Effects of Land-Use Changes on Vegetation Structure and Composition Along the Rongai River in Nakuru County, Kenya

Changes in land-use practices have affected the integrity and quality of water resources worldwide [1]. Changes in land and water use have also resulted from the human population increase, posing a growing threat to biodiversity and ecosystem services [2]. Physico-chemical and biological diversity are very important to the health of an aquatic ecosystem like rivers and other freshwater systems [3]. Despite the importance of riparian vegetation in the provision of ecosystem services and maintenance of regional biodiversity, the impact of anthropogenic activities on vegetation diversity is still poorly understood [4]. Riparian vegetation exhibits unique characteristics such as hydric soils, floral and faunal composition, and community structural relationships and provides an ideal habitat for floral and faunal communities. A healthy riparian ecosystem is essential to the maintenance of water quality and biological integrity in surface water systems, and their destruction often leads to the degradation of adjoining aquatic ecosystems [5]. Despite their importance, the anthropogenic impact on this vegetation type is still poorly understood more so in developing countries [6].

Although the effects of land-use changes are well documented for aquatic ecosystems, few studies have specifically assessed riparian vegetation [7]. Changes in riparian vegetation can affect water quality and aquatic communities. Indeed, agriculture, pasture, and urban land-use changes often result in decreased diversity and shifts in relative abundance among the functional feeding groups (FFGs) of aquatic macroinvertebrate assemblages [8]. In contrast, sites of good riparian quality present higher densities of scrapers, predators, and collector-gatherers. The natural factors that determine the structure of riparian vegetation include natural disturbances for example periodic flooding or tree fall, altitudinal climate shift and geographically unique channel processes [9]. Riparian vegetation is especially vulnerable to environmental changes and might be the first component of the landscape to show a decline in these crucial services brought on by processes related to land use change [10]. Particularly in Neotropical societies, little is known about how urbanization and agricultural practices affect riparian vegetation assemblages and the environmental benefits they provide. Keeping these places in a relatively well-conserved state, like those found close to the sites, is crucial for this reason. This type of vegetation is widely acknowledged to be one of the most ecologically diverse, dynamic, and complex ecosystems, and as such, it serves as an essential refuge for biodiversity [11].

Through its beneficial effects on soil microorganisms, higher plant variety modifies the breakdown dynamics of fresh residue inputs, which may have positive effects on soil nutrient cycling and SOM dynamics. For instance, in grassland that has been converted to eutrophic soil, plant diversity increases the carbon use efficiency of soil microbes without affecting the exudation efficiency of phosphorous-acquiring enzymes (phosphatase). This suggests that efficient C, rather than P, cycling is necessary to support plant species richness and phosphatase activity [12].

Riparian degradation occurs because of disturbances from anthropogenic activities that occur at different levels, frequencies, and intensities exerting pressure on the riparian vegetation and biodiversity. Livestock grazing, agriculture, and hydrological modification are a few of the disturbances brought on by land-use change that have an impact on riparian populations along River Rongai. Increased erosion, decreased plant vigour, altered plant age structure, and reduced species diversity are all effects of unrestricted cattle grazing. However, animals can also contribute to a rise in the variety of plant species by spreading seeds from far-off places and/or from nearby riparian vegetation [13].

Agricultural expansion has shifted between regions over time; this followed the general development of civilizations, economies, and increasing populations [14]. The communities located around the Rongai River rely on it for domestic usage, irrigation, and other human activities. All of these activities reduce the amount of water in the river, resulting in a loss of biodiversity, including floral species along the river [15]. However, there is a lack of evidence on the extent/amount of the effects of land use change along the river on riparian vegetation. Rapid development in the Rongai area has resulted in changes in land use, which has resulted in the establishment of impermeable surfaces. Riparian vegetation is essential for preserving regional biodiversity and delivering environmental services. As a result, knowing the impact of human activities on stream flow was critical. Therefore, the study aims to contribute to the characterization and documentation of the effects of land-use change through the assessment of the changes on vegetation structure and composition along Rongai River. This will increase more information about Rongai River.

In River Rongai riparian zone, overall species diversity values (H') based on the Shannon Diversity Index were high in the grazing areas, followed by the forest areas and lowest in the agricultural areas recording diversity values of 0.85, 0.61, and 0.32 respectively (Figure 2). These diversity index differences mean that grazing areas has more diverse species, followed by forest areas, then agricultural areas with least diverse of species. The study revealed that it is because the land is more degraded in agricultural areas. ANOVA showed that there was no significant difference in the plant abundance and diversity among the different land uses in the Rongai River (p=0.39).

2.png

Figure 2. Riparian vegetation structure and Shannon Diversity Index across different land uses along River Rongai

3.3 Land use system with a major threat to the sustainability of Rongai River

The majority of respondents (63.3%), cited open-field agriculture farming as the biggest threat to the sustainability of the River Rongai, while 36.7% answered that greenhouse flower cultivation near the river poses the greatest harm (Table 6).

Table 6. Land use system with a major threat to the sustainability of River Rongai

3.4 Effects of selected land use system on the sustainability of River Rongai

The majority of respondents (63.3%), indicated that they contribute to water reduction when asked how their chosen land use system affects the sustainability of the Rongai River, Meanwhile, 23.3% of respondents indicated that they use water abstraction for irrigation, and 13.3% of respondents indicated that they pollute the water (Table 7).

Table 7. Effects of selected land use system on the sustainability of River Rongai

The study revealed that the river’s poor riparian biological conditions in agricultural regions were the case, supporting the theory that intensive land uses have a detrimental impact on riparian habitat conditions. Furthermore, it was evident that a larger diversity of species and structural complexity in the community were observed as the quality of the riparian environment increased. The possibility for conserving plant species, the complexity of structure, and function in riparian ecosystems were greatest in the wooded areas. A good stream’s water quality is significantly influenced by riparian vegetation. The study revealed that the loss of riparian vegetation along the river Rongai downstream is due to anthropogenic activities like clearing, livestock grazing, agricultural practices, and settlements, which result in a reduction in the overall health of the stream as well as a loss of the advantages provided by riparian ecosystems.

Since the overhead irrigation system that open-field farmers currently use is not sustainable in terms of water conservation but also causes soil leaching and surface run-off, it is necessary to encourage them to organize into community-based organizations and associations to provide capacity building on sustainable irrigation practices that promote water use efficiency. Environmental conservation officers in conjunction with administrative organs from both the national and county government need to create awareness of the importance of conserving riparian land. This sensitization exercise will reduce riparian land encroachment and destruction by farmers who clear the vegetation intending to expand arable land.