Invasive Prosopis competes with the native Vachellia erioloba (Camel Thorn) for
groundwater, increasing the likelihood of competive exclusion of Camel Thorn trees
when water resources are limiting (Schachtschneider and February 2013).
Other than for the genus Prosopis, there are very few studies that have quantified
the impact of alien plants on biodiversity in South Africa. Richardson et al. (1989)
reviewed published and unpublished data on plant species richness in the Fynbos
Biome with different levels of invasion by alien trees and shrubs in the genera Pinus,
Hakea and Acacia. They found significant declines in native plant species richness at
the scale of the sample quadrats used in their study (4–256 m
2 ), notably when the
cover of alien plants exceeded 50%. Similarly, Yapi et al. (2018) recorded marked
declines in the cover of native grass species with increases in the canopy cover of
alien Acacia mearnsii (Black Wattle) trees in the Eastern Cape. Such declines in the
abundance and richness of native plant species, and associated faunal species, are
likely to take place where any alien plant species becomes dominant. Given that
many alien plant species have recently entered a phase of rapid spread, it can be
expected that these kinds of impacts will increase. For example, Henderson and
Wilson (2017) showed that the number of quarter degree grid cells occupied by alien
plants in South Africa increased by ~50% between 2000 and 2016, with at least nine
species having expanded rapidly over the past decade (see also van Wilgen et al.
2020a, Chap. 21).
Besides trees, some grasses have been shown to affect native biodiversity in
South Africa. In addition to the examples mentioned earlier, Ammophila arenaria
(European Beach Grass) has changed native dune communities (Hertling and Lubke
1999). Avena barbata (Slender Wild Oat) has invaded some lowland fynbos and can
dominate old field habitats (Heelemann et al. 2013), and a recent study on Paspalum
quadrifarium (Tussock Paspalum) showed its ability to affect native species communities (Nkuna 2018).
17.2.2.2 Invertebrates
Many alien invertebrates are major pests of agriculture, but here we focus on taxa
that have negatively impacted native biodiversity. There are 629 alien freshwater and
terrestrial invertebrates that are known to occur outside of captivity in South Africa
(van Wilgen and Wilson 2018). For a majority (74%) of these, their impacts on
biodiversity have not been evaluated. Of the remainder, 25% were assessed using
expert opinion as low-impact species, and less than 2% had major to severe impacts
(Table 17.2). Five species of terrestrial invertebrates that purportedly have major to
severe impacts include Cornu aspersum (Common Garden Snail), Deroceras
invadens (Tramp Slug), Linepithema humile (Argentine Ant), Thebia pisana and
Trogoderma granarium (Khapra Beetle). Linepithema humile has invaded fynbos,
where it displaces native ants. The displacement of native ants is a direct impact on
biodiversity, but the role that native ant species play in the dispersal of the seeds of
fynbos plants has also been disrupted. Bond and Slingsby (1984) found that L.
humile differed from native ants in moving seeds shorter distances, and in failing to
17 An Evaluation of the Impacts of Alien Species on Biodiversity
501
groundwater, increasing the likelihood of competive exclusion of Camel Thorn trees
when water resources are limiting (Schachtschneider and February 2013).
Other than for the genus Prosopis, there are very few studies that have quantified
the impact of alien plants on biodiversity in South Africa. Richardson et al. (1989)
reviewed published and unpublished data on plant species richness in the Fynbos
Biome with different levels of invasion by alien trees and shrubs in the genera Pinus,
Hakea and Acacia. They found significant declines in native plant species richness at
the scale of the sample quadrats used in their study (4–256 m
2 ), notably when the
cover of alien plants exceeded 50%. Similarly, Yapi et al. (2018) recorded marked
declines in the cover of native grass species with increases in the canopy cover of
alien Acacia mearnsii (Black Wattle) trees in the Eastern Cape. Such declines in the
abundance and richness of native plant species, and associated faunal species, are
likely to take place where any alien plant species becomes dominant. Given that
many alien plant species have recently entered a phase of rapid spread, it can be
expected that these kinds of impacts will increase. For example, Henderson and
Wilson (2017) showed that the number of quarter degree grid cells occupied by alien
plants in South Africa increased by ~50% between 2000 and 2016, with at least nine
species having expanded rapidly over the past decade (see also van Wilgen et al.
2020a, Chap. 21).
Besides trees, some grasses have been shown to affect native biodiversity in
South Africa. In addition to the examples mentioned earlier, Ammophila arenaria
(European Beach Grass) has changed native dune communities (Hertling and Lubke
1999). Avena barbata (Slender Wild Oat) has invaded some lowland fynbos and can
dominate old field habitats (Heelemann et al. 2013), and a recent study on Paspalum
quadrifarium (Tussock Paspalum) showed its ability to affect native species communities (Nkuna 2018).
17.2.2.2 Invertebrates
Many alien invertebrates are major pests of agriculture, but here we focus on taxa
that have negatively impacted native biodiversity. There are 629 alien freshwater and
terrestrial invertebrates that are known to occur outside of captivity in South Africa
(van Wilgen and Wilson 2018). For a majority (74%) of these, their impacts on
biodiversity have not been evaluated. Of the remainder, 25% were assessed using
expert opinion as low-impact species, and less than 2% had major to severe impacts
(Table 17.2). Five species of terrestrial invertebrates that purportedly have major to
severe impacts include Cornu aspersum (Common Garden Snail), Deroceras
invadens (Tramp Slug), Linepithema humile (Argentine Ant), Thebia pisana and
Trogoderma granarium (Khapra Beetle). Linepithema humile has invaded fynbos,
where it displaces native ants. The displacement of native ants is a direct impact on
biodiversity, but the role that native ant species play in the dispersal of the seeds of
fynbos plants has also been disrupted. Bond and Slingsby (1984) found that L.
humile differed from native ants in moving seeds shorter distances, and in failing to
17 An Evaluation of the Impacts of Alien Species on Biodiversity
501