1. Introduction
Ecosystems provide a variety of direct and indirect products and services for human survival, health and welfare [
1], which form the foundation of human society [
2,
3,
4]. The quality and quantity of provisions generated by ecosystem services depend on the structure, process and function of the surrounding natural ecosystem [
5]. Yet, global population growth, economic development and urban expansion have placed pressure on ecosystems, resulting in their being substantially degraded, destroyed or transformed. As a result, the effectiveness of ecosystem services has been impeded at multiple scales. Studies on ecosystem services have become increasingly popular to evaluate environmental change, resource management and sustainable development, because these services incorporate society benefits [
6]. Assessing the value of ecosystem services that influence human well-being by market price or non-market value is regarded as an important tool to promote the importance of ecosystems and encourage sustainable economic growth.
Costanza
et al. [
1] provided the first classification of the global biosphere into 16 types of ecosystems and 17 types of service functions, from which they estimated the ecosystem service values (ESV) for each category. Subsequently, Costanza
et al. [
7] updated their ecosystem service value coefficient table using current ESV units and land cover change estimates based on data collected between 1997 and 2011. These coefficient values have been widely applied in estimating ESV. Xie
et al. [
8] modified the coefficient values based on those of Costanza
et al. [
1] in 1997 to formulate an accurate ESV-per-hectare for the terrestrial ecosystems in China by surveying approximately 200 Chinese ecologists. The coefficient values were directly applied to several ecosystem service studies in China, in addition to being adjusted for the unique land cover types of China’s internal regions [
9,
10,
11,
12]. Most studies evaluating ESV in China have been primarily based on the method of Costanza
et al. [
1] and Xie
et al. [
8].
In the past five years, most studies have evaluated the ESV of one type of single [
13,
14] or whole ecosystem in relation to changes at global [
15,
16], national [
17,
18] and local [
19,
20] scales. Several studies have used land cover types as proxies for ecosystems, by matching the land cover types to equivalent biomes, because different biomes have different land cover benefit transfer values [
21,
22]. At large scales, land cover change represents one of the clearest and most informative indicators of ecosystem impacts at state- and province-level scales [
23]. Consequently, several studies have focused on the relationship between ESV and land cover types to determine how land cover change affects the provision of ecosystem services [
24,
25]. These studies have validated that land cover affects ESV globally, especially in urbanized regions. For instance, land cover change in urban sites is detrimental for several ecosystems [
26,
27].
Human activities have an unavoidable impact on natural and semi-natural ecosystems in urban agglomerations. Examples of such activities include urban expansion, industrialization and economic growth. As rapid urbanization by the Chinese population continues [
28,
29], a large number of urban agglomerations have emerged. Urban agglomeration is a modern spatial pattern formed by functional connections between cities or metropolitan areas [
30]. It has become the principal economic and urbanization unit for countries to participate in globalization. Unfortunately, some urban agglomerations have caused major changes to land cover due to urban expansion, leading to ecological damage, especially during the process of urbanization in China. Critics have warned that urban expansion has created a Chinese version of urban sprawl and the loss of natural ecosystems [
31]. Indeed, a large proportion of land made up of natural ecosystems has been converted to artificial uses to meet the demands of housing, industry and commerce around the cities of China [
32], particularly in economically-developed coastal urban agglomerations [
33,
34].
Our study focuses on the three urban agglomerations along the coast of the Bohai Rim, China. These cities were selected because increasingly noticeable and rapid changes in land cover have had important ramifications on coastal urban agglomerations [
35]. The Bohai Rim is the third-largest economic zone in China, after the Yangtze River Delta and the Pearl River Delta. The three urban agglomerations of this study are affected by excessive land reclamation and natural resource exploitation, along with rapid urbanization. For example, groundwater was exploited excessively for domestic, industrial use and agriculture in the three agglomeration region, which have resulted in seriously short supply of water resources and affected the sustainability of the three study areas. All three urban agglomerations are of economic and ecological significance; however, differences exist in the characteristics of land cover change and associated ESV changes. However, few studies have investigated such differences among cities. We aimed to: (1) characterize and compare the change in land cover for these three coastal urban agglomerations from 2000 to 2010 in the context of rapid urbanization; (2) evaluate and compare changes in ESV caused by land cover changes; and (3) provide suggestions for policymaking to mitigate ESV loss by adjusting land cover composition and encouraging sustainable development of coastal urban agglomerations.
4. Discussion
Here, we detected noticeable changes in land cover and ESV among three urban agglomerations in China. The total ESV of LP slightly increased, due to an increase in forest, grassland and water body cover, the higher coefficient values of which offset most of the decreases caused by the loss of cultivated land and wetland to artificial surfaces. The degradation of coastal wetlands primarily occurred on the southwestern shore of Panjin Municipality, where some scattered wetland patches were converted to cultivated land. However, the decrease in total wetland area in LP was minimal, because the main wetland area is located in the Shuangtaihekou National Nature Reserve in the Liaohe River Delta. This reserve was established in 1988, with the aim to protect the coastal estuary wetland ecosystem. Thus, the wetland area in LP is protected by effective natural reserve laws and policies. The increase in artificial surfaces due to large-scale urban expansion occurred at the expense of water bodies and grasslands, primarily in Shenyang, Panjin and Yingkou municipalities along the coastal area.
JJJ had a net loss in total ESV, because of the largest absolute increase in artificial surface area in the three study areas. The conversion of cultivated land to artificial surfaces due to urban expansion was the strongest driver in JJJ. Beijing and Tianjin were the largest hotspots for urban expansion in JJJ between 2000 and 2010. The cultivated land on the periphery of Beijing and Tianjin in 2000 gradually disappeared and almost completely vanished with the expansion of Beijing and Tianjin until 2010. This expansion occurred because there was an urgent need to accommodate the increasing population and growing economy. In addition to artificial surfaces occupying cultivated land around the city, they also occurred as a result of land reclamation in the coastal area of the Caofeidian District and New Binhai District in Tianjin, mainly due to the construction of ports and harbor industry [
40]. This phenomenon resulted in the loss of wetlands at the shoals on the coast of the Caofeidian District and New Binhai District. Consequently, land reclamation contributed to the loss of a proportion of coastal wetland areas in JJJ [
41]. The expansion of artificial surfaces around the city and in the urban clusters in the interior of JJJ should be strictly controlled to avoid ESV loss. Coastal areas should receive more ecological protection; however, to date, the focus has been on economic development.
The greatest reduction in ESV occurred in SP, due to the largest absolute loss of wetland coverage. The coastal wetland was substantially degraded and disappeared because of conversion to artificial water bodies dominated by saltpans and aquaculture, especially in the coastal area of Laizhou Bay [
42]. Because Laizhou Bay is an important extensive saltpan and aquaculture zone in China, local economic income is highly dependent on the salt industry and aquaculture and has been subject to targeted state-led industrialization and economic growth policy pressure. In addition, local residents had low awareness about wetland ecosystem services, resulting in intertidal-mudflat wetlands being extensively and excessively utilized for saltpans and aquaculture. Consequently, pressure for economic growth by the local community has heavily influenced land use policies and decision-making processes. As a result, wetlands were primarily converted to other land cover types, including water bodies (aquaculture and saltpans), cultivated land and grassland. Wetlands urgently require protection and validation of their importance as ecosystem services to improve the quality of the ecological environment in SP [
43].
We detected the main negative and positive factors associated with the trends in changing land cover. These factors are interrelated and often have a synergistic impact on ecosystem services. Urban expansion and industrial construction had a strong negative impact on the ecosystem service values estimated in this study. The conversion of cultivated land to artificial surfaces was the most recurrent land cover change. Urban land increased by 8.3%, 24.3% and 26.6% in the three study areas, respectively. In addition, new saltpans and aquaculture construction occurred at the cost of coastal wetlands, leading to a substantial decline in wetland coverage. While the amount of wetland loss varied among the three study areas, it was universally a major contributor to total ESV loss. This is because wetland ecosystems have the highest estimated coefficient values. Consequently, the decline in cultivated land and wetland coverage contributed the most to ESV loss. However, China’s national green infrastructure programs, which aim to protect the environment, represented highly relevant positive drivers for ESV gains. The National Forest Conservation Program, the Grain for Green Program and the Great Wall of China Program are implemented by the Chinese government to increase forested and grassland areas. The programs are widespread and were initiated in 2000 [
44]. The hilly and mountainous areas of eastern LP and the western and northern areas of JJJ are included in the Three-North Shelterbelt Development Program and the JJJ Sand Prevention Program implemented by national forestry key programs [
45]. These long-term and large-scale projects are beneficial to decelerate total ESV losses.
Under current economic development pressures, many natural landscapes have been lost or degraded. Similar declines in ESV have been detected during the urbanization of different parts of China. Our understanding about the reliability of ESV will be aided by comparing our findings with those obtained in other urban areas. Wang
et al. [
12] applied the coefficients proposed by Xie
et al. [
8] to estimate spatio-temporal variations of ESV in China. Their calculations indicated that the total ESV of China decreased by 243.4 billion Yuan from the 1980s to 2010 due to land use changes. This decrease was mostly attributed to the loss of important woodland and water areas. Jan Haas
et al. [
38] employed the method proposed by Xie
et al. [
8] to estimate environmental impacts using ESV in JJJ, the Yangtze River Delta and the Pearl River Delta. The decrease in ESV in JJJ from 1990 to 2010 was calculated to be 9045 million Yuan, with losses being attributed to an increase in built-up areas. At the urban scale, Li
et al. [
39] detected a decrease of 19.3% in the value of ecosystem services provided by Chang Zhou’s natural and semi-natural land from 1991 to 2006, with the 1.3% annual decrease being attributed to the conversion of farmland to other uses. Our results support all of these preceding studies; specifically, the reduction in natural and semi-natural ecosystems, ecosystem services and functions is the result of changes in land cover.
The method adopted here has several limitations. For instance, our ESV estimates are based on the usefulness of the links between land cover types and ecosystem service categories. This method has been criticized as having low resolution, high uncertainty and high variation arising from the complex, dynamic and nonlinear properties of ecosystems [
46,
47]. However, accurately calculated coefficients have less impact on dynamic analyses over time than on cross-sectional analyses, because the value of the coefficients tend to influence estimates of directional change to a lesser extent than they affect estimates of the magnitude of ecosystem values at specific points in time [
24]. Our study focused on variations in ecosystem services that influence human well-being over time. To this information, we added high-resolution land cover data (30 m) to calculate land cover and to assess ESV to obtain further precise spatial results. Therefore, the ESV unit in our study is credible at a regional scale; however, it may not provide an accurate assessment of the true value of ecosystem services.
In addition, we were not able to estimate monetized ESV for artificial surfaces, because we assumed that the well-known valuation of built-up land was zero. In fact, the levels of supply and demand of ecosystem services in urban areas are highly heterogeneous [
48]. The developed land produces increasing levels of significant negative ecosystem service values because of air, water, solid waste and other pollution. However, the green urban areas play a relevant and positive role in the provision of ecosystem services [
49]. For example, parks in urban areas provide aesthetic values for residents [
50]. Ecosystems in urban and rural areas have important ecological roles in urban and regional studies and are expected to be a valuable asset in future studies of urbanization, providing insights about the interaction between natural ecosystems and human households. Therefore, it is necessary to extract reliable coefficient values for urban areas in future studies.
5. Conclusions
We explored the similarities and differences in ESV in three urban agglomerations by evaluating the impacts of land cover change on ecosystem service values over a 10-year period (2000 to 2010). The changes in land cover trends were similar in all three coastal urban agglomerations. Changes were dominated by urban expansion to convert natural and semi-natural land cover to urban land, with cultivated land being largely occupied in the peri-urban areas of the major cities. Meanwhile, differences in the magnitude and rate of impact of land cover change on ESV were really obvious.
This study demonstrated that urban expansion in coastal areas may not necessarily lead to a net decline in ESV, if there are substantial increases in natural land with greater estimated coefficients. For instance, like SP and JJJ, there was a rapid increase in artificial surfaces in LP due to urban expansion; however, the total ESV in LP increased because of the effective national sustainable forestry (national green infrastructures) and natural wetland reserve policies. Thus, the negative impacts of urban expansion on ecosystem services could be offset by positive changes to natural landscapes, because a change in ESV depends on the interaction of changes of various land cover types over time.
This study highlights the important links between land cover change and impacts on ecosystem service values. The transformation of land cover types leads to a change in the structure and function of ecosystem services. However, when the capacity of the ecosystem to deliver ecosystem service functions is depleted because of land cover change, the intermediate- and long-term gains from economic growth may exceed the short-term gains, because degraded environmental quality may lead to economic losses. In China, while natural land should be used sustainably in the process of rapid urbanization, the need to balance economic, social and ecological benefits is becoming increasingly urgent. Therefore, land use and land cover policymaking processes should be aimed at balancing ecological resources with economic growth.