1. Introduction
In recent years, global climate change and enhanced human land development activities have led to increased risks of negative ecosystem impacts [
1], such as: loss of biodiversity [
2], serious environmental pollution [
3], and loss of natural resources [
4]. Therefore, timely prevention of potential human-induced or naturally occurring damage to ecosystems is becoming increasingly important. Ecological risk can be assessed by determining the probability of occurrence and degree of harm due to one or more natural factors or human activities that adversely affect ecosystem functions and structures [
5,
6]. Ecological risk assessments enable construction of whole-process, multi-level ecological risk prevention systems for application to human activities for the purpose of promoting ecosystem stability, use of virtuous cycles, and sustainable resource development [
7,
8]. Early ecological risk assessments predominantly addressed single risk sources and risk receptors. However, since that time such assessments have been rapidly replaced with ecological risk assessment methodologies that are no longer limited to assessing single risk sources and risk receptors, enabling scaling-up of assessment scopes from ecosystems to landscapes [
9].
A landscape refers to an area of spatial heterogeneity consisting of interacting land units or ecosystems [
10,
11] that recur in a similar form. Landscape heterogeneity is closely related to a landscape’s capacity to recover from disturbances (resilience), as well as its stability and ecosystem diversity that together underscore the importance of the landscape as a key ecological risk assessment element [
12]. Landscape ecological risk refers to possible adverse effects resulting from interactions between landscape patterns and ecological processes with natural or anthropogenic factors [
13]. As compared with traditional ecological risk assessment methodologies, landscape ecological risk assessments are based on identification and analysis of coupled correlations as they relate to ecological processes and spatial patterns within landscape ecological systems. Ultimately, these assessment methodologies focus on spatial and temporal heterogeneity of risks [
14] and entail comprehensive characterization and spatial visualization of multi-source risks [
15].
At present, scholars have selected various assessment indicators, methods, and models for use in studies conducted in different regions with different assessment goals, all of which have yielded excellent results. In terms of assessment objects, hotspot areas for landscape ecological risk assessment research have mainly included urban areas [
16], watersheds [
17], administrative regions [
18], nature reserves [
19], etc. In terms of assessment units, landscape ecological risk assessments with different goals conducted to evaluate risks in different regions require rational selection of assessment units in order to optimize risk assessment results. There are three existing methods for dividing the assessment units: administrative districts [
18], natural geographical boundaries [
20], and risk cells [
21]. In terms of assessment methods, landscape ecological risk assessments can be categorized into two types of methods based on risk source sinks [
22] or on landscape patterns [
20]. The risk source sink-based method mainly assesses risk exposures and hazards by identifying risk sources and conducting receptor analysis. By contrast, the landscape pattern method breaks away from the inherent model of traditional ecosystem assessment [
23] and assesses landscape ecological risks directly from spatial patterns on a regional scale, while emphasizing integration of multi-source ecological characteristics.
Ecological risk assessment methods based on landscape patterns tend to identify and directly assess ecological risks quantitatively from the perspective of spatial landscape patterns caused by land-use changes. Land use is viewed as a comprehensive reflection of direct impacts of human economic and social activities on surface resources and the natural environment [
24], whereby spatio-temporal heterogeneity of land use is influenced by regional topographic and geomorphic features that are closely tied to spatio-temporal changes in landscape patterns and landscape ecological risks [
25]. Currently, development of ecological risk assessment models based on land-use changes is a research hotspot [
26]. For example, Xie et al. [
27] analyzed ecological risks associated with landscape patterns within the region of Poyang Lake from 2010 to 2018 and ranked landscape pattern risks in descending order as watershed, construction land, unused land, woodland, grassland, and arable land. As another example, Zhang et al. [
28] analyzed land-use changes and dynamic characteristics of coastal cities in China using a land-use transfer matrix and other tools so as to further analyze the impact of land-use changes on landscape ecological risks.
Guangxi possesses a relatively fragile natural ecological environment characterized by high mountains and steep slopes, poor soil, severe soil erosion, and frequent natural disasters [
29]. In order to reduce environmental risks, Guangxi has been vigorously implementing seven key national ecological projects, including afforestation, pollution prevention, and control and comprehensive management of stone desertification. In addition, efforts have been made to continuously improve natural ecosystem balance through the implementation of a series of institutional reforms and policies. Nevertheless, rapid industrialization and urbanization in that region [
30] have seriously damaged arable land, woodland, and water bodies and continue to undermine attainment of ecological security in Guangxi [
31]. Therefore, it is particularly important that ecological risk assessments are conducted in Guangxi in order to guide efforts toward establishing a systematic and effective ecological environment monitoring and early warning system.
This study constructs an ecological risk assessment model according to both the landscape disturbance index and landscape fragility index, while also taking into account land-use changes toward the goal of comprehensively describing the overall spatial pattern characteristics of ecological risk in Guangxi. At the same time, in order to monitor land-use changes as they occur with time, a geographic detector is used to quantitatively analyze the driving causes of landscape ecological risk changes from both natural and socio-economic perspectives. Moreover, in order to guide ecologically sensitive land development in Guangxi, magnitudes of driving forces for each ecological risk factor and results of ecological risk autocorrelation analysis are taken into account.
4. Discussion
4.1. Analysis of Land-Use Change and Landscape Characteristics
Due to the combined influence of natural and human factors, land use in each region of Guangxi has been dynamically changing [
49]. According to third-generation reanalysis data provided by the ERA-Interim website of the European Centre for Medium-Range Weather Forecasting, the annual average temperature of Guangxi increased by 0.5 °C and the annual average amount of precipitation increased by 259.16 mm from 2000 to 2018, with such changes expected to drive many natural factor changes. In addition, according to the Guangxi Statistical Yearbook, it is known that the total population of Guangxi in 2000 was 47.51 million and the per capita gross regional product was RMB 4652 as compared to the total population in 2018 of 56.59 million and the per capita gross regional product of RMB 40,012. Such increases in social and economic activities are also expected to impact natural factors, since drivers of land-use change are closely related to natural environmental changes and human activities. Moreover, these results demonstrate how land-use change studies can be used to visually reveal environmental impacts of human activities [
50].
Different land-use types have different forms and rates of change due to their different uses and functions [
51]. The large area of steep-slope arable land in Guangxi and the existence of a large amount of rock-desert arable land and arable land undergoing soil erosion [
52] have led to an increased ecological risk of arable land in the region. In addition, due to effects of economic and social development and occurrences of natural disasters, arable land is being gradually replaced with construction land, resulting in a significantly decreased regional arable land area, an increase in arable land patch number and increased arable land fragmentation. Meanwhile, woodland has various ecological functions, such as water conservation [
53], soil and water conservation [
54], and species protection [
55], etc. Guangxi woodland covers a large area that is mostly concentrated in patches, with significant dominance trends and small change trend that keep ecological risk low–medium. Nevertheless, in the 21st century, peak periods of industrialization, urbanization, and infrastructure development have occurred in Guangxi that have increased the demand for land development, which has remained high for quite a long time [
56]. In fact, construction land area increased by 1306.99 km
2 from 2000 to 2018, due to development of large amounts of original woodland and grassland areas. Moreover, construction land area is growing significantly faster than the number of patches, due to rapid development of the area that has resulted in the formation of patches in a concentrated and contiguous manner that has led to enhanced patch aggregation and increased internal stability. Meanwhile, water areas serve an important ecological function by maintaining the balance and stability of ecosystems. There are numerous rivers in Guangxi that belong to four major watersheds, namely, Pearl River, Yangtze River, Baedu River, and Binhai that have increased in total area due to effective management at the local level.
Different types of land use in each region determine different forms of local economic development and ecological protection [
57,
58]. Guangxi is mainly dominated by forest land, arable land, and grassland. Forest land and grassland are mainly distributed in patches in Baise and Hechi in northwestern Guangxi and Hezhou, Wuzhou, and Yulin in eastern Guangxi. These areas are mainly dominated by forestry and pastoralism such that forest land and grassland play important roles in the economic development and ecological stability of the region. Northwestern Guangxi is rich in mineral resources, but the region is relatively economically poor and has serious rock desertification problems that should be controlled; thus, in this region active development of unused land should be promoted to provide land for urban construction. Arable land and construction land are mainly distributed in Nanning, Guigang, and Laibin in central Guangxi, and in Beihai and Qinzhou in southern Guangxi, which are mainly agricultural and industrial areas with leading roles in the economic development of Guangxi. These areas are also are rich in water resources and have supported moderate land development, but these areas should be improved and maintained as arable land.
4.2. Ecological Risk Change Analysis
Ecological risk levels of areas of Guangxi in 2000–2018 mainly included low, low–medium, and medium risk levels, with overall increases in risk levels observed. However, spatial distributions vary significantly for each risk level, with risk levels that are high in the south, low in the north, high in the middle, and low in all directions outward from the middle. Areas with high ecological risk areas are concentrated in Laibin, Guigang, and Nanning in central Guangxi, Qinzhou, Beihai, and Fangchenggang and other areas in southern Guangxi, while areas with low ecological risk are mainly concentrated in northern Baise, Hechi, and other areas in the north. Importantly, the spatial distribution of ecological risks is closely related to regional natural conditions and socio-economic conditions [
59], with different natural conditions determining which industries are suitable for regional development that, in turn, will shape land use and human activities in that region [
60]. The southern and central regions of Guangxi have medium–high temperatures, low levels of precipitation, low elevation and gradual slopes, high population, high GDP, and high distributions of construction land and unused land. These factors contribute to the medium–high ecological risk in Guangxi, with the southern region possessing especially high ecological risk due to its status as a coastal region. As compared to 2000, areas with high and medium–high risk within Nanning and Qinzhou and other places underwent rapid expansion by 2010 that may have due to economic developmental pressures that led to a continuous increase in construction land with associated high ecological risk. It is worth noting that land areas with high ecological risk in Nanning in 2010 decreased in size significantly by 2018, with reductions in land areas concentrated in Jiangnan District and Xixiangtang District of Nanning. These changes are due to good progress made in those regions since 2010 in implementing ecological initiatives, such as “ecological village” construction, afforestation, water environment management, and village greening.
In order to scientifically maintain the ecological security of Guangxi, Guangxi is divided into ecological risk key control zones, strict control zones, and general control zones in which high-risk areas and medium–high-risk areas correspond to key control zones, while areas with medium and low–medium risk correspond to strict control zones and low-risk areas correspond to general control zones (
Table 6).
Ecological risk key control zone: This area is mainly concentrated in the central and southern part of Guangxi, with low elevations and gradual slopes, low levels of precipitation and high temperatures. Due to rapid economic development and GDP growth, these areas contain high proportions of construction land that are rapidly increasing, resulting in increased ecological pressure on the local ecological environment that causes ecological risk to continually rise. Therefore, in these areas, it is necessary to develop reasonable ecological red line policies, raise awareness of local governments and residents regarding the importance of ecological risk mitigation, strictly control the scale of urban land development, promote transformation of land-use types, strengthen water pollution control, and enhance protection of ecological systems such as forests, grasslands, and high-quality arable land.
Strictly controlled ecological risk zones: Distribution of various land-use types in this zone are balanced and impacts of natural and socio-economic factors are low as compared to their impacts in other zones. Therefore, these zones should play a leading role in land-use planning by coordinating current land-use distribution patterns, strengthening optimization and integration of construction land and by rationally developing land according to resource availability to promote economic development and reduce ecological risks at the same time.
General control ecological risk zones: These areas are mainly scattered along the margins of Guangxi and are characterized by high mountains and steep slopes, high levels of precipitation, low temperatures, a relatively low economic level, predominance of woodland and grassland, relatively little human activity, and low ecological risk. Therefore, the effective arable land area should be increased, the quality of arable land should be improved, construction land should be developed moderately and reasonably, and infrastructure development should be strengthened. In addition, advantages of local woodland distribution should be maintained, while comprehensive management of stone desertification and karst areas should be vigorously carried out to repair and improve the ecological health of the environment.
4.3. Study Shortcomings and Recommend Processed Improvements
First, the validity of the evaluation used in this study is uncertain, due to differences in data quality [
61], risk cell selection [
20], and weight assignments [
62] that all increased uncertainty. Accuracy and quality of remote sensing images and the accuracy of interpretation largely affect data quality, with different criteria used to define risk cells producing different scale effects and variable evaluation results due to use of different weight assignment methods. This study adopted the equally spaced grid method to divide risk cells, a method that is conducive to spatial interpolation, which is necessary to generate a risk spatial distribution map. However, this method disrupts the continuity of the original natural landscape. Secondly, when constructing the comprehensive ecological risk index model of Guangxi, assessment results obtained using different assignment methods will differ. Therefore, future research is needed to develop more powerful uncertainty analysis methods that focus on analyzing the uncertainty of each link in the assessment process for use in improving the reliability of assessment results.
Second, evaluation indicators and evaluation criteria are not uniform and therefore cannot serve as unified evaluation indicators for use in an ecological risk assessment, as selection of evaluation indicators that vary significantly would lead to different selection of assessment objects. At the same time, although the classification of ecological risk levels is generally not dependent on research needs, distribution ranges of ecological risk values and classification intervals differ greatly among different research disciplines. Therefore, ecological risk assessment results are relative and can only be used to assess relatively high and low regional ecological risks.
Finally, ecological risk management research efforts have been relatively weak [
63], due to the lack of a mature ecological risk management framework system. Meanwhile, research to date has mainly focused on risk assessment itself and not on the formulation of implementable recommendations based on assessment studies geared toward mitigating regional ecological risk.
5. Conclusions
(1) Guangxi land-use types mainly include woodland, arable land, and grassland. Woodland and grassland are distributed in patches within marginal areas of Guangxi, while arable land and construction land types are mainly concentrated within the middle zone. Due to influences of natural factors and human activities, large areas of arable land, woodland, and grassland have been developed, such that each patch of construction land tends to be located within larger areas of concentrated land patches that tend to rapidly increase in area with time.
(2) The overall ecological risk level of Guangxi is low, but differences in spatial distribution of ecological risks are significant; the northwest half of Guangxi shows a trend of gradually increasing ecological risk when analyzing land areas from the edges toward the central area, while the southern half that is adjacent to the South China Sea shows steadily increasing ecological risk. In terms of spatial distribution, areas of high risk and medium–high risk gradually expand to form a continuous distribution. Combining land-use types, woodland is mainly distributed in low-risk areas, while arable land and grassland are mainly distributed in areas with medium ecological risk, and construction land, water, and unused land are distributed in areas with highest ecological risk.
(3) Ecological risks in Guangxi from 2000 to 2018 show positive correlations, with Moran’s I index values rising then falling and the spatial aggregation and spatial differentiation of ecological risks diminishing overall. In terms of spatial distribution, high-high aggregation is mainly concentrated in the central and southern coastal areas and tends to be focally concentrated, while low-low aggregation is distributed sporadically throughout peripheral areas.
(4) From 2000 to 2018, influences of socio-economic factors on ecological risk were greater than influences of natural factors, with GDP and population density acting as important drivers of changes in spatial and temporal distributions of ecological risk across the landscape of Guangxi.