327 plant taxa, most of which are invasive, are listed in national legislation. We
collated records of 759 plant taxa in 126 families and 418 genera that have
naturalised in natural and semi-natural ecosystems. Over half of these naturalised
taxa are trees or shrubs, just under a tenth are in the families Fabaceae (73 taxa) and
Asteraceae (64); genera with the most species are Eucalyptus, Acacia, and Opuntia.
The southern African Plant Invaders Atlas (SAPIA) provides the best data for
assessing the extent of invasions at the national scale. SAPIA data show that
naturalised plants occur in 83% of quarter-degree grid cells in the country. While
SAPIA data highlight general distribution patterns (high alien plant species richness
in areas with high native plant species richness and around the main human
settlements), an accurate, repeatable method for estimating the area invaded by
plants is lacking. Introductions and dissemination of alien plants over more than
three centuries, and invasions over at least 120 years (and especially in the last
50 years) have shaped the distribution of alien plants in South Africa. Distribution
patterns of naturalised and invasive plants define four ecologically-meaningful
clusters or “alien plant species assemblage zones”, each with signature alien plant
taxa for which trait-environment interactions can be postulated as strong determinants of success. Some widespread invasive taxa occur in high frequencies across
multiple zones; these taxa occur mainly in riparian zones and other azonal habitats,
or depend on human-mediated disturbance, which weakens or overcomes the factors
that determine specificity to any biogeographical region.
3.1 Introduction
South Africa has a rich diversity of environmental conditions, biota, and a unique
socio-political situation. This makes it a fascinating place to explore the many
interacting factors that have mediated the introduction and dissemination of particular plant species, and their interactions with resident biota and prevailing environmental conditions that determine their performance as alien species (Richardson
et al. 1997, 2011a; Le Roux et al. 2020, Chap. 14; van Wilgen et al. 2020a, Chap. 1;
Wilson et al. 2020, Chap. 13). Terrestrial ecosystems in South Africa have been
invaded by hundreds of alien plant species. Some of these have very large adventive
ranges, and some of these have transformed invaded ecosystems. These invasions
pose a major threat to the country’s biodiversity, impact negatively on the capacity
of ecosystems to deliver goods and services, and in some cases severely threaten
human livelihoods (Richardson and van Wilgen 2004; Le Maitre et al. 2020, Chap.
15; O’Connor and van Wilgen 2020, Chap. 16; Potgieter et al. 2020, Chap. 11;
Zengeya et al. 2020, Chap. 17).
This chapter focusses on the biogeography of terrestrial plant invasions in the
country. It: (1) presents a brief history of alien plant invasions; (2) summarises
information on which alien plants are naturalised and invasive; (3) reviews the extent
68
D. M. Richardson et al.
collated records of 759 plant taxa in 126 families and 418 genera that have
naturalised in natural and semi-natural ecosystems. Over half of these naturalised
taxa are trees or shrubs, just under a tenth are in the families Fabaceae (73 taxa) and
Asteraceae (64); genera with the most species are Eucalyptus, Acacia, and Opuntia.
The southern African Plant Invaders Atlas (SAPIA) provides the best data for
assessing the extent of invasions at the national scale. SAPIA data show that
naturalised plants occur in 83% of quarter-degree grid cells in the country. While
SAPIA data highlight general distribution patterns (high alien plant species richness
in areas with high native plant species richness and around the main human
settlements), an accurate, repeatable method for estimating the area invaded by
plants is lacking. Introductions and dissemination of alien plants over more than
three centuries, and invasions over at least 120 years (and especially in the last
50 years) have shaped the distribution of alien plants in South Africa. Distribution
patterns of naturalised and invasive plants define four ecologically-meaningful
clusters or “alien plant species assemblage zones”, each with signature alien plant
taxa for which trait-environment interactions can be postulated as strong determinants of success. Some widespread invasive taxa occur in high frequencies across
multiple zones; these taxa occur mainly in riparian zones and other azonal habitats,
or depend on human-mediated disturbance, which weakens or overcomes the factors
that determine specificity to any biogeographical region.
3.1 Introduction
South Africa has a rich diversity of environmental conditions, biota, and a unique
socio-political situation. This makes it a fascinating place to explore the many
interacting factors that have mediated the introduction and dissemination of particular plant species, and their interactions with resident biota and prevailing environmental conditions that determine their performance as alien species (Richardson
et al. 1997, 2011a; Le Roux et al. 2020, Chap. 14; van Wilgen et al. 2020a, Chap. 1;
Wilson et al. 2020, Chap. 13). Terrestrial ecosystems in South Africa have been
invaded by hundreds of alien plant species. Some of these have very large adventive
ranges, and some of these have transformed invaded ecosystems. These invasions
pose a major threat to the country’s biodiversity, impact negatively on the capacity
of ecosystems to deliver goods and services, and in some cases severely threaten
human livelihoods (Richardson and van Wilgen 2004; Le Maitre et al. 2020, Chap.
15; O’Connor and van Wilgen 2020, Chap. 16; Potgieter et al. 2020, Chap. 11;
Zengeya et al. 2020, Chap. 17).
This chapter focusses on the biogeography of terrestrial plant invasions in the
country. It: (1) presents a brief history of alien plant invasions; (2) summarises
information on which alien plants are naturalised and invasive; (3) reviews the extent
68
D. M. Richardson et al.