interaction reassembly pathways (novel associations, co-introduction, and co-xenic)
may differently impact invasion outcomes, but also caution that these effects may be
specific to the system studied. Nevertheless, the diversity of multi-trophic level
networks that incorporate insects in South Africa makes them great model systems
to test invasion hypotheses invoking biotic interactions.
Understanding the role of biotic interactions in mediating invasions is complicated and not a trivial task. The expectation that individual biotic interactions, or
even interaction guilds (e.g. dispersal, pollination, parasitism), can mediate the
outcomes of invasions may be unrealistic under many circumstances. Mollusc
invasions in South Africa exemplify this. Despite the obvious and severe negative
impacts imposed by shell parasites on some of these species, their unabated spread in
South Africa probably reflects the fact that other biotic and abiotic interactions, in
combination with unique life-history traits, aid their invasiveness. On the other hand,
one or two interaction types can have massive, and often unforeseen, consequences
for invasions and their ecological impacts. Predation on breeding seabird colonies by
invasive cats and mice on South Africa’s Marion Island is an example of such
unforeseen impacts on multi-trophic interactions, where a decline in birds led to
altered soil nutrient cycling, in turn, reducing plant cover. Reduced plant cover led to
reduced habitat of other marine animals. This serves as a powerful example of how a
keystone species can be indirectly impacted by the presence of one or two invasive
species, with multiple knock-on effects on native species interactions.
The biodiversity consequences of co-introduction, ecological fitting, and co-xenic
associations, as different pathways for interaction reassembly needs urgent attention,
not only in South Africa, but globally. Using legume-rhizobium associations, Le
Roux et al. (2017) recently hypothesised that the severity and rate of accrual of
impacts will be higher on native plants when invasive plants are co-introduced with
their co-evolved mutualists. Testing these theoretical expectations across various
plant-mutualism types provide exciting future research opportunities. On the other
hand, the prevalence of ecological fitting for some biotic interactions suggest that
certain life-history traits predispose invasive species to infiltrate native ecological
interaction networks. For example, most alien plants are readily integrated into
plant–pollinator and plant-seed disperser networks as generalists. Moreover, high
levels of selfing might explain why non-native plants rarely experience pollen
limitation (van Kleunen et al. 2018). These general trends appear to hold for
South Africa, but there are many opportunities to compare aspects of the reproductive biology of invasive species in their native and introduced ranges. A number of
well-studied plant genera that are invasive in South Africa lend themselves to such
studies, including Acacia, Banksia, Eucalyptus, Hakea, and Melaleuca.
While much research has been focused on biotic interactions during invasion in
South Africa, there are still major gaps in our understanding. In particular, little is
known of the contribution of soil organisms and microbes to South African invasions, but the experiences from elsewhere, e.g. from invasive earthworms altering
forest dynamics (Bohlen et al. 2004) to the widespread loss of biodiversity due to
fungal infections (e.g. Kilpatrick et al. 2010), suggest that we have but scratched the
surface of this fascinating topic (see Janion-Scheepers and Griffiths 2020, Chap. 7).
14 Biotic Interactions as Mediators of Biological Invasions
415
may differently impact invasion outcomes, but also caution that these effects may be
specific to the system studied. Nevertheless, the diversity of multi-trophic level
networks that incorporate insects in South Africa makes them great model systems
to test invasion hypotheses invoking biotic interactions.
Understanding the role of biotic interactions in mediating invasions is complicated and not a trivial task. The expectation that individual biotic interactions, or
even interaction guilds (e.g. dispersal, pollination, parasitism), can mediate the
outcomes of invasions may be unrealistic under many circumstances. Mollusc
invasions in South Africa exemplify this. Despite the obvious and severe negative
impacts imposed by shell parasites on some of these species, their unabated spread in
South Africa probably reflects the fact that other biotic and abiotic interactions, in
combination with unique life-history traits, aid their invasiveness. On the other hand,
one or two interaction types can have massive, and often unforeseen, consequences
for invasions and their ecological impacts. Predation on breeding seabird colonies by
invasive cats and mice on South Africa’s Marion Island is an example of such
unforeseen impacts on multi-trophic interactions, where a decline in birds led to
altered soil nutrient cycling, in turn, reducing plant cover. Reduced plant cover led to
reduced habitat of other marine animals. This serves as a powerful example of how a
keystone species can be indirectly impacted by the presence of one or two invasive
species, with multiple knock-on effects on native species interactions.
The biodiversity consequences of co-introduction, ecological fitting, and co-xenic
associations, as different pathways for interaction reassembly needs urgent attention,
not only in South Africa, but globally. Using legume-rhizobium associations, Le
Roux et al. (2017) recently hypothesised that the severity and rate of accrual of
impacts will be higher on native plants when invasive plants are co-introduced with
their co-evolved mutualists. Testing these theoretical expectations across various
plant-mutualism types provide exciting future research opportunities. On the other
hand, the prevalence of ecological fitting for some biotic interactions suggest that
certain life-history traits predispose invasive species to infiltrate native ecological
interaction networks. For example, most alien plants are readily integrated into
plant–pollinator and plant-seed disperser networks as generalists. Moreover, high
levels of selfing might explain why non-native plants rarely experience pollen
limitation (van Kleunen et al. 2018). These general trends appear to hold for
South Africa, but there are many opportunities to compare aspects of the reproductive biology of invasive species in their native and introduced ranges. A number of
well-studied plant genera that are invasive in South Africa lend themselves to such
studies, including Acacia, Banksia, Eucalyptus, Hakea, and Melaleuca.
While much research has been focused on biotic interactions during invasion in
South Africa, there are still major gaps in our understanding. In particular, little is
known of the contribution of soil organisms and microbes to South African invasions, but the experiences from elsewhere, e.g. from invasive earthworms altering
forest dynamics (Bohlen et al. 2004) to the widespread loss of biodiversity due to
fungal infections (e.g. Kilpatrick et al. 2010), suggest that we have but scratched the
surface of this fascinating topic (see Janion-Scheepers and Griffiths 2020, Chap. 7).
14 Biotic Interactions as Mediators of Biological Invasions
415