humans have played an important role. Patterns of spatial occupancy of invaders
suggest that invasions radiate out from human structures (viz. the research base and
the field huts) (le Roux et al. 2013b). Additionally, disturbance caused by human
trampling provides an opportunity for invaders to establish, increasing their cover
and abundance (Gremmen et al. 2003). Disturbances along with nutrient addition
that are associated with seal colonies further increase suitability for invasion (Haussmann et al. 2013). Coastal vegetation thus tends to be more invaded than inland
vegetation (Greve et al. 2017). Birds also play a role: some invading plants, such as
the grass P. annua, are associated with the burrows and nests of seabirds (Ryan et al.
2003), and it is thought that two (S. procumbens and C. fontanum) of the three
invasive plants on Prince Edward Island were introduced from Marion Island with
natural vectors—either by seabirds or by wind (Ryan et al. 2003).
Little is known about the impacts of plant invaders on Marion Island. Only the
impact of A. stolonifera has been rigorously assessed (Gremmen et al. 1998). This
grass species especially dominates drainage lines and slopes, where it is outcompeting
native species, although it is not thought to threaten any native species with extinction
(Gremmen et al. 1998). A more recent study that compared the plant and springtail
communities associated with S. procumbens with those associated with two native
plants that were being overgrown by S. procumbens, showed that epiphytic plant
communities did not differ between the native and invasive host species. However,
S. procumbens appeared to facilitate a higher richness and biomass (though not
abundance) of invasive Collembola than did the native plant species (Twala 2018).
8.3.4 Microbes
Although microbes are some of the most readily transported, and thus most frequently introduced, group of organisms (Mallon et al. 2015), not much is known
about their invasion ecology in the Antarctic region (Hughes et al. 2015). The
microbiology of the PEIs has received little attention (Sanyika et al. 2012), and to
date only one fungus, which is presumed to be invasive, has been recorded from
Marion Island. Botryotinia fuckeliana is a fungal pathogen that attacks the leaves of
the native Pringlea antiscorbutica (Kerguelen Cabbage), and is thought to have been
introduced to Marion Island in fresh produce (Kloppers and Smith 1998).
8.4 Freshwater Invaders
Two species of trout, the Rainbow Trout (Oncorhynchus mykiss) and the Brown
Trout (Salmo trutta), are the only non-native freshwater species that are known to
have been introduced, and survived, on the PEIs (Watkins and Cooper 1986; Cooper
et al. 1992). Both species were introduced to Marion Island, O. mykiss in 1959, and
S. trutta in 1964. Neither are thought to have reproduced and both species are now
extinct on the island (Watkins and Cooper 1986; Cooper et al. 1992). Stomach
contents of the Brown Trout (S. trutta) revealed that the species had a fairly
8 Biological Invasions in South Africa’s Offshore Sub-Antarctic Territories
215
suggest that invasions radiate out from human structures (viz. the research base and
the field huts) (le Roux et al. 2013b). Additionally, disturbance caused by human
trampling provides an opportunity for invaders to establish, increasing their cover
and abundance (Gremmen et al. 2003). Disturbances along with nutrient addition
that are associated with seal colonies further increase suitability for invasion (Haussmann et al. 2013). Coastal vegetation thus tends to be more invaded than inland
vegetation (Greve et al. 2017). Birds also play a role: some invading plants, such as
the grass P. annua, are associated with the burrows and nests of seabirds (Ryan et al.
2003), and it is thought that two (S. procumbens and C. fontanum) of the three
invasive plants on Prince Edward Island were introduced from Marion Island with
natural vectors—either by seabirds or by wind (Ryan et al. 2003).
Little is known about the impacts of plant invaders on Marion Island. Only the
impact of A. stolonifera has been rigorously assessed (Gremmen et al. 1998). This
grass species especially dominates drainage lines and slopes, where it is outcompeting
native species, although it is not thought to threaten any native species with extinction
(Gremmen et al. 1998). A more recent study that compared the plant and springtail
communities associated with S. procumbens with those associated with two native
plants that were being overgrown by S. procumbens, showed that epiphytic plant
communities did not differ between the native and invasive host species. However,
S. procumbens appeared to facilitate a higher richness and biomass (though not
abundance) of invasive Collembola than did the native plant species (Twala 2018).
8.3.4 Microbes
Although microbes are some of the most readily transported, and thus most frequently introduced, group of organisms (Mallon et al. 2015), not much is known
about their invasion ecology in the Antarctic region (Hughes et al. 2015). The
microbiology of the PEIs has received little attention (Sanyika et al. 2012), and to
date only one fungus, which is presumed to be invasive, has been recorded from
Marion Island. Botryotinia fuckeliana is a fungal pathogen that attacks the leaves of
the native Pringlea antiscorbutica (Kerguelen Cabbage), and is thought to have been
introduced to Marion Island in fresh produce (Kloppers and Smith 1998).
8.4 Freshwater Invaders
Two species of trout, the Rainbow Trout (Oncorhynchus mykiss) and the Brown
Trout (Salmo trutta), are the only non-native freshwater species that are known to
have been introduced, and survived, on the PEIs (Watkins and Cooper 1986; Cooper
et al. 1992). Both species were introduced to Marion Island, O. mykiss in 1959, and
S. trutta in 1964. Neither are thought to have reproduced and both species are now
extinct on the island (Watkins and Cooper 1986; Cooper et al. 1992). Stomach
contents of the Brown Trout (S. trutta) revealed that the species had a fairly
8 Biological Invasions in South Africa’s Offshore Sub-Antarctic Territories
215