While it is possible to provide such broad generalisations, the phases of introduction are taxon-specific. Visser et al. (2017) assessed the pathways of introduction
of 256 alien grass species to South Africa. They found that introduction to supplement forage for livestock was by far the dominant pathway, accounting for 62% of
species introductions. Horticulture and soil and stabilisation were the next most
common reasons for introductions, followed by the categories “food and beverage”
and “raw materials”. The cumulative number of alien grass species in South Africa
has increased steadily since the early 1800s and shows no signs of slowing (Visser
et al. 2017). As in other parts of the world, new pasture taxa (including species,
subspecies, varieties, cultivars, and new plant-endophyte combinations) are increasingly being introduced to South Africa (Driscoll et al. 2014). Although many of the
grass species involved are already in the country, the novel genetic material and
endophyte variations are changing the risk of such introductions producing invasions
with major impacts.
A detailed assessment of the history of introduction of bamboo species (Poaceae
subfamily Bambusoideae) to South Africa revealed five main phases of introduction
and dissemination. These were associated with (1) intra-African migration of people;
(2) the arrival of Europeans; (3) growth of the agricultural and forestry sectors;
(4) small-scale domestic use by landowners; and (5) the rise of the “green economy”
(Canavan et al. 2019). Each phase created new opportunities for particular uses of
bamboo species.
By contrast, there have been only two main phases of Cactaceae introductions.
Initial introductions of a few species for agriculture in the nineteenth century (for
food, cochineal, and as barrier plants); and in the last few decades the introduction of
many species for ornamental horticulture (Kaplan et al. 2017; Novoa et al. 2017).
Interestingly, due to correlations between growth forms, life-history traits and
usages, most cactus species suitable for agriculture are invasive whereas many of
the taxa widely used in horticulture pose minimal risk (Novoa et al. 2015).
This link between reasons for introduction and invasiveness is particularly interesting. The role of forestry in launching and sustaining invasions is well-established
(Richardson 1998; Rouget et al. 2002; van Wilgen and Richardson 2012; Donaldson
et al. 2014; McConnachie et al. 2015). Many non-woody invasive plants were also
introduced, mainly for ornamental horticulture, and the configuration and persistence of plantings has left a strong imprint on invasion patterns (e.g. Foxcroft et al.
2008). Wilson et al. (2007) assessed the spread rates of 62 alien plant species in
South Africa, and found that species planted as ornamentals had spread faster than
those used for other purposes. In a related analysis, Thuiller et al. (2006) found that
the spatial pattern of invasive plants in South Africa was driven by, among other
factors, human uses. Many widespread invaders were accidentally introduced and
disseminated; important examples are Chromolaena odorata (Triffid Weed), Datura
innoxia (Downy Thorn Apple), Tagetes minuta (Khaki Bush) and Xanthium
spinosum (Spiny Cocklebur). The current extent and patterns of alien plant invasions
are due to interactions between species traits, environmental features, residence time,
and the ways in which reasons for introduction have facilitated spread within the
3 The Biogeography of South African Terrestrial Plant Invasions
71
of 256 alien grass species to South Africa. They found that introduction to supplement forage for livestock was by far the dominant pathway, accounting for 62% of
species introductions. Horticulture and soil and stabilisation were the next most
common reasons for introductions, followed by the categories “food and beverage”
and “raw materials”. The cumulative number of alien grass species in South Africa
has increased steadily since the early 1800s and shows no signs of slowing (Visser
et al. 2017). As in other parts of the world, new pasture taxa (including species,
subspecies, varieties, cultivars, and new plant-endophyte combinations) are increasingly being introduced to South Africa (Driscoll et al. 2014). Although many of the
grass species involved are already in the country, the novel genetic material and
endophyte variations are changing the risk of such introductions producing invasions
with major impacts.
A detailed assessment of the history of introduction of bamboo species (Poaceae
subfamily Bambusoideae) to South Africa revealed five main phases of introduction
and dissemination. These were associated with (1) intra-African migration of people;
(2) the arrival of Europeans; (3) growth of the agricultural and forestry sectors;
(4) small-scale domestic use by landowners; and (5) the rise of the “green economy”
(Canavan et al. 2019). Each phase created new opportunities for particular uses of
bamboo species.
By contrast, there have been only two main phases of Cactaceae introductions.
Initial introductions of a few species for agriculture in the nineteenth century (for
food, cochineal, and as barrier plants); and in the last few decades the introduction of
many species for ornamental horticulture (Kaplan et al. 2017; Novoa et al. 2017).
Interestingly, due to correlations between growth forms, life-history traits and
usages, most cactus species suitable for agriculture are invasive whereas many of
the taxa widely used in horticulture pose minimal risk (Novoa et al. 2015).
This link between reasons for introduction and invasiveness is particularly interesting. The role of forestry in launching and sustaining invasions is well-established
(Richardson 1998; Rouget et al. 2002; van Wilgen and Richardson 2012; Donaldson
et al. 2014; McConnachie et al. 2015). Many non-woody invasive plants were also
introduced, mainly for ornamental horticulture, and the configuration and persistence of plantings has left a strong imprint on invasion patterns (e.g. Foxcroft et al.
2008). Wilson et al. (2007) assessed the spread rates of 62 alien plant species in
South Africa, and found that species planted as ornamentals had spread faster than
those used for other purposes. In a related analysis, Thuiller et al. (2006) found that
the spatial pattern of invasive plants in South Africa was driven by, among other
factors, human uses. Many widespread invaders were accidentally introduced and
disseminated; important examples are Chromolaena odorata (Triffid Weed), Datura
innoxia (Downy Thorn Apple), Tagetes minuta (Khaki Bush) and Xanthium
spinosum (Spiny Cocklebur). The current extent and patterns of alien plant invasions
are due to interactions between species traits, environmental features, residence time,
and the ways in which reasons for introduction have facilitated spread within the
3 The Biogeography of South African Terrestrial Plant Invasions
71