Council mainly during 2007 (Kotzé et al., 2010). This assessment focussed on
28 invasive taxa (mainly trees and shrubs) that are the main targets of the Working
for Water programme. The sampling method involved defining homogeneous mapping units, allocating point samples, conducting aerial surveys of those points, and
then extrapolating the point data to the mapping unit. (Kotzé et al. 2019). The
assessment focused on the mesic parts of the country, and excluded a very large
proportion of arid South Africa.
Versfeld et al. (1998) found that invasions were extensive (1.76 million ha) and
had significant impacts (6.7% reduction in the mean annual runoff). The National
Invasive Alien Plant Survey found that invasions by a number of high-impact taxa
(wattles, pines, and especially eucalypts) were far more extensive than previously
thought, and that invasions in the Eastern Cape were far more extensive and denser
than previously estimated.
The most comprehensive and accessible source of field data for the whole country
is the southern African Plant Invaders Atlas (SAPIA; see Henderson 2001 for a fieldguide, and Henderson and Wilson 2017 for a recent update). SAPIA is based on
roadside surveys conducted by Lesley Henderson starting in 1979, and was
formalised in 1994 by incorporating observations from participants (adopting
many of the citizen science elements of the South African Bird Atlas Project and
other such initiatives). As an atlas project, SAPIA is well suited for describing broadscale biogeographical patterns, but it was neither intended nor designed to provide
in-depth estimates of the extent of invasions, the efficacy of management interventions, or abundance. It has provided insights into all these aspects and more. SAPIA
data are often summarised to show the frequencies of naturalised plant taxa in
quarter-degree grid cells (QDGCs), although most data were collected at a finer
resolution. SAPIA (accessed May 2018) contains data on 739 terrestrial naturalised
plant taxa (note: the list is not the same as that in Appendix 3.1) and shows that
naturalised plants have been recorded in 82% of the 1804 QDGCs in South Africa
(Fig. 3.2), with alien plant species richness varying from 1–172 species per QDGC.
SAPIA has been very useful for illustrating the national scale of plant invasions (Nel
et al. 2004; van Wilgen and Wilson 2018), for elucidating broad-scale drivers of
invasions (e.g. Foxcroft et al. 2007; Wilson et al. 2007; Donaldson et al. 2014;
Moodley et al. 2014), and for demonstrating the efficacy of control measures
(including biological control, Henderson and Wilson 2017).
We used SAPIA and data on native plant species richness at the QDGC scale
from the Botanical Database of Southern Africa (BODATSA; accessed December
2018) to compare naturalised and native plant species richness patterns (Fig. 3.2). As
in a previous analysis using data in SAPIA collated up to 2004 (Richardson et al.
2005), naturalised plant species richness is highest in the southwest, eastwards along
the coast and into the north-eastern corner of the country. However, these patterns
are driven by a relatively few widespread species, around a quarter of all naturalised
alien plant taxa in SAPIA occur in only one QDGC, and many at only one or a few
sites (Fig. 3.3). In many cases, this is not due to climatic restrictions, the lack of
detailed surveys, or the limited time to sample potentially invasible habitats, but is
rather an artefact of where species were introduced. Morevoer, most widespread
invasive plant species are still increasing their ranges (Henderson and Wilson 2017).
3 The Biogeography of South African Terrestrial Plant Invasions
79
28 invasive taxa (mainly trees and shrubs) that are the main targets of the Working
for Water programme. The sampling method involved defining homogeneous mapping units, allocating point samples, conducting aerial surveys of those points, and
then extrapolating the point data to the mapping unit. (Kotzé et al. 2019). The
assessment focused on the mesic parts of the country, and excluded a very large
proportion of arid South Africa.
Versfeld et al. (1998) found that invasions were extensive (1.76 million ha) and
had significant impacts (6.7% reduction in the mean annual runoff). The National
Invasive Alien Plant Survey found that invasions by a number of high-impact taxa
(wattles, pines, and especially eucalypts) were far more extensive than previously
thought, and that invasions in the Eastern Cape were far more extensive and denser
than previously estimated.
The most comprehensive and accessible source of field data for the whole country
is the southern African Plant Invaders Atlas (SAPIA; see Henderson 2001 for a fieldguide, and Henderson and Wilson 2017 for a recent update). SAPIA is based on
roadside surveys conducted by Lesley Henderson starting in 1979, and was
formalised in 1994 by incorporating observations from participants (adopting
many of the citizen science elements of the South African Bird Atlas Project and
other such initiatives). As an atlas project, SAPIA is well suited for describing broadscale biogeographical patterns, but it was neither intended nor designed to provide
in-depth estimates of the extent of invasions, the efficacy of management interventions, or abundance. It has provided insights into all these aspects and more. SAPIA
data are often summarised to show the frequencies of naturalised plant taxa in
quarter-degree grid cells (QDGCs), although most data were collected at a finer
resolution. SAPIA (accessed May 2018) contains data on 739 terrestrial naturalised
plant taxa (note: the list is not the same as that in Appendix 3.1) and shows that
naturalised plants have been recorded in 82% of the 1804 QDGCs in South Africa
(Fig. 3.2), with alien plant species richness varying from 1–172 species per QDGC.
SAPIA has been very useful for illustrating the national scale of plant invasions (Nel
et al. 2004; van Wilgen and Wilson 2018), for elucidating broad-scale drivers of
invasions (e.g. Foxcroft et al. 2007; Wilson et al. 2007; Donaldson et al. 2014;
Moodley et al. 2014), and for demonstrating the efficacy of control measures
(including biological control, Henderson and Wilson 2017).
We used SAPIA and data on native plant species richness at the QDGC scale
from the Botanical Database of Southern Africa (BODATSA; accessed December
2018) to compare naturalised and native plant species richness patterns (Fig. 3.2). As
in a previous analysis using data in SAPIA collated up to 2004 (Richardson et al.
2005), naturalised plant species richness is highest in the southwest, eastwards along
the coast and into the north-eastern corner of the country. However, these patterns
are driven by a relatively few widespread species, around a quarter of all naturalised
alien plant taxa in SAPIA occur in only one QDGC, and many at only one or a few
sites (Fig. 3.3). In many cases, this is not due to climatic restrictions, the lack of
detailed surveys, or the limited time to sample potentially invasible habitats, but is
rather an artefact of where species were introduced. Morevoer, most widespread
invasive plant species are still increasing their ranges (Henderson and Wilson 2017).
3 The Biogeography of South African Terrestrial Plant Invasions
79