8.3.1 Vertebrates
Only one mammalian invader is currently present on the PEIs, namely M. musculus.
The rodent occurs only on Marion Island, where it was introduced by sealers during
the 1800s (Cooper 2008). Mus musculus is absent from Prince Edward Island.
Mus musculus on Marion Island has shown an increase in population density by
about 430% over 20 years (McClelland et al. 2018), ostensibly due in part to the
eradication of the feral cats (Felis catus) on Marion island (see below), but also
because of an earlier onset of breeding season brought about by a reduction in winter
rainfall (McClelland et al. 2018).
Of all invaders on the PEIs, M. musculus has the most severe, and best-studied,
impacts (Zengeya et al. 2020, Chap. 17, Sect. 17.3). Several impacts on individual taxa
have been recorded. These include impacts on a number of native plant species. The
seeds of at least six native vascular plant species are consumed by M. musculus (Smith
et al. 2002), with some species’ seeds being taken at almost 100%, resulting in reduced
reproductive output of these species (Chown and Smith 1993). Mus musculus also
show a preference for creating the entrances to their burrows in the cushion-shaped
keystone plant, Azorella selago (Avenant and Smith 2003). Such burrows can cause
extensive damage to, and in some cases lead to mortality of, A. selago cushions (Phiri
et al. 2009). Although mouse damage to A. selago cushions decreases with altitude,
damage has been observed at relatively high altitudes (548 m) within almost 100 m of
the altitudinal limit of A. selago on Marion Island (Phiri et al. 2009).
Invertebrates constitute the majority of M. musculus’ diet on Marion Island
(Smith et al. 2002; McClelland et al. 2018). It has been estimated that M. musculus
has reduced total invertebrate biomass by more than 85% (McClelland et al. 2018).
Although limited comparisons with mouse-free Prince Edward Island have shown no
evidence of lower invertebrate populations on Marion Island (Hugo et al. 2006), it is
thought that preferential consumption of large individuals by M. musculus has resulted
in the body size of weevils on Prince Edward being significantly larger than on Marion
Island (Chown and Smith 1993; Treasure and Chown 2014).
Most recently, M. musculus has been observed feeding on the live chicks of
surface-nesting (Dilley et al. 2016) and on burrowing (Dilley et al. 2018) seabirds
on Marion Island (Fig. 8.1b). The first such occurrence on Marion Island was only
observed in 2003, where attacks on surface-nesting seabirds started, seemingly independently, at different sites simultaneously across the island (Dilley et al. 2016). The
incidence of M. musculus attacks on affected populations of four seabird species was
recorded to be high, with up to 9% chick mortality (once an attack has taken place) in
surface-nesting species, and up to 100% mortality in burrowing species (Dilley et al.
2016, 2018) because chicks do not defend themselves against M. musculus attacks
(Wanless et al. 2007). However, the occurrence of feathers in the gut content of
M. musculus was recorded as early as the early 1990s and was initially put down to
scavenging (Smith et al. 2002); it may well have been an earlier indication of active
predation of seabirds by M. musculus (Smith 2008)—perhaps of the burrowing petrels.
Beyond affecting individual species, M. musculus also has impacts on ecosystem
processes. It has been suggested that, especially due to their heavy predation on
8 Biological Invasions in South Africa’s Offshore Sub-Antarctic Territories
211
Only one mammalian invader is currently present on the PEIs, namely M. musculus.
The rodent occurs only on Marion Island, where it was introduced by sealers during
the 1800s (Cooper 2008). Mus musculus is absent from Prince Edward Island.
Mus musculus on Marion Island has shown an increase in population density by
about 430% over 20 years (McClelland et al. 2018), ostensibly due in part to the
eradication of the feral cats (Felis catus) on Marion island (see below), but also
because of an earlier onset of breeding season brought about by a reduction in winter
rainfall (McClelland et al. 2018).
Of all invaders on the PEIs, M. musculus has the most severe, and best-studied,
impacts (Zengeya et al. 2020, Chap. 17, Sect. 17.3). Several impacts on individual taxa
have been recorded. These include impacts on a number of native plant species. The
seeds of at least six native vascular plant species are consumed by M. musculus (Smith
et al. 2002), with some species’ seeds being taken at almost 100%, resulting in reduced
reproductive output of these species (Chown and Smith 1993). Mus musculus also
show a preference for creating the entrances to their burrows in the cushion-shaped
keystone plant, Azorella selago (Avenant and Smith 2003). Such burrows can cause
extensive damage to, and in some cases lead to mortality of, A. selago cushions (Phiri
et al. 2009). Although mouse damage to A. selago cushions decreases with altitude,
damage has been observed at relatively high altitudes (548 m) within almost 100 m of
the altitudinal limit of A. selago on Marion Island (Phiri et al. 2009).
Invertebrates constitute the majority of M. musculus’ diet on Marion Island
(Smith et al. 2002; McClelland et al. 2018). It has been estimated that M. musculus
has reduced total invertebrate biomass by more than 85% (McClelland et al. 2018).
Although limited comparisons with mouse-free Prince Edward Island have shown no
evidence of lower invertebrate populations on Marion Island (Hugo et al. 2006), it is
thought that preferential consumption of large individuals by M. musculus has resulted
in the body size of weevils on Prince Edward being significantly larger than on Marion
Island (Chown and Smith 1993; Treasure and Chown 2014).
Most recently, M. musculus has been observed feeding on the live chicks of
surface-nesting (Dilley et al. 2016) and on burrowing (Dilley et al. 2018) seabirds
on Marion Island (Fig. 8.1b). The first such occurrence on Marion Island was only
observed in 2003, where attacks on surface-nesting seabirds started, seemingly independently, at different sites simultaneously across the island (Dilley et al. 2016). The
incidence of M. musculus attacks on affected populations of four seabird species was
recorded to be high, with up to 9% chick mortality (once an attack has taken place) in
surface-nesting species, and up to 100% mortality in burrowing species (Dilley et al.
2016, 2018) because chicks do not defend themselves against M. musculus attacks
(Wanless et al. 2007). However, the occurrence of feathers in the gut content of
M. musculus was recorded as early as the early 1990s and was initially put down to
scavenging (Smith et al. 2002); it may well have been an earlier indication of active
predation of seabirds by M. musculus (Smith 2008)—perhaps of the burrowing petrels.
Beyond affecting individual species, M. musculus also has impacts on ecosystem
processes. It has been suggested that, especially due to their heavy predation on
8 Biological Invasions in South Africa’s Offshore Sub-Antarctic Territories
211