dormant for up to 50 years, and a rapid sapling growth rate that promotes
overtopping of native vegetation (Richardson and Kluge 2008).
16.5.4 Parthenium hysterophorus (Parthenium)
Parthenium hysterophorus is an annual herb native to tropical and subtropical
America that has increased alarmingly between 2000 and 2016 (Table 16.2). It has
negative impacts on livestock production, and is also a serious threat to environmental and human health because of its ability to produce chemicals that cause
severe dermatitis, allergy and toxicity in humans and animals (Terblanche et al.
2016). In a study of impacts on subsistence and commercial farmers, Wise et al.
(2007) found that unchecked spread reduced the economic returns to small-scale
farmers by between 26 and 41%, but that these losses could be offset if the levels of
invasion were reduced by at least 50%. However, although 80% mortality can be
achieved using chemical or manual control, density of P. hysterophorus will return
to pre-control levels within a year owing to the size of the seed bank, but density will
decline if follow-up clearing is maintained (Goodall et al. 2010).
16.5.5 Chromolaena odorata (Triffid Weed)
Chromolaena odorata is a tall, bushy, scrambling shrub that was introduced to
Africa from the Caribbean and has spread rapidly to occupy most of the climatically
suitable habitats for the species in the eastern part of South Africa (Rouget et al.
2004). It has negative impacts on agricultural practices and on biodiversity. Studies
of the magnitude of impacts are rare, but Wise et al. (2007) found that the introduction of a mechanical control programme saw annual returns from cattle sales increase
by between 7 and 34%, depending on the area of the initial invasion, in the
Ntambanana district in KwaZulu-Natal, South Africa.
16.5.6 Opuntia ficus-indica (Mission Prickly Pear)
The invasive cactus Opuntia ficus-indica provides an example of the devastating
impact that invasive cacti can have on arid rangelands. The species was responsible
for severe degradation in parts of the Karoo, but has now been dramatically reduced
in density over most of its distribution by effective biological control (Hill et al.
2020, Chap. 19). At the height of its dominance at the start of the twentieth century,
O. ficus-indica covered almost 1 million ha of semi-arid rangelands. There were no
formal ecological studies that assessed its impact at the time, but several historical
overviews have recorded that these impacts were dramatic. Annecke and Moran
16 The Impact of Invasive Alien Plants on Rangelands in South Africa
479
overtopping of native vegetation (Richardson and Kluge 2008).
16.5.4 Parthenium hysterophorus (Parthenium)
Parthenium hysterophorus is an annual herb native to tropical and subtropical
America that has increased alarmingly between 2000 and 2016 (Table 16.2). It has
negative impacts on livestock production, and is also a serious threat to environmental and human health because of its ability to produce chemicals that cause
severe dermatitis, allergy and toxicity in humans and animals (Terblanche et al.
2016). In a study of impacts on subsistence and commercial farmers, Wise et al.
(2007) found that unchecked spread reduced the economic returns to small-scale
farmers by between 26 and 41%, but that these losses could be offset if the levels of
invasion were reduced by at least 50%. However, although 80% mortality can be
achieved using chemical or manual control, density of P. hysterophorus will return
to pre-control levels within a year owing to the size of the seed bank, but density will
decline if follow-up clearing is maintained (Goodall et al. 2010).
16.5.5 Chromolaena odorata (Triffid Weed)
Chromolaena odorata is a tall, bushy, scrambling shrub that was introduced to
Africa from the Caribbean and has spread rapidly to occupy most of the climatically
suitable habitats for the species in the eastern part of South Africa (Rouget et al.
2004). It has negative impacts on agricultural practices and on biodiversity. Studies
of the magnitude of impacts are rare, but Wise et al. (2007) found that the introduction of a mechanical control programme saw annual returns from cattle sales increase
by between 7 and 34%, depending on the area of the initial invasion, in the
Ntambanana district in KwaZulu-Natal, South Africa.
16.5.6 Opuntia ficus-indica (Mission Prickly Pear)
The invasive cactus Opuntia ficus-indica provides an example of the devastating
impact that invasive cacti can have on arid rangelands. The species was responsible
for severe degradation in parts of the Karoo, but has now been dramatically reduced
in density over most of its distribution by effective biological control (Hill et al.
2020, Chap. 19). At the height of its dominance at the start of the twentieth century,
O. ficus-indica covered almost 1 million ha of semi-arid rangelands. There were no
formal ecological studies that assessed its impact at the time, but several historical
overviews have recorded that these impacts were dramatic. Annecke and Moran
16 The Impact of Invasive Alien Plants on Rangelands in South Africa
479