and Shackleton 2004). Australian Acacia species are the primary source of fuel wood
for many rural communities located in the Grassland Biome (Agripa 2016), in the
absence of which native forest may be harvested for fuel wood (Geldenhuys and
Cawe 2011). By contrast, Prosopis species are sparingly used as fuel wood, owing to
most infestations being distant from settlements (van den Berg 2010), and also due to
the inferior quality of mesquite wood compared with that of native species (Shackleton et al. 2015).
The medicinal plant trade in South Africa is a commercial enterprise valued at
ZAR 2.9 billion annually and it involves 27 million consumers who consume 20,000
tonnes of plant material annually comprising 771 native species of which most are
derived from rangeland (von Ahlefeldt et al. 2003). By contrast, only a few alien
species are used for medicinal purposes. Examples include common use of Acacia
dealbata in the Eastern Cape (Agripa 2016), Senna occidentalis (Stinking Weed)
(Henderson 2001), and the naturalised perennial wetland herb Acorus calamus
(Sweet Flag) in the Indian Ocean Coastal Belt (von Ahlefeldt et al. 2003). In most
cases, rangeland lost to invasive aliens is expected to result in a loss of medicinal
plants. Alien species introduced for providing edible fruits for humans include
Opuntia species, Rubus cuneifolius (American Bramble), and Psidium guajava
(Guava), all now very successful invaders, but all these are used incidentally as a
food source. Eucalyptus species are important for supporting native bee species
which pollinate most of the insect-pollinated crops in South Africa that are worth an
estimated ZAR 10.3 billion annually (SANBI 2018). In extensively transformed
landscapes, Eucalyptus plantations and stands serve as their primary source of food.
16.5 Negative Impacts of Invasions on Rangelands
16.5.1 Physical and Economic Impacts
This section assesses the influence of invasive alien plants on animal production
through affecting the amount and quality of forage, producing harmful chemicals
that may be ingested, reducing accessibility to forage, and inflicting physical harm to
animals. Their effect on forage abundance depends on whether alien plants are more,
equally, or less desirable for animals than the native vegetation they supplant, with
the severity of this effect depending on the total area invaded. Van Wilgen et al.
(2008) estimated that (at the time of their study) alien plants had caused a 1%
decrease in livestock production, but predicted that this may increase to as much
as 71% in the future. The forage value of a grass sward invaded by alien grasses is
expected to be maintained in most instances because the majority of the 256 alien
grass species in the country, of which 122 species have become naturalised and
41 species have become invasive or have had an economic impact, were introduced
at pasture research stations for screening of their potential mainly as fodder plants
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T. G. O’Connor and B. W. van Wilgen
for many rural communities located in the Grassland Biome (Agripa 2016), in the
absence of which native forest may be harvested for fuel wood (Geldenhuys and
Cawe 2011). By contrast, Prosopis species are sparingly used as fuel wood, owing to
most infestations being distant from settlements (van den Berg 2010), and also due to
the inferior quality of mesquite wood compared with that of native species (Shackleton et al. 2015).
The medicinal plant trade in South Africa is a commercial enterprise valued at
ZAR 2.9 billion annually and it involves 27 million consumers who consume 20,000
tonnes of plant material annually comprising 771 native species of which most are
derived from rangeland (von Ahlefeldt et al. 2003). By contrast, only a few alien
species are used for medicinal purposes. Examples include common use of Acacia
dealbata in the Eastern Cape (Agripa 2016), Senna occidentalis (Stinking Weed)
(Henderson 2001), and the naturalised perennial wetland herb Acorus calamus
(Sweet Flag) in the Indian Ocean Coastal Belt (von Ahlefeldt et al. 2003). In most
cases, rangeland lost to invasive aliens is expected to result in a loss of medicinal
plants. Alien species introduced for providing edible fruits for humans include
Opuntia species, Rubus cuneifolius (American Bramble), and Psidium guajava
(Guava), all now very successful invaders, but all these are used incidentally as a
food source. Eucalyptus species are important for supporting native bee species
which pollinate most of the insect-pollinated crops in South Africa that are worth an
estimated ZAR 10.3 billion annually (SANBI 2018). In extensively transformed
landscapes, Eucalyptus plantations and stands serve as their primary source of food.
16.5 Negative Impacts of Invasions on Rangelands
16.5.1 Physical and Economic Impacts
This section assesses the influence of invasive alien plants on animal production
through affecting the amount and quality of forage, producing harmful chemicals
that may be ingested, reducing accessibility to forage, and inflicting physical harm to
animals. Their effect on forage abundance depends on whether alien plants are more,
equally, or less desirable for animals than the native vegetation they supplant, with
the severity of this effect depending on the total area invaded. Van Wilgen et al.
(2008) estimated that (at the time of their study) alien plants had caused a 1%
decrease in livestock production, but predicted that this may increase to as much
as 71% in the future. The forage value of a grass sward invaded by alien grasses is
expected to be maintained in most instances because the majority of the 256 alien
grass species in the country, of which 122 species have become naturalised and
41 species have become invasive or have had an economic impact, were introduced
at pasture research stations for screening of their potential mainly as fodder plants
474
T. G. O’Connor and B. W. van Wilgen