One of the ways to assess the economic impacts is the loss of production capacity due
to water-use by invasive alien plants. The value of water is then estimated as the partial
derivative of a sector or commodity’s production function relative to water (Moolman
et al. 2006) and is typically much higher than water tariffs or delivery costs. Not much
research has been done on this topic, because research has focused on determining
whether invasive alien plant control is the most cost-effective option for a given water
supply scheme. This has generally been done using the unit reference value, which
provides an estimate of the unit cost of supplying a cubic meter of water at the required
assurance over the portion of the water management or augmentation project’s lifespan
during which it produces economic benefits for society (van Niekerk 2013). It has been
used to show that clearing invasive alien plants can be more cost-effective than other
water supply options such as building a dam (van Wilgen et al. 1997; Larsen et al. 2001;
Hosking and Preez 2002; Marais and Wannenburgh 2008; Blignaut et al. 2010; DWS
2014; Preston 2015; Vundla et al. 2016; Morokong et al. 2016; Mander et al. 2017;
Nkambule et al. 2017). None of these studies assessed the financial impacts of impacts
on yields (Table 15.2) so this is an opportunity for further research.
Another approach is based on a recent change in international accounting rules
which recognises natural capital as an asset, i.e. a stock generating a flow (IASB
2018). Any restoration or improvement of natural capital is, therefore, seen as an
investment and not merely an expense. A study using this approach found that a
game farm had considerably more value to the company than they thought
(Mudavanhu et al. 2017a). Research is needed to evaluate its application to restoration (e.g. clearing of invasions), but this approach could be used to value municipal
assets and request transfer payments from National Treasury for management of
natural capital—including controlling of invasive alien plants.
15.10 Conflicts and Controversies Relating to Their
Hydrological Impacts and the Impacts of Clearing
Alien tree species undoubtedly have been of benefit to South Africa—the native
forests would have been lost if there had been no plantations to meet the timber and
fibre requirements of the country. The industry sustains numerous jobs and a
profitable commercial forestry industry, not to mention other services, such as
fuelwood and fodder for bees (pollination services) (Forsyth et al. 2004; Shackleton
et al. 2007). The location and the extent of plantations managed as commercial forest
plantations has been regulated to limit their impacts on water resources since the
1970s, while attempting to meet the country’s timber needs (van der Zel 1975).
Historically, little has been done to require the industry to address the spread of the
same tree species beyond their landholdings, but amendments to the Forestry
Stewardship Council’s certification systems could change this situation in future
(Christine Colvin pers. comm. 2018).
15 Impacts of Plant Invasions on Terrestrial Water Flows in South Africa
449
to water-use by invasive alien plants. The value of water is then estimated as the partial
derivative of a sector or commodity’s production function relative to water (Moolman
et al. 2006) and is typically much higher than water tariffs or delivery costs. Not much
research has been done on this topic, because research has focused on determining
whether invasive alien plant control is the most cost-effective option for a given water
supply scheme. This has generally been done using the unit reference value, which
provides an estimate of the unit cost of supplying a cubic meter of water at the required
assurance over the portion of the water management or augmentation project’s lifespan
during which it produces economic benefits for society (van Niekerk 2013). It has been
used to show that clearing invasive alien plants can be more cost-effective than other
water supply options such as building a dam (van Wilgen et al. 1997; Larsen et al. 2001;
Hosking and Preez 2002; Marais and Wannenburgh 2008; Blignaut et al. 2010; DWS
2014; Preston 2015; Vundla et al. 2016; Morokong et al. 2016; Mander et al. 2017;
Nkambule et al. 2017). None of these studies assessed the financial impacts of impacts
on yields (Table 15.2) so this is an opportunity for further research.
Another approach is based on a recent change in international accounting rules
which recognises natural capital as an asset, i.e. a stock generating a flow (IASB
2018). Any restoration or improvement of natural capital is, therefore, seen as an
investment and not merely an expense. A study using this approach found that a
game farm had considerably more value to the company than they thought
(Mudavanhu et al. 2017a). Research is needed to evaluate its application to restoration (e.g. clearing of invasions), but this approach could be used to value municipal
assets and request transfer payments from National Treasury for management of
natural capital—including controlling of invasive alien plants.
15.10 Conflicts and Controversies Relating to Their
Hydrological Impacts and the Impacts of Clearing
Alien tree species undoubtedly have been of benefit to South Africa—the native
forests would have been lost if there had been no plantations to meet the timber and
fibre requirements of the country. The industry sustains numerous jobs and a
profitable commercial forestry industry, not to mention other services, such as
fuelwood and fodder for bees (pollination services) (Forsyth et al. 2004; Shackleton
et al. 2007). The location and the extent of plantations managed as commercial forest
plantations has been regulated to limit their impacts on water resources since the
1970s, while attempting to meet the country’s timber needs (van der Zel 1975).
Historically, little has been done to require the industry to address the spread of the
same tree species beyond their landholdings, but amendments to the Forestry
Stewardship Council’s certification systems could change this situation in future
(Christine Colvin pers. comm. 2018).
15 Impacts of Plant Invasions on Terrestrial Water Flows in South Africa
449