Special Feature: Aliens in the KNP
by Llewellyn Foxcroft
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 few hundred years. Worldwide, there is an increasing realisation of the ecological costs of biological invasion in terms of irretrievable loss of native biodiversity. The Kruger National Park (KNP) faces this same global challenge. What follows is a brief look at Invasive Alien Plant (IAP) control in the KNP over the last seven decades.
In any conservation management programme, long-term ecological and economic sustainability ultimately determines the outcome. Invasive alien plant species, first identified in the in 1937, are now recognised as the single greatest threat to the biodiversity of the KNP.
Over the last seven decades, the KNP, while largely a road-less 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.
Scientific research now supports his practical understanding of the situation. Unfortunately, little was done to combat the problem of IAPs until the 1950’s, and even then, at insufficient intensity to prevent or curtail the invasions. 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 372 alien plant taxa in the KNP.
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 1960’s 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. This attracted the majority of funding with large herbicide companies sponsoring the control of alien plants. However, the trials often failed to provide adequate control of the plants and in some areas resulted in erosion and other impacts, especially along firebreaks and fences, and chemical control was ultimately not sustainable.
In 1985 ten 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, more likely, a lack of continuity in control efforts, 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. This is only 16 % of the 590 295 hectares of IAP infestation, with most of the areas constituting follow-up control.
Although these areas are small they are essential to the long-term success of the initial control work. 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, largely from sponsors. Invasive plant population abundances are now nearing maintenance levels, with annual follow-up operations 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. Management plans also now recognise the role staff played in the introduction and spread of IAPs. Efforts were made to stop the cultivation of alien species for ornamental and other uses in all park rest camps, but implementation was initially poor. Now, in addition to the KNP Code of Conduct (which stipulates that introducing alien species is prohibited), the KNP Management Committee approved a policy in November 2001, documenting and attempting to address the serious problem of invasive plants and listing species that would be phased out of cultivation. Most staff comply with regulations and importation of alien plant species is minimal. However, the transferring of plants from one garden or area to another still continues through lack of awareness or understanding rather than direct contravention of KNP policy. Control of IAPs in the KNP rest camps and personnel villages is carried out by the IAS section, particularly in Skukuza, and by rangers in the camps in their respective sections.
IAPs in the KNP are currently controlled by a combined effort involving the KNP IAS section, Working for Water, and KNP rangers. Recognition of the main vectors and pathways of dispersal are critical. The invasion of riparian habitats, which far exceeds any other habitat invaded in the KNP, is directly linked to the extensive invasions in the KNP's catchments. Of the 27 species with moderate to high impact on the KNP in 1988, 17 species were dispersed primarily by water. With the increase in development in the Lowveld and escarpment areas, including commercial farming and forestry, the catchments of all rivers entering the KNP have been heavily invaded by a wide range of species, complicating IAP control efforts further. Management of the invasions along the rivers, within and a few kilometres upstream of the KNP, is carried out by the WFW programme. Collaboration with provincial WFW structures remains essential in coordinating operations aimed at eventually clearing entire catchments. In addition, initial and follow-up chemical control work has been done on the O. stricta invasion in the Skukuza region.
The KNP has recently developed a biological control rearing facility in Skukuza, primarily for rearing the cochineal scale insect (Dactylopius opuntiae). WFW control teams have adapted to this new control procedure as opposed to spraying herbicides, which marks a significant development in the long-term effective management of O. stricta in the KNP, and in the expanding breadth of WFW operating procedures. Large areas adjacent to the KNP are also being targeted by the WFW teams.
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.
The costs of the slow response to the increasing invasions over the last seven decades have, and will still result in a costly and continuous clearing and/or maintenance programme. In only the last 10 years has a gradual increase in awareness and support been witnessed. This is not however a problem unique to the KNP.
Thankfully, KNP recognises that the setting of detailed objectives, drafting of management plans, setting out of 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.
For more information on KNP Flora and the KNP stategy on Invasive Alien Plants, read here