recent times. Such traits are also associated with reproductive success and efficient
dispersal and allow species to establish and spread into new environments (Aronson
et al. 2007; Moodley et al. 2013). For example, following a lag phase of several
decades, several paperbark (Melaleuca spp.) species (introduced for ornamental
purposes) are now emerging as invasive (Jacobs et al. 2017). This trend in human
preference for particular plant traits has led to an increase in the proportion of
invasive alien trees and shrubs in many urban areas due to the spread of species
introduced for ornamentation (Potgieter et al. 2017). Indeed, some invasive plant
species are still available in nurseries around the country (Cronin et al. 2017).
Many alien plant species also hybridise with other alien and native congeners
introduced by horticulturalists. This process can compromise the genetic integrity of
native taxa and/or enhance the invasive ability of their hybrid offspring (Williamson
and Fitter 1996). An example in South Africa is the genus Celtis, where the
introduced C. sinensis (Chinese Hackberry) hybridises with the native C. africana
(White Stinkwood; Siebert et al. 2018). Invasive populations of Celtis species in
South Africa are almost certainly hybrids (Milton et al. 2007). Problems with
identification have exacerbated invasions in some cases. For example, C. sinensis
is often incorrectly identified, labelled, sold and disseminated as the native
C. africana (Siebert et al. 2018).
11.2.2 Vertebrates
Aside from domestic pets and agricultural livestock, most alien terrestrial vertebrates
in South Africa were introduced as novelties for private collections, for game
viewing, or for hunting (Richardson et al. 2003; van Rensburg et al. 2011; Measey
et al. 2020, Chap. 5). Trading in animals between landowners provides opportunities
for invasion, or for genetic contamination of native species through hybridisation
(Spear and Chown 2009a, b). Most alien bird taxa that arrived in South Africa (apart
from those intentionally introduced by C.J. Rhodes between 1853 and 1902, and the
House Crow Corvus splendens), appear to have been imported for aviaries.
Few alien mammals have been intentionally introduced into urban South Africa.
The Grey Squirrel (Sciurus carolinensis) which was introduced to the Cape Peninsula by C.J. Rhodes as part of his programme to “improve” the amenities at the Cape
(Brooke et al. 1986; Picker and Griffiths 2017). This species has persisted in urban
environments and spread in areas where alien pines and oaks occur, but cannot
colonise widely separated patches of these trees, except with assistance of humans
(Smithers 1983). The Black Rat (Rattus rattus) was probably introduced accidentally
by Arab traders moving down the east coast, with subsequent additional undocumented arrivals (Brooke et al. 1986). The Brown Rat (R. norvegicus) and the House
Mouse (Mus musculus) arrived as stowaways on ships from Europe, while the Asian
House Rat (Rattus tanezumi) from South-East Asia was first recorded in
South Africa in 2005 (Bastos et al. 2011). Some naturalised populations of alien
mammals originated from escapees from zoological collections (e.g. Himalayan
11 Biological Invasions in South Africa’s Urban Ecosystems
283
dispersal and allow species to establish and spread into new environments (Aronson
et al. 2007; Moodley et al. 2013). For example, following a lag phase of several
decades, several paperbark (Melaleuca spp.) species (introduced for ornamental
purposes) are now emerging as invasive (Jacobs et al. 2017). This trend in human
preference for particular plant traits has led to an increase in the proportion of
invasive alien trees and shrubs in many urban areas due to the spread of species
introduced for ornamentation (Potgieter et al. 2017). Indeed, some invasive plant
species are still available in nurseries around the country (Cronin et al. 2017).
Many alien plant species also hybridise with other alien and native congeners
introduced by horticulturalists. This process can compromise the genetic integrity of
native taxa and/or enhance the invasive ability of their hybrid offspring (Williamson
and Fitter 1996). An example in South Africa is the genus Celtis, where the
introduced C. sinensis (Chinese Hackberry) hybridises with the native C. africana
(White Stinkwood; Siebert et al. 2018). Invasive populations of Celtis species in
South Africa are almost certainly hybrids (Milton et al. 2007). Problems with
identification have exacerbated invasions in some cases. For example, C. sinensis
is often incorrectly identified, labelled, sold and disseminated as the native
C. africana (Siebert et al. 2018).
11.2.2 Vertebrates
Aside from domestic pets and agricultural livestock, most alien terrestrial vertebrates
in South Africa were introduced as novelties for private collections, for game
viewing, or for hunting (Richardson et al. 2003; van Rensburg et al. 2011; Measey
et al. 2020, Chap. 5). Trading in animals between landowners provides opportunities
for invasion, or for genetic contamination of native species through hybridisation
(Spear and Chown 2009a, b). Most alien bird taxa that arrived in South Africa (apart
from those intentionally introduced by C.J. Rhodes between 1853 and 1902, and the
House Crow Corvus splendens), appear to have been imported for aviaries.
Few alien mammals have been intentionally introduced into urban South Africa.
The Grey Squirrel (Sciurus carolinensis) which was introduced to the Cape Peninsula by C.J. Rhodes as part of his programme to “improve” the amenities at the Cape
(Brooke et al. 1986; Picker and Griffiths 2017). This species has persisted in urban
environments and spread in areas where alien pines and oaks occur, but cannot
colonise widely separated patches of these trees, except with assistance of humans
(Smithers 1983). The Black Rat (Rattus rattus) was probably introduced accidentally
by Arab traders moving down the east coast, with subsequent additional undocumented arrivals (Brooke et al. 1986). The Brown Rat (R. norvegicus) and the House
Mouse (Mus musculus) arrived as stowaways on ships from Europe, while the Asian
House Rat (Rattus tanezumi) from South-East Asia was first recorded in
South Africa in 2005 (Bastos et al. 2011). Some naturalised populations of alien
mammals originated from escapees from zoological collections (e.g. Himalayan
11 Biological Invasions in South Africa’s Urban Ecosystems
283