Scheepers and Griffiths 2020, Chap. 7, Sect. 7.2), the Lepidoptera are the invertebrate group with the highest number of invasive species, followed by the Diptera. An
additional 15 species that have been recorded from the PEIs have not become
naturalised on the islands. The number of invasive species is probably an underestimate, as the earthworms, nematodes and tardigrades have not been adequately
sampled. As with other invasive taxa, Marion Island has more invasive terrestrial
invertebrate species than neighbouring Prince Edward Island due to the strict
regulations for visiting the latter island. Nevertheless, the potential for invasive
invertebrates to be introduced to Prince Edward Island from Marion Island by
means of birds or wind exists (Ryan et al. 2003).
Known pathways for introductions of invertebrates to the PEIs include as contaminants in fresh fruit and vegetables (no longer allowed ashore at either island), in
dry-food stores, and in packing containers and building material (Smith 1992; Hänel
et al. 1998; Slabber and Chown 2002). Evidence from invasive springtails (Fig. 8.1c)
suggests that only a few individuals of a species are required for introductions to be
successful (Myburgh et al. 2007).
The spread of invasive terrestrial invertebrates can vary substantially. For example, the Parasitic Wasp (Aphidius matriciae), first introduced in about 2001, spread at
a rate of 3–5 km year
À1 and currently occurs across the island. Within 5 years,
abundances of adults doubled whilst the percentage of parasitism in its host,
Rhopalosiphum padi (Bird Cherry-oat Aphid), increased from about 7% to 30%
(Lee and Chown 2016). On the other hand, it has been estimated that
Pogonognathelllus flavescens (Springtail), first recorded in 1993, will take centuries
to spread around the island (Treasure and Chown 2013), and it is currently only
known from a few localities.
The impacts of invasive terrestrial invertebrates are difficult to measure, but
examples on other sub-Antarctic islands suggest that the high abundance of an
invasive species can result in the displacement of native species (Convey et al.
1999; Terauds et al. 2011). On Marion Island, for example, the midge Limnophyes
minimus significantly alters nutrient cycling in areas where it is very abundant
(Hänel and Chown 1998). New interactions can also form among invasive species.
For example, A. matriciae became a parasitoid of R. padi (Lee and Chown 2016).
The distribution of many invasive invertebrate species seems to be restricted to
lower altitudes (Gabriel et al. 2001; Lee et al. 2007). This may be due to physiological or microclimate restrictions. For example, Deroceras panormitanum only occurs at
altitudes up to 300 m, above which it is physiologically limited by low temperatures
(Lee et al. 2007). However, as temperatures continue to increase on the PEIs (le Roux
and McGeoch 2008), invasive invertebrate species are expected to expand to higher
altitudes, either because they are able to cope physiologically, or because their host
plants are also expanding their ranges in response to a milder climate.
Due to their size, abundance and wide distribution, the eradication of widespread
invasive terrestrial invertebrates on the PEIs is not currently considered feasible.
However, Porcellio scaber (Common Rough Woodlouse), which was restricted to
the immediate vicinity of the old meteorological station, has been controlled with an
insecticide since it was first discovered on Marion Island in 2012 (D. Muir, pers.
comm). Ongoing monitoring will be needed to confirm its eradication.
8 Biological Invasions in South Africa’s Offshore Sub-Antarctic Territories
213
additional 15 species that have been recorded from the PEIs have not become
naturalised on the islands. The number of invasive species is probably an underestimate, as the earthworms, nematodes and tardigrades have not been adequately
sampled. As with other invasive taxa, Marion Island has more invasive terrestrial
invertebrate species than neighbouring Prince Edward Island due to the strict
regulations for visiting the latter island. Nevertheless, the potential for invasive
invertebrates to be introduced to Prince Edward Island from Marion Island by
means of birds or wind exists (Ryan et al. 2003).
Known pathways for introductions of invertebrates to the PEIs include as contaminants in fresh fruit and vegetables (no longer allowed ashore at either island), in
dry-food stores, and in packing containers and building material (Smith 1992; Hänel
et al. 1998; Slabber and Chown 2002). Evidence from invasive springtails (Fig. 8.1c)
suggests that only a few individuals of a species are required for introductions to be
successful (Myburgh et al. 2007).
The spread of invasive terrestrial invertebrates can vary substantially. For example, the Parasitic Wasp (Aphidius matriciae), first introduced in about 2001, spread at
a rate of 3–5 km year
À1 and currently occurs across the island. Within 5 years,
abundances of adults doubled whilst the percentage of parasitism in its host,
Rhopalosiphum padi (Bird Cherry-oat Aphid), increased from about 7% to 30%
(Lee and Chown 2016). On the other hand, it has been estimated that
Pogonognathelllus flavescens (Springtail), first recorded in 1993, will take centuries
to spread around the island (Treasure and Chown 2013), and it is currently only
known from a few localities.
The impacts of invasive terrestrial invertebrates are difficult to measure, but
examples on other sub-Antarctic islands suggest that the high abundance of an
invasive species can result in the displacement of native species (Convey et al.
1999; Terauds et al. 2011). On Marion Island, for example, the midge Limnophyes
minimus significantly alters nutrient cycling in areas where it is very abundant
(Hänel and Chown 1998). New interactions can also form among invasive species.
For example, A. matriciae became a parasitoid of R. padi (Lee and Chown 2016).
The distribution of many invasive invertebrate species seems to be restricted to
lower altitudes (Gabriel et al. 2001; Lee et al. 2007). This may be due to physiological or microclimate restrictions. For example, Deroceras panormitanum only occurs at
altitudes up to 300 m, above which it is physiologically limited by low temperatures
(Lee et al. 2007). However, as temperatures continue to increase on the PEIs (le Roux
and McGeoch 2008), invasive invertebrate species are expected to expand to higher
altitudes, either because they are able to cope physiologically, or because their host
plants are also expanding their ranges in response to a milder climate.
Due to their size, abundance and wide distribution, the eradication of widespread
invasive terrestrial invertebrates on the PEIs is not currently considered feasible.
However, Porcellio scaber (Common Rough Woodlouse), which was restricted to
the immediate vicinity of the old meteorological station, has been controlled with an
insecticide since it was first discovered on Marion Island in 2012 (D. Muir, pers.
comm). Ongoing monitoring will be needed to confirm its eradication.
8 Biological Invasions in South Africa’s Offshore Sub-Antarctic Territories
213