1. Introduction
Ecosystems provide humans with a diverse range of goods and services that support their survival and development [
1]. Around the world today, the magnitude and rates of ecosystem change are undermining the sustainability of some fragile and impoverished areas that rely directly on ecosystem services for their livelihoods [
2,
3]. Ecosystem services are the benefits that humans obtain from ecosystems [
4]. They refer to life support products and services directly or indirectly obtained through the structure, process, and function of the ecosystem, and they form and maintain the natural environment conditions and utility on which human survival and development depend [
5,
6]. The Millennium Ecological Assessment divides ecological services into provisioning, regulating, cultural, and supporting services [
7]. In recent years, ecosystem service value has become a hotspot in sustainable development research [
8,
9]. In regions relying on natural resources for production and development, areas with a high supply of ecosystem services tend to have low livelihood levels [
10,
11]. An obvious relationship exists between the vulnerability of the ecological environment or the importance of the ecological environment and the livelihood of residents [
12,
13,
14].
“Livelihood” is defined in the dictionary as “a means or way of living”, which is the basic concept of the sustainable livelihood analysis framework [
15,
16,
17]. Livelihoods consist of abilities, assets (including physical and social resources), and actions required for living [
18]. At present, sustainable livelihood analysis frameworks are of three main types [
19], namely, the sustainable livelihood analysis framework established by the Department for International Development (DFID) of the United Kingdom [
15], the farmers’ livelihood security framework proposed by the Cooperative for American Remittances to Everywhere [
20], and the sustainable livelihood approach proposed by the United Nations Development Programme [
21]. Among these sustainable livelihood frameworks, the framework developed by the DFID is the most widely used and the most typical [
22,
23]. DFID’s conceptual framework consists of five parts: vulnerability background, livelihood capital, organizational structure and policy system, livelihood strategy, and livelihood status [
15,
23]. In the framework, livelihood capitals refer to the resource base of a community and of different categories of residents, and they are grouped into human, natural, financial, physical, and social capitals [
15,
24]. DFID’s framework is a good approach to sustainable livelihood analysis that provides a checklist of important issues for development and poverty research and emphasizes the multiplicity of interactions among the different factors that affect life [
15]. Scholars have employed different approaches to explore the influencing factors of livelihoods, such as the structural equation model [
25], livelihood resilience evaluation model [
26], gray correlation model [
27], participatory method [
28,
29], and coupling model [
30]. Through these methods, the relationships between livelihoods and the factors (i.e., land use, ecological conservation policy, poverty alleviation) were analyzed in different ecologically vulnerable, sensitive, and economically poor areas. The above-mentioned research has shown that the relationship between ecosystem services and residents’ livelihood is very close, so deeply analyzing and exploring the relationship between the two is necessary.
Understanding the relationship between livelihood and ecosystem service is important not only for the purpose of scientific research but also to inform policy and practice [
3,
31,
32,
33]. Existing research has explored the relationships between residents’ livelihoods and ecosystem services. Some conceptual models have been developed to study the interaction between livelihood and ecosystem services, such as ecosystem services—livelihood adaptation framework [
2], spatial targets and sustainable livelihoods analytic framework of payment for ecosystem service program [
34], framework for cost–benefit analysis of ecosystem service providers and beneficiaries [
35], landscape framework for food and livelihood security and ecosystem services [
36], and framework of ecosystem services, human well-being, and poverty alleviation [
37]. These conceptual models provide a practical basis to study the ecosystem services and residents’ livelihood. From the perspective of livelihoods, some studies have focused on the impact of changes in ecosystem services on livelihoods. They found the changes in grassland [
38,
39], forest [
40,
41], mangrove [
42], homegardens [
43,
44], and wetland [
45,
46] ecosystem services because of climate change, land use change, or human factors produced an important impact on residents’ livelihood. The land use change process affects the transformation of the ecosystem structure and function to a certain extent, and land use change is an important cause of ecosystem service changes [
47,
48,
49]. Changes in land use lead to changes in ecosystem services, which also affect people’s livelihoods [
50,
51]. Some other studies have tried to quantify the contribution of ecosystem services to livelihoods [
52,
53,
54,
55] in different landscapes and clarify the impacts of payment for ecosystem services on local livelihoods [
56,
57,
58,
59]. Unreasonable utilization of ecosystem services cannot sustain life effectively and is an important cause of ecological degradation [
17]. During the formulation and implementation of ecological compensation policies, if the relationship between ecosystem services and livelihoods is not fully considered, its effects will be greatly affected [
17,
60,
61]. From the perspective of livelihoods, some studies have focused on the livelihood dependence on ecosystem services [
62,
63] and tried to improve livelihoods by resilient livelihood strategies to cope with changes in ecosystem service provisions [
64,
65,
66]. Ecosystem services are the basic life guarantee for residents’ survival and development [
1]. The improvement of residents’ livelihood depends on the improvement of ecosystem services, and livelihoods react to the composition and structure of ecosystems, resulting in changes in ecosystem services [
30,
67,
68]. Therefore, the key to improving the livelihood of residents and protecting the ecological environment is to scientifically understand the coupling relationship between the livelihood of residents and ecosystem services. Only with a full understanding of the interaction between the two can we maintain a livelihood and ecological balance and achieve regional sustainable development.
Within the previous literature on livelihoods and ecosystem services, research on the coupling relationship between livelihoods and ecosystem services remains insufficient. Few research studies consider the interaction between livelihoods and ecosystem services from a systematic integration perspective. To close the gap, there is a great need for research that employs a coupling model to assess livelihoods and ecosystem services. Coupling is a physical concept that is used to describe the state of interaction and coordination between two or more systems [
69]. Coordination degree aims to quantitatively reflect the coordinated development of multiple systems, that is, whether they are in a state of imbalance or coordination, and control measures are timely taken according to the changing trend of coordination degree. The coupling coordination model has obvious advantages in analyzing the complex system relation composed of multiple factors and levels, and it can better reflect the structure and function of complex systems [
70,
71]. Human, natural, financial, physical, and social capital are involved in the improvement of livelihoods. Therefore, livelihoods and ecosystem services are not one-dimensional, and they are relatively complex systems. The coupling model can be employed to quantify the coupling level and coupling coordination level between livelihoods and land ecosystem services. Some research has used the coupling model to identify the relationship between ecosystem services and residents’ income [
72] and between ecological compensation and farmers’ livelihood [
27], which provides an important reference for the coupling coordination between livelihoods and land ecosystem services. By studying the evolution law of residents’ livelihood and land ecosystem services, the coupling relationship between the two systems can be revealed, thus providing a reference for improving residents’ livelihood and protecting the ecological environment.
As the main body of the Qinghai–Tibet Plateau, Tibet’s ecological value is mainly manifested by its huge gene pool on Earth; it is an important initiator of the world’s climate, the solid reservoirs in the surrounding areas, the source of rivers, and an ecological barrier in eastern China [
73,
74]. However, with the acceleration of urbanization and industrialization and the disorderly development and utilization of natural resources, the ecosystem of Tibet has been severely damaged [
75]. The “One River and Two Tributaries” region (ORTTR) in Tibet is a fragile ecological environment, and it is a typical interlaced area of valley-type planting, agriculture, and animal husbandry [
76]. Tibet has harsh natural conditions [
77]. Affected by the alpine natural geographical environment and the harsh conditions for the development and utilization of agricultural resources, agricultural resources are mainly distributed in the ORTTR with low altitude, good water and heat conditions, and fertile soil [
78]. The farmers and herdsmen in the study area converted the primary industry to secondary and tertiary industries through labor and successfully diversified their livelihoods, and improved their living standards [
79]. This area is the political, economic, religious, and cultural center of Tibet. It is an area characterized by early development and rapid economic development in the Tibet Autonomous Region. Local farmers and herdsmen utilize the products and services provided by the ecosystem to maintain their livelihoods and exert various impacts on the structure and function of the ecosystem; meanwhile, the ecosystem maintains the ecological security of the basin by providing water conservation, soil conservation, and other services and affects the improvement of agriculture, animal husbandry, and people’s livelihood. Hence, they have a close coupling relationship. Investigating the relationship between residents’ livelihoods and land ecosystem services in the ORTTR is the premise of promoting regional sustainable development. Against this background, our work explores the following questions: (1) how to assess the coupling relationships between residents’ livelihoods and land ecosystem services in the ORTTR; (2) are the coupling relationships improved in recent twenty years, and what are the reasons; (3) and what is the policy implications of the coupling coordination results between residents’ livelihoods and land ecosystem services in the ORTTR. To answer these questions, we employ the coupling model to quantify the coupling degree and the coupling coordination degree between livelihoods and land ecosystem services from 2000 to 2020. A relative development model is adopted to identify the lead–lag relationship between livelihoods and land ecosystem services and zone the county-level ecological management. By studying the evolution laws of residents’ livelihood and land ecosystem service value in the ORTTR, this work provides a reference for improving residents’ livelihood level and protecting the ecological environment. Related policy suggestions are proposed for supporting ecological protection and high-quality sustainable development in the ORTTR.
4. Discussion
4.1. Coupling Relationship between Residents’ Livelihoods and Land Ecosystem Services
Identifying the relationships between livelihoods and ecosystem services and incorporating effective results into management practices were conducive to achieving region sustainability [
11,
31,
62,
95]. The existing research focused more on the contribution of ecosystem services to the residents’ livelihood [
52,
53,
54,
55] or the impact of livelihood activities on ecosystem services [
96,
97,
98]. The interaction (i.e., coupling degree, coupling coordinate degree, and relative development index) between livelihoods and ecosystem services failed to be identified from a systematic integration perspective. The spatiotemporal dynamics of coupling relationships could not be detected because of the limitations of methods and data [
33,
64]. Existing research mainly applied a questionnaire survey [
50,
54,
57,
62,
99] to quantify the livelihoods. However, the methods are unable to analyze the dynamic changes of the relationships between the two systems because the long-term data were difficult to be accumulated, thereby introducing challenges in identifying spatiotemporal dynamics of coupling relationships under a complex background (e.g., urbanization and ecological environmental change). Our research in the ORTTR of Tibet explored the coupling relationship between residents’ livelihoods and land ecosystem services using a coupled model from 2000 to 2020. The livelihood capital index was used to quantify the residents’ livelihood level. Ecosystem service value was utilized to quantify the land ecosystem service level. Our study sheds light on the relationships between residents’ livelihoods and land ecosystem services and their dynamics by quantifying the coupling degree, coupling coordinate degree, and relative development degree between the two from 2000 to 2020 on the county scale. The research revealed three major findings that could be important for further exploration of the coupling relationships between livelihood and ecosystem service in other similar areas. The findings of the research can be summarized in the following major points: (i) the coupling degree and coupling coordinate degree between livelihood and ecosystem service continued to increase from 2000 to 2020, (ii) the coupling coordination degree showed more revealing results than the coupling degree in time scale, and (iii) identifying the relative development types by relative development index between livelihood and ecosystem service was conducive to ecosystem management zoning of the study area.
Our results have shown that the residents’ livelihood capital index doubled from 0.14 to 0.28 during the study period. The continuous improvement of the financial and social capital provided strong support for the development of the residents’ livelihood. Actually, in the whole of China, the total scale of livelihood capital has increased greatly and has maintained a trend of continuous increase with the development of land use. The rate of change in the western region is lower than that in the central and eastern regions of China, and the imbalance in the development of national livelihood capital has intensified [
94]. Poverty livelihoods mainly occurred in the Qinghai–Tibet Plateau [
27], and our study in the ORTTR of Tibet has important policy value. During the study period, the coupling degree between residents’ livelihood and land ecosystem service value in the ORTTR of Tibet remained at the medium–high coupling stage, and an intense interaction exists between the residents’ livelihood and the land ecosystem services. Although a series of large-scale or super-large ecological protection projects have achieved remarkable results in the field of ecological protection in the past 20 years [
72,
100], a win–win situation between livelihood improvement and ecological conservation remained challenging. In the ORTTR of Tibet, ecological protection measures need to consider the dual goals of safeguarding critical ecosystem services and improving residents’ livelihoods to achieve harmonious coexistence between man and nature and attain regional sustainable development. Some other relevant studies [
101,
102] also confirmed that. Human well-being will suffer if people’s livelihoods are not considered. Although the coupling coordination degree of residents’ livelihoods and land ecosystem services increased, the relative development degree of the two systems in most districts and counties was of the residents’ livelihood lagging type, which introduced challenges about how to achieve win–win of livelihoods and land ecosystem services in the ORTTR of Tibet in the future research.
Our study applied an effective method for assessing the coupling relationship between residents’ livelihoods and land ecosystem services. However, there are still some limitations in the selection of measurement indicators. In our study, the evaluation factors of livelihood capitals were selected from five aspects: natural capital, human capital, physical capital, financial capital, and social capital [
15]. The selection of evaluation indicators followed the principles of indicator diversity and data availability. Some indicators (e.g., expenditures on social interaction, loan amounts, and income structure) were relevant to the livelihood capital [
81,
82,
83,
84,
85,
86], but the data availability could not be guaranteed. Therefore, those indicators were not selected in the evaluation system. Although our indicators have covered all the capital types, the impact of the incomplete indicators still need to be attention. In the following research, we will conduct research on some other availability data. Residents’ livelihoods of the ORTTR can be more fully and convincingly assessed in the future.
4.2. Influencing Factors
This study used a coupling model to identify the spatiotemporal changes in the coupling relationship between residents’ livelihoods and land ecosystem services in the ORTTR from 2000 to 2020. Overall, the coupling degree and coupling coordination degree between residents’ livelihoods and land ecosystem services increased. The coupling degree increased from 0.72 in 2000 to 0.82 in 2020 (an increase of 13.75%), and the coupling coordination degree increased from 0.57 to 0.72. The proportion of districts and counties in recession and imbalance decreased from 76.47% to 41.18%. The direct reason is that during the study period, the residents’ livelihood and the land ecosystem service value in the ORTTR were improved.
The ORTTR has a flat terrain and excellent agricultural planting conditions [
76]. Its area of arable land accounts for more than half of the total area of arable land in Tibet [
78]. It is the birthplace of Tibetan culture. China has included the comprehensive development of the ORTTR as a key construction project in the National Eighth Five-Year Plan and Ten-Year Plan. A total of USD 0.16 billion was allotted to support Tibet. The central basin of the ORTTR is an agriculturally comprehensive area with the highest investment and the largest development scale. It is the largest comprehensive agricultural and animal husbandry development project in Tibet’s history that includes water conservancy, planting, animal husbandry, and forestry in more than 200 key development and construction projects. This project has made the three rivers the “Golden Triangle” of Tibet’s economic development, benefiting 800,000 people. The completion of numerous basic agricultural and animal husbandry projects has greatly improved the production conditions of agriculture and animal husbandry in Tibet. The production and lifestyle of farmers and herdsmen relying on the sky for food and raising animals are becoming history, and agricultural and animal husbandry production has begun to move toward modernization. The supportive policies have laid a good foundation for the future development of Tibet’s ORTTR and improved the infrastructure conditions of the Tibet Autonomous Region. Livelihood capital has been continuously improved. For example, the grain and meat outputs in the ORTTR and the per capita income of farmers and herdsmen have increased significantly in the past 20 years. In the past 20 years, the urban and rural medical and health service systems have been further improved, and free compulsory education has been implemented. All of these provide an effective guarantee for the improvement of the livelihood of residents in the ORTTR.
The enhancement of the land ecosystem service value is directly affected by land use change. The type of land use directly determines the value of ecosystem services, and the effect of different land use type changes on ecosystem services value is different. Some scholars [
103,
104,
105] found urbanization processes could produce a negative impact on ecosystem services. In some ecological sensitive areas, ecosystem service value could be improved because of ecological protection or restoration [
106,
107,
108]. In the ORTTR, ecosystem service value has changed because of the changes in different land use types. From 2000 to 2020, the main land use types in Tibet’s ORTTR were grassland, unused land, and forest land, followed by cultivated land, water area, and construction land. From 2005 to 2010, the area of land use types in the ORTTR underwent great changes, and the area of forest land, water area, construction land, and unused land increased rapidly. The newly added forest land area is 944,200 hectares, the unused land area is 749,600 hectares, the water area is 48,300 hectares, and the construction land area is 285,100 hectares. The areas of cultivated land and grassland showed a decreasing trend. The cultivated land area decreased by 5300 hectares, and the grassland area decreased the most, namely, 944,200 hectares. The reason for the increase in the area of construction land may be that in 2014, county-level Shigatse was changed to Sangzhuzi District. In 2015, Duilongdeqing was removed from the county level and changed into a district. In 2016, Naidong was removed from the county level and transferred to the district level. To meet the needs of urban development, the urbanization process has accelerated, and cultivated land, forest land, and grassland have been occupied and converted into construction land, resulting in an increase in the area of construction land. The greatest decrease in the grassland ecosystem service value was due to the prominent contradiction between grass and livestock in the river valley. Eighty percent of livestock are concentrated in the river valley of the ORTTR, resulting in the overgrazing of grassland, a decline in the proportion of fine grass, the depletion of water sources, and the intensification of desertification. Although grassland reduction and urbanization process produced a negative impact on ecosystem service value, woodlands increase markedly improved ecosystem service value. Moreover, the newly added wetlands and water areas made up for the decline of the ecosystem service value caused by the reduction of grassland, increased the diversity of ecosystem services, and improved the ecosystem service value.
According to the calculation results of the correlation coefficient (
Table 10), land use area changes were the major factors that affected the coupling coordinate degree between residents’ livelihoods and land ecosystem services. Woodland area and urbanization level exhibited a positive correlation with the coupling coordinate degree. That is, the higher the woodland area and urbanization level, the higher the coupling coordinate degree. The major factors that exhibited a negative correlation were cultivated land area and grassland area. During the study period, the coupling coordinate degree between residents’ livelihoods and land ecosystem services increased with the decrease in cultivated land area and grassland area, which might mean a decline in human activities. With the enhancement of humans’ awareness of environmental protection, China has begun to issue a series of ecological protection policies, and the ecological protection policies and measures for Tibet’s ORTTR have been promoted. Since 2005, Shannan of the ORTTR has been listed as a National Comprehensive Demonstration Zone for Sand Prevention and Control by the National Forestry Administration. In the same year, the Planning of the Comprehensive Demonstration Zone for Sand Prevention and Control in the Shannan Area of Tibet (2005–2010) was compiled. From 2005 to 2010, the forest land area of the five districts and counties in Shannan increased significantly, which promoted the improvement of ecosystem service value from 2005 to 2010. The overall implementation of afforestation and shelter forest construction projects in Lhasa and its surrounding areas has been good, increasing the forest area and greatly improving the ecological environment of the study area. In 2015, the afforestation and greening project in the “Two Rivers and Four Tributaries” watershed in Tibet was launched, and Nanmulin, as the demonstration and starting area of the “Two Rivers and Four Tributaries” project, effectively promoted the implementation of the ORTTR ecological protection construction project. In 2017, Shannan developed a sand control ecological poverty alleviation project, which played an important role in ecological protection construction. The wetland area in the study area increased. In 2011, the policy titled “Regulations on Wetland Protection of the Tibet Autonomous Region” was passed, focusing on the protection of wetlands, protection of wetland ecological functions and biodiversity, and promotion of sustainable resource utilization; Tibet’s ORTTR was included in this policy. Overall, land use change caused by a series of ecological policies was the main driving force of the improvement in the coupling coordinate degree between residents’ livelihoods and land ecosystem services.
4.3. Policy Enlightenment
Although the coupling coordination degree between some residents’ livelihoods and land ecosystem services is on the rise, the current results on the relative development degree showed that residents’ livelihoods in the ORTTR lag behind ecosystem services. In 2020, the livelihoods of residents in most districts and counties lagged behind ecosystem services. The relative development index can accurately identify the leading–lagging relationship between residents’ livelihoods and land ecosystem services in each district and county. Therefore, on the basis of the relative development types of residents’ livelihood and ecosystem services, ecological management zoning can be conducted. In 2020, most of the districts and counties in the study area were dominated by residents’ livelihood lag (
Figure 6). By considering the physical geography and socio-economic characteristics and by taking the lag of ecosystem services as a reference for ecological management zoning, the counties and districts in the ORTTR are divided into three types of ecological management zones: ecological conservation areas, ecological restoration areas, and ecological reconstruction areas.
Districts and counties with lagging residents’ livelihoods, such as Xietongmen and Nanmulin in the northwest and Mozhugongka in the east, are designated as ecological conservation areas. These counties are high in altitude and sparsely populated. The grassland vegetation coverage is high, with the characteristics of minimal human interference, fragile ecology, and difficult recovery. A comprehensive ban on custodial breeding or rotational grazing should be implemented to reduce human interference and focus on prevention and protection. The government should delimit a strict ecological protection red line. To improve the residents’ livelihoods, land ecosystem service value must be realized in various ways, such as compensation for grassland ecological conservation and ecological tourism development. On the basis of existing ecological vegetation, the counties should plan to build demonstration sites for ecological tourism to create a visual offering of rural and ecological landscapes. The in-depth integration and development of ecological and cultural tourism should be actively promoted.
Bailang and Jiangzi, which are characterized by the simultaneous development of residents’ livelihoods and land ecosystem services, and Lazi, Nimu, Linzhou, Gongga, Zharang, Qiongjie, and Sangri, which are characterized by relatively lagging residents’ livelihoods, are designated as ecological restoration areas. Ecological restoration areas have a low altitude and relatively flat terrain. The vegetation type is mainly alpine meadows. The vegetation coverage is generally high, the water and heat conditions are good, and the ecosystem has a certain self-healing ability. Enclosure management in the key ecological area should be implemented to reduce livestock disturbance and consolidate the existing governance effects. In these areas, the river valley is wide, the soil is fertile, the sunshine is sufficient, and the temperature difference between day and night is large. The fruit industry in this area should be listed as the key industry with the most characteristics, the most potential, and the most promising development prospects. Accelerating the transformation and development of the agricultural industry and realizing agricultural modernization can effectively meet the growing development needs of farmers, encourage overall economic and social progress and improve residents’ livelihoods.
Sangzhuzi, Duilongdeqing, Qushui, Dazi, and Naidong, with lagging ecosystem services, are designated as ecological reconstruction areas, and the population, production, and life of the study area are concentrated in this region. With the acceleration of urbanization and frequent infrastructure construction activities in those areas, soil erosion and land desertification have become serious, and soil and water conservation in the region should be strengthened. In this area, comprehensive control measures that combine engineering measures with forest and grass measures should be adopted, and bank protection and embankment construction should be carried out for the channel. Grids and grass-checkered sand barriers should be built for debris flow channels, the slopes should be changed to ladders, and soil conservation for sloping farmland should be promoted.