17.2.3 Impacts Identified During Red-Listing Processes
Some recent studies have used data from the IUCN Red List of Threatened Species
(Mace et al. 2008) to assess the role of alien species as drivers of recent extinctions
(Bellard et al. 2016a), and global patterns of threats to vertebrates posed by biological
invasions (Bellard et al. 2016b). In South Africa, several taxa have undergone formal
red list assessments and these include plants, dragonflies, freshwater fishes, amphibians, reptiles, birds and mammals (Child et al. 2015; Taylor et al. 2015; IUCN 2018;
SANBI 2019). Following a similar approach as Bellard et al. (2016b), we assessed
whether and how species listed under the IUCN Red List in South Africa were
affected by alien species. We calculated the proportion of threatened native species,
i.e. those in the upper tier of extinction risk (Vulnerable, Endangered, and Critically
endangered), where alien species were indicated as a threat (Table 17.3).
A total of 23,609 native species were assessed, of which 17% had alien species as
a major threat to their extinction risk. The proportion of threatened species that are
imperilled by alien species varied across threat categories, being higher for
Endagered (60%, N ¼ 609 out of 1007) and Vulnerable species (48%, N ¼ 788
out of 1641) and lower for Critically Endangered (40%, N ¼ 276 out of 688) (Table
17.3). This trend is also reflected when comparions are made across taxa in each
threat category (Table 17.3). More than half of the taxa assessed as Endangered (five
out of eight), Critically Endangered (three out of five) and Vulnerable (three out of
five) were threatened by alien species. Across all three threat categories, the proportion of species that are being threatened by alien species was highest for fishes,
amphibians and plants. The Red List assessments identified and classified 12 major
threats to the persistence of a species, and alien species were rarely considered to be
the sole extinction risk for most species. For example, most of the fish and amphibian
species listed as Critically Endangered have small distributional ranges, experience a
continuous decline in habitat quality through several anthropogenic activities (e.g.
pollution, excessive water abstraction and altered flow regimes), and are additionally
threatened by invasive species through predation, competition and physical alteration of ecosystems (van Wilgen et al. 2020b, Chap. 29).
17.2.4 Impacts on Biodiversity at a Biome Scale
A limited number of studies have quantifed the impact of invasive species on ecosystem services at a landscape scale. Several studies have either focused on a particular
ecosystem service (e.g. surface water supplies, Le Maitre et al. 2000), or on a single
species (e.g. A. mearnsii, De Wit et al. 2001). This has led to problems in extraplolating
the results generated by the different studies to make broad conclusions about the
entirety of impacts of invasions on an ecosystem. Van Wilgen et al. (2008), however,
assessed current and potential impacts of alien plants on selected ecosystem services
(surface water runoff, ground water recharge, livestock production and biodiversity) in
17 An Evaluation of the Impacts of Alien Species on Biodiversity
503
Some recent studies have used data from the IUCN Red List of Threatened Species
(Mace et al. 2008) to assess the role of alien species as drivers of recent extinctions
(Bellard et al. 2016a), and global patterns of threats to vertebrates posed by biological
invasions (Bellard et al. 2016b). In South Africa, several taxa have undergone formal
red list assessments and these include plants, dragonflies, freshwater fishes, amphibians, reptiles, birds and mammals (Child et al. 2015; Taylor et al. 2015; IUCN 2018;
SANBI 2019). Following a similar approach as Bellard et al. (2016b), we assessed
whether and how species listed under the IUCN Red List in South Africa were
affected by alien species. We calculated the proportion of threatened native species,
i.e. those in the upper tier of extinction risk (Vulnerable, Endangered, and Critically
endangered), where alien species were indicated as a threat (Table 17.3).
A total of 23,609 native species were assessed, of which 17% had alien species as
a major threat to their extinction risk. The proportion of threatened species that are
imperilled by alien species varied across threat categories, being higher for
Endagered (60%, N ¼ 609 out of 1007) and Vulnerable species (48%, N ¼ 788
out of 1641) and lower for Critically Endangered (40%, N ¼ 276 out of 688) (Table
17.3). This trend is also reflected when comparions are made across taxa in each
threat category (Table 17.3). More than half of the taxa assessed as Endangered (five
out of eight), Critically Endangered (three out of five) and Vulnerable (three out of
five) were threatened by alien species. Across all three threat categories, the proportion of species that are being threatened by alien species was highest for fishes,
amphibians and plants. The Red List assessments identified and classified 12 major
threats to the persistence of a species, and alien species were rarely considered to be
the sole extinction risk for most species. For example, most of the fish and amphibian
species listed as Critically Endangered have small distributional ranges, experience a
continuous decline in habitat quality through several anthropogenic activities (e.g.
pollution, excessive water abstraction and altered flow regimes), and are additionally
threatened by invasive species through predation, competition and physical alteration of ecosystems (van Wilgen et al. 2020b, Chap. 29).
17.2.4 Impacts on Biodiversity at a Biome Scale
A limited number of studies have quantifed the impact of invasive species on ecosystem services at a landscape scale. Several studies have either focused on a particular
ecosystem service (e.g. surface water supplies, Le Maitre et al. 2000), or on a single
species (e.g. A. mearnsii, De Wit et al. 2001). This has led to problems in extraplolating
the results generated by the different studies to make broad conclusions about the
entirety of impacts of invasions on an ecosystem. Van Wilgen et al. (2008), however,
assessed current and potential impacts of alien plants on selected ecosystem services
(surface water runoff, ground water recharge, livestock production and biodiversity) in
17 An Evaluation of the Impacts of Alien Species on Biodiversity
503