Plants that are taken to new environments often thrive and become invasive. This, together with habitat destruction, has been a major cause of extinction of native species throughout the world in the past one hundred years. Worldwide, there is an increasing realisation of the ecological costs of biological invasions in terms of irretrievable loss of native biodiversity.
Moreover, the economic losses are shown to reach into billions of US$. South African National Parks (SANParks) face the same global challenge in protecting its estate from the additional impacts of Invasive Alien Plants (IAPs). In any conservation management programme, long-term ecological and economic sustainability ultimately determines the outcome. Invasive alien plant species, first identified in 1937, are now recognised as one of the most severe threats to the biodiversity of the KNP.
Over the last seven decades, the KNP, while largely a roadless wilderness area under protection for the last century, has not escaped the increasing international threats of biological invasions. IAPs, probably already present in the KNP region prior to its declaration in 1898, have established and dispersed along all major rivers and vast areas of upland vegetation. Stevenson-Hamilton, the first warden of the KNP, recognised IAPs as a concern almost seven decades ago.
The first six alien plants recorded in KNP in 1937 were "troublesome weeds" (Chenopodium ambrosioides, Tagetes minuta, Argemone mexicana, Gomphrena celosioides, Boerhavia diffusa and Cocculus hirsutus). This list has been periodically updated thanks to general botanical and specific IAP surveys, as well as an increased awareness of the problem.
It is estimated that there are now 373 alien plant species in the KNP. This list includes aquatic plants (e.g. water hyacinth), ornamental plants (eg. mother of millions), agricultural type weeds (eg. Mexican poppy) and plants that invade the savanna (eg. sour prickly pear).
Early control efforts in the KNP were aimed at the eradication of species such as Melia azedarach on the Sabie, Crocodile and Nsikasi Rivers. An attempt was made in 1956 to eradicate M. azedarach by boring holes into the trunks and filling these with paraffin and in 1957 through mechanical means. Historically, management of invasive plants developed through the use of registered herbicides, and after a slow start in the 1950s, this formed the main IAP control method and focus of research.
During the 1960s the herbicide KOP 250 was tested and proved effective resulting in the formal introduction of herbicides into the control of alien plants in the KNP. Numerous herbicide trials followed over the ensuing years with herbicides being used to control woody shrubs, aquatic weeds, bush thickening and road verge encroachment.
In 1985, 10 species were thought to have been successfully eradicated, with the eradication of a further 14 species considered possible. However, one of these plants was again found near the Skukuza conservation services offices as recently as 1998, 17 years after its ‘eradication', thus ongoing control efforts are needed. Whether failure to eradicate these species was due to re-introduction of the species or through other reasons is not documented.
Eradication is however highly difficult to achieve under any scenario. From 1982 until 1995, IAP control was conducted by the KNP alien plant clearing team of 10 people, with occasional assistance from rangers' labour teams. Work performance was measured by recording the number of IAP stems removed (focusing mainly on L. camara and O. stricta) and although assumed to be a coarse estimate, 6 889 515 stems (~ plants) were controlled.
Between 1996 and 1999, a further 8 579 314 stems were removed by mechanical herbicide applications. During the period 1996 and 2000 the first use of Geographic Information System technology captured the extent of clearing operations, which amounted to approximately, 92 688 hectares.
In 1997 the National Working for Water (WFW) programme launched its first project in the KNP. Up to a 1000 previously unemployed people in the KNP were able to focus on clearing invasive alien plants. The project has continued to the present, with a total of around R60 million being invested in this project. Invasive plant population abundances are now nearing maintenance levels, with annual follow-up operations being implemented. However, should these follow-up operations lapse, the system will revert to its former densely invaded state.
The introduction of the first biological control agent (Neohydronomus affinis) by the Agricultural Research Council's Plant Protection Research Institute for the control of Pistia stratiotes at Dakamila Pan in the far northern Pafuri region in 1985 was another significant milestone for managing IAPs in KNP. This has led to the development of integrated control programmes in the KNP. Since 1985, 16 biological control agents have been introduced into the KNP for the control of seven alien plants, namely Azolla filiculoides, Eichhornia crassipes, Lantana camara, Opuntia stricta, Pistia stratiotes, Salvinia molesta and Sesbania punicea.
Research has predominantly focused on the long-term post-release evaluation of many species under biological control, allowing a greater understanding of the insect-host interactions. This has been largely facilitated through a partnership between the KNP, the Plant Protection Research Institute and other institutions such as the University of Cape Town, which used research opportunities to evaluate biological control.
The introduction of biological control has not been without problems. The often slow build up of insects, as well as the cyclic seasonal fluctuations have received criticism from those not conversant with the mechanism/functioning of biological control. There have been pressures to prefer "quick fix" solutions for example at favoured tourist sites, such as Sunset dam near Lower Sabie. However, biological control provides long-term management options which save the KNP substantial financial and other resources.
The future challenge in many areas of weed control however lies in integrating biological control, chemical, and other control operations, to maximise the benefit and cost effectiveness of overall control efforts.
Movement of propagules by animals and birds is also challenging. Preventing it means that all flowering and fruiting populations of plants need to be controlled and the populations followed-up prior to the next season of setting seed. This management aspect is explicit in the control of O. stricta, as long-range dispersal of seeds by elephants and baboons is a key concern.
Invasive Alien Plants impact the structural diversity of an ecosystem, namely the landscape, habitat, population and genetic structure. The KNP landscape is structurally an island amidst vast and varying landscape uses and has been invaded to varying extents by numerous alien plant species. Landscape patterns fluctuate but alien species tend to cause alternative patterns to emerge in landscape structures, frequently with severe knock-on effects.
SANParks has developed management plans that set detailed management approaches and objectives, setting out long-term monitoring programmes and research into key issues need to be fully implemented, in order to develop an overall, institutional approach to biological invasions.
Careful integration of control methods is essential to ensure the most appropriate use of techniques to deliver the best possible results from the resources available and achieve longer-term sustainability. Ongoing clearing operations which respond quickly to new and serious invasions are aimed at preventing the build up of IAPs to the extent that extinction of indigenous species occurs.
By Dr Llewellyn Foxcroft