1. Introduction
Urban expansion is not new, neither is its effect on various ecosystems and land cover types. However, the alarming rates at which these lands are converted for urban uses require a deeper understanding of the conversion processes, especially when examined at small scales, such as the sub-watershed scale, which have not been studied in depth [
1,
2]. The ability to carry out this assessment at smaller scale will provide the opportunity to investigate urban expansion impacts that would have been lost when examined at larger scales. Rapid urban expansion in different parts of the world has led to significant changes on the types of land use and cover, including agricultural lands [
3,
4] and grasslands or prairie [
5,
6]. Urban expansion presents extreme problems to prairie and agricultural lands in the immediate vicinities of large urban areas. In particular, the conversion of farmlands near cities for other human uses is a global trend that challenges our long-term capacity to provide food, fiber, and ecosystem services to a growing world population [
7], increasingly living in major cities around the world [
2]. It is estimated that the population living in urban areas has increased from 30% in the 1950s to 54% in 2014 [
8], with a projected increase to over 65% by 2050 [
9]. This growth will continue to put pressure on the urban ecosystem, including grass and agricultural lands. Although most people recognize the effects that growing scarcity of fossil fuels and water will have on agriculture, there is less awareness of the impact of other factors such as loss of prime agricultural land and prairie on other human pursuits [
10]. There is even less concern among the general population about essential ecosystem services that are provided by farmland and natural areas, such as providing clean water, reducing soil erosion, mitigating the impact of severe weather, preserving biodiversity, and maintaining open land for recreation [
10], which support the effective functioning of the urban ecosystem. Thus, many even consider these lands, especially prairie, as wastelands. The conversion of agricultural lands for other urban uses tends to be very attractive in the short term because of the high economic returns from the sale of these lands, and the general perception that increased urbanization is a good index of economic growth. However, in the long term, these attractive benefits will present a challenge to future food production and ecosystem services that should support urban life.
Land is a basic resource for the survival and continued expansion of cities [
11]. Nevertheless, as cities continue to expand, more units of land are converted from their natural state into urban areas. This process will eventually lead to land-use-related problems, which may include the development into a non-contiguous or “leapfrog” pattern or conversion from native land cover to designed land cover [
12], including the introduction of non-native invasive species. Many case studies already exist on how urbanization has led to ecosystem degradation and loss of ecosystem services [
13], especially provisioning services [
14], which are mostly provided by prairies. Therefore, understanding urban expansion processes at a local scale and their impact on other types of land use that provide support for the urbanization process will help us cope with the emerging problems associated with urban development, and to ensure both environmental and socio-economic sustainability for the ever-growing urban population [
15].
Urban expansion takes place through different forms: the redevelopment of built-up areas at higher densities, the infill of remaining open spaces in already built-up areas, or the development of greenbelt land around cities [
16]. While urban expansion may take different forms across different cities across the globe as a result of differences in socio-economic, cultural, historical, and environmental conditions, as well as preferences [
17], its effects on the natural and urban agricultural lands are similar, namely, mostly loss in grass/agricultural lands and ecosystem fragmentation. As a result, many rapidly urbanizing communities are struggling with the question of whether or not the loss of agricultural lands and the ecosystem degradation are inevitable in the expansion of their cities [
7]. While many communities in the U.S. and Europe may be able to marginally balance expanding populations and commerce with a desire to maximize the remaining open space [
7], other less developed regions of the world may suffer a huge direct loss of these lands [
18]. This situation will eventually affect their ability to produce food [
19] and maintain urban ecosystem health. However, for U.S. cities to achieve this balance, we must strive to understand the historical urban expansion processes that account for these effects. This will ultimately serve as a tool for balancing an expanding urban population with the sustainability of the natural environment and urban agricultural lands. As part of broader efforts to achieve this balance, this study identified one of the major sprawling cities in the U.S., namely, Kansas City [
20], which lies in both Missouri and Kansas States. The goal was to assess the impact of urban expansion on the loss of prairie and agricultural lands in six sub-watersheds in the study area between 1992 and 2014, using SPOT satellite images. SPOT imagery is a product of remote sensing that is commonly used for studying urban growth [
2,
6,
21,
22]. SPOT satellite images are often preferred for this type of application because of their relatively higher spatial resolution when compared with products of similar medium resolution satellites, which allows for easier separation of feature categories during the classification process. Hence, SPOT imagery was adopted in this study.
4. Discussion
As shown in the results, agricultural lands and grasslands reduced considerably in all six sub-watersheds. This result is consistent with what others have reported in the literature when classifying the LULC of many urban centers and their immediate vicinity [
2,
25,
32,
33]. This consistent result suggests that this a common phenomenon around the world. However, by investigating this loss in the specific six sub-watersheds, specific mitigation strategies can be adopted and implemented in each area. The dynamics of change in wetland were balanced between losses and gains in the six sub-watersheds. For instance, while wetlands decreased from 0.71% to 0.3% in the Headwaters Indian Creek sub-watershed, they increased in the East Fork Little Blue River and Blue Creek-Missouri River sub-watersheds. This pattern of loss and increase can be observed in the other three sub-watersheds as well. Urbanized areas increased in all six sub-watersheds, which is normal considering results from similar studies in the study area and elsewhere [
2,
32,
33,
34,
35].
Forestland cover increased in all six sub-watersheds except for the East Fork Little Blue River sub-watershed, in which it decreased from 29.58% to 28.55%. This pattern is consistent with the work of Ji et al. [
32], whose findings from investigating urban wetlands dynamics in the area indicated that forestland increased from 13.49% in 1992 to 23.56% in 2008. Ji et al. [
20] suggested that the increase in forestland across the area at large could be the result of tree planting as part of suburban development. Recent studies also showed that forest cover generally increased in both the states of Missouri and Kansas [
36,
37], which is the general trend in the U.S. since the 1990s [
38].
The East Fork Little Blue River sub-watershed experienced the highest growth in urbanized areas between 1992 and 2014. A previous study by Mid America Regional Council (MARC) [
24] showed that the population increased in the area, with an estimated growth rate of about 11.3% between 2000 and 2010. This growth encouraged the continued expansion of urbanized areas in the study location. The rate of expansion is similar to the pattern of growth in the sub-watersheds. For instance, East Fork Little Blue River had the highest rate of expansion followed by Upper Shoal Creek-Missouri River. Urban expansion in all the sub-watersheds took place at the expense of the other LULC types. Many types of LULC, particularly agricultural land/grassland, were converted for urban uses. In a report for the U.S. Environmental Protection Agency, Region 7, Rouse [
39] showed that although the East Fork Little Blue River sub-watershed’s land use was predominantly farming in the past, within the last 20 years, urban growth from the Kansas City metropolitan area has spread into eastern Jackson County, prompting significant commercial and residential development. Today, the cities of Lee’s Summit and Blue Springs account for most of the growth within the East Fork Little Blue River sub-watershed [
39].
Our study also showed that agricultural land/grassland decreased in the East Fork Little Blue River by 20%, with 92.02% of this loss being due to urban expansion. In the Headwaters Indian Creek sub-watershed, 2299.8 ha of grassland/agricultural land were lost between 1992 and 2014. Of this loss, 96.77% was due to urban expansion. Similar results were found in the other sub-watersheds. This study thus revealed the alarming impact of urban expansion on the loss of agricultural lands and grasslands in the six sub-watersheds.
5. Conclusions
The impact of urban expansion on agricultural land/grassland in six sub-watersheds was investigated between 1992 and 2014. In order to achieve this, the types of LULC of the study area were produced by classifying SPOT satellite images of the sub-watersheds. The supervised maximum likelihood classification approach, popularly used for thematic mapping, was used for classifying the SPOT images. The maps produced from this classification process were used to investigate the loss of prairies and agricultural lands to urban expansion in six sub-watersheds in the Kansas City metropolitan area of the States of Kansas and Missouri in the U.S. Based on the classified maps, we calculated the magnitude and rate of urban expansion, and the proportion of loss in prairies and agricultural lands that was a result of urban expansion. Over the 22-year study period, urbanized areas increased at an accelerated rate. The East Fork Little Blue River and the Upper Shoal Creek Missouri River experienced the highest growth rates, with 16.8 and 14.4, respectively. Agricultural land/grassland decreased in all six sub-watersheds, while wetlands and forestlands had different patterns of gains and losses. However, beyond just revealing the dynamics of change in the six sub-watersheds, this study revealed that a greater proportion of the loss experienced in agricultural land/grassland was due to urbanization, which is similar to what others have reported elsewhere. For instance, three of the six sub-watersheds (Headwaters Indian Creek, East Fork Little Blue River, and Buckeye Creek-Missouri River) had proportions of loss to agricultural land/grassland higher than 90%, which is very high by all standards. While this may not be viewed negatively from an economic standpoint, ecologically speaking, it is negative [
26]. The question then arises, should urbanization be halted or reduced in sub-watersheds to minimize the loss to agricultural lands/grasslands? While this is not a realistic goal, city planners can focus on planning urban expansion to minimize sprawl, which has been previously documented [
20]. Sprawl creates many patches, which is not good for biodiversity because it fragments the landscape. Compact development with infills rather than leapfrog development should be encouraged. This study revealed that a rapid assessment of the magnitude of agricultural lands/grasslands that have been converted for urban uses over a period of time is achievable. Satellite images thus provide a quick and effective way of monitoring the impact of urban expansion on the natural environment. Results from this study can serve as baseline information for policy makers and planners at the local level of administration in ensuring that urban expansion in each of the sub-watershed studied is smart and sustainable. Future work will examine the implications of the loss in prairies and farmlands on food cost and biodiversity in the study area.