et al. (1997), “Features of the riparian environment that promote invasions include
the easier access to moisture (which reduces any drought stresses imposed by
prevailing features that delimit the biomes), and periodic disturbances in the form
of floods that disperse seeds, prepare them for germination, provide seed beds, and
remove competing plants”. Invasive woody trees like Weeping Willow (Salix
babylonica) and Red Sesbania (Sesbania punicea) can form dense stands, obstruct
flow, alter watercourses, and convert well-defined rivers into diffuse systems of
shallow streamlets and trickles. An important finding of Galatowitsch and Richardson
(2005), working on the Eerste River in the Western Cape, was that “seed regeneration
of indigenous trees in these headwater rivers is not disturbance-triggered.” This is in
contrast to the major invaders of such rivers, for which germination is typically very
clearly disturbance-related. With major alterations of the flow regimes in such rivers
(Meek et al. 2010), flooding is more common, which provides abundant opportunities for regeneration of the invasive species. These processes, and the impacts
caused, can be exacerbated by violent thunder-storms that typify some parts of
South Africa (e.g. much of the Grassland Biome).
13.5.2 Droughts
Droughts induce extreme stress conditions that can reduce the biotic resistance of a
community over time. If alien species survive longer, and respond quicker once a
drought lifts, they will have significant opportunities post-drought (Diez et al. 2012).
Prosopis species were introduced to South Africa in the late 1880s, and widely
distributed as they provide fodder and shade when water is scarce. It is now
estimated that invasive Prosopis populations cover 1.8 million ha in South Africa
(Shackleton et al. 2015). Similarly, the Peruvian Pepper Tree (Schinus molle) was
selected for its drought tolerance and widely planted along roadsides in arid areas
over the past 60 years (Iponga et al. 2008). It out-competes native species and is
increasing in abundance (Iponga et al. 2009). Cactaceae species, in particular
Mission Prickly Pear (Opuntia ficus-indica), were widely planted in some arid
regions for a variety of benefits, and have invaded at least 900,000 ha, displacing
natural vegetation (Annecke and Moran 1978). Although the importance of drought
in facilitating these invasions is not clear, their competitive ability under drought
conditions clearly played a role in their ability to establish, persist, and dominate.
13.6 Fire
A combination of periods of hot, dry weather, flammable vegetation, and abundant
sources of ignition means that fires are a regular feature of many (but not all) of the
terrestrial landscapes in South Africa (see Table 13.2 for examples; van Wilgen and
Scholes 1997). Specifically the Fynbos, Grassland and Savanna Biomes have
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the easier access to moisture (which reduces any drought stresses imposed by
prevailing features that delimit the biomes), and periodic disturbances in the form
of floods that disperse seeds, prepare them for germination, provide seed beds, and
remove competing plants”. Invasive woody trees like Weeping Willow (Salix
babylonica) and Red Sesbania (Sesbania punicea) can form dense stands, obstruct
flow, alter watercourses, and convert well-defined rivers into diffuse systems of
shallow streamlets and trickles. An important finding of Galatowitsch and Richardson
(2005), working on the Eerste River in the Western Cape, was that “seed regeneration
of indigenous trees in these headwater rivers is not disturbance-triggered.” This is in
contrast to the major invaders of such rivers, for which germination is typically very
clearly disturbance-related. With major alterations of the flow regimes in such rivers
(Meek et al. 2010), flooding is more common, which provides abundant opportunities for regeneration of the invasive species. These processes, and the impacts
caused, can be exacerbated by violent thunder-storms that typify some parts of
South Africa (e.g. much of the Grassland Biome).
13.5.2 Droughts
Droughts induce extreme stress conditions that can reduce the biotic resistance of a
community over time. If alien species survive longer, and respond quicker once a
drought lifts, they will have significant opportunities post-drought (Diez et al. 2012).
Prosopis species were introduced to South Africa in the late 1880s, and widely
distributed as they provide fodder and shade when water is scarce. It is now
estimated that invasive Prosopis populations cover 1.8 million ha in South Africa
(Shackleton et al. 2015). Similarly, the Peruvian Pepper Tree (Schinus molle) was
selected for its drought tolerance and widely planted along roadsides in arid areas
over the past 60 years (Iponga et al. 2008). It out-competes native species and is
increasing in abundance (Iponga et al. 2009). Cactaceae species, in particular
Mission Prickly Pear (Opuntia ficus-indica), were widely planted in some arid
regions for a variety of benefits, and have invaded at least 900,000 ha, displacing
natural vegetation (Annecke and Moran 1978). Although the importance of drought
in facilitating these invasions is not clear, their competitive ability under drought
conditions clearly played a role in their ability to establish, persist, and dominate.
13.6 Fire
A combination of periods of hot, dry weather, flammable vegetation, and abundant
sources of ignition means that fires are a regular feature of many (but not all) of the
terrestrial landscapes in South Africa (see Table 13.2 for examples; van Wilgen and
Scholes 1997). Specifically the Fynbos, Grassland and Savanna Biomes have
370
J. R. Wilson et al.