This indicates both that South Africa has a substantial invasion debt (Rouget
et al. 2016), and that there are many opportunities for pro-active management
(i.e. incursion response, Wilson et al. 2013, 2017b; van Wilgen et al. 2020b,
Chap. 21). For example, Richardson et al. (2015) produced a graph similar to
Fig. 3.3, but only for Australian wattles. Four of the six most widespread invasive
wattle species had been introduced for forestry; species introduced for dune
stabilisation and as ornamentals had intermediate distributions. Species only found
in a few QDGCs had only ever been planted in experimental trials at one or a few
sites. Clearing such experimental plantings will likely go a long way to reducing the
risk of future invasions (Wilson et al. 2013).
Despite the strong long-lasting signal of introduction effort and the likely
dynamic nature of the extent of invasions, patterns of species richness at the scale
Fig. 3.2 Species richness of (a) native and (b) naturalised plants in quarter-degree grid cells in
South Africa. Data for native species are from the Botanical Database of Southern Africa (accessed
3 December 2018) and data for alien species are from the Southern African Plant Invaders Atlas
(accessed May 2018). (c) Shows the relationship between native and naturalised plant species
richness [log(naturalised richness) = 0.45 x log(native richness); p-value <2e-16; Pearson R = 0.60]
and (d) shows residuals (cells shaded in blue have fewer alien species than predicted from native
species richness; red shading denotes higher alien richness than expected)
80
D. M. Richardson et al.
et al. 2016), and that there are many opportunities for pro-active management
(i.e. incursion response, Wilson et al. 2013, 2017b; van Wilgen et al. 2020b,
Chap. 21). For example, Richardson et al. (2015) produced a graph similar to
Fig. 3.3, but only for Australian wattles. Four of the six most widespread invasive
wattle species had been introduced for forestry; species introduced for dune
stabilisation and as ornamentals had intermediate distributions. Species only found
in a few QDGCs had only ever been planted in experimental trials at one or a few
sites. Clearing such experimental plantings will likely go a long way to reducing the
risk of future invasions (Wilson et al. 2013).
Despite the strong long-lasting signal of introduction effort and the likely
dynamic nature of the extent of invasions, patterns of species richness at the scale
Fig. 3.2 Species richness of (a) native and (b) naturalised plants in quarter-degree grid cells in
South Africa. Data for native species are from the Botanical Database of Southern Africa (accessed
3 December 2018) and data for alien species are from the Southern African Plant Invaders Atlas
(accessed May 2018). (c) Shows the relationship between native and naturalised plant species
richness [log(naturalised richness) = 0.45 x log(native richness); p-value <2e-16; Pearson R = 0.60]
and (d) shows residuals (cells shaded in blue have fewer alien species than predicted from native
species richness; red shading denotes higher alien richness than expected)
80
D. M. Richardson et al.