The outcome that was projected in 1997 had therefore not been realised, because
control methods were not effectively applied, and because the control costs were
underestimated (Fill et al. 2017).
Finally, the potential returns on investment from invasive plant control operations
have been the subject of several recent studies that compared the outcomes of
various management scenarios (Vundla et al. 2016; Mudavanhu et al. 2016;
Morokong et al. 2016; Nkambule et al. 2017). The scenarios included different
rates of spread, included or excluded value-added products using biomass from
invasive plants, and included or excluded private sector co-funding. The inclusion
of co-funding and value-added products delivered more favourable returns on
investment, and a failure to intervene at all would deliver negative outcomes.
These operations could therefore be financially viable, but the accuracy of the
predictions depends on whether or not the underlying assumptions will hold.
These assumptions include that effective and professional clearing would continue
into the future; that co-financing would be available; that due compensation for
the services rendered and the value-added products produced would be realised;
that the extent of the invasions was accurately known; and that the costs had been
accurately estimated. Most, or even all, of the above assumptions will not hold,
however, because alien plant invasions are rarely accurately mapped (see, for
example, Cheney et al. 2018); the costs of control are routinely under-estimated
by a factor of 3–7 times; there are low levels of efficiency associated with control
work; and including value-added products could lead to unintended consequences.
Consequently, there can only be a low level of confidence in these predictions of the
return on investment from control projects.
21.10 Synthesis
There are a number of points that can be made with regard to the effectiveness of
mechanical and chemical control measures. Firstly, as widespread invasions by alien
plants can bring about substantial costs, it would obviously be beneficial to reduce
invasions as far as possible. In South Africa, the largest proportion of funding for
control operations comes from the Working for Water programme (WfW) within the
Department of Environmental Affairs. Between 1995 and 2017, WfW spent ZAR
15 billion (unadjusted for inflation) on alien plant control, but this has only been
enough to deal with between 2 and 5% of the estimated extent of invasions each
year, and so the most important invasive species continue to spread (van Wilgen
et al. 2012; Henderson and Wilson 2017).
Control interventions have nonetheless succeeded in reducing the extent of
invasions in some areas. Early work demonstrated that the systematic implementation of a careful plan resulted in the reduction of populations of invasive alien
trees and shrubs to maintenance levels (Macdonald et al. 1989). Where concerted
efforts have been made to remove invasive trees from fynbos catchment areas,
marked declines in the density have been achieved (Fill et al. 2017; van Rensburg
21 The Extent and Effectiveness of Alien Plant Control Projects in South Africa
621
control methods were not effectively applied, and because the control costs were
underestimated (Fill et al. 2017).
Finally, the potential returns on investment from invasive plant control operations
have been the subject of several recent studies that compared the outcomes of
various management scenarios (Vundla et al. 2016; Mudavanhu et al. 2016;
Morokong et al. 2016; Nkambule et al. 2017). The scenarios included different
rates of spread, included or excluded value-added products using biomass from
invasive plants, and included or excluded private sector co-funding. The inclusion
of co-funding and value-added products delivered more favourable returns on
investment, and a failure to intervene at all would deliver negative outcomes.
These operations could therefore be financially viable, but the accuracy of the
predictions depends on whether or not the underlying assumptions will hold.
These assumptions include that effective and professional clearing would continue
into the future; that co-financing would be available; that due compensation for
the services rendered and the value-added products produced would be realised;
that the extent of the invasions was accurately known; and that the costs had been
accurately estimated. Most, or even all, of the above assumptions will not hold,
however, because alien plant invasions are rarely accurately mapped (see, for
example, Cheney et al. 2018); the costs of control are routinely under-estimated
by a factor of 3–7 times; there are low levels of efficiency associated with control
work; and including value-added products could lead to unintended consequences.
Consequently, there can only be a low level of confidence in these predictions of the
return on investment from control projects.
21.10 Synthesis
There are a number of points that can be made with regard to the effectiveness of
mechanical and chemical control measures. Firstly, as widespread invasions by alien
plants can bring about substantial costs, it would obviously be beneficial to reduce
invasions as far as possible. In South Africa, the largest proportion of funding for
control operations comes from the Working for Water programme (WfW) within the
Department of Environmental Affairs. Between 1995 and 2017, WfW spent ZAR
15 billion (unadjusted for inflation) on alien plant control, but this has only been
enough to deal with between 2 and 5% of the estimated extent of invasions each
year, and so the most important invasive species continue to spread (van Wilgen
et al. 2012; Henderson and Wilson 2017).
Control interventions have nonetheless succeeded in reducing the extent of
invasions in some areas. Early work demonstrated that the systematic implementation of a careful plan resulted in the reduction of populations of invasive alien
trees and shrubs to maintenance levels (Macdonald et al. 1989). Where concerted
efforts have been made to remove invasive trees from fynbos catchment areas,
marked declines in the density have been achieved (Fill et al. 2017; van Rensburg
21 The Extent and Effectiveness of Alien Plant Control Projects in South Africa
621