Three practical approaches – mechanical, chemical and biological – are used in controlling IAPs, but often prove ineffective once alien invasion has gained maturity. Mechanical or manual clearing of alien plants has been identified as the eco-friendliest and frequently used method, but is labour intensive (Mack et al. 2000). Other mechanical approaches include manual-pulling, hand-cutting and fire-burning, but such methods are also limited by their small-scale application.
In agriculture, chemical methods are most commonly used in combating weed population, but this is limited for broad-scale application due to its cost. Adverse effects on human health and non- target crops also hinders its usage; however contemporary herbicides tend to be user- and environmentally- friendly in terms of toxicity, residence time and specificity (Hobbs & Humphries 1995). Biological control methods have emerged due to the limitations of chemical and mechanical control methods when immediate intervention and remediation are required. Reducing the impact by removal of alien plants through skilful introduction of specialist natural enemies such as bio- control agents can replace chemical and mechanical methods (Olckers, Zimmermann & Hoffman 1998). However, non-specificity of bio-control agents to the target host can cause extinction of non-target species (Simberloff & Stiling 1996; Howarth 1991), this being the major pitfall of this method. However, the parsimonious costs of operation, safety operation, extensive natural dispersal, easy integration with other management approaches and self-sustaining of biological control methods has outweighed the criticisms against this method.
Strategizing for sustainability and conservation, the national Working for Water programme (WfW) was established to bridge the gap between socio-economic development and ecological health. WfW, administered previously through the Department of Water Affairs and Forestry and now the Department of Environmental Affairs, was established in 1995 with initial allocation of R25 million for development (Macdonald 2004). The WfW programme attempts to maximise the sustainability of natural, ecological, social, economic and political activities between man and natural resources, plainly stated, managing and saving water resources and ecosystem services from IAPs (Macdonald 2004). Since the inception of WfW, large area masses of riparian zones invaded by mostly woody plants have undergone clearing and follow-up treatment. For instance, Sabie River and Kei River catchments have been frequently cleared (Levendal et al. 2008) and documentation of the clearing history of both catchments have been kept.
Many studies focus on control approaches such as removal of IAPs, while recovery of invaded habitats to pristine states are carried out through processes of natural attenuation without further
management procedures (Reid et al. 2009). Although vegetation regeneration will occur, it is often impaired by the re-sprouting of removed plant species. Such passive restorations may defeat the purpose of the initial control measures applied, exacerbate the invasion degree and alter the already designed management strategy. In order to avoid greater habitat loss and degradation, active ecosystem restoration is a highly valuable tool in achieving conservation and eco-sustainability (Cairns 2002; Hobbs & Harris 2001).
Objectively, ecosystem restoration aims at returning a degenerated habitat to its natural condition prior to degradation, as well as restoring the functionality and self-regulatory ecosystem processes. This theoretical restoration goal was readily broadened to inculcate ‘recovery assistance and ecological management’ as the most crucial but critical processes in achieving active ecosystem restoration (Society for Ecological Restoration 1996). However, it may be extremely difficult to return a degraded habitat to its pristine state after a disturbance regime, as evolved discontinuity in resident pools, dominance shifts, trophic connectivity and biogeochemical processes during invasion may impair the restoration strategy and efforts (Suding et al. 2004). Physical, chemical and biological phases of an ecosystem often limit ecosystem repair, therefore it is imperative to understand the re-vegetation process, reinvasion impact on regeneration, key species that drive regeneration coupled with vegetation structure and restoration process (Beater 2006).
Exclosures are, a commonly used rehabilitation approach in arid and semi-arid zones, to prevent livestock grazing in degraded sites. The term ‘exclosures’ refer to protected land units from a particular class of animals using barriers, quite opposite of ‘enclosures’ but often interchangeably used in literature (Aerts, Nyssen & Haile 2009). This is to allow for regeneration of native species in order to increase water infiltration and woody biomass as well as to reduce soil disturbance and erosion. Many studies have proven the effectiveness of exclosures in rejuvenating degraded soils that occur as a result of overgrazing (Mekuria et al 2007), in addition to the restoration of native vegetation in communal grazing lands (Yayneshet, Eik & Moe 2009) and the recovery of woody vegetation in a degraded dryland (Mengistu et al. 2005). Exclosures have also been established to be effective in recovering native plant species community, richness, diversity and general vegetation cover and structure in degraded rangelands (Yayneshet, Eik & Moe 2009).
Visser, Botha & Hardy (2004) evaluated various rehabilitation methods and treatment procedures (mosaic of seeding, tilling and branches) to re-vegetate bare patches of degraded rangeland as a result of overgrazing in Nama Karoo. A combination of different restoration techniques such as brush packing, ripping over sowing and organic matter addition were used as a trial test on a farm
denuded with brackish soil and was confirmed as an effective restoration treatment if applied in degraded rangelands (Van den Berg & Kellner 2005). A different set of restoration methods was employed by Snyman (2003).in a semi-arid rangeland, over sowing and mechanical inputs. Successful re-establishing of some species were recorded after 10 years, but slow restoration in semi-arid rangelands were registered as a result of unreliable rainfall.
As discussed in Chapter 1, passive approaches for restoring functionality in degraded ecosystems (Gaertner et al. 2012; Le Maître et al. 2011) often fail due to reinvasion of the removed invader or another invader (Loo et al. 2009; Zavaleta et al. 2001). Active restoration, including additional restoration protocols after IAPs removal, has proven to be effective when restoring ecosystems degraded by alien invasion (Van den Berg & Kellner 2005; Visser, Botha & Hardy 2004). Therefore, the restoration protocol for this study is assumed to adopt similar rehabilitation programmes according to the outcome of the soil analysis to achieve active restoration and sustainability.