Intensive land-cover change (LCC) is one of the most important factors leading to increasing concerns over global environmental change and sustainability across many cityscapes [1
]. The rate and intensity of LCC pose several sustainability challenges particularly related to ensuring ecological sustainability, resource availability and management. The loss of ecological assets through the processes of land-cover changes is driven by rapid population growth and spatial expansion of cities, among other socio-economic drivers such as technology development. These factors lead to a decline in land-cover types with high ecological values, thereby thwarting their capacity to sustain livelihoods and the well-being of people [2
]. Global changes in land cover due to population growth urbanisation and expansion of human settlements have been estimated to be 4.89% between 1992 and 2015 [2
]. This has resulted in a significant decline in the estimated value of major ecosystem assets, including their services and resources, ranging from US$
4.3 to 20.2 trillion per year globally [4
Analysis, modelling and prediction of the dynamics of LCC through remote sensing and geographic information system (GIS) tools and techniques and the utilisation of satellite-based and ground-truth data provides fundamental information for a better understanding of spatial change and transformation processes and trajectories [3
]. In sustainability debates, up-to-date information on LCC is particularly relevant for identifying areas of vulnerability where long-term ecosystem functions and biodiversity are important for sustainability [2
]. The lost land is often that used for ecological assets which help ensure a sustainable environment [1
]. Thus, LCC often leads to the destruction of ecological landscapes, loss of biodiversity, diminished land productivity, land degradation and a depletion of freshwater and forest resources. Sustainable land use implies that land-related resources should be exploited to produce goods and services in such a way that, over the long term, the natural resource base is not damaged and that future human needs can be met [9
]. Changes in the ecological landscape through human-nature interactions, particularly the influence of the former on the latter are a good indicator for sustainability [10
The availability of the biosphere’s resources, ecosystems and services is an important guarantee for sustainability. Current trends of rapid spatial changes, however, have led to increasing appropriation and degradation of ecological systems, which potentially undermine their capacity to sustain food production and natural resources. Since land-cover represents the resource base of our ecosystem, the dynamics of LCC invariably leads to the conversion of a natural ecological system into a social ecosystem [5
]. In the light of this, assessing the implications of past, current and future LCC for ecological change and sustainability is becoming increasingly necessary.
The literature has generally identified two components of LCC relevant for understanding the implications of landscape changes for ecological sustainability [12
]. Firstly, recent studies of LCC have largely built on the consensus that the magnitude of quantitative changes in land-cover types can seriously compromise the sustainability of a particular ecosystem [5
]. Such changes lead to desiccation of the land, which can affect the quantity of biodiversity. In the context of African cities, land conversion especially through rapid urban sprawl characterised by uncontrolled real estate development, incremental building of single-family homes, and inner-city redevelopment (i.e., densification through high rise building) are some of the critical contributory factors of quantitative LCC [20
]. A second line of research has focused on understanding qualitative changes taking place within stable land-cover elements that are critical for the sustainability of ecological systems and communities [1
]. These include inter alia, the processes through which changes occur and the underlying mechanisms for ecosystem function and provision of services [13
]. In the context of African cities, such processes can include population growth, urban farming, informal developments, waste disposal and encroachment onto wetlands among others [16
]. These two strands of thought collectively draw attention to the fact that changes in the functions, resources and services provided by ecological systems can occur through both quantitative and qualitative changes in land-cover types [13
Within this context, urbanisation constitutes one among many important forms of land-cover changes in major fast-growing African cities such as the Greater Accra Metropolitan Area (GAMA), Ghana. During the last decade, a rapid increase in population has taken place in GAMA [26
]. Accordingly, there is a growing demand for vacant undeveloped land for housing development [27
]. Subsequently, rapid expansion of rural and urban built-up land areas as well as infrastructure construction have occurred across formerly vegetation areas [27
]. The patterns and distribution of this spatial growth, characterised by unplanned, uncontrolled, scattered and sprawling built-up areas, exert tremendous pressure on the natural landscape. The conversion of the natural landscape towards built-up areas in GAMA underscore the influences of LCC on ecological balance. The higher the level of land cover and land use changes, the more destructive the effects on ecological sustainability. Consequently, spatio-temporal changes in land cover and the impacts on ecological assets, systems and potential resource deficiency risk have become subjects of great concern [29
There is a lack of research regarding how current and potential changes in land cover affects ecological sustainability within the GAMA region. Currently, a few satellite-based remote sensing studies have analysed land-use/cover (LULC) change dynamics in GAMA [20
], revealing that that forest, grasslands, agricultural land, water bodies and wetlands are being rapidly replaced by contiguous built-up areas. Up until now, research which targets analysis of quantitative and qualitative LCC to unravel the implications of such changes for ecological sustainability is still absent. The studies shown in references [20
] have only warned, based on their findings, that ongoing spatio-temporal changes in LULC may lead to resource deterioration and consequently have significant impacts on ecosystem services. A comprehensive analysis of both current and potential future spatial patterns and processes of LCC to show how such changes will affect ecological balance and sustainability is urgently needed.
Building on our previous research [20
], the present study aimed at bridging the gap in knowledge by primarily analysing, modelling and predicting the spatio-temporal dynamics of LCC and subsequently highlighting the key implications of the changes for ecological sustainability of the GAMA region. We address these research objectives by integrating remote sensing, GIS, land accounting techniques and utilisation of very high spatial resolution (VHSR) imagery for 2008 and 2017. After mapping past LCC, future LCC is predicted for 2030. The statistical data derived from actual and predicted LCC is utilised to construct a land cover account [25
] in order to evaluate the processes of land consumption and formation, vulnerability and/or persistence and the turnover rates of changing ecological land-cover types. These data will aid in exploring the key implications of actual and predicted LCC for ecological sustainability of the GAMA regions. The outcomes of this study are expected to provide information on the state of ecological degradation, which is critical for defining policies targeted at ensuring proactive land management and spatial planning for sustainable trajectories of urban development and ecological land conservation.
The context and rationale of this study forms a key component of the WaterPower project’s in-depth research on the collision of mega-trends in a West African coastal city [34
]. With the main aim of contributing to current debates on society-nature relations by mapping, analysing and understanding processes that unfold in the urban water sphere, the WaterPower project (01.04.2014–30.09.2019) explored the intersections and dynamics of urbanisation, urban land expansion, the (mis-)allocation of resources and climate change, drawing on the example of water scarcity in the coastal city of Accra.
This section presents outputs of LCC analysis as well as the land stock and flow account which highlight how land was transferred or exchanged between the different cover categories. This is particularly useful in answering some of the key questions that arise in the context of sustainability. Figure 2
below shows land-cover maps derived from classified images for the years 2008 and 2017 and land-cover change prediction modelling results for 2030.
below illustrates the spatial area distribution of the land-cover types identified in the study area. Grassland/farmland was the dominant land-cover type as of 2008, whilst the built-up area increased by 277 km2
(67%) within the time period. This increase was largely due to the conversion of 83 km2
(54%) of open space and 194 km2
(27%) of grassland/farmland. Furthermore, the area of forestland and water body/wetland increased and decreased slightly by 5%. According to the predicted LCC distribution between 2017 and 2030, urban built-up area is likely to experience an increase of 408 km2
(59%). On the contrary, open space, forestland, grassland/farmland and water body/wetland may decrease by 65 km2
(90%), 86 km2
(46%), 253 km2
(49%) and 6 km2
The Sankey diagrams in Figure 4
a,b below were used for visualising the size and direction of land-cover stocks and flows. The stacked bars represent the relative stocks or size of the land area of each cover category in 2008, 2017 and 2030, as shown in Figure 3
. The height of each cover category in the vertically stacked bars is proportional to the relative stock of the represented land-cover category in the study area, and categories are also arranged vertically by spatial extent, from largest on top to smallest at the bottom (Figure 4
a,b). The slightly darker shade of colour in the diagram represents transition lines for each land-cover category. Thus, Figure 4
] illustrate transitions showing the extent of each land-cover category as well as the size and change trajectories of the five land-cover categories.
As depicted in Figure 4
a, between 2008 and 2017 the extent of grassland/farmland declined from 713.02 km2
to 518.74 km2
. The area covered by grassland/farmland transformed mainly into urban built-up area (239.04 km2
) and forestland (90.11 km2
). Despite this net increase, the area forestland had been converted. As much as 72.60 km2
and 26.61 km2
of forested area transitioned to grassland/farmland and urban built-up area respectively. Thirdly, the area covered by open space reduced substantially. About 65.02 km2
of open space transitioned to grassland/farmland, whilst 60.78 km2
were converted to urban built-up area. The obvious increase in urban built-up area is largely due to the conversion of grassland/farmland (239.04 km2
), open space (60.78 km2
) and forestland (26.61 km2
). Quite interestingly, a total area of 50.9 km2
of urban built-up area transitioned into grassland (34.08 km2
), open space (11.51 km2
) and forestland (5.29 km2
). Finally, although two-thirds of the water body/wetland area remained unchanged, a marginal extent (2.23 km2
) transitioned into urban built-up.
b illustrates how the predicted land-cover transitioned from 2017 to 2030. It is observed that urban built-up may expand by 408.11 km2
. This can be attributed mainly to the conversion of 290.77 km2
of grassland/farmland, 70.29 km2
of forestland, 59.79 km2
of open space and 4.28 km2
of water body/wetland. It is also interesting to note that by 2030, 71.89 km2
of forestland and 13.65 km2
of open space may transition into grassland/farmland. Overall, the results give an indication that between 2017 and 2030, rapid urban expansion may occupy a large area of ecological lands.
Having identified the dominant patterns of land-cover transitions across the study area, it was important to assess whether the consumption or loss of a given land-cover type is compensated for by the formation of new areas of the same type. This effectively presents information on land-cover persistence and vulnerability over time and space.
From Table 2
, it can be observed that quite a substantial stock of most ecological land-cover types in 2008 was largely converted to urban built-up area as of 2017. It is interesting to note, however, that although open space, grassland/farmland and water body maintained some proportion of their original stock and also gained additional area, the proportion of the original stock that was consumed in each case were not compensated for by the formation of new areas. On the contrary, the original stock of forestland and urban built-up area that was lost from 2008 were fully compensated for in 2017. A marginal net formation and a much larger proportional increase in forestland and urban built-up area were observed. The importance of net formation is emphasised by the total amount of turnover for each land-cover type. The analysis of turnover revealed that with the exception of water body/wetland recording a low turnover, all the other land-cover types recorded relatively high turnover, implying the vulnerability of such land-cover types.
From the data, the area stock recorded for open space, forestland, grassland/farmland and water body/wetland in 2017 may decrease by 2030. As can be seen, the area stock each of these land cover categories may gain during the period might not be compensated for at all by 2030. In terms of stock turnover, open space and forestland may record a little over 100%, constituting a very high turnover. With a turnover of 85%, the farmland/grassland category is also expected to experience a relatively high stock turnover. These expected turnover rates are an indication of the high vulnerability of these land-cover categories. All things being equal, the water bodies and wetlands category may experience a relatively low stock turnover (17%), indicating relative resistance to change.
Overall, the results reveal that ecological land cover lost a greater proportion of area compared with built-up land. Similarly, the changes experienced over the last decade are likely to continue into the future whereby urbanisation and urban growth may increase in GAMA by 2030. This suggests that as urban and rural settlements grow, the quantum of built-up areas increases. In constrast, ecologically significant land areas decrease, i.e., total land area – built-up area. Overall, the spatial distribution patterns of the predicted LCC are in accordance with the fact that the majority of current and future urban growth is concentrated around and expanding upon ecological areas.
This study explored the spatio-temporal dynamics of LCC and its implications for ecological sustainability in the GAMA. Following the analysis, modelling and accounting of actual LCC (2008-2017) and predicted LCC (2030), the study produced the following relevant findings. Firstly, the results revealed that between 2008 and 2017, there was a pervasive conversion and/or loss of ecological land including semi-vegetated open space, forestland, grassland/farmland, and water bodies/wetland due to a substantial increase in the urban built-up area. The general patterns and tendency of LCC in the region reveal that the trend of changes experienced between 2008 and 2017 is expected to continue into the future as per the predicted land-cover map for 2030. Overall, these results clearly showed a high degree of loss and vulnerability of ecological lands and by extension ecosystem assets, resources and services. This is a clear indication that the changing dynamics of land cover in the region does not represent a trend towards sustainability. While previous studies warned that ecological land deterioration could place the sustainability of many regions under threat, the findings of this study confirms the case of the GAMA region. Perhaps the predicted future LCC maps can serve as an early warning system for understanding the potential implications of LCC. The study’s findings form the basis for proactive land use plans, policies and decisions that lay emphasis on sustainable management of current patterns of LULC change, resources and protection of ecologically sensitive areas. Thus, policy and decision makers, urban planning, land management organisations and natural resources managers should plan alternative conservation actions to develop improved LULC management practices for balanced sustainable trajectories of urban development and ecological land conservation.
The study reveals information that addresses one of the specific research questions of the WaterPower project on how the ‘urbanscape’ of rapidly growing coastal cities are at risk from local environmental burdens enforced by urbanisation and environmental change. The study synthesises impacts from urbanisation-driven LCC processes taking place on the coast environment of Accra and provides an understanding of the nexus between coastal urbanisation processes and a transformation towards sustainability.