have been found in association with alien trees (Procheş et al. 2008). For example,
the native Keurboom (Virgilia divaricata) shares up to 30% of its associated
arthropod community with the confamilial invasive Black Wattle, Acacia mearnsii
(van der Colff et al. 2015), while the native polyphagous moth, Imbrasia cytherea
(Pine Tree Emperor Moth), is a common pest on introduced pines (Roux et al. 2012).
Similarly, the native seed-feeding alydid bug, Zulubius acaciaphagus, is commonly
found feeding on the invasive Rooikrans, Acacia cyclops (Holmes and Rebelo
1988).
In some instances, co-xenic associations may exacerbate invader ecosystem
impacts or even facilitate secondary invasions by other species, i.e. Invasional
Meltdown (Simberloff and Von Holle 1999). For example, Puccinia psidii, a myrtle
rust fungus native to South and Central America (Coutinho et al. 1998) is now
commonly associated with alien Myrtaceae taxa, including eucalypts (Glen et al.
2007). This pathogen has now spilled over onto native forest Myrtaceae species in
South Africa (Roux et al. 2015).
14.2.2 Marine Ecosystems
In South Africa, alien marine molluscs are often parasitised by endolithic bacteria
resulting in bioerosion and causing severe shell damage (Prenter et al. 2004), often
leading to lethal and sub-lethal impacts (Kaehler and McQuaid 1999). Along the
South African coastline, high rates of endolithic parasitism have been reported in the
widespread invasive Mediterranean Mussel, Mytilus galloprovincialis (Fig. 14.3d;
Zardi et al. 2009; Marquet et al. 2013). Studies comparing the effects of endoliths
between the native South African Green Mussel, Perna perna, and
M. galloprovincialis found infected individuals of the latter to be more negatively
impacted (Zardi et al. 2009), with both higher endolith incidence and greater
reductions in shell thickness, shell strength, and overall condition (Zardi et al.
2009). Infected mussels also have lower attachment strength, probably because
more energy is being directed toward shell repair and away from the production of
byssus threads that are responsible for securing them to substrates (Kaehler and
McQuaid 1999). This, and other mechanisms such as wave action and emersion
stress (see Rius and McQuaid 2006, 2009), are thought to mediate competition,
promoting co-existence between P. perna and M. galloprovincialis on South African
rocky shores. In particular, wave action favours the abundance of P. perna on the
low shore, while P. perna facilitates the establishment of M. galloprovincialis in the
mid shore (resulting in mixed mussel beds). On the high shore, P. perna is excluded
due to emersion stress, leaving M. galloprovincialis to dominate (Rius and McQuaid
2006, 2009).
Although four out of the seven endolithic species parasitising
M. galloprovincialis in South Africa are also found in the species’ native range
(Marquet et al. 2013), it is unlikely that they were co-introduced with their host
398
J. J. Le Roux et al.
the native Keurboom (Virgilia divaricata) shares up to 30% of its associated
arthropod community with the confamilial invasive Black Wattle, Acacia mearnsii
(van der Colff et al. 2015), while the native polyphagous moth, Imbrasia cytherea
(Pine Tree Emperor Moth), is a common pest on introduced pines (Roux et al. 2012).
Similarly, the native seed-feeding alydid bug, Zulubius acaciaphagus, is commonly
found feeding on the invasive Rooikrans, Acacia cyclops (Holmes and Rebelo
1988).
In some instances, co-xenic associations may exacerbate invader ecosystem
impacts or even facilitate secondary invasions by other species, i.e. Invasional
Meltdown (Simberloff and Von Holle 1999). For example, Puccinia psidii, a myrtle
rust fungus native to South and Central America (Coutinho et al. 1998) is now
commonly associated with alien Myrtaceae taxa, including eucalypts (Glen et al.
2007). This pathogen has now spilled over onto native forest Myrtaceae species in
South Africa (Roux et al. 2015).
14.2.2 Marine Ecosystems
In South Africa, alien marine molluscs are often parasitised by endolithic bacteria
resulting in bioerosion and causing severe shell damage (Prenter et al. 2004), often
leading to lethal and sub-lethal impacts (Kaehler and McQuaid 1999). Along the
South African coastline, high rates of endolithic parasitism have been reported in the
widespread invasive Mediterranean Mussel, Mytilus galloprovincialis (Fig. 14.3d;
Zardi et al. 2009; Marquet et al. 2013). Studies comparing the effects of endoliths
between the native South African Green Mussel, Perna perna, and
M. galloprovincialis found infected individuals of the latter to be more negatively
impacted (Zardi et al. 2009), with both higher endolith incidence and greater
reductions in shell thickness, shell strength, and overall condition (Zardi et al.
2009). Infected mussels also have lower attachment strength, probably because
more energy is being directed toward shell repair and away from the production of
byssus threads that are responsible for securing them to substrates (Kaehler and
McQuaid 1999). This, and other mechanisms such as wave action and emersion
stress (see Rius and McQuaid 2006, 2009), are thought to mediate competition,
promoting co-existence between P. perna and M. galloprovincialis on South African
rocky shores. In particular, wave action favours the abundance of P. perna on the
low shore, while P. perna facilitates the establishment of M. galloprovincialis in the
mid shore (resulting in mixed mussel beds). On the high shore, P. perna is excluded
due to emersion stress, leaving M. galloprovincialis to dominate (Rius and McQuaid
2006, 2009).
Although four out of the seven endolithic species parasitising
M. galloprovincialis in South Africa are also found in the species’ native range
(Marquet et al. 2013), it is unlikely that they were co-introduced with their host
398
J. J. Le Roux et al.