severely affected seabird populations, especially burrowing species such as petrels
(Procellariidae), leading to decreased breeding success, population declines and the
local extinction of at least one species (van Rensburg 1983; Bester et al. 2002). Felis
catus was eventually eradicated from the island in 1991 (Bester et al. 2002; Davies et
al. 2020, Chap. 22), but the population of summer-breeding burrowing petrels is still
to recover (Cerfonteyn and Ryan 2016).
Rattus norvegicus, R. rattus, S. scrofa and C. hircus have all caused massive
impacts in other alien ranges (Hagen and Kumschick 2018), but their impacts in
South Africa have not been studied apart from a few unpublished reports on wild
boar impacts on vegetation and soil erosion (as mentioned in Spear and Chown
2009). The two rat species were unintentionally introduced into South Africa
through the shipping trade (Long 2003). These two species are amongst the most
invasive Rattus species with a global distribution in urban areas (Aplin et al. 2011).
Rattus rattus is believed to be widely distributed but restricted by the drier parts of
South Africa, while R. norvegicus is assumed to be limited to coastal areas of the
country (De Graaf 1981). The two rat species are widely regarded as pests; in
South Africa, specifically, they damage infrastructure, contaminate foodstuffs, and
act as reservoirs of zoonotic diseases (e.g. Jassat et al. 2013; Julius et al. 2018;
Potgieter et al. 2020, Chap. 11). Sus scrofa damages some critically endangered
plants in the Western Cape, affecting succession and facilitating alien plant spread
(Picker and Griffiths 2011). Capra hircus grazing has reduced the cover and density
of endemic geophytes and succulents shrubs in thicket vegetation, and conservation
of this endemic-rich flora is seriously threatened (Moolman and Cowling 1994).
17.2.2 Expert Opinion Assessments
Given that few taxa have been formally evaluated for the impacts using EICAT in
South Africa, a classification based on expert opinion rather than on published
literature was used as an interim measure in South Africa’s first national status
report on biological invasions (van Wilgen and Wilson 2018). Here, experts scored
species according to their perceived ecological impacts and their socio-economic
impacts (separately for negative and positive impacts) (see Zengeya et al. 2017 for
more details). Using this scheme, 25 species were assessed as having severe impacts,
and 82 as having major impacts (Table 17.2), the majority of these being terrestrial
and freshwater plants (80 species). Here, using selected examples we highlight
impacts on biodiversity of some of these alien species.
17.2.2.1 Plants
There are 893 alien plants species that are known to occur outside of cultivation in
South Africa (van Wilgen and Wilson 2018). For a majority (56%) of these plant
species, their impact on biodiversity has not been evaluated (Table 17.2). Of the 379
17 An Evaluation of the Impacts of Alien Species on Biodiversity
499
(Procellariidae), leading to decreased breeding success, population declines and the
local extinction of at least one species (van Rensburg 1983; Bester et al. 2002). Felis
catus was eventually eradicated from the island in 1991 (Bester et al. 2002; Davies et
al. 2020, Chap. 22), but the population of summer-breeding burrowing petrels is still
to recover (Cerfonteyn and Ryan 2016).
Rattus norvegicus, R. rattus, S. scrofa and C. hircus have all caused massive
impacts in other alien ranges (Hagen and Kumschick 2018), but their impacts in
South Africa have not been studied apart from a few unpublished reports on wild
boar impacts on vegetation and soil erosion (as mentioned in Spear and Chown
2009). The two rat species were unintentionally introduced into South Africa
through the shipping trade (Long 2003). These two species are amongst the most
invasive Rattus species with a global distribution in urban areas (Aplin et al. 2011).
Rattus rattus is believed to be widely distributed but restricted by the drier parts of
South Africa, while R. norvegicus is assumed to be limited to coastal areas of the
country (De Graaf 1981). The two rat species are widely regarded as pests; in
South Africa, specifically, they damage infrastructure, contaminate foodstuffs, and
act as reservoirs of zoonotic diseases (e.g. Jassat et al. 2013; Julius et al. 2018;
Potgieter et al. 2020, Chap. 11). Sus scrofa damages some critically endangered
plants in the Western Cape, affecting succession and facilitating alien plant spread
(Picker and Griffiths 2011). Capra hircus grazing has reduced the cover and density
of endemic geophytes and succulents shrubs in thicket vegetation, and conservation
of this endemic-rich flora is seriously threatened (Moolman and Cowling 1994).
17.2.2 Expert Opinion Assessments
Given that few taxa have been formally evaluated for the impacts using EICAT in
South Africa, a classification based on expert opinion rather than on published
literature was used as an interim measure in South Africa’s first national status
report on biological invasions (van Wilgen and Wilson 2018). Here, experts scored
species according to their perceived ecological impacts and their socio-economic
impacts (separately for negative and positive impacts) (see Zengeya et al. 2017 for
more details). Using this scheme, 25 species were assessed as having severe impacts,
and 82 as having major impacts (Table 17.2), the majority of these being terrestrial
and freshwater plants (80 species). Here, using selected examples we highlight
impacts on biodiversity of some of these alien species.
17.2.2.1 Plants
There are 893 alien plants species that are known to occur outside of cultivation in
South Africa (van Wilgen and Wilson 2018). For a majority (56%) of these plant
species, their impact on biodiversity has not been evaluated (Table 17.2). Of the 379
17 An Evaluation of the Impacts of Alien Species on Biodiversity
499