven indicators with the highest average barrier degree were identified as the main obstacle factors, as shown in Table 3.

In accordance with the ranking of the barrier degree of main obstacle factors, the first obstacle factor was the per capita GDP (A1), and the second one was the population urbanization rate (B1), with barrier degrees of 12.96% and 12.51%, respectively. Their change from rising to falling indicated that the difference in the basic economy and population was an important shortcoming that hindered the urban–rural coordinated integration. However, with continuous promotion of urbanization and rural rejuvenation, the rural economic level and population urbanization level were significantly improved, so their limiting role gradually weakened. Especially in 2020, the average population urbanization rate in the research area was 75.59%, which increased by 28.38% compared with 2005. At that time, the population had gathered to a certain extent, and its barrier degree ranking was overtaken by the urban–rural basic pension insurance coverage (B9) and the faculty–student ratio in general elementary and middle schools (B5). The two main obstacle factors all belonged to the public service field of the social integration subsystem, reflecting the comprehensive level of social security and basic education in various areas of the YRD region. Therefore, insufficient supply of public services greatly hindered the coordinated development. The fifth obstacle factor was the per capita park green land (C2), which reflected the regional ecology, while the fertilizer application per unit area of sown land (C4) in the secondary obstacle factors reflected the environmental pollution situation in rural areas. If we neglected the cultivation of the region’s sustainable development capability, the most basic environmental problems could also become a bottleneck for the coordinated integration. The road network density (B3) represented the accessibility of transportation in a region. A low value inhibited the circulation of various factors and hindered the gradual integrated development of the urban–rural subsystems.

5. Discussion and Conclusions

5.1. Discussion

The comprehensive evaluation and obstacle factor analysis of the coupling coordination degree of an urban–rural integration system in the YRD region could help to accurately grasp its development status and dynamic trends and the specific direction of urban–rural development. Based on the theory of urban–rural continuum and composite ecosystem, this study clarified the connotation of urban–rural integration in the new era. According to the calculation of its comprehensive development level, the coupling coordination model and barrier degree model were used to explore the coordinated development level of urban–rural economic, social, and ecological integration subsystems and their main obstacle factors, enriching the current research content of urban–rural sustainable development. The research has important reference value for promoting rural revitalization and common prosperity.

The study found that the development level of urban–rural integration steadily increased from 2005 to 2020, in line with the research results of Guan Shu [15], Zheng Yuhan et al. [16], and Luo Wanlu et al. [52]. With the support of a series of policy measures such as beautiful countryside, new urbanization, rural revitalization, and urban–rural integration, the overall urban–rural integration development in the YRD region maintains a good momentum. However, there existed development gaps in the integration rates of its economy, society, and ecology, with a phenomenon of economic upgrading at the sacrifice of ecological environment, leading to some coordination problems. Qian Zhengyuan et al. [53] showed that the practical direction of urban–rural ecological integration should be studied under the green framework of the integrated development of the urban–rural “ecological-economic-social” system, Shi Jiangang [54] and Duan Kaifeng et al. [55] analyzed the structure of the urban–rural integration system under the overall framework of sustainability science, pointing out that the operation of the urban–rural integration system should mainly achieve the sustainable development of the environment, economy, and society in urban and rural areas and focus on the balanced relationship between the urban–rural subsystems in each dimension. To respond to such calls and fill the research gap related to the coordinated development of subsystems, we analyzed the features of both time and space evolution of the coupling coordination level; it was found that the degree of coupling coordination is increasing year by year, and its regional development differences continued to decrease, but the spatial patterns became mildly solidified, with the low-value areas stabilizing in the longitudinal ends, and the high-value areas concentrating in the western and central-northern parts, basically in line with regional location advantages and resource endowments. It was further known from the diagnosis of the barrier degree that the key obstacle factors of urban–rural “economy–society–ecology” coordination included per capita GDP, population urbanization rate, urban and rural basic pension insurance coverage, faculty–student ratio in urban and rural primary and middle schools, per capita park green land, road network density, and fertilizer application per unit area of sown. Although there is less literature on the study of obstacles to the coordination of urban–rural composite system, it can be explained and supported by the results of related research on some specialized topics. Zhang Xinlin [56] pointed out that there existed an “economic development-urbanisation” driving mode for urban–rural integration development, and the bottleneck of promoting urban–rural integration through raising the level of population urbanization and per capita GDP is relatively high and not easy to break through. Jiang Manqi et al. [57] showed that human capital is a necessary condition to promote urban–rural coordination, and that the fundamental change of the status quo of rural resource shortage needs to open up the channels for rural labors to go to the city to work, and, on the other hand, it is necessary to improve the quality of the labor force through education. Gao Jing et al. [47] and Wang Fei [58] pointed out that the gap between urban and rural social security and education development levels was large and suggested that, when promoting the equalization of these public service elements, too much reliance on government support and neglect of social support have resulted in a single support force and insufficient endogenous motivation. Wang Shuo et al. [59] thought that the lack of transport network construction inhibits rural laborers from seeking urban employment opportunities while increasing the cost of factor mobility and hindering industrial and social exchanges between urban and rural areas. From the above research, it can be seen that scholars may have paid more attention to the influence of economic and social factors on the development level of urban–rural integration in previous studies. However, in the process of exploring the obstacles to urban–rural “economy–society–ecology” coordination, we found that ecological indicators also have a pivotal influence on the coordinated development of the system, and that the construction and maintenance of the environment and the emission and management of pollution from agricultural sources deserve attention, opening up new ideas for promoting urban–rural sustainable development.

5.2. Conclusions and Recommendations

Based on the theory of the urban–rural continuum and composite ecosystem, the study clarified the connotation of urban–rural integration in the YRD region and constructed the evaluation index system, which calculated the development level of economic, social, and ecological integration subsystems through the entropy weight method. Then we further measured the level of their coupling and coordination by the coupling and coordination model, explored the characteristics of their spatial–temporal evolution, and diagnosed the main obstacle factors. The specific conclusions are as follows:

(1) From 2005 to 2020, the urban–rural integration in the YRD region showed a good momentum, and the development levels of the ecological, social, and economic dimension all significantly improved, while there were differences in their growth rates. The economic integration subsystem had the lowest initial value but the fastest increase, while the ecological integration subsystem was the opposite, leading to a narrowing and then increasing development gap among the three subsystems, resulting in a certain degree of development imbalance.

(2) From temporal changes, the coupling coordination degree of urban–rural ecological, social, and ecological integration in the YRD region had kept increasing, and the coordination status had been transformed from borderline imbalance to moderate coordination, accompanied by a slowdown in the pace of improvement after 2010. The difference of regional coordinated development was narrowing, but there was a multiplier-developing trend, and a certain number of regions were still at a relatively low level. From spatial changes, the distribution of the coupling coordination level was relatively scattered, with the high value areas from a “Π” shape to a “>” shape, while the low-value areas were fixed at both longitudinal ends in the YRD region. The east–west curve shifted from an approximate “-” shape to a positive U shape, while the north and south direction showed a stable inverted U shape, with an overall spatial pattern of “being lower in the west than the east, being higher in the north than the south”.

(3) The results of barrier degree diagnosis showed that the top seven key obstacle factors in the average ranking were per capita GDP, population urbanization rate, urban and rural basic pension insurance coverage, faculty–student ratio in urban and rural primary and middle schools, per capita park green land, road network density, and fertilizer application per unit area of sown. Among them, the barrier degree of the indicators of the economic integration subsystem had decreased, while the indicators of the social integration subsystem occupied the main position. It means that promotion of urban–rural coordination should focus on the development of social integration and also take into account ecological integration.

Therefore, the following recommendations are given in regard to the conclusions:

(1) Adjust development priorities to local conditions. For regions with better development foundations, their central urban areas have advantages in terms of conditions and more frequent flow of resources and can make full use of urban spillover impact, so the development focus should be on promoting rural urbanization and modernization while strengthening social and livelihood construction and ecological environment management, filling the urban–rural gap in all aspects, and realizing harmonious and common progress. The most urgent task for lagging regions is the development of economy and people’s livelihood, which requires continuous improvement of various service functions and development strength of central urban areas, broadening the channels of urban–rural exchanges and stimulating the interior growth. While reflecting on the many contradictions existing in urbanization and rural construction, we should also avoid falling into the “double low” misunderstanding that urban development slows down or stagnates, forming the illusion of high-level coordination.

(2) Expand urban–rural communication channels. Highway is an important medium for the integration of urban–rural resource elements, and special attention should be paid to the quality of rural roads to enhance the convenient circulation of labors and physical resources between urban and rural areas. However, the sinking and feedback of non-physical factors such as industry and technology are equally important. We should accelerate the construction of rural informatization and strengthen the industrial connection. We should also try to promote the diversification of urban–rural connection channels with the agricultural and sideline products-processing industry and rural characteristic tourism as the entry point.

(3) Promote the equalization of urban–rural public services to clarify the responsibilities of subjects at all levels in the supply of public services. Firstly, the higher-level government should incorporate the equal development of public services into the local government performance evaluation system and formulate unified urban and rural public service quality evaluation standards to ensure scientific and effective supervision and supervision of supply entities. Secondly, local governments should fully tap into the focus of urban and rural demand, identify the diverse needs of different levels of residents for different types of public services, and achieve targeted docking and precise assistance. Finally, it is necessary to amplify the participation rights of various entities, including grassroots mass autonomous organizations and individual urban and rural residents, in the equalization of public service construction and expand democratic participation channels. Additionally, it is necessary to build the “Internet plus public service” model. We should take the construction of digital rural areas as an opportunity to promote the extension of high-quality urban public services to counties and townships through information technology construction, forming a comprehensive urban–rural cooperation mechanism that spans the existing resource gap and alleviating the current situation of insufficient supply and low quality of rural public services.

(4) Guide the urban–rural ecological relationship from opposition to complementarity. Firstly, it is necessary to establish a complete ecological responsibility mechanism, eliminate non-compliant pollutant emissions, and clarify the corresponding consequences that local government departments should bear if their governance work is not in place, in order to improve their environmental awareness and initiative. Secondly, a reasonable ecological benefit compensation mechanism should be designed, based on the “four who principle”, to break away from the thinking of urban one-way relief for rural areas, encourage cities to provide more funds or technical support to compensate for the past resource sacrifice and ecological contribution of rural areas, and promote the regeneration of urban production energy and waste reuse by maintaining the rural ecological environment, thus enhancing their own ecological resilience. Thirdly, it is necessary to increase investment in rural environmental protection. In addition to urban assistance, the government needs to provide necessary policy and institutional support, actively seeking cooperation from non-governmental organizations and commercial enterprises through contract contracting, government subsidies, PPP projects, and other social forces. At the same time, there is a need to actively cultivate public ecological awareness and participation in governance and form a benign ecological partnership of tripartite cooperation.

6. Limitations and Prospects

On the basis of the existing research, this paper integrates from the dimensions of economy, social life, and ecological environment, analyzes the spatiotemporal evolutionary traits of urban–rural integrated development in the YRD region under tripartite coordination, and summarizes its main restrictive factors. This, to a certain extent, enriches the relevant research results, but there are still some shortcomings. Firstly, the assessment indicators for appraising the urban–rural integration level are complicated, so there is no unified measurement standard. This paper constructs a comprehensive index system consisting of three subsystems, but due to the limited coverage of indexes and available data, it is impossible to make an absolutely comprehensive evaluation. Secondly, although we have chosen the classic indicator evaluation method to ensure the accuracy of the results, it still has inevitable limitations, such as ignoring the subjective intentions of decision makers. Thirdly, because the article selected the representative core area of the YRD region as the research object, it may not be in-depth enough to explore the diversities of various cities. Therefore, our study will further search more indexes directly related to urban–rural integration to improve the evaluation system, and then evaluate the urban–rural coordinated integration in conjunction with other analysis methods and explore more external influencing factors to combine with internal restrictive factors. In addition, our future study will expand its scope to the entire YRD region and even the whole country, which will help to achieve a sustainable urban–rural development path with universal applicability.