when compared to a protected area, resulted in the dominant native shrubs Euclea
undulata (Common Guarri) and Portulacaria afra (Spekboom) being replaced by
the native Putterlickia pyracantha (False Spike-thorn) and the invasive alien Opuntia ficus-indica (Mission Prickly Pear) (Hoffman and Cowling 1990).
Graziers commonly implement a long-term burning regime (where applicable),
and a system of grazing management involving various combinations of stocking
rate, animal type, and grazing intensity. The effect of these on the abundance of alien
plants was assessed by compiling a novel data set of the results of long-term
management experiments and long-standing fence-line contrasts of management
systems (Supplementary Appendix 16.1). Box 16.1 describes the value of this
database in illustrating some key management principles. First, exclusion or very
infrequent fire promotes the establishment and often dominance of alien woody
species in moist grassland or mesic savanna, an effect which is not apparent in
regularly-burnt rangeland. By contrast, alien species are not conspicuous in semiarid savanna despite the fact that they seldom burn. Secondly, commercial dryland
rangeland under prescribed grazing and fire management was largely resistant to
invasive alien plants with only small amounts of a few usually herbaceous species
being recorded. Alien plants increased in response to heavy grazing pressure.
Available evidence indicates that effective use of fire in particular can be used for
controlling invasive plants. However, riparian habitats are vulnerable in part because
they are not usually subjected to planned fires.
Biological control has been implemented for a small proportion of the 71 prominent species; specifically, 3 are considered to be completely controlled, 16 are under
substantial control, 16 are under negligible control, 4 are under investigation, 5 have
not been determined, and the remainder are subject to no biological control
(Table 16.2) (Hill et al. 2020, Chap. 19).
16.7 Prognosis
The threat posed by invasive alien plants to rangelands is expected to increase with
time because many species have not yet spread to occupy their potential climatic
range, whereas others that have, such as Australian Acacia species, can be expected
to increase in density (Rouget et al. 2004). Many currently quiescent species are
expected to become invasive once an exponential phase of population growth
commences (Hui and Richardson 2017). An example of this is the recent expansion
of Campuloclinium macrocephalum (Pompom Weed) following a long period where
it was only localised (Wilson et al. 2013). Despite increased effort to control the
introduction of alien plant species into the country, the pool of alien species is
growing (van Wilgen and Wilson 2018) and the irruption of new invasive species is
almost inevitable.
Direct actions such as clearing and biological control will continue to provide a
foundation for containing invasive alien plants, but management of rangelands and
16 The Impact of Invasive Alien Plants on Rangelands in South Africa
481
undulata (Common Guarri) and Portulacaria afra (Spekboom) being replaced by
the native Putterlickia pyracantha (False Spike-thorn) and the invasive alien Opuntia ficus-indica (Mission Prickly Pear) (Hoffman and Cowling 1990).
Graziers commonly implement a long-term burning regime (where applicable),
and a system of grazing management involving various combinations of stocking
rate, animal type, and grazing intensity. The effect of these on the abundance of alien
plants was assessed by compiling a novel data set of the results of long-term
management experiments and long-standing fence-line contrasts of management
systems (Supplementary Appendix 16.1). Box 16.1 describes the value of this
database in illustrating some key management principles. First, exclusion or very
infrequent fire promotes the establishment and often dominance of alien woody
species in moist grassland or mesic savanna, an effect which is not apparent in
regularly-burnt rangeland. By contrast, alien species are not conspicuous in semiarid savanna despite the fact that they seldom burn. Secondly, commercial dryland
rangeland under prescribed grazing and fire management was largely resistant to
invasive alien plants with only small amounts of a few usually herbaceous species
being recorded. Alien plants increased in response to heavy grazing pressure.
Available evidence indicates that effective use of fire in particular can be used for
controlling invasive plants. However, riparian habitats are vulnerable in part because
they are not usually subjected to planned fires.
Biological control has been implemented for a small proportion of the 71 prominent species; specifically, 3 are considered to be completely controlled, 16 are under
substantial control, 16 are under negligible control, 4 are under investigation, 5 have
not been determined, and the remainder are subject to no biological control
(Table 16.2) (Hill et al. 2020, Chap. 19).
16.7 Prognosis
The threat posed by invasive alien plants to rangelands is expected to increase with
time because many species have not yet spread to occupy their potential climatic
range, whereas others that have, such as Australian Acacia species, can be expected
to increase in density (Rouget et al. 2004). Many currently quiescent species are
expected to become invasive once an exponential phase of population growth
commences (Hui and Richardson 2017). An example of this is the recent expansion
of Campuloclinium macrocephalum (Pompom Weed) following a long period where
it was only localised (Wilson et al. 2013). Despite increased effort to control the
introduction of alien plant species into the country, the pool of alien species is
growing (van Wilgen and Wilson 2018) and the irruption of new invasive species is
almost inevitable.
Direct actions such as clearing and biological control will continue to provide a
foundation for containing invasive alien plants, but management of rangelands and
16 The Impact of Invasive Alien Plants on Rangelands in South Africa
481