growing global effort to assess all alien species with standardised protocols to
alleviate the problem of comparing impacts measured using different approaches.
Formal assessments have been done for a few alien species in South Africa, but most
naturalised and invasive species have not been evaluated, and, we suspect, for most
alien species there has been no attempt, as yet, to document their impacts. However,
red-listing processes found that alien species were frequently included as a significant extinction risk for several native species of fish, amphibians, and plants. There
are very few studies that cover the combined impacts of co-occurring alien species in
particular areas, and these studies could provide the rationale for regulation and
management, which is often absent. While reductions due to alien species in the
value of ecosystem services, the productivity of rangelands, and biodiversity intactness are relatively low at present these impacts are expected to grow rapidly as more
invasive species enter a stage of exponential growth.
17.1 Introduction
South Africa occupies only 2% of the world’s land area but it is one of the most
biologically diverse countries globally (Mittermeier et al. 1997). For example, it is
estimated that 7% of the world’s vascular plants, 5% of mammals and 7% of birds
are found in South Africa (Le Roux 2002). In addition, more than half of the plants,
butterflies, amphibians and reptiles native to South Africa are endemic (Colville et
al. 2014). This is partly because of the country’s diverse environmental conditons
that have resulted not only in high species diversity and endemism, but also in a
diversity of terrestrial, freshwater and marine ecosystems (van Wilgen et al. 2020a,
Chap. 1; Wilson et al. 2020, Chap. 13). The country’s terrestrial vegetation types can
be broadly grouped into nine biomes that range from deserts and Mediterraneanclimate shrublands (fynbos) in the west to grasslands and savanna in the eastern
interior, and evergreen forests in high rainfall areas along the east coast (Mucina and
Rutherford 2006). Three areas within these biomes—the Cape Floristic Region, the
Succulent Karoo, and the Maputaland-Pondoland-Albany region—have been designated global biodiversity hotspots (Myers et al. 2000; Mittermeier et al. 2004). The
marine realm includes several ecosystems in the surrounding Indian and Atlantic
oceans and offshore islands (van Wilgen et al. 2020a, Chap. 1). The marine
ecosystems are also diverse with over 12,000 species that represent 15% of all
known coastal marine species worldwide (Le Roux 2002; Griffiths et al. 2010).
Alien species can change the composition, structure and functioning of biodiversity in many ways. Examples include hybridisation with native species (e.g. Tracey
et al. 2008), extirpation of native species through predation (e.g. Rodda and Fritts
1992) or disease transmission (Hulme 2014), alterations to the structure of vegetation by introducing novel growth forms (van Wilgen and Richardson 1985), and
changes to ecosystem processes like fire (Brooks et al. 2004) and hydrology (Le
Maitre et al. 2015). A recent study concluded, based on data from the IUCN Red
List, that alien species were the most common threat associated with extinctions of
490
T. A. Zengeya et al.
alleviate the problem of comparing impacts measured using different approaches.
Formal assessments have been done for a few alien species in South Africa, but most
naturalised and invasive species have not been evaluated, and, we suspect, for most
alien species there has been no attempt, as yet, to document their impacts. However,
red-listing processes found that alien species were frequently included as a significant extinction risk for several native species of fish, amphibians, and plants. There
are very few studies that cover the combined impacts of co-occurring alien species in
particular areas, and these studies could provide the rationale for regulation and
management, which is often absent. While reductions due to alien species in the
value of ecosystem services, the productivity of rangelands, and biodiversity intactness are relatively low at present these impacts are expected to grow rapidly as more
invasive species enter a stage of exponential growth.
17.1 Introduction
South Africa occupies only 2% of the world’s land area but it is one of the most
biologically diverse countries globally (Mittermeier et al. 1997). For example, it is
estimated that 7% of the world’s vascular plants, 5% of mammals and 7% of birds
are found in South Africa (Le Roux 2002). In addition, more than half of the plants,
butterflies, amphibians and reptiles native to South Africa are endemic (Colville et
al. 2014). This is partly because of the country’s diverse environmental conditons
that have resulted not only in high species diversity and endemism, but also in a
diversity of terrestrial, freshwater and marine ecosystems (van Wilgen et al. 2020a,
Chap. 1; Wilson et al. 2020, Chap. 13). The country’s terrestrial vegetation types can
be broadly grouped into nine biomes that range from deserts and Mediterraneanclimate shrublands (fynbos) in the west to grasslands and savanna in the eastern
interior, and evergreen forests in high rainfall areas along the east coast (Mucina and
Rutherford 2006). Three areas within these biomes—the Cape Floristic Region, the
Succulent Karoo, and the Maputaland-Pondoland-Albany region—have been designated global biodiversity hotspots (Myers et al. 2000; Mittermeier et al. 2004). The
marine realm includes several ecosystems in the surrounding Indian and Atlantic
oceans and offshore islands (van Wilgen et al. 2020a, Chap. 1). The marine
ecosystems are also diverse with over 12,000 species that represent 15% of all
known coastal marine species worldwide (Le Roux 2002; Griffiths et al. 2010).
Alien species can change the composition, structure and functioning of biodiversity in many ways. Examples include hybridisation with native species (e.g. Tracey
et al. 2008), extirpation of native species through predation (e.g. Rodda and Fritts
1992) or disease transmission (Hulme 2014), alterations to the structure of vegetation by introducing novel growth forms (van Wilgen and Richardson 1985), and
changes to ecosystem processes like fire (Brooks et al. 2004) and hydrology (Le
Maitre et al. 2015). A recent study concluded, based on data from the IUCN Red
List, that alien species were the most common threat associated with extinctions of
490
T. A. Zengeya et al.