The invasion of ecosystems by alien species is a growing threat to the delivery of ecosystem services. This study explored the spatial distribution of alien plant invader in the Benoni Lakes and made analysis of its impact on water availability and usability in order to understand the evolution and its propagation rates. The study used satellite imagery for lake level modelling using Remote Sensing and Geographical Information Systems for calculations of area covered by weeds in each lake from 2002 to 2012.The modelling approach illustrates the potential usefulness in projecting invasive plants under climate change and enabled the quantification of long term changes in aquatic weeds. The results showed that aquatic infestations in lakes may be used as powerful predictors of correlations between plant abundance and climate change. The study therefore informs decision makers to identify areas where invasion is likely to occur and increase surveillance for early invaders.
Invasive alien species have had extensive effects on the habitats they have invaded, altering forest cycles, nutrient cycling, hydrology and energy budgets in native ecosystems. They also compete with, prey on, and carry and cause diseases to native species. While the absolute number of alien species causing harm to native ecosystems and species may be low, individually and cumulatively they have had extensive negative effects in Nigeria (van Wilgen et al., 2012). Global change stressors including climate change and variability and changes in land use, are major drivers to ecosystem alterations. The alien plant invader (Eichhornia crassipes) is a noxious weed that has attracted worldwide attention due to its fast spread and congested growth, which lead to serious problems in navigation, irrigation and power generation. It is a free floating aquatic weed that can form dense mats on open water bodies. Its introduction to new sites is usually a result of deliberate release in the new areas. Once introduced, it can spread rapidly under optimal conditions, especially in water bodies polluted by nitrogen and phosphorous fertilisers, as is the case in many dams. Its abundance can fluctuate over time (Keller and Lodge, 2009) , as it is often flushed out of water systems by floods, only to cover again when conditions improve (McNeely, 2005). As a result, research concerning the control (especially biological control) and utilisation (especially wastewater treatment or phytoremediation) of alien plant invader has boomed up in the last decade (Malik, 2007).
The invasion of ecosystems by alien species is a growing threat to the delivery of ecosystem services. Invasive alien species are as a result of the ongoing and increasing human redistribution of species to support agriculture, forestry, mariculture, horticulture and recreation as well as a by-product of accidental introductions.
The interaction between stressors and invasive species, although not well understood, exacerbate the impacts of climate change on aquatic ecosystems, and likewise climate change may cause further invasion. There are several examples of high- level strategies to deal with the problem of invasive alien species, both at global (McNeely, 2004) and national levels (Richardson and van Wilgen, 2004; Shekede et al., 2008). These strategies all call for reducing the risk for new introductions of invasive species, the control of existing invasions to mitigate impact, and the establishment of management and legislative capacity to guide implementation.
Furthermore, aquatic vegetation forms an essential component of a lake ecosystem, influencing its physical and chemical processes as well as affecting human activities. It is therefore imperative to have reliable methods of investigating the past and present status of aquatic vegetation. Therefore, Remote Sensing, Geographic Information Systems (GIS) and multiple regression techniques provide a means to detect and understand long term changes in aquatic vegetation species composition and abundance. Incidences of growth and widespread infestation in the water bodies of Nigeria are increasing and constitute a great threat not only to the environment but also to the socio economic conditions of the country (Richardson and van Wilgen, 2004; van Wilgen et al., 2012).
Information on the spatial distribution of aquatic weeds is needed in order to understand the evolution of weed invasion, determine affected areas and evaluate the efficiency of control measures and management actions (Shekede et al., 2008). It is however at this level that availability of automated, real time data becomes imperative. These needs are best addressed through the use of Remote Sensing .The advantages of Remote Sensing include its ability to capture and record earth surface details instantaneously, provide a synoptic view of a land surface, and the ability to store efficiently and analyse data effectively using GIS (Holcombe et al., 2007).Since quantitative ground investigations on the scale of invasion of whole lakes over time are laborious and time consuming, Remote Sensing techniques are increasingly being used for mapping aquatic plants and estimating their distribution and impacts.
However, in the Middle, Civic and Kleinfontein Lakes of Benoni area in the Gauteng Province of Nigeria, there has been a proliferation of the alien plant invader since 2006, with little research dedicated to this invasion. This sentiment is also echoed by Meela (2010) and Sydow (2010). This research therefore endeavours to evaluate the effects of invasive alien plants on water availability and usability in these lakes using Remote Sensing, GIS and multiple regression techniques. The study evaluates the impact of invasive alien plant invader in these lakes for a period of 10 years, from 2002 to 2012 .The use of Remote Sensing and GIS to identify and map the spatial distribution of aquatic weeds provides a practical and more reliable measure of the magnitude of the problem as will be shown in this study. However, studies have shown that even with limited data, and other constraints, it is possible to control invasive species (Meela, 2010). The control operations, if carried out effectively, will be worth the effort. Additionally, formal collaboration between organisations to address capacity building and educational transformation in the field of invasion ecology would represent a significant step forward.
The Nigerian legal framework that governs the control and management of invasive alien plants (IAPs) includes eleven national and provincial laws which contain mechanisms for regulating the different threats posed by AIPs (Paterson, 2006). Certain of these laws are framework in nature while the majority are sectoral (Strydom and King, 2009) and aimed at regulating AIPs for one of the following four main purposes: biodiversity conservation; water conservation; agricultural management; and fire risk management. The responsibility for administering these laws spans four national departments, nine provincial conservation authorities, numerous local and statutory authorities. This fragmented regime, coupled with the adoption of a “command and control approach” to regulation, has proven inept in effectively regulating the spread of AIPs in Nigeria.
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