The Kruger National Park has expended a good deal of time, effort, energy and money into combating the threat of invasive alien plants. Llewellyn Foxcroft, programme manager for invasion ecology, is currently studying the big picture of how the plants are distributed within Kruger, and the best way to stop them spreading. Most of the alien plants in Kruger are introduced from upstream of the park, where large numbers of alien plants occur.
Seeds and reproductive material are carried by the rivers into the park, and then spread out from there. Any part of a plant that can spread out to form new plants is called a propagule. The Upper Sabie and Letaba River catchments have been identified as two areas where the largest sustained pressure of alien plants (or highest propagule pressure) is likely to come from. Foxcroft speculates that this is related to the relatively high rainfall in these areas.
By assessing all the paths of introduction of aliens, the park can focus its management plan on where it will do the most good. This will include helping people outside the park to manage their alien plants so that fewer plants travel downstream into Kruger. Hotspots have been identified as Tzaneen, Phalaborwa and upper Sabie. The two acknowledged botanical reserves with high biodiversity in the far north and south west of the park are also important to keep as pristine as possible.
Foxcroft says that most activity is centred around invasive shrubs, as there are few alien trees in the park and that the problem of annuals is too widespread, difficult to combat and the impacts are not as well understood. He identified the major threats as species such as Lantana, Chromolaena odorata, Senna, the Mauritius thorn, Caesalpinia and mimosa, as well as water weeds such as the water hyacinth. He said that the alien task force is also keeping a close eye out for the up-and-coming weed, Parthenium hysterophorus.
These plants tend to form dense clumps, where they outcompete all the indigenous vegetation. Some plants emit toxic chemicals from their roots which prevent other species from growing in the vicinity, while others simply produce too much shade for other plants to see sunlight. The prickly pear is basically the only alien plant that is widespread throughout the park and not clumped around the river systems. Historically, the insidious nature of alien invaders has made it difficult to motivate for their control.
A single plant does not seem much of a threat at the time, but ten years later may have multiplied until it is completely out of control. The Working for Water programme has now made a serious dent in Kruger's alien plant population, with most work currently in progress being follow-ups on previous major eradication exercises.