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Plant invasions affect virtually all ecosystems on earth, even those parcels of land set aside as protected areas and mandated with the conservation of biological diversity. Protected areas are islands in a sea of different types and sources of propagules, including those of many non-native species. These propagules can enter into the protected areas in many ways, continuously bombarding what are often the last bastions of natural habitats.

Colleagues from the C·I·B and South African National Parks have been studying the dynamics of plant invasions of the largest protected area in South Africa, namely the Kruger National Park (KNP). Their efforts focus on determining the processes and pathways of invasion and exploring the implications for long-term management of plant invasions in the park.

The most highly invaded areas in the KNP are the river landscapes and riparian vegetation along the banks of most of the major rivers. Two main pathways appear to have contributed significantly to the current levels of invasion in the park, namely dispersal along rivers from the upper catchments of the main rivers (Fig. 1) and the use of alien species in ornamental gardens in the tourist camps and personnel villages (Fig. 2).

To determine the role of alien plants in the upper catchments of the parks’ rivers serving as a source of invasions, a framework was developed to evaluate the risk of spread of alien plants from different parts of the catchments into the KNP. The framework combines species- and landscape-level approaches and has five key components:

  • definition of the geographical area of interest (“the domain”),
  • delineation of the domain into ecologically meaningful zones,
  • identification of the appropriate landscape units,
  • categorization of alien species and mapping of their distribution and abundance, and
  • definition of management options.

The framework guides the determination of species distribution and abundance through successive, easily followed steps, providing the means for the assessment of areas of concern.

A total of 231 invasive alien plant species (of which 79 have potential to transform the character and form of the environment) were recorded in the study area. The KNP is facing increasing pressure from alien species in the upper regions of the drainage areas of neighbouring watersheds. On the basis of the climatic modelling, we showed that a large number of the most invasive riparian species have the ability to spread across the KNP should they be transported down the rivers. With this information, KNP managers can identify areas for proactive intervention, monitoring, and resource allocation. Even for a very large protected area such as the KNP, sustainable management of biodiversity will depend heavily on the response of land managers upstream managing alien plants.

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Figure 1. The Sabie River, which originates in the high lying areas to the west of the Kruger National Park, and flows through the southern region of the park and into Mozambique. Bottom: typical invasive plant species along the Sabie River (from left to right) Lantana camara, Chromolaena odorata, and Senna occidentalis.

Within the park, a detailed survey was done of alien plants in the 36 tourist camps, staff villages and other infrastructures over a four year period. 257 alien plant species were recorded, of which at least 85 taxa are well know invasive aliens in South Africa or elsewhere in the world. The most widespread and common species include: Alpinia zerumbet (shell ginger), Bryophyllum daigremontianum (good luck plant), Bryophyllum delagoense (chandelier plant/mother of millions), Callisia repens (striped creeping inch plant), Nephrolepis exaltata (sword fern/Boston fern), Sphagneticola trilobata ( Singapore daisy), Syngoniumpodophyllum (goose-foot plant/ arrowhead-vine), Tradescantiapallida (purple wandering Jew/purpleheart), and Tradescantiaspathacea (boat plant/ oyster plant). Introductions of ornamental plants were also responsible for some of the most prominent alien plant problems in the park; including Opuntiastricta (sour prickly pear) and Pistiastratiotes (water lettuce).

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The identification of these two important conduits of invasion into the park has assisted in developing more holistic long-term strategies for the management of alien plants. In the case of the riparian invasions, the national Working for Water programme, which also has a number of projects in South African National Parks, carries out control measures along the most invaded rivers in the park. Using the information gained through the risk assessment process, this enabled the project to assess areas of increased importance, both in terms of alien species of priority and areas of priority, and then prioritise these for clearing operations. Internally, policy has been adopted that prohibits the use of alien plants as ornamental species, and regulates the phased removal of existing species. Currently all tourist camps and personnel villages have been cleared of alien plants at least once, with follow-up operations ongoing

Figure 2. Five commonly used alien ornamental plants, also found in the Kruger National park. (a) Bryophyllum delagoense (Eckl. & Zeyh.) Schinz. (= Kalanchoe delagoense, K. tubiflora) chandelier plant/mother of millions, (b) Zingiber zerumbet (L.) Sm. Zingiberaceae wild ginger/pinecone ginger, (c) Stachytarpheta mutabilis (Jacq.) Vahl Verbenaceae pink porterweed, (d) Pontederia cordata L. Pontederiaceae pontederia/pickerel weed, (e) Thevetia peruviana (Pers.) K. Schum. Apocynaceae yellow oleander.