important. For example, invasive legumes may not cause legume-rhizobium associations of native host plants to collapse but may impact on the identity and effectiveness of the rhizobia they associate with. In Portugal for example, the performance of
Acacia longifolia which was co-introduced with its bradyrhizobia (RodríguezEcheverría 2010) was much higher than it would have been had it relied on
Portuguese bradyrhizobia (Rodríguez-Echeverría et al. 2012). These invasive rhizobial strains may outcompete native strains for associations with native legumes
(e.g. Rodríguez-Echeverría et al. 2012), and may result in a reduction in the
performance of these legumes (Rodríguez-Echeverría et al. 2012). Similar data are
scarce for South Africa. Recently, Warrington et al. (2019) confirmed that invasive
acacias in South Africa associate with a bradyrhizobial strains that were co-introduced from Australia. Le Roux et al. (2018) also found that acacia invasions affect
both the diversity and structure of whole soil rhizobial communities in CFR soils by
lowering diversity and homogenising community structure in invaded compared to
uninvaded soils. They also found that overall acacia-induced changes to soil abiotic
conditions further benefit their invasive performance. These changes may impact
co-occurring native species in a similar way to what has been previously
documented in Portugal. Such impacts may explain Le Roux et al.’s (2016) observations that native CFR legumes and invasive wattles interact with distinct rhizobial
assemblages, most likely due to the phylogenetic distance between these host plant
groups and the co-introduction of acacias and their symbionts (Warrington et al.
2019). Moreover, rhizobia associated with native CFR legumes sampled from
wattle-invaded and uninvaded sites showed strong compositional turnover.
Specialised natives appear unable to persist in wattle-invaded areas, while generalist
natives could persist, but only in association with compositionally different rhizobia.
This South African example illustrates that specialist native legumes may be more
severely impacted by invasive acacias than generalist native congeners. Whether
these perceived impacts by acacias translate into lowered symbiotic effectiveness
(i.e. nitrogen fixation) of native legumes remain unknown.
The legume-rhizobium example above illustrates how invasive species can interrupt mutualistic interactions of native species. A more dramatic example comes from
the disruption of ant mutualist interactions with native myrmecochorous CFR
species. Bond and Slingsby (1984) found that when native ants are outcompeted
and displaced by invasive Argentine Ants, the overall recruitment of
myrmecochorous native plants (Mimetes cucullatus, M. pauciflorus and
Leucospermum glabrum) were severely impacted. Unlike native ants, Argentine
Ants are slow to discover the seeds of these plants, move them over shorter
distances, and do not store them in below-ground nests. This leads to the majority
of seeds being consumed by rodents. This, in turn, translated to a 50-fold reduction
in the seedling emergence of Mimetes cucullatus (Red-crested Pagoda) compared to
areas where no Argentine Ants were present (Bond and Slingsby 1984). It has
subsequently been shown that these impacts can cause shifts in CFR plant community composition, owing to a disproportionate reduction in the densities of largeseeded species that are not being dispersed by Argentine Ants (Christian 2001). Lach
(2007) also found novel mutualistic associations between invasive Argentine Ants
14 Biotic Interactions as Mediators of Biological Invasions
411
Acacia longifolia which was co-introduced with its bradyrhizobia (RodríguezEcheverría 2010) was much higher than it would have been had it relied on
Portuguese bradyrhizobia (Rodríguez-Echeverría et al. 2012). These invasive rhizobial strains may outcompete native strains for associations with native legumes
(e.g. Rodríguez-Echeverría et al. 2012), and may result in a reduction in the
performance of these legumes (Rodríguez-Echeverría et al. 2012). Similar data are
scarce for South Africa. Recently, Warrington et al. (2019) confirmed that invasive
acacias in South Africa associate with a bradyrhizobial strains that were co-introduced from Australia. Le Roux et al. (2018) also found that acacia invasions affect
both the diversity and structure of whole soil rhizobial communities in CFR soils by
lowering diversity and homogenising community structure in invaded compared to
uninvaded soils. They also found that overall acacia-induced changes to soil abiotic
conditions further benefit their invasive performance. These changes may impact
co-occurring native species in a similar way to what has been previously
documented in Portugal. Such impacts may explain Le Roux et al.’s (2016) observations that native CFR legumes and invasive wattles interact with distinct rhizobial
assemblages, most likely due to the phylogenetic distance between these host plant
groups and the co-introduction of acacias and their symbionts (Warrington et al.
2019). Moreover, rhizobia associated with native CFR legumes sampled from
wattle-invaded and uninvaded sites showed strong compositional turnover.
Specialised natives appear unable to persist in wattle-invaded areas, while generalist
natives could persist, but only in association with compositionally different rhizobia.
This South African example illustrates that specialist native legumes may be more
severely impacted by invasive acacias than generalist native congeners. Whether
these perceived impacts by acacias translate into lowered symbiotic effectiveness
(i.e. nitrogen fixation) of native legumes remain unknown.
The legume-rhizobium example above illustrates how invasive species can interrupt mutualistic interactions of native species. A more dramatic example comes from
the disruption of ant mutualist interactions with native myrmecochorous CFR
species. Bond and Slingsby (1984) found that when native ants are outcompeted
and displaced by invasive Argentine Ants, the overall recruitment of
myrmecochorous native plants (Mimetes cucullatus, M. pauciflorus and
Leucospermum glabrum) were severely impacted. Unlike native ants, Argentine
Ants are slow to discover the seeds of these plants, move them over shorter
distances, and do not store them in below-ground nests. This leads to the majority
of seeds being consumed by rodents. This, in turn, translated to a 50-fold reduction
in the seedling emergence of Mimetes cucullatus (Red-crested Pagoda) compared to
areas where no Argentine Ants were present (Bond and Slingsby 1984). It has
subsequently been shown that these impacts can cause shifts in CFR plant community composition, owing to a disproportionate reduction in the densities of largeseeded species that are not being dispersed by Argentine Ants (Christian 2001). Lach
(2007) also found novel mutualistic associations between invasive Argentine Ants
14 Biotic Interactions as Mediators of Biological Invasions
411