Impacts associated with the new suite of aquatic invasive species are yet to be
manifest themselves, particularly those of wetland invaders such as S. platyphylla
and I. pseudacorus whose distributions are increasing exponentially across
South Africa (Box 4.1). Reductions in wetland floral and faunal biodiversity are
expected. The extent of the alteration to sedimentation processes, hydrology and
subsequent wetland ecosystem service provisioning are not known, but are likely to
be significant.
Box 4.1 Spread of Delta Arrowhead in South Africa
Sagittaria platyphylla Engelm. (Alismataceae; Delta Arrowhead) is a freshwater aquatic macrophyte that has become an important invasive species
in freshwater ecosystems in South Africa. The plant was first discovered
in the Kranzskloof Nature Reserve, KwaZulu-Natal, in 2008, followed by
identification of invasions in the Eastern Cape in Makhanda (Grahamstown)
Botanical Gardens and Maden Dam near Stutterheim, and Jonkershoek trout
hatchery near Stellenbosch in the Western Cape, in 2009. These invasions are
assumed to be the result of unintentional introductions via dumping of fish
tank contents, and intentional planting for trout fry.
Sagittaria platyphylla is now regarded as one of the fastest-spreading
invasive species in the country (Henderson and Wilson 2017). It is also
invasive in Australia where its invasion biology and spread has been studied
extensively. The plant’s ability to reproduce sexually and asexually contributes to its rapid ability to spread. Each S. platyphylla plant produces numerous
inflorescences every few weeks, with approximately 70,000 achenes produced
per inflorescence (Adair et al. 2012; Broadhurst and Chong 2011). Therefore,
even a small population of S. platyphylla could produce hundreds of thousands
of viable achenes every few weeks. Achenes are able to disperse to new sites
via wind and water dispersal, and attachment to recreational equipment and
water birds (Adair et al. 2012). Asexual reproduction occurs via vegetative
propagules, such as underground stem fragments, daughter plants (runners),
stolons and tubers (Broadhurst and Chong 2011). The underground tubers
allow the plant to survive through drought, water drawdown, frost and chemical and mechanical management (Adair et al. 2012).
Annual surveys conducted to monitor the spread, density and distribution
of the plant in South Africa, showed an increase in the number of invaded sites
from a single site in 2008, to 16 sites by 2009, 19 sites in 2013, and over
33 sites in 2017 (first figure below). Sagittaria platyphylla has been successfully eradicated from two sites in South Africa through the South African
National Biodiversity Institute’s Biological Invasions Directorate, but it has
spread from a number of sites. Six populations have been monitored since
2008, and results show that the plant has spread on average 11.4 Æ 4.6 km
from each site (second figure below), at an average of 1.4 km per year (MPH,
(continued)
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M. P. Hill et al.
manifest themselves, particularly those of wetland invaders such as S. platyphylla
and I. pseudacorus whose distributions are increasing exponentially across
South Africa (Box 4.1). Reductions in wetland floral and faunal biodiversity are
expected. The extent of the alteration to sedimentation processes, hydrology and
subsequent wetland ecosystem service provisioning are not known, but are likely to
be significant.
Box 4.1 Spread of Delta Arrowhead in South Africa
Sagittaria platyphylla Engelm. (Alismataceae; Delta Arrowhead) is a freshwater aquatic macrophyte that has become an important invasive species
in freshwater ecosystems in South Africa. The plant was first discovered
in the Kranzskloof Nature Reserve, KwaZulu-Natal, in 2008, followed by
identification of invasions in the Eastern Cape in Makhanda (Grahamstown)
Botanical Gardens and Maden Dam near Stutterheim, and Jonkershoek trout
hatchery near Stellenbosch in the Western Cape, in 2009. These invasions are
assumed to be the result of unintentional introductions via dumping of fish
tank contents, and intentional planting for trout fry.
Sagittaria platyphylla is now regarded as one of the fastest-spreading
invasive species in the country (Henderson and Wilson 2017). It is also
invasive in Australia where its invasion biology and spread has been studied
extensively. The plant’s ability to reproduce sexually and asexually contributes to its rapid ability to spread. Each S. platyphylla plant produces numerous
inflorescences every few weeks, with approximately 70,000 achenes produced
per inflorescence (Adair et al. 2012; Broadhurst and Chong 2011). Therefore,
even a small population of S. platyphylla could produce hundreds of thousands
of viable achenes every few weeks. Achenes are able to disperse to new sites
via wind and water dispersal, and attachment to recreational equipment and
water birds (Adair et al. 2012). Asexual reproduction occurs via vegetative
propagules, such as underground stem fragments, daughter plants (runners),
stolons and tubers (Broadhurst and Chong 2011). The underground tubers
allow the plant to survive through drought, water drawdown, frost and chemical and mechanical management (Adair et al. 2012).
Annual surveys conducted to monitor the spread, density and distribution
of the plant in South Africa, showed an increase in the number of invaded sites
from a single site in 2008, to 16 sites by 2009, 19 sites in 2013, and over
33 sites in 2017 (first figure below). Sagittaria platyphylla has been successfully eradicated from two sites in South Africa through the South African
National Biodiversity Institute’s Biological Invasions Directorate, but it has
spread from a number of sites. Six populations have been monitored since
2008, and results show that the plant has spread on average 11.4 Æ 4.6 km
from each site (second figure below), at an average of 1.4 km per year (MPH,
(continued)
104
M. P. Hill et al.