Major tropical storms or tsunamis are not common in South Africa, nor are there
major snowstorms. However, global climate change is expected to lead to a greater
frequency of cyclones that affect the east of the country, and these might lead to
dramatic increases in the extent of some invasive species. For example,
McConnachie et al. (2011) noted that Parthenium Weed (Parthenium
hysterophorus) “. . .reportedly became common and invasive after Cyclone Demoina
caused extensive flooding along the east coast of southern Africa in 1984”. Further
occurrences such as the March–April 2019 Cyclones Idai and Kenneth, and subsequent flooding events in eastern Mozambique, will probably have similar impacts on
the spread of invasive species in this area. In the next sections we discuss how floods
and droughts (which are common in the region) have major impacts on a wide range
of biological invasions.
13.5.1 Floods
Alien species can often take advantage of flood-induced disturbance, particularly if
they tolerate wider environmental conditions (Dukes and Mooney 1999), and/or
exhibit traits that facilitate rapid resource acquisition, growth, and colonisation
(Pyšek and Richardson 2007). By contrast, large infrequent floods can also act to
remove invasions. For example, the flood in the Sabie River in Kruger National Park
in 2000 [which had an estimated return interval of 90–200 years depending on the
position in the catchment (Smithers et al. 2001)] removed all vegetation and
restructured the physical template (Parsons et al. 2006). By 2004 few alien species
had re-established—only 19 of 119 herbaceous species and nine of 136 woody
species (Foxcroft et al. 2008)—and their abundance was low—6% of all herbaceous
species and 3% of all woody species. This state was not stable, however, and a
survey of the same sites in 2015 revealed that the number of alien species present had
increased (to 40), herbaceous alien species had densified (to 70–80% of the total
density), although woody species remained at very low densities (TE Sibiya,
unpublished data).
Recruitment of native species in arid regions is frequently linked to rare rainfall
events, and it is likely that alien species use the same strategy. Milton and Dean
(2010) reported that seedlings of Pink Tamarisk (Tamarix ramosissima) occasionally recruited in large numbers following floods in dry riverbeds and dams. Prosopis
species (Mesquite) in the Karoo appeared to spread substantially following large
floods in 1970s and 1980s (Harding and Bate 1991), with a fourfold increase
between 1974 and 1991 during above-average rainfall years (Hoffman et al.
1995). Floods disperse Giant Reed (Arundo donax) rhizomes, which take advantage
and establish in bare disturbed rivers, from where it rapidly invades (Guthrie 2007).
A large amount of research in South Africa has focussed on the impacts of plant
invasions on ecosystem services [e.g. on surface water loss (Le Maitre et al. 2000)
and on the risk of flood damage (Le Maitre et al. 2014)]. The role that floods play in
the invasion process, however, requires further attention. As argued by Richardson
13 The Role of Environmental Factors in Promoting and Limiting Invasions
369
major snowstorms. However, global climate change is expected to lead to a greater
frequency of cyclones that affect the east of the country, and these might lead to
dramatic increases in the extent of some invasive species. For example,
McConnachie et al. (2011) noted that Parthenium Weed (Parthenium
hysterophorus) “. . .reportedly became common and invasive after Cyclone Demoina
caused extensive flooding along the east coast of southern Africa in 1984”. Further
occurrences such as the March–April 2019 Cyclones Idai and Kenneth, and subsequent flooding events in eastern Mozambique, will probably have similar impacts on
the spread of invasive species in this area. In the next sections we discuss how floods
and droughts (which are common in the region) have major impacts on a wide range
of biological invasions.
13.5.1 Floods
Alien species can often take advantage of flood-induced disturbance, particularly if
they tolerate wider environmental conditions (Dukes and Mooney 1999), and/or
exhibit traits that facilitate rapid resource acquisition, growth, and colonisation
(Pyšek and Richardson 2007). By contrast, large infrequent floods can also act to
remove invasions. For example, the flood in the Sabie River in Kruger National Park
in 2000 [which had an estimated return interval of 90–200 years depending on the
position in the catchment (Smithers et al. 2001)] removed all vegetation and
restructured the physical template (Parsons et al. 2006). By 2004 few alien species
had re-established—only 19 of 119 herbaceous species and nine of 136 woody
species (Foxcroft et al. 2008)—and their abundance was low—6% of all herbaceous
species and 3% of all woody species. This state was not stable, however, and a
survey of the same sites in 2015 revealed that the number of alien species present had
increased (to 40), herbaceous alien species had densified (to 70–80% of the total
density), although woody species remained at very low densities (TE Sibiya,
unpublished data).
Recruitment of native species in arid regions is frequently linked to rare rainfall
events, and it is likely that alien species use the same strategy. Milton and Dean
(2010) reported that seedlings of Pink Tamarisk (Tamarix ramosissima) occasionally recruited in large numbers following floods in dry riverbeds and dams. Prosopis
species (Mesquite) in the Karoo appeared to spread substantially following large
floods in 1970s and 1980s (Harding and Bate 1991), with a fourfold increase
between 1974 and 1991 during above-average rainfall years (Hoffman et al.
1995). Floods disperse Giant Reed (Arundo donax) rhizomes, which take advantage
and establish in bare disturbed rivers, from where it rapidly invades (Guthrie 2007).
A large amount of research in South Africa has focussed on the impacts of plant
invasions on ecosystem services [e.g. on surface water loss (Le Maitre et al. 2000)
and on the risk of flood damage (Le Maitre et al. 2014)]. The role that floods play in
the invasion process, however, requires further attention. As argued by Richardson
13 The Role of Environmental Factors in Promoting and Limiting Invasions
369