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The bad group are those trees and plants which have been imported into South Africa and which simply do not know how to behave in our environment.  Also known as aliens.

Check out the Department of Agriculture list of aliens.
Link: Department of Agriculture

Another problem plant is the strong growth of mistletoe on many of the Acacia karroo trees.  This mistletoe species is a Viscum species which are spread by fruit eating birds, including the yellow fronted tinker barbet, redfaced moosebird, blackcollared barbet, pied barbet.  The tinker barbets are the most important dispersers of mistletoe seeds in Africa.
Viscum species flower all year round and also produce seeds all year.  A close examination of the Acacia karroo trees in the Magaliesberg will reveal tiny insect pollinated flowers, orange berries and clumps of regurgitated or defecated mistletoe seeds stuck to branches and which are already germinating.  After germination, the ‘root’ or haustoria will penetrate the branch of the Acacia and form a permanent bond.
Viscum spp are indigenous.  There are 17 Viscum spp species in Southern Africa, so there could be more than one in the Magaliesberg area.
It is a hemi-parasite and can photosynthesize for itself; it does not take all its nutrients from the host.  It would take water and nitrates, and would use the plant for support and does not normally destroy its host plant.  When the host tree is heavily stressed, i.e. during a dry winter, or when the tree is parasitized by many mistletoe plants, one would imagine that the tree might be killed, but this is not documented.  The drought stress would probably kill the host tree before the mistletoe did.