the total for South Africa stands at 1422 alien species that are naturalised or invasive in
the country.
1.3 Estimating the Cost of Invasions to South Africa
Biological invasions have economic consequences because they can substantially
reduce the flow of ecosystem services from invaded areas. According to one
estimate, the cost of invasive species amounts to more than US$300 billion per
year in the United States, British Isles, Australia, Europe, South Africa, India and
Brazil alone (Pimentel 2011). Preventing these losses, or restoring the flow of
services by removing the alien species concerned, also has a cost because the control
measures have to be paid for. Ideally, these parameters should be known, and the
decision to initiate control measures should take these into account by assessing
what the return on investment from control would be; in other words, control should
ideally be undertaken only where the estimated value of avoided or restored costs
exceeds the estimated cost of control.
Understanding the magnitude of impacts of invasive species would be a first step
towards estimating their costs. However, impacts have in most cases been poorly
quantified, and it is necessary to make assumptions when extrapolating to larger
spatial scales. Several South African studies have followed this approach. An early
South African example is provided by Higgins et al. (1997), who estimated that
ecosystem services arising from a hypothetical 4 km
2 area of mountain fynbos would
be worth US$3 million with no management of invasive species, compared to US
$50 million with effective alien plant management. Other studies followed (see Le
Maitre et al. 2011 for the most recent comprehensive review), but it was the
prediction that alien plant invasions would lead to substantial reductions in water
runoff from catchment areas (Le Maitre et al. 1996) that provided the economic
motivation to initiate large-scale alien plant control operations (van Wilgen and
Wannenburgh 2016). At the time, it was estimated that more water could be
delivered, at a lower unit cost, by integrating alien plant control with the maintenance of water supply infrastructure, than without control (van Wilgen et al. 1996).
While further studies that quantified the economic impact of invasive species on
ecosystem services and returns on investment from control were subsequently
undertaken, they were all either focussed on a relatively small area (e.g. Hosking
and du Preez 2004 for selected project sites), or on a single species [e.g. De Wit et al.
2001 for Black Wattle (Acacia mearnsii); McConnachie et al. 2003 for Red Water
Fern (Azolla filiculoides); and Wise et al. 2012 for Mesquite (Prosopis species)].
In 2010, De Lange and van Wilgen (2010) attempted a national-scale estimation
of the economic losses due to invasive alien plants, with a focus on the value of water
resources, rangeland productivity, and biodiversity. These ecosystem services were
chosen because data were available to make the estimates possible. Their study
suggested that the value of annual losses of water resources amounted to US$773
million per year, and that the loss of livestock production from invaded natural
rangelands amounted to US$45 million annually. The losses due to reductions in
1 Biological Invasions in South Africa: An Overview
21
the country.
1.3 Estimating the Cost of Invasions to South Africa
Biological invasions have economic consequences because they can substantially
reduce the flow of ecosystem services from invaded areas. According to one
estimate, the cost of invasive species amounts to more than US$300 billion per
year in the United States, British Isles, Australia, Europe, South Africa, India and
Brazil alone (Pimentel 2011). Preventing these losses, or restoring the flow of
services by removing the alien species concerned, also has a cost because the control
measures have to be paid for. Ideally, these parameters should be known, and the
decision to initiate control measures should take these into account by assessing
what the return on investment from control would be; in other words, control should
ideally be undertaken only where the estimated value of avoided or restored costs
exceeds the estimated cost of control.
Understanding the magnitude of impacts of invasive species would be a first step
towards estimating their costs. However, impacts have in most cases been poorly
quantified, and it is necessary to make assumptions when extrapolating to larger
spatial scales. Several South African studies have followed this approach. An early
South African example is provided by Higgins et al. (1997), who estimated that
ecosystem services arising from a hypothetical 4 km
2 area of mountain fynbos would
be worth US$3 million with no management of invasive species, compared to US
$50 million with effective alien plant management. Other studies followed (see Le
Maitre et al. 2011 for the most recent comprehensive review), but it was the
prediction that alien plant invasions would lead to substantial reductions in water
runoff from catchment areas (Le Maitre et al. 1996) that provided the economic
motivation to initiate large-scale alien plant control operations (van Wilgen and
Wannenburgh 2016). At the time, it was estimated that more water could be
delivered, at a lower unit cost, by integrating alien plant control with the maintenance of water supply infrastructure, than without control (van Wilgen et al. 1996).
While further studies that quantified the economic impact of invasive species on
ecosystem services and returns on investment from control were subsequently
undertaken, they were all either focussed on a relatively small area (e.g. Hosking
and du Preez 2004 for selected project sites), or on a single species [e.g. De Wit et al.
2001 for Black Wattle (Acacia mearnsii); McConnachie et al. 2003 for Red Water
Fern (Azolla filiculoides); and Wise et al. 2012 for Mesquite (Prosopis species)].
In 2010, De Lange and van Wilgen (2010) attempted a national-scale estimation
of the economic losses due to invasive alien plants, with a focus on the value of water
resources, rangeland productivity, and biodiversity. These ecosystem services were
chosen because data were available to make the estimates possible. Their study
suggested that the value of annual losses of water resources amounted to US$773
million per year, and that the loss of livestock production from invaded natural
rangelands amounted to US$45 million annually. The losses due to reductions in
1 Biological Invasions in South Africa: An Overview
21