large continuous mats that significantly diminish the potential to utilise waterbodies,
and reduce aquatic biodiversity and ecosystem functioning (Hill 2003). In large river
systems in South Africa, such as the Vaal River and several inland impoundments
(e.g. the Hartebeespoort and Roodeplaat dams), invasive populations block access to
sporting and recreational areas and decrease waterfront property values
(McConnachie et al. 2003). Such impacts harm the economies of communities that
depend upon fishing, tourism and water sports for revenue. Losses to the agricultural
community involve the replacement costs of irrigation pumps that block and burnt
out, the drowning of livestock (McConnachie et al. 2003) and water loss (Fraser
et al. 2016; Arp et al. 2017).
Dense mats of floating invasive plants reduce light to submerged plants, thus
depleting dissolved oxygen in aquatic communities. The consequent reduction in
phytoplankton alters the composition of invertebrate communities, with knock-on
effects at lower and higher trophic levels. For example, Midgley et al. (2006) and
Coetzee et al. (2014) showed that Water Hyacinth mats significantly reduced the
diversity and abundance of benthic invertebrates in impoundments in a temperate
and subtropical region of South Africa, respectively.
The cost to control freshwater invasive macrophytes is also significant. The
Department of Environmental Affairs spent some ZAR 42 million (approx. US$3
million) between 2010 and 2018, mainly on herbicide control of Water Hyacinth at a
cost of ZAR 1800 per hectare (approx. US$130) (A. Wannenburgh, pers. comm.).
However, the cost of control varies depending on the locality and application
required. For example, van Wyk and van Wilgen (2002) compared the costs of
controlling Water Hyacinth under herbicide application, biological control, and
integrated control. The most expensive method was herbicidal control (US$250
per ha), while a biological control approach was much less expensive (US$44 per
ha), but the best return of investment was provided by integrated methods (US$39
per ha). McConnachie et al. (2003) showed that Nett Present Value (NPV) of
avoided impacts arising from the biological control of Red Water Fern in
South Africa between 1995 and 2000 amounted to US$206 million, which converted
to a benefit–cost ratio of 2.5:1 for the year 2000, increasing to 13:1 in 2005, and 15:1
in 2010, and although not calculated is still accruing as the weed remains under
complete control. While these examples show the economic benefit of an intervention such as biological control, it is in contrast to manual removal, where for
example, some EUR 14,680,000 was spent between 2005 and 2008 to remove
nearly 200,000 tons of Water Hyacinth from the Guadiana River, Spain (75 km of
river) (Ruiz Téllez et al. 2008). However, in this example, Water Hyacinth
re-invaded the river, most likely from seed, or scattered plants that the mechanical
harvesting had missed, and in 2010, an additional 5 tons of the weed was removed,
followed by >51,000 tons, and then 170,000 tons in 2012 and 2016 respectively. In
10 years of control (2005–2015), up to EUR 26,000,000 was spent (Duarte 2017).
Despite this effort, scattered populations of Water Hyacinth has spread along 150 km
of the river, almost reaching Portugal and Alqueva, the largest Reservoir in Europe,
and this management option has thus failed.
4 Invasive Alien Aquatic Plants in South African Freshwater Ecosystems
103
and reduce aquatic biodiversity and ecosystem functioning (Hill 2003). In large river
systems in South Africa, such as the Vaal River and several inland impoundments
(e.g. the Hartebeespoort and Roodeplaat dams), invasive populations block access to
sporting and recreational areas and decrease waterfront property values
(McConnachie et al. 2003). Such impacts harm the economies of communities that
depend upon fishing, tourism and water sports for revenue. Losses to the agricultural
community involve the replacement costs of irrigation pumps that block and burnt
out, the drowning of livestock (McConnachie et al. 2003) and water loss (Fraser
et al. 2016; Arp et al. 2017).
Dense mats of floating invasive plants reduce light to submerged plants, thus
depleting dissolved oxygen in aquatic communities. The consequent reduction in
phytoplankton alters the composition of invertebrate communities, with knock-on
effects at lower and higher trophic levels. For example, Midgley et al. (2006) and
Coetzee et al. (2014) showed that Water Hyacinth mats significantly reduced the
diversity and abundance of benthic invertebrates in impoundments in a temperate
and subtropical region of South Africa, respectively.
The cost to control freshwater invasive macrophytes is also significant. The
Department of Environmental Affairs spent some ZAR 42 million (approx. US$3
million) between 2010 and 2018, mainly on herbicide control of Water Hyacinth at a
cost of ZAR 1800 per hectare (approx. US$130) (A. Wannenburgh, pers. comm.).
However, the cost of control varies depending on the locality and application
required. For example, van Wyk and van Wilgen (2002) compared the costs of
controlling Water Hyacinth under herbicide application, biological control, and
integrated control. The most expensive method was herbicidal control (US$250
per ha), while a biological control approach was much less expensive (US$44 per
ha), but the best return of investment was provided by integrated methods (US$39
per ha). McConnachie et al. (2003) showed that Nett Present Value (NPV) of
avoided impacts arising from the biological control of Red Water Fern in
South Africa between 1995 and 2000 amounted to US$206 million, which converted
to a benefit–cost ratio of 2.5:1 for the year 2000, increasing to 13:1 in 2005, and 15:1
in 2010, and although not calculated is still accruing as the weed remains under
complete control. While these examples show the economic benefit of an intervention such as biological control, it is in contrast to manual removal, where for
example, some EUR 14,680,000 was spent between 2005 and 2008 to remove
nearly 200,000 tons of Water Hyacinth from the Guadiana River, Spain (75 km of
river) (Ruiz Téllez et al. 2008). However, in this example, Water Hyacinth
re-invaded the river, most likely from seed, or scattered plants that the mechanical
harvesting had missed, and in 2010, an additional 5 tons of the weed was removed,
followed by >51,000 tons, and then 170,000 tons in 2012 and 2016 respectively. In
10 years of control (2005–2015), up to EUR 26,000,000 was spent (Duarte 2017).
Despite this effort, scattered populations of Water Hyacinth has spread along 150 km
of the river, almost reaching Portugal and Alqueva, the largest Reservoir in Europe,
and this management option has thus failed.
4 Invasive Alien Aquatic Plants in South African Freshwater Ecosystems
103