Two major river systems, the Limpopo and the Orange, both forming part of
South Africa’s northern border, contain more than 85% of South Africa’s freshwater.
Most other catchments are relatively small, and there are no large lakes. South Africa
has a 2798 km long coastline, with estuaries making up a small but important
component of South Africa’s ecosystems [with a total of 465 estuaries, including
the largest estuary in Africa, the St Lucia estuary in iSimangaliso Wetland Park in
northern KwaZulu-Natal (Allanson and Baird 1999)]. South Africa has a significant
marine exclusive economic zone (not including the sub-Antarctic islands) of just
over 1 million km
2 , a large proportion of which is on the continental shelf. There are
two main oceanic currents, the warm Agulhas current that runs from the north-east to
the south-west, and the cold nutrient-rich Benguela current that runs northward
along South Africa’s west coast. In this chapter we discuss flooding, but the impact
of environmental factors on freshwater and marine invasions is discussed elsewhere
(Robinson et al. 2020, Chap. 9; Weyl et al. 2020, Chap. 6). Urban ecosystems (and
the particular climatic conditions they represent) are also discussed elsewhere
(Potgieter et al. 2020, Chap. 11).
The key environmental features of South Africa are summarised in Fig. 13.1. The
influence of these conditions on establishment, spread, and the impact of invasives is
summarised in Table 13.1, and discussed in more detail using case-studies in the
sections that follow.
13.2 Geomorphology
South Africa has a fascinating range of landscapes, and, as a result, a complex array
of potential biogeographical barriers. There is substantial variation in elevation—
43% of the country is under 1000 m asl; 55% 1000–2000 m asl; and while only 2% is
over 2000 m asl, the maximum is 3450 m asl. Elevation in itself is unlikely to have a
significant effect on invasions, though due to correlations with remoteness, pathways
of introduction, and the impact of road development, the study of elevational
patterns of invasions and how these change over time has been (and should continue
to be) a valuable topic of applied research in South Africa (e.g. Kalwij et al. 2015).
Relief is sharp in many parts of the country, with “topographic roughness” an
important correlate of naturalised plant species richness (Richardson et al. 2005).
In comparison to the diversity of landscapes, there is relatively little seismic
activity, no volcanoes, and few earthquakes. As in other countries, over-grazing,
fires, and injudicious control of bush-thickening vegetation can result in landslides,
but relative to other countries, landslides are not a major source of disturbance. The
extraction of water through boreholes, the collapse of old mines, and the possibility
of fracking, could lead to an increase in the frequency of disturbances
(e.g. sinkholes), but it seems unlikely that these will provide major opportunities
for invasions.
13 The Role of Environmental Factors in Promoting and Limiting Invasions
361
South Africa’s northern border, contain more than 85% of South Africa’s freshwater.
Most other catchments are relatively small, and there are no large lakes. South Africa
has a 2798 km long coastline, with estuaries making up a small but important
component of South Africa’s ecosystems [with a total of 465 estuaries, including
the largest estuary in Africa, the St Lucia estuary in iSimangaliso Wetland Park in
northern KwaZulu-Natal (Allanson and Baird 1999)]. South Africa has a significant
marine exclusive economic zone (not including the sub-Antarctic islands) of just
over 1 million km
2 , a large proportion of which is on the continental shelf. There are
two main oceanic currents, the warm Agulhas current that runs from the north-east to
the south-west, and the cold nutrient-rich Benguela current that runs northward
along South Africa’s west coast. In this chapter we discuss flooding, but the impact
of environmental factors on freshwater and marine invasions is discussed elsewhere
(Robinson et al. 2020, Chap. 9; Weyl et al. 2020, Chap. 6). Urban ecosystems (and
the particular climatic conditions they represent) are also discussed elsewhere
(Potgieter et al. 2020, Chap. 11).
The key environmental features of South Africa are summarised in Fig. 13.1. The
influence of these conditions on establishment, spread, and the impact of invasives is
summarised in Table 13.1, and discussed in more detail using case-studies in the
sections that follow.
13.2 Geomorphology
South Africa has a fascinating range of landscapes, and, as a result, a complex array
of potential biogeographical barriers. There is substantial variation in elevation—
43% of the country is under 1000 m asl; 55% 1000–2000 m asl; and while only 2% is
over 2000 m asl, the maximum is 3450 m asl. Elevation in itself is unlikely to have a
significant effect on invasions, though due to correlations with remoteness, pathways
of introduction, and the impact of road development, the study of elevational
patterns of invasions and how these change over time has been (and should continue
to be) a valuable topic of applied research in South Africa (e.g. Kalwij et al. 2015).
Relief is sharp in many parts of the country, with “topographic roughness” an
important correlate of naturalised plant species richness (Richardson et al. 2005).
In comparison to the diversity of landscapes, there is relatively little seismic
activity, no volcanoes, and few earthquakes. As in other countries, over-grazing,
fires, and injudicious control of bush-thickening vegetation can result in landslides,
but relative to other countries, landslides are not a major source of disturbance. The
extraction of water through boreholes, the collapse of old mines, and the possibility
of fracking, could lead to an increase in the frequency of disturbances
(e.g. sinkholes), but it seems unlikely that these will provide major opportunities
for invasions.
13 The Role of Environmental Factors in Promoting and Limiting Invasions
361