Bruchid Beetle, Acanthoscelides macrophthalmus, a biological control agent introduced to South Africa in 1999 against the River Tamarind shrub, Leucaena
leucocephala (Fabaceae) (Olckers 2011; Sharrat and Olckers 2012; Ramanand and
Olckers 2013). Native bruchinid beetles that target native mimosoid Fabaceae in
South Africa are also known to host native parasitoid communities (Impson et al.
1999), thus parasitism of the introduced host by native parasitoids may reflect
parasitoid eco-evolutionary experience to mimosoid-associated beetles. However,
it should be cautioned that, in general, there is insufficient knowledge and quantification of the extent of shared parasitoids between alien and native hosts and of foodweb interactions in these systems to confidently invoke eco-evolutionary processes.
It is clear, however, that biotic resistance has played a major role in limiting the
ability of some candidate biological control agents from establishing populations in
South Africa (see Hill et al. 2020, Chap. 19).
14.3 Mutualism
14.3.1 Plants and Soil Bacteria
Mutualisms can play key roles in mediating not only the establishment success of
alien species (Richardson et al. 2000a), but also their ecological impacts once they
become invasive (Traveset and Richardson 2006, 2011). Legumes (family Fabaceae)
are over-represented in regional invasive floras in many parts of the world (Pyšek
et al. 2017), and 73 legume species are naturalised in South Africa (Richardson et al.
2020, Chap. 3, Sect. 3.3). The widespread success of legumes as invasive species has
been attributed partly to their ability to form symbioses with soil bacteria known as
rhizobia (Parker 2001). Rhizobia are bacteria capable of forming specialised structures called root nodules on the roots of most legumes. Rhizobia fix atmospheric
nitrogen into ammonium that legumes can utilise. In return, legumes provide
rhizobia with various sources of carbon through photosynthate. This symbiosis
allows legumes to colonise nutrient-poor environments and often impacts these
environments through nitrogen enrichment of soils (Parker 2001; Yelenik et al.
2004).
Invasive Australian wattles in South Africa’s Cape Floristic Region (CFR), a
global biodiversity hotspot, are a good study system for exploring how interaction
reassembly pathway (i.e. co-introduction vs. ecological fitting vs. co-xenic) and
interaction specialisation affect the way in which invaders infiltrate ecological
networks (Fig. 14.2) and their subsequent impacts on native species. Molecular
evidence has revealed that invasive wattles in the CFR have often been
co-introduced with their rhizobia, primarily from the genus Bradyrhizobium
(Fig. 14.3b; Ndlovu et al. 2013; Le Roux et al. 2016; Warrington et al. 2019).
However, this is not the case for all alien wattles in South Africa. A recent survey of
rhizobial communities associated with 19 invasive Acacia species in South Africa
showed that wattles often share highly abundant Bradyrhizobium strains across wide
404
J. J. Le Roux et al.
leucocephala (Fabaceae) (Olckers 2011; Sharrat and Olckers 2012; Ramanand and
Olckers 2013). Native bruchinid beetles that target native mimosoid Fabaceae in
South Africa are also known to host native parasitoid communities (Impson et al.
1999), thus parasitism of the introduced host by native parasitoids may reflect
parasitoid eco-evolutionary experience to mimosoid-associated beetles. However,
it should be cautioned that, in general, there is insufficient knowledge and quantification of the extent of shared parasitoids between alien and native hosts and of foodweb interactions in these systems to confidently invoke eco-evolutionary processes.
It is clear, however, that biotic resistance has played a major role in limiting the
ability of some candidate biological control agents from establishing populations in
South Africa (see Hill et al. 2020, Chap. 19).
14.3 Mutualism
14.3.1 Plants and Soil Bacteria
Mutualisms can play key roles in mediating not only the establishment success of
alien species (Richardson et al. 2000a), but also their ecological impacts once they
become invasive (Traveset and Richardson 2006, 2011). Legumes (family Fabaceae)
are over-represented in regional invasive floras in many parts of the world (Pyšek
et al. 2017), and 73 legume species are naturalised in South Africa (Richardson et al.
2020, Chap. 3, Sect. 3.3). The widespread success of legumes as invasive species has
been attributed partly to their ability to form symbioses with soil bacteria known as
rhizobia (Parker 2001). Rhizobia are bacteria capable of forming specialised structures called root nodules on the roots of most legumes. Rhizobia fix atmospheric
nitrogen into ammonium that legumes can utilise. In return, legumes provide
rhizobia with various sources of carbon through photosynthate. This symbiosis
allows legumes to colonise nutrient-poor environments and often impacts these
environments through nitrogen enrichment of soils (Parker 2001; Yelenik et al.
2004).
Invasive Australian wattles in South Africa’s Cape Floristic Region (CFR), a
global biodiversity hotspot, are a good study system for exploring how interaction
reassembly pathway (i.e. co-introduction vs. ecological fitting vs. co-xenic) and
interaction specialisation affect the way in which invaders infiltrate ecological
networks (Fig. 14.2) and their subsequent impacts on native species. Molecular
evidence has revealed that invasive wattles in the CFR have often been
co-introduced with their rhizobia, primarily from the genus Bradyrhizobium
(Fig. 14.3b; Ndlovu et al. 2013; Le Roux et al. 2016; Warrington et al. 2019).
However, this is not the case for all alien wattles in South Africa. A recent survey of
rhizobial communities associated with 19 invasive Acacia species in South Africa
showed that wattles often share highly abundant Bradyrhizobium strains across wide
404
J. J. Le Roux et al.