trees. The extent or severity of transformation of native vegetation has not been well
described, with a few exceptions (e.g. Prosopis spp.; van den Berg 2010; Shackleton
et al. 2015), but impacts are widely evident, especially in riparian environments.
Growth forms other than trees, shrubs, and succulent shrubs are not usually important invasive alien species (Table 16.2). Despite an updated national list of 256 alien
grass species (Visser et al. 2017), only three grass species were prominent. However,
a number of species which have more than doubled their distribution between 2000
and 2016 were herbaceous; these include Campuloclinium macrocephalum and
Parthenium hysterophorus (Table 16.2; Fig. 16.1). Although many species occurred
in more than one biome, a biome supports a distinctive community of alien plants,
which suggests that a biome-specific approach to the control of alien plants would be
appropriate (Rouget et al. 2015; see also Richardson et al. 2020, Chap. 3). Half of the
important species impact both riparian and dryland habitat, whilst 19 and 16 species
tend to be restricted to riparian or dryland habitat, respectively (Table 16.2). In the
dry Karoo biomes, however, 65% of the 101 alien species are found in riparian
habitat (Milton and Dean 2010).
16.4 Benefits of Alien Plants to Rangelands
Many of the alien species that have become invasive were introduced for a specific
purpose (Richardson et al. 2020, Chap. 3). For example, Milton and Dean (2010)
determined that 54% of the 101 alien species recorded in arid parts of South Africa
(<250 mm mean annual precipitation, MAP) are cultivated for utility products or
services, 34% as ornamentals, and only 12% were accidental introductions. Valuable
services provided by alien plants include forage and shelter for livestock, soil
protection, fuel and building materials, barrier plants, medicines, and pollination.
However, their relative value depends on what is displaced. A species may provide
multiple services, for example Acacia dealbata (Silver Wattle) in the Eastern Cape
Province is used by 100% of households for firewood, 73% for fencing, 18% as a
medicine, and 79% for livestock fodder (Agripa 2016).
Alien plants may contribute to soil stability in the face of chronic grazing if they
can withstand trampling and grazing, as observed for the small, mat-growing alien
herb Richardia brasiliensis (Tropical Richardia), a sub-dominant plant (13% cover)
on communal grassland in the moist southern Drakensberg (O’Connor 2005). Its
mat-like growth serves to intercept rainfall energy, promote infiltration, reduce
lateral flow, and contain soil wash. This widespread species only becomes conspicuous in degraded swards but can be displaced by improved grazing management. By
contrast, the widespread invasive alien perennial grass Pennisetum clandestinum
(Kikuyu Grass) fulfils the same function but is not easily displaced once established.
Fuelwood is the primary source of energy in rural areas under communal tenure; a
rural household consumes on average 5.3 tonnes of firewood annually (Shackleton
16 The Impact of Invasive Alien Plants on Rangelands in South Africa
473
described, with a few exceptions (e.g. Prosopis spp.; van den Berg 2010; Shackleton
et al. 2015), but impacts are widely evident, especially in riparian environments.
Growth forms other than trees, shrubs, and succulent shrubs are not usually important invasive alien species (Table 16.2). Despite an updated national list of 256 alien
grass species (Visser et al. 2017), only three grass species were prominent. However,
a number of species which have more than doubled their distribution between 2000
and 2016 were herbaceous; these include Campuloclinium macrocephalum and
Parthenium hysterophorus (Table 16.2; Fig. 16.1). Although many species occurred
in more than one biome, a biome supports a distinctive community of alien plants,
which suggests that a biome-specific approach to the control of alien plants would be
appropriate (Rouget et al. 2015; see also Richardson et al. 2020, Chap. 3). Half of the
important species impact both riparian and dryland habitat, whilst 19 and 16 species
tend to be restricted to riparian or dryland habitat, respectively (Table 16.2). In the
dry Karoo biomes, however, 65% of the 101 alien species are found in riparian
habitat (Milton and Dean 2010).
16.4 Benefits of Alien Plants to Rangelands
Many of the alien species that have become invasive were introduced for a specific
purpose (Richardson et al. 2020, Chap. 3). For example, Milton and Dean (2010)
determined that 54% of the 101 alien species recorded in arid parts of South Africa
(<250 mm mean annual precipitation, MAP) are cultivated for utility products or
services, 34% as ornamentals, and only 12% were accidental introductions. Valuable
services provided by alien plants include forage and shelter for livestock, soil
protection, fuel and building materials, barrier plants, medicines, and pollination.
However, their relative value depends on what is displaced. A species may provide
multiple services, for example Acacia dealbata (Silver Wattle) in the Eastern Cape
Province is used by 100% of households for firewood, 73% for fencing, 18% as a
medicine, and 79% for livestock fodder (Agripa 2016).
Alien plants may contribute to soil stability in the face of chronic grazing if they
can withstand trampling and grazing, as observed for the small, mat-growing alien
herb Richardia brasiliensis (Tropical Richardia), a sub-dominant plant (13% cover)
on communal grassland in the moist southern Drakensberg (O’Connor 2005). Its
mat-like growth serves to intercept rainfall energy, promote infiltration, reduce
lateral flow, and contain soil wash. This widespread species only becomes conspicuous in degraded swards but can be displaced by improved grazing management. By
contrast, the widespread invasive alien perennial grass Pennisetum clandestinum
(Kikuyu Grass) fulfils the same function but is not easily displaced once established.
Fuelwood is the primary source of energy in rural areas under communal tenure; a
rural household consumes on average 5.3 tonnes of firewood annually (Shackleton
16 The Impact of Invasive Alien Plants on Rangelands in South Africa
473