early as the 1950s, and there were widespread invasions by the 1960s (Bhagat et al.
2012). The spread of Campuloclinium macrocephalum (Pompom Weed) was first
noted in the 1960s around Pretoria, whereafter it spread to other parts of the country
(Goodall et al. 2011). Prosopis species (mesquite) began spreading in the arid
interior of South Africa in the 1970s and 1980s some 60 years after major plantings
(Harding and Bate 1991). Rapid mesquite expansion followed several years of
above-average rainfall in the Karoo that created conditions suitable for seed dispersal
and seedling establishment. Another, similarly rapid, expansion of mesquite
occurred in the 2000s (van den Berg et al. 2013). The 1980s also saw the rapid
invasion of Opuntia stricta (Australian Pest Pear) in the Kruger National Park where
major invasions grew from scattered foci around Skukuza, where the species was
grown as an ornamental plant in tourist villages in the 1950s (Foxcroft et al. 2004).
Although first reported in South Africa in 1880, at Inanda in KwaZulu-Natal,
Parthenium hysterophorus (Parthenium Weed) remained uncommon until the
1980s when its populations expanded rapidly after Cyclone Demoina caused extensive flooding along the east coast of southern Africa in 1984 (McConnachie et al.
2011). Since then its range has increased rapidly and it is now a major invader over
large parts of mesic savannas in eastern South Africa (Terblanche et al. 2016).
Similarly, Pyracantha angustifolia only began invading the Grassland Biome in
the early 1980s (the first herbarium record for the species is dated 1970 from the
Ficksburg district of the Free State); it then spread very rapidly and dense stands of
this shrub now occur in many high-altitude grasslands.
3.3 How Many Taxa? South Africa’s Alien, Naturalised
and Invasive Flora
3.3.1 A National List of the Alien Flora?
No comprehensive list of the alien flora of South Africa exists, but several publications have made estimations of between 8750 and 9000 alien plant taxa (Le Maitre
et al. 2011; Richardson et al. 2011b; Irlich et al. 2014; van Wilgen and Wilson 2018).
These estimations seem to be based largely on insights from Glen’s (2002) book on
the “Cultivated plants of southern Africa”. Glen’s list was based on herbarium
specimens, nursery catalogues and records from plant breeders’ rights. It does not
include naturalised species that have not been cultivated, such as those introduced as
seed contaminants. Discussions with many botanists suggest that the estimate of
8750–9000 alien taxa is conservative. Glen and van Wyk (2016) estimated that there
were around 2000 alien tree species in South Africa.
The challenges associated with compiling a definitive alien flora for South Africa,
and deciding which taxa reside in different “introduction status” categories (based on
their position along the introduction-naturalisation-invasion continuum; Blackburn
3 The Biogeography of South African Terrestrial Plant Invasions
73
2012). The spread of Campuloclinium macrocephalum (Pompom Weed) was first
noted in the 1960s around Pretoria, whereafter it spread to other parts of the country
(Goodall et al. 2011). Prosopis species (mesquite) began spreading in the arid
interior of South Africa in the 1970s and 1980s some 60 years after major plantings
(Harding and Bate 1991). Rapid mesquite expansion followed several years of
above-average rainfall in the Karoo that created conditions suitable for seed dispersal
and seedling establishment. Another, similarly rapid, expansion of mesquite
occurred in the 2000s (van den Berg et al. 2013). The 1980s also saw the rapid
invasion of Opuntia stricta (Australian Pest Pear) in the Kruger National Park where
major invasions grew from scattered foci around Skukuza, where the species was
grown as an ornamental plant in tourist villages in the 1950s (Foxcroft et al. 2004).
Although first reported in South Africa in 1880, at Inanda in KwaZulu-Natal,
Parthenium hysterophorus (Parthenium Weed) remained uncommon until the
1980s when its populations expanded rapidly after Cyclone Demoina caused extensive flooding along the east coast of southern Africa in 1984 (McConnachie et al.
2011). Since then its range has increased rapidly and it is now a major invader over
large parts of mesic savannas in eastern South Africa (Terblanche et al. 2016).
Similarly, Pyracantha angustifolia only began invading the Grassland Biome in
the early 1980s (the first herbarium record for the species is dated 1970 from the
Ficksburg district of the Free State); it then spread very rapidly and dense stands of
this shrub now occur in many high-altitude grasslands.
3.3 How Many Taxa? South Africa’s Alien, Naturalised
and Invasive Flora
3.3.1 A National List of the Alien Flora?
No comprehensive list of the alien flora of South Africa exists, but several publications have made estimations of between 8750 and 9000 alien plant taxa (Le Maitre
et al. 2011; Richardson et al. 2011b; Irlich et al. 2014; van Wilgen and Wilson 2018).
These estimations seem to be based largely on insights from Glen’s (2002) book on
the “Cultivated plants of southern Africa”. Glen’s list was based on herbarium
specimens, nursery catalogues and records from plant breeders’ rights. It does not
include naturalised species that have not been cultivated, such as those introduced as
seed contaminants. Discussions with many botanists suggest that the estimate of
8750–9000 alien taxa is conservative. Glen and van Wyk (2016) estimated that there
were around 2000 alien tree species in South Africa.
The challenges associated with compiling a definitive alien flora for South Africa,
and deciding which taxa reside in different “introduction status” categories (based on
their position along the introduction-naturalisation-invasion continuum; Blackburn
3 The Biogeography of South African Terrestrial Plant Invasions
73