humans occurred, often leading to notable invasion episodes (Richardson et al.
2003). The last third of the twentieth century saw substantial social transformations
in South Africa, leading to significant changes in human demographics, micro- and
macro-economic climates, and in the country’s role in the global economy. These
factors all continue to influence the relationship between South African societies and
alien species and consequently the vectors and pathways for alien taxa (Richardson
et al. 2003; Le Maitre et al. 2004). Below we discuss key drivers of invasions of alien
plant and vertebrate species in urban areas of South Africa.
11.2.1 Plants
The history of plant introductions has been crucial for driving invasions of alien
plants in many South African cities, and demonstrates in part why invasions in cities
are very different to those in natural or semi-natural areas (where most alien plants
were introduced for purposes other than ornamental horticulture) (Fig. 11.1).
While there are similarities in the process between urban and rural or natural
invasions (e.g. lag phase), the dynamics and characteristics of the receiving environments differ. The high heterogeneity of the urban landscape, altered disturbance
regimes, and increased resource availability associated with concentrated human
activities provide opportunities for the establishment, reproduction and proliferation
of many alien plant species even in marginal sites (Figs. 11.1a and 11.2a). The
horticultural industry has been a particularly important pathway for the introduction
of alien plants to South Africa, and the escape of ornamental plants from cultivation
and gardens has resulted in some of the most extensive biological invasions in the
country (Figs. 11.1b and 11.2b, c; Richardson et al. 2003; Foxcroft et al. 2008;
Geerts et al. 2013, 2017; Holmes et al. 2018).
Invasibility is strongly influenced by propagule pressure—massive propagule
pressure (many large trees) ensures that even suboptimal microsites are invaded
(overcoming abiotic barriers and biotic resistance) (Rejmánek et al. 2005).
Donaldson et al. (2014) show that the number of trees introduced into urban areas
was the most important parameter influencing abundance and extent of invasive
Australian Acacia populations. A survey along the Eerste River in Stellenbosch
found that areas along the river bordered by urban land had the highest numbers of
alien plant species (Fig. 11.2d; Meek et al. 2010). Another notable example is the
invasion of Metrosideros excelsa (New Zealand Christmas Tree) in Betty’s Bay,
Western Cape—where relatively large areas of natural vegetation within the town’s
border are dotted with ‘islands’ of human habitation in the form of single residences
(Figs. 11.1c and 11.2e). In the late 1960s, horticulturists recommended the planting
of M. excelsa as a “safe” replacement hedge plant for the highly invasive
Leptospermum laevigatum (Australian Myrtle). Today, M. excelsa is a serious
invader in and around several coastal towns in the Western Cape (Richardson and
Rejmánek 1998).
The influence of propagule pressure on invasibility is also evident in smaller
settlements. While large cities are usually the first sites of introduction, small human
11 Biological Invasions in South Africa’s Urban Ecosystems
279
2003). The last third of the twentieth century saw substantial social transformations
in South Africa, leading to significant changes in human demographics, micro- and
macro-economic climates, and in the country’s role in the global economy. These
factors all continue to influence the relationship between South African societies and
alien species and consequently the vectors and pathways for alien taxa (Richardson
et al. 2003; Le Maitre et al. 2004). Below we discuss key drivers of invasions of alien
plant and vertebrate species in urban areas of South Africa.
11.2.1 Plants
The history of plant introductions has been crucial for driving invasions of alien
plants in many South African cities, and demonstrates in part why invasions in cities
are very different to those in natural or semi-natural areas (where most alien plants
were introduced for purposes other than ornamental horticulture) (Fig. 11.1).
While there are similarities in the process between urban and rural or natural
invasions (e.g. lag phase), the dynamics and characteristics of the receiving environments differ. The high heterogeneity of the urban landscape, altered disturbance
regimes, and increased resource availability associated with concentrated human
activities provide opportunities for the establishment, reproduction and proliferation
of many alien plant species even in marginal sites (Figs. 11.1a and 11.2a). The
horticultural industry has been a particularly important pathway for the introduction
of alien plants to South Africa, and the escape of ornamental plants from cultivation
and gardens has resulted in some of the most extensive biological invasions in the
country (Figs. 11.1b and 11.2b, c; Richardson et al. 2003; Foxcroft et al. 2008;
Geerts et al. 2013, 2017; Holmes et al. 2018).
Invasibility is strongly influenced by propagule pressure—massive propagule
pressure (many large trees) ensures that even suboptimal microsites are invaded
(overcoming abiotic barriers and biotic resistance) (Rejmánek et al. 2005).
Donaldson et al. (2014) show that the number of trees introduced into urban areas
was the most important parameter influencing abundance and extent of invasive
Australian Acacia populations. A survey along the Eerste River in Stellenbosch
found that areas along the river bordered by urban land had the highest numbers of
alien plant species (Fig. 11.2d; Meek et al. 2010). Another notable example is the
invasion of Metrosideros excelsa (New Zealand Christmas Tree) in Betty’s Bay,
Western Cape—where relatively large areas of natural vegetation within the town’s
border are dotted with ‘islands’ of human habitation in the form of single residences
(Figs. 11.1c and 11.2e). In the late 1960s, horticulturists recommended the planting
of M. excelsa as a “safe” replacement hedge plant for the highly invasive
Leptospermum laevigatum (Australian Myrtle). Today, M. excelsa is a serious
invader in and around several coastal towns in the Western Cape (Richardson and
Rejmánek 1998).
The influence of propagule pressure on invasibility is also evident in smaller
settlements. While large cities are usually the first sites of introduction, small human
11 Biological Invasions in South Africa’s Urban Ecosystems
279