Alien invader plants in South Africa: management and challenges

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.