benefits mainly managed honeybees, which are used for crop pollination during
spring (de Lange et al. 2013). The impacts of alien pollinators on plant communities
in South Africa remains largely unknown. However, impacts are conceivable. For
example, social bumblebees from the genus Bombus are often used for agricultural
pollination services. These bees are similar to native honeybees in that they are
super-generalists and will be effective pollinators of many plants in South Africa,
including invasive species like Paterson’s Curse (Echium plantagineum). Furthermore, if they were to escape into natural environments, bumblebees are likely to
compete with functionally similar native carpenter bees (Xylocopa spp.), which
could disrupt native plant communities through inter-specific competition (Pauw
2013).
It is now also becoming evident that familiar associations under novel environmental conditions may lead to altered native species interactions. Veldtman et al.
(2011) found that two gall-forming biological control agents released in
South Africa against Acacia longifolia and A. saligna can affect native species
interactions. Galls formed on these two invaders accumulated multi-trophic food
chain links (with South African inquilines, parasitoids, and hyperparasitoids) similar
to those observed in their native Australian range. Theoretically, these novel interactions can lead to apparent competition and losses of native biodiversity if the
biological control agent shares these natural enemies (predators and parasitoids) with
herbivores of native plants, as has been found elsewhere (Carvalheiro et al. 2008).
However, it might be more appropriate to classify these interactions as commensal,
whereby the introduction of the biological control agents has created a resource
allowing for greater population sizes than would otherwise be maintained. Lastly, in
some instances biological control might also facilitate novel co-xenic associations,
such as the important agricultural pest False Codling Moth (Thaumatotibia
leucotreta) utilising the galls formed on A. saligna as a larval food resource in
agricultural ecosystems in South Africa (Seymour and Veldtman 2010). Notably, if
biological control were to provide complete control, such cases are expected to
decline over time.
Oceanic islands often suffer more severe ecological impacts from invasive
species than mainland areas (see Greve et al. 2020, Chap. 8). This can, in part, be
explained by the isolation and evolutionary naivety of island biotas to novel biotic
interactions, e.g. extensive grazing by large herbivores or predation by
mesopredators. South Africa’s sub-Antarctic Marion Island not only provides an
example of this, but also illustrates the complexity and unforeseen outcomes of these
novel interactions. House Mice (Mus musculus) reached Marion Island some time
before 1818 (Watkins and Cooper 1986), and Domestic Cats (Felis catus) were
intentionally introduced in 1951 to control them (Anderson and Condy 1974). The
cat population grew rapidly, with an estimated population size of 2100 by 1970 (van
Aarde 1979). Cats found burrowing seabirds to be easier prey than mice, and in the
mid-1970s they were killing an estimated minimum of 635,000 petrels and prions
each year (van Aarde 1980). This led to decreased breeding success of these birds
and caused the local extinction of one species (Berruti et al. 1981). These impacts on
birds may have also led to changes in soil nutrient fluxes (through bird manuring), in
14 Biotic Interactions as Mediators of Biological Invasions
413
spring (de Lange et al. 2013). The impacts of alien pollinators on plant communities
in South Africa remains largely unknown. However, impacts are conceivable. For
example, social bumblebees from the genus Bombus are often used for agricultural
pollination services. These bees are similar to native honeybees in that they are
super-generalists and will be effective pollinators of many plants in South Africa,
including invasive species like Paterson’s Curse (Echium plantagineum). Furthermore, if they were to escape into natural environments, bumblebees are likely to
compete with functionally similar native carpenter bees (Xylocopa spp.), which
could disrupt native plant communities through inter-specific competition (Pauw
2013).
It is now also becoming evident that familiar associations under novel environmental conditions may lead to altered native species interactions. Veldtman et al.
(2011) found that two gall-forming biological control agents released in
South Africa against Acacia longifolia and A. saligna can affect native species
interactions. Galls formed on these two invaders accumulated multi-trophic food
chain links (with South African inquilines, parasitoids, and hyperparasitoids) similar
to those observed in their native Australian range. Theoretically, these novel interactions can lead to apparent competition and losses of native biodiversity if the
biological control agent shares these natural enemies (predators and parasitoids) with
herbivores of native plants, as has been found elsewhere (Carvalheiro et al. 2008).
However, it might be more appropriate to classify these interactions as commensal,
whereby the introduction of the biological control agents has created a resource
allowing for greater population sizes than would otherwise be maintained. Lastly, in
some instances biological control might also facilitate novel co-xenic associations,
such as the important agricultural pest False Codling Moth (Thaumatotibia
leucotreta) utilising the galls formed on A. saligna as a larval food resource in
agricultural ecosystems in South Africa (Seymour and Veldtman 2010). Notably, if
biological control were to provide complete control, such cases are expected to
decline over time.
Oceanic islands often suffer more severe ecological impacts from invasive
species than mainland areas (see Greve et al. 2020, Chap. 8). This can, in part, be
explained by the isolation and evolutionary naivety of island biotas to novel biotic
interactions, e.g. extensive grazing by large herbivores or predation by
mesopredators. South Africa’s sub-Antarctic Marion Island not only provides an
example of this, but also illustrates the complexity and unforeseen outcomes of these
novel interactions. House Mice (Mus musculus) reached Marion Island some time
before 1818 (Watkins and Cooper 1986), and Domestic Cats (Felis catus) were
intentionally introduced in 1951 to control them (Anderson and Condy 1974). The
cat population grew rapidly, with an estimated population size of 2100 by 1970 (van
Aarde 1979). Cats found burrowing seabirds to be easier prey than mice, and in the
mid-1970s they were killing an estimated minimum of 635,000 petrels and prions
each year (van Aarde 1980). This led to decreased breeding success of these birds
and caused the local extinction of one species (Berruti et al. 1981). These impacts on
birds may have also led to changes in soil nutrient fluxes (through bird manuring), in
14 Biotic Interactions as Mediators of Biological Invasions
413