ALIEN INVADER PLANTS IN SOUTH AFRICA: MANAGEMENT AND CHALLENGES

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1 20 Alien invader plants in South Africa: Management and challenges International Symposium: Current Trends in Plant Protection UDK: 574(680) Procedings (680) ALIEN INVADER PLANTS IN SOUTH AFRICA: MANAGEMENT AND CHALLENGES CARL REINHARDT Department of Plant Production and Soil Science, University of Pretoria, South Africa, Invasive alien plants impact negatively on agriculture, forestry, natural ecosystems, human health, and biodiversity. These global threats also apply to South Africa, which basically is a waterstressed country that cannot afford wastage of water by any means, least of all through alien invasive plants. The level of understanding of the origins, biology, ecology, and impacts of alien invasive plants is high for certain species but for many others the knowledge that matters is either at a low level or non-existent. Human activities have contributed much to the general problem, especially through injudicious practices such as deliberate introduction of foreign species, crop production in marginal areas that are prone to soil erosion, overgrazing of grassland, mining activities that disrupt or destroy ecosystems, etc. Negative impacts of alien invasive plants that drive control efforts include the aggressive nature of many species, disruption and displacement of native biodiversity, water conservation, fire intensity, and soil stability. Research on the control of invasive plants is mainly focused on biological control by means of natural enemies (mainly insects, and some pathogens) that are procured in countries from where the plants originated. Chemical control involving herbicides is in many cases arguably the best option for adequate control but there are real or perceived risks of environmental pollution and non-selectivity of herbicides that need to be considered. Political support in the country for the management of alien invasive plants is considerable in the form of policies, legislation and control programmes. Conflicts of interest sometimes arise because of the usefulness of certain species (e.g. sources of fruit, animal feed, firewood, building material, etc), and due to the perception of people that plants in general do more good than harm. Opposition of this kind can complicate strategies for the effective and sustainable management of invasive alien species. Key words: alien, invader, plants, South Africa, INTRODUCTION In South Africa there are three distinct vegetation types: (1) the southern fynbos (shrubland or heathland within areas with a Mediterranean climate) and forest of the winter rainfall region; (2) the northern tropical forest, savannah and grassland of the summer rainfall region; (3) the Karoo that developed between the afore-mentioned floras, and is characterized by semi-desert to near-desert conditions (rainfall of less than 250 mm per annum). Irrespective of the eastern side of the country being wetter than the drier central and western parts (Acocks, 1988), all vegetation types and regions of the country are prone to invasion by a wide variety of alien plant species, which thanks to peculiar adaptive abilities and growth habits, can thrive in diverse environments. Indigenous plant

2 Carl Reinhardt 21 communities that are particularly susceptible to invasion by alien plants are those where deterioration of the environment has occurred, either due to human activities, e.g. farming, mining, road-building, or as a result of natural events such as floods or soil erosion caused by water or wind. Climatic changes that occurred over periods of tens of thousands, or hundreds of thousands of years probably caused changes in the composition and distribution of the basic vegetation types of South Africa. But the changes that have been observed in the last 100 to 150 years are considered far too great to be attributed to only fluctuations of climate that apparently occur with a periodicity of about 200 years (Brooks, 1926; Acocks, 1988). It is generally accepted that crop and animal production have been the main causes of the rapid deterioration in vegetation type, species abundance and biodiversity that have been experienced on the sub-continent in recent times (c ). This happened despite the fact that, in comparison to Europe, human pressure on the environment of the sub-continent of Africa started late because farming with crops and livestock only intensified to a marked extent after colonization by Europeans, which started in 1652 with a small Dutch settlement around Cape Town at the southernmost tip of the country. Degradation of natural vegetation escalated with the introduction in the 1920s of the tractor which revolutionized row-crop production. Extensive, indiscriminate tillage of soil and the keeping of large herds of livestock put great pressure on the tropical grasses, which through ploughing and/or over-grazing caused the soil across vast tracts of land to become denuded of vegetation that gave protection against wind and water erosion. Reduction and elimination of the grass component lead to the development of bare areas (devoid of vegetation) which creates an ideal environment for the establishment of pioneer plant species such as weeds, in particular alien invasive species. Another consequence of the degradation of grassland is reduction of natural fuel (grass biomass) needed for hot fires which is important for preventing the woody component of the savannah from becoming unnaturally dense. This phenomenon whereby indigenous woody types become overly dense is known as densification or bush encroachment. Such species are the target of plant control programmes that, in terms of effort and cost, can easily match those on alien invasive species. Management of bush encroachment is mostly done by private landowners who seek to provide as wide a variety of plant species as possible for wild animals and livestock to feed on. Farming with wild animals has developed into an important industry in South Africa, to such an extent that in certain regions it can rival livestock production as regards profitability. The mechanisms through which alien invasive plants are able to displace native species and thrive in foreign environments at the latter s expense are well documented (van Andel, 2005; Knapp and Kühn, 2012), and therefore, it is not discussed in any depth here. Instead, the overview given here will deal with those alien plants that are considered the most noxious under South African conditions, their impacts and how they are controlled in certain instances successfully and in others not adequately, or not all. Although the impacts of alien plants differ from species to species and according to the habitat in which they occur, their impacts can be generalized as follows: reduction in biodiversity; displacement and even extinction of indigenous species; reduction in productivity of natural grazing; reduction in land value; promotes soil erosion; loss of water; economic loss; promotes diseases such as malaria and bilharzia. Despite it having regions that receive 1000 to 2000 mm rain per annum, South Africa is for the greater part a water-stressed country. Most regions receive <750 mm rain, and then mostly concentrated in either the summer or the winter season, and usually in the form of low frequency rain events. Therefore, competition for water and nutrients, N in particular, because its availability for plant uptake

3 22 Alien invader plants in South Africa: Management and challenges is closely linked to soil moisture content, is a major factor governing inter-species interference. The other factor involved in inter-species interference is allelopathy, thus interference = competition + allelopathy. Few competition studies involving alien invader plants have been done in South Africa, and to date even less research has been done on the allelopathic potential of these species (Van der Laan et al., 2008; Belz et al., 2009). In contrast, considerable research has been done on biological control (Moran et al., 2011) and water-use (water-wastage!) by alien invasive plants (Versveld et al., 1998). POLICY BACKGROUND The first national policy and legislative instrument that directly addressed the management of invasive plants in South Africa came about under the Conservation of Agricultural Resources Act [(CARA)] Act 43 of 1983, amended in 2001). Regulations under this act classify 368 species as declared weeds and alien invasive plants, and stipulate that they must be controlled to certain degrees based on their problem status (Bromilow, 2010). Based on the 1983 regulations, 57 alien plants were declared noxious weeds that must be controlled where they occur, and their spread had to be prevented. In 2001, the list of noxious weeds was expanded to include 198 species. From then on weeds were classified into three categories: (1) weeds of no value control is required and trade is banned; (2) recognized weeds that have commercial value permits are required for their cultivation, and trade in plants and products is permitted (e.g. fruit and forestry species); and (3) recognized weeds that have ornamental but no commercial value permits are needed to keep them, but further cultivation and sales are prohibited. A more recent development is the National Environment Management: Biodiversity Act (NEMBA] Act 10 of 2004) that is in the process of being finalized, and will probably introduce similar categories which will complement those of CARA. ORIGINS AND IMPACTS OF INVASIVE ALIEN PLANTS About 50% of the 316 alien invasive species in South Africa originated in South, Central and Tropical America, about 25% came from Europe, Asia and the Mediterranean, and about 13% from Australia (Moran et al., 2011). Because South Africa s indigenous plant species (approx ) far outnumber the exotic or alien types, it may appear as if the latter species could not have a large influence on the natural (indigenous) vegetation. In fact, the impacts on natural vegetation are often environment (soil and rainfall) and speciesspecific, in that certain alien plants cause significant direct and/or indirect damage to particular natural plant communities in certain locations. Certainly, the level of infestation that an alien invasive species has attained at a given point in time will also play an important role with regard to impact high infestation levels and high impact usually takes considerable time to develop, depending on the adaptability of the invader and the resistance against invasion put up by the threatened native vegetation. Low infestation levels of an alien invader plant could be indicative of it experiencing resistance (interspecies interference) from the native species, but on the other hand, low numbers of the alien species may signify recent infestation which may or may not escalate over time. The alien weed problem in South Africa is characterized by a high proportion of woody species, most of which were introduced for good reasons such as stabilization of sand dunes (Acacia spp from Australia), commercial forestry (Acacia spp and Eucalyptyus spp from Australia, and Pinus spp from North America and Europe), agroforestry (e.g. Prosopis spp from Texas, USA), horticulture (e.g. Jacaranda mimosifolia from South

4 Carl Reinhardt 23 America, Melia azedarach from India, and Lantana camara from many parts of the tropics), and fruit production (e.g. Psidium guajava and Opuntia ficus-indica). Same as for many other alien invader species, all the afore-mentioned species have already become naturalized in South Africa, and therefore, control measures can only hope to maintain their levels below the economic threshold that can be set with regard to economic and environmental impact. Predicting the further spread of already naturalized alien species is an important science in South Africa (Hui et al., 2011). The Southern African Plant Invaders Atlas (SAPIA) project has as its focus the recording of alien plant distribution and the identification of emerging or dormant species that have the potential to attain problem status (Henderson, 2011). South Africa became Party to the Convention on Biodiversity in November South Africa is increasingly involved in solving alien plant problems in the rest of Africa, and there have been and still are many collaborative projects with countries in various parts of the world, notably on the exchange of organisms or agents for biocontrol. Most of the research focused on the control of alien invasive plants in South Africa has been on biological control which exploits a plant s natural enemies as means for control (Moran et al., 2011). Failure of biocontrol agents to adapt to environments in which they are introduced has proved to be a major stumbling block, and recent research has shown that morphological traits and the biochemical profile of target plants can play important roles (Ghebremariam et al., 2012). Research on chemical control methods are virtually exclusively done by chemical companies in the process of registering herbicides. Use of herbicides gets a lot of opposition, rightly or wrongly, from those concerned about risks of environmental pollution. Considerably less effort has thus far gone into researching the biology and ecology of alien invader species, and rarely is the combined use of different methods of control (IWM) employed. In fact, there is the perception that proponents of biological control are averse to the use of herbicides, or at least regards chemical control as a threat to the success of biological control efforts. The reason for this perception is that biocontrol agents (insects, pathogens), which invariably are 100% host-specific, require live plants as hosts and herbicides are meant to kill those very plants. Since 1996 there exists a national Working for Water programme that seeks to provide all South Africans with readily available and sufficient potable water, and hence, the programme has at its core the control of alien plants that constitute a risk to water resources (van Wilgen, 2012). About 7% (3300 million m 3 ) of South Africa s mean annual water runoff is lost through transpiration of woody alien species that have invaded water catchments, riparian zones and wetlands (Versveld et al., 1998). Ten alien aquatic weeds also contribute to water loss from surface waters (rivers, lakes, reservoirs). Other important driving forces behind the Working for Water programme is job creation through involving local people in the control of alien plants, as well as the realization that controlling alien plants is more cost-effective than building new water reservoirs. KRUGER NATIONAL PARK: EXAMPLE OF A CONSERVATION AREA THREATENED BY INVASIVE ALIEN PLANTS The development and management of strategies for the control of those alien species that cause negative impacts is one of the major tasks of managers in many protected areas. The management of invasive alien plants in South Africa s Kruger National Park (area: km 2 ) has been supported by research that includes: studies on the determinants and dynamics of spread of key species (Foxcroft et al., 2008), examination of the importance of

5 24 Alien invader plants in South Africa: Management and challenges issues pertaining to spatial scale in designing management plans (Foxcroft et al., 2009), and the effectiveness of the park boundary as a filter for invasive species (Foxcroft et al., 2011). Alien invasive species in KNP (top 28 arranged in descending order of abundance) Opuntia stricta Ageratum spp. Lantana camara Ricinus communis Opuntia spp. Argemone mexicana Chromolaena odorata Catharanthus roseus Pistia stratiotes Arundo donax Parthenium hysterophorus Datura inoxia Eichhornia crassipes Datura stramonium Xanthium strumarium Ageratum conyzoides Azolla filiculoides Melia azedarach Argemone spp. Senna occidentalis Senna spp. Datura ferox Xanthium spp. Zinnia peruviana Nicotiana glauca Cardiospermum halicacabum Psidium guajava Argemone ochroleuca Fig. 1. Mean annual river run off in Kruger National Park (KNP), pattern of non-native plant records relative to KNP boundary and segments (lower left inset), and location of KNP within South Africa upper left inset (from Foxcroft et al., 2010).

6 Carl Reinhardt 25 CONCERNS REGARDING THE CONTROL OF INVASIVE ALIEN SPECIES In South Africa, an important obstacle to efforts for controlling alien invasive species is the issue of conflicts of interest which arise when an alien invader plant has some or other useful purpose (e.g. wood for fuel and building material, fruits as food, plants as fodder, etc). Many encourage the use of alien plants for small business enterprises (e.g. charcoal, fruit, wood fuel, furniture, ornaments, etc), whereas others believe that promotion of such uses will only perpetuate the problem. According to van Wilgen (2012), other concerns that often are raised as arguments against the wholesale destruction of alien invasive populations include the following: (1) the perception that forest cover in general is the best way of protecting soil against erosion, and ensures sustained water flow from catchments; (2) habitat destruction through removal of alien plants is regarded as further damage to ecosystems already under stress; (3) some alien plants are aesthetically pleasing and are enjoyed by people for recreational purposes, especially where trees are rare; (4) the removal of vegetation, trees in particular, is seen by many as counter-intuitive in light of growing awareness of global war333ming and its mitigating factors, such as carbon sequestration in plants and soil. REFERENCES Acocks, J. P. H. (1988): Veld types of South Africa. In: O.A. Leistner (ed). Memoirs of the Botanical Society of South Africa No. 57 (3 rd edn,). Botanical Research Institute, South Africa. Belz, R. G., Van der Laan, M., Reinhardt, C. F., Hurle, K. (2009): Soil degradation of parthenin does it contradict the role of allelopathy in the invasive weed Parthenium hysterophorus? Journal Chemical Ecology, 35, Bromilow, C. (2010): Problem plants and alien invader weeds of South Africa. Briza Publications, Pretoria, South Africa. Brooks, C. E. P. (1926): Climate through the ages. Benn, London. Foxcroft, L. C., Jarosˇic, V., Pysˇek, P., Richardson, D. M., Rouget, M. (2011): Protected-area boundaries as filters of plant invasions. Conservation Biology, 25, Foxcroft, L.C., Richardson, D. M., Wilson, J. R. U. (2008): Ornamental plants as invasive aliens: problems and solutions in the Kruger National Park, South Africa. Environmental Management, 41, Foxcroft, L. C., Richardson, D. M., Rouget, M., MacFadyen, S. (2009): Patterns of alien plant distribution at multiple spatial scales in a large national park: implications for ecology, management and monitoring. Diversity and Distributions, 15, Ghebremariam, T., Reinhardt, C. F., Krüger, K. (2012): Unpublished research from a PhD study conducted in the Department of Entomology and Zoology, University of Pretoria, South Africa. Henderson, L. (2011): Mapping of invasive alien plants: the contribution of the Southern African Plant Invaders Atlas (SAPIA) to biological weed control. African Entomology, 19, Hui, C., Foxcroft, L. C., Richardson, D. M., MacFadyen, S. (2011): Defining optimal sampling effort for large-scale monitoring of invasive alien plants: a Bayesian method for estimating 1abundance and distribution. Journal of Applied Ecology, 48, Knapp, S., Kühn, I. (2012): Origin matters: widely distributed native and non-native species benefit from different functional traits. Ecology Letters, 15,

7 26 Alien invader plants in South Africa: Management and challenges Moran, V. C., Hoffmann, J. H., Hill, M. P. (2011): A context for the 2011 compilation of reviews on the biological control of invasive alien plants in South Africa. African Entomology, 19, Van Andel, J. (2005): Species interactions structuring plant communities. In: E. van der Maarel (ed.). Vegetation ecology. Blackwell Science Ltd, UK. Van der Laan, M., Reinhardt, C. F., Belz, R. G., Truter, W. F., Foxcroft, L. C., Hurle, K. (2008): Interference potential of the perennial grasses Eragrostis curvula, Panicum maximum and Digitaria eriantha with Parthenium hysterophorus. Tropical Grasslands, 42, Van Wilgen, B. (2012): Evidence, perceptions, and trade-offs associated with invasive alien plant control in the Table Mountain National Park, South Africa. Ecology and Society, 17, Versveld, D. B., Le Maitre, D. C., Chapman, R. A. (1998): Alien invading plants and water resources in South Africa: A preliminary assessment. WRC Report no. TT 99/98, CSIR Division of Water, Environmentvand Forestry Technology, Stellenbosch, South Africa.