8.3.3 Plants
Seventeen alien plant species are currently established on the Prince Edward Islands,
of which all occur on Marion Island, and only three on Prince Edward Island. The
first alien plants are thought to have been introduced to the PEIs by sealers.
However, most introductions were probably associated with the importation of
building material to Marion Island for the construction of the station and other
infrastructure, and with fodder imported for sheep and chickens between the late
1940s and early 1970s (Gremmen and Smith 1999; Greve et al. 2017, Cooper et al.,
pers. comm.), though propagules of some alien species may well have been introduced with clothing and other outdoor equipment (Lee and Chown 2009).
Of the alien plants that have been introduced to Marion Island, some never
naturalised, i.e. they were casual invaders and no longer occur on the island
(Gremmen and Smith 1999). Other species remain localised in their distribution,
despite the fact that several have been on the island for more than 50 years (le Roux
et al. 2013b). It could be that these localised species are poorly suited to the
sub-Antarctic environment; indeed, non-invasive and invasive alien species show
consistent differences in their traits, which could support this explanation
(Mathakutha et al. 2019). However, the possibility that these species are still in the
lag phase of the invasion process (Crooks and Soulé 1999), and may spread in future,
cannot be ruled out. Several of these localised alien species [e.g. Festuca rubra
(Creeping Red Fescue) and Rumex acetosella (Sheep Sorrel)] are widespread across
the sub-Antarctic islands (Shaw 2013). Given their success across the region, these
species could spread more widely on Marion Island if control measures are not
carried out (four populations of localised species on Marion Island are now regularly
controlled with herbicides; Department of Environmental Affairs Directorate: Antarctica and Islands 2010). A single shrub of Ochetophila trinervis (Floating-heart),
native to the South American Andes, is thought to have been introduced on Marion
Island through natural dispersal by vagrant birds (and should thus be considered a
native species) (Kalwij et al. 2019).
Of the 17 introduced plant species on the PEIs, 8 of the species on Marion Island
and three on Prince Edward Island have become established and spread over substantial distances from likely sites of introduction (Greve et al. 2017), and are considered
invasive (sensu Richardson et al. 2000). The invasive plants of the PEIs are of
European origin and widespread across the sub-Antarctic region, occurring on several
other islands (Shaw 2013). The invasive plants of Marion Island include three species
in the Poaceae [Agrostis stolonifera (Creeping Bent Grass), Poa annua (Annual
Meadow Grass, also present on Prince Edward Island, Fig. 8.1d) and Poa pratensis
(Kentucky Bluegrass)], and three in the Carophyllaceae [Cerastium fontanum (Common Mouse-ear Chickweed), also on Prince Edward Island], Sagina procumbens
(Birdeye Pearlwort, also on Prince Edward Island, Fig. 8.1d) and Stellaria media
(Common Chickweed)] (Greve et al. 2017).
The spread rates of invasive plant species on the PEIs have been estimated to vary
between 0.13 and a fairly rapid 2.36 km
2 year
À1 (le Roux et al. 2013b). The spread of
invasive plants on the PEIs is enhanced by a number of factors. On Marion Island,
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M. Greve et al.
Seventeen alien plant species are currently established on the Prince Edward Islands,
of which all occur on Marion Island, and only three on Prince Edward Island. The
first alien plants are thought to have been introduced to the PEIs by sealers.
However, most introductions were probably associated with the importation of
building material to Marion Island for the construction of the station and other
infrastructure, and with fodder imported for sheep and chickens between the late
1940s and early 1970s (Gremmen and Smith 1999; Greve et al. 2017, Cooper et al.,
pers. comm.), though propagules of some alien species may well have been introduced with clothing and other outdoor equipment (Lee and Chown 2009).
Of the alien plants that have been introduced to Marion Island, some never
naturalised, i.e. they were casual invaders and no longer occur on the island
(Gremmen and Smith 1999). Other species remain localised in their distribution,
despite the fact that several have been on the island for more than 50 years (le Roux
et al. 2013b). It could be that these localised species are poorly suited to the
sub-Antarctic environment; indeed, non-invasive and invasive alien species show
consistent differences in their traits, which could support this explanation
(Mathakutha et al. 2019). However, the possibility that these species are still in the
lag phase of the invasion process (Crooks and Soulé 1999), and may spread in future,
cannot be ruled out. Several of these localised alien species [e.g. Festuca rubra
(Creeping Red Fescue) and Rumex acetosella (Sheep Sorrel)] are widespread across
the sub-Antarctic islands (Shaw 2013). Given their success across the region, these
species could spread more widely on Marion Island if control measures are not
carried out (four populations of localised species on Marion Island are now regularly
controlled with herbicides; Department of Environmental Affairs Directorate: Antarctica and Islands 2010). A single shrub of Ochetophila trinervis (Floating-heart),
native to the South American Andes, is thought to have been introduced on Marion
Island through natural dispersal by vagrant birds (and should thus be considered a
native species) (Kalwij et al. 2019).
Of the 17 introduced plant species on the PEIs, 8 of the species on Marion Island
and three on Prince Edward Island have become established and spread over substantial distances from likely sites of introduction (Greve et al. 2017), and are considered
invasive (sensu Richardson et al. 2000). The invasive plants of the PEIs are of
European origin and widespread across the sub-Antarctic region, occurring on several
other islands (Shaw 2013). The invasive plants of Marion Island include three species
in the Poaceae [Agrostis stolonifera (Creeping Bent Grass), Poa annua (Annual
Meadow Grass, also present on Prince Edward Island, Fig. 8.1d) and Poa pratensis
(Kentucky Bluegrass)], and three in the Carophyllaceae [Cerastium fontanum (Common Mouse-ear Chickweed), also on Prince Edward Island], Sagina procumbens
(Birdeye Pearlwort, also on Prince Edward Island, Fig. 8.1d) and Stellaria media
(Common Chickweed)] (Greve et al. 2017).
The spread rates of invasive plant species on the PEIs have been estimated to vary
between 0.13 and a fairly rapid 2.36 km
2 year
À1 (le Roux et al. 2013b). The spread of
invasive plants on the PEIs is enhanced by a number of factors. On Marion Island,
214
M. Greve et al.