CHAPTER 1: GENERAL INTRODUCTION AND LITERATURE REVIEW 1.1 Rationale of the study Alien plant invasions are a global problem and, after direct habitat destruction, is the second most important threat to biodiversity (Randall, 1996). Southern Africa is severely affected by alien invasions, and has one of the biggest problems with invasions of any area in the world. Large areas have been transformed as a result of these invasions, and many negative impacts on the economy, in sectors such as health, agriculture, water supply and tourism, have resulted (Macdonald, 1989). The threat posed by alien invasions to biodiversity is significant as biodiversity is a fundamental property of ecological communities. It influences productivity, soil nutrient availability, and invasion resistance, as well as system stability and reliability (Purvis and Hector, 2000; Knops et al., 2001). When biodiversity is reduced, greater rates of loss of limiting soil nutrients through leaching results, which ultimately decreases soil fertility, thus lowering plant productivity (Tilman, 2000). Clearing infestations of invading alien plants will have many benefits. These include increasing the available surface and underground water, preventing the loss of biodiversity, reducing fire hazard, stabilizing catchment areas and preventing erosion (Le Maitre et al., 2002). Riparian ecosystems constitute the interface between aquatic and terrestrial ecosystems (Gregory et al., 1991), and are particularly prone to alien plant invasions due to the efficient dispersal mechanism, water, which spreads the seeds of aquatic and riparian weeds downstream (Pysek and Prach, 1993; DeFerrari an Naiman, 1994). The frequency of disturbance in this ecosystem is also a contributing factor increasing the invasibility (Rejmanek, 1989; Hobbs and Huenneke, 1992). Disturbances increase the invasibility of an ecosystem due to the creation of new microhabitats and niches for invading species (Carlton 1996, 2000), the direct removal or decrease in populations of indigenous competitor species thereby making them less capable of controlling or resisting a growing population of invading species (Davis et al., 2000), and the introduction of alien species’ propagules into areas that were inaccessible to the propagules on their own (Lonsdale, 1999). The Sabie River catchment in the Mpumalanga province of South Africa, is important from both economic and eco-tourism perspectives. However, the Sabie River riparian ecosystem has been severely affected by invasive alien plants. As a result, the Working for Water (WfW) alien plant clearing programme, a programme initiated in 1995 by the Department of Water Affairs and Forestry (DWAF) in response to the massive threat posed by invasive alien woody plants to ecosystems, has been clearing alien plants along this river over the past 10 years, i.e. from 1995 to 2005. Thus, the main aim of this 2005 study was to measure the ecosystem repair of the Sabie River (which traverses through both the grassland and savanna biomes) riparian environment in response to the clearing of alien plants by WfW. This was done in order to assess the effectiveness of the WfW clearing on the Sabie River riparian plant community composition and associated environmental factors. Although “effectiveness” can be assessed in various ways, such as determining whether there is an increase in indigenous species richness and/or a decrease in alien species richness, in this study it also includes determining whether there is a reduction 1 in the invasion intensity (defined as the percentage aerial cover of woody alien plants) after clearing. This study forms part of a national project in which targets for ecosystem repair in riparian ecosystems in the fynbos, grassland and savanna biomes will be developed (Holmes et al., 2003). This study will develop targets for the grassland and savanna biomes as the Sabie River traverses through both biomes. Ecosystem recovery studies assess the degree of repair that has occurred as a result of alien clearing, based on suitable benchmarks, and an analysis of the information then clarifies whether biotic or abiotic thresholds in the environment have been passed (Whisenant, 1999). It can then be determined whether biotic or abiotic components need to be manipulated in order to facilitate the recovery of the indigenous plant community, which then facilitates the development of achievable targets for ecosystem repair. For example, if an abiotic threshold has been passed, it will be necessary to repair the relevant components of the physical environment before manipulating the vegetation components. If the inputs of physical energy are the dominating forces in structuring an ecosystem, e.g. water and wind movement in riparian zones (Planty-Tabbachi et al., 1996), then manipulating these abiotic components is then particularly important in ecosystem repair (Ehrenfeld, 2000). Data were collected and compared with a similar study conducted in 1996/1997 (Garner, 2005) in order to measure the ecosystem repair of the Sabie River riparian community, and hence the WfW effectiveness. In 1996/1997, 40 permanent Modified Whittaker nested plots were first surveyed along the Sabie River and several variables were measured, such as the plant species composition, diversity and vegetation structure, as well as environmental variables (Garner, 2005). In this 2005 study, these same factors were measured and compared to the 1996 study. The specific aims and objectives are listed on page 33 of this chapter. The results of this 2005 study can then be used to develop suitable targets for ecosystem repair in both biomes. As stated previously, “effectiveness” in this study includes a reduction in the invasion intensity after clearing. In this study, the percentage aerial cover of woody alien plants was used as a measure of the invasion intensity as the aerial cover gives a true reflection of the effects of the woody alien plants on the plant community (as opposed to, for example, using basal density of alien plants as a measure of invasion intensity which does not take into account the effects of the full canopy cover). This measure of invasion intensity was also used in order to be consistent with the 1996 study (Garner, 2005), which used the same measure. The invasion intensity was then used as a measure of the degree of alien plant invasion in this 2005 study. The national ecosystem repair project (Holmes et al., 2003) has developed key questions relating to the ecosystem repair in the three biomes, and this study will aim to answer these questions in relation to the grassland and savanna biomes. Any relevant gaps in our knowledge or understanding will be identified and prioritised. These questions are: 1) What level of ecosystem repair has been achieved in each of the different situations studied? 2) Are the thresholds derived from ecological theory applicable in practice? a) In what situations have biotic thresholds been passed? b) In what situations have abiotic thresholds been passed? 3) What is achievable in each of the different situations studied? 2 4) What could be improved? 5) Have any important ecosystem drivers or keystone species (to facilitate recovery) been identified? 6) What are the realistic goals for the different situations, particularly in relation to vegetation type, river order and level of ecosystem degradation? 1.2 The Working for Water (WfW) alien plant clearing programme The Working for Water (WfW) alien plant clearing programme focuses on controlling woody alien plants that invade riverine areas (Macdonald, 2004). Because South Africa has such an enormous problem with alien invasive plants, and because it is a semi-arid country with a highly variable rainfall and hydrology, the primary aim of the programme is to increase water supplies by controlling the alien plants. One of the main goals of the programme is to enhance social development. An increasingly important benefit of this programme is maintenance of biodiversity over the long- term, which can be achieved via the restoration of indigenous ecosystems. The WfW programme is one of the world’s biggest programmes dealing with invasive alien species, and is one of the most effective poverty eradication programmes in South Africa (Van Wilgen, 2004). The success of the programme is a result of its ability to gain local and international funding and continuing political support (Richardson and Van Wilgen, 2004). Historically, the removal of invasive alien woody plants in riparian zones by WfW involved felling all alien trees and shrubs, and then treating the stumps of the coppicing species with herbicide (Holmes et al., 2005). The felled material was then either removed from the river corridor or burnt in slash stacks (Holmes et al., 2005). The current method is to use frilling or ring barking to kill the larger trees (> 200 mm basal diameter) as felling and timber removal is too expensive (Holmes et al., 2005). Reinvading aliens are regularly removed by follow-up treatments, which are perceived to be a temporary practice (i.e. 2 – 4 treatments) intended to deplete remnant propagule reserves (Galatowitsch and Richardson, 2004). Satisfactory, sustainable and affordable management of invasive alien plants will be impossible without the integration of biological control as a supplement to other management practices (Zimmermann et al., 2004). As a result much of the progress achieved in recent years in biocontrol has been facilitated by the WfW programme (Olckers, 2004). An example of a WfW clearing contract (for the Barbeton area) is given in Appendix 20, and an example of a WFW worker’s contract is given in Appendix 21. There is a contractual agreement between WfW and the landowners, and details of this contract are given in Appendix 23. 1.2.1 History of the WfW programme In 1995/1996, the WfW programme obtained an initial grant of R25 million from the government’s funds for reconstruction and development (Macdonald, 2004). The programme then succeeded in obtaining considerable further funding, largely from the government’s poverty relief budget, once it had proven its significant employment potential (Magadlela and Mdzeke, 2004).