21.8.4 Using Sterile Cultivars
The use of sterile cultivars (for example in the case of Pinus and Australian Acacia
species used in commercial forestry) is sometimes proposed as a solution to the problem
of invasions that originate from commercial plantations, but there are no documented
cases of where this has been successful in South Africa. It has also been shown that large
reductions in fecundity do not necessarily adequately reduce the population growth
rates of long-lived species, which remain an invasion threat (Knight et al. 2011). In
addition, while modern technologies such as genetic modification may be used to
develop sterile varieties (Miao et al. 2012), forestry companies stand to lose environmental certification status as certifying bodies prohibit the use of genetically modified
organisms (van Wilgen and Richardson 2012). Similarly, the use of sterile cultivars of
horticultural species has been proposed as a means of reducing conflicts. There are,
however, still several open research questions as to the nature and stability of sterility
required to sufficiently reduce the risk, and whether sterility on its own would be
sufficient to prevent invasions (Richardson and Petit 2005).
21.9 Returns on Investment from Control Measures
The economic costs of plant invasions, and the economic benefits of control, have
also been the subject of a few studies in South Africa. One study (De Lange and van
Wilgen 2010) suggested that the cost of some impacts (lost water, grazing and
biodiversity) was currently about ZAR 6.5 billion per annum, but would become
much higher as invasions grow. In the case of biological control of invasive plants,
all studies have estimated very high returns on investment. By comparing the costs
of biological control research and implementation to the benefits of restored ecosystem services, or avoided ecosystem degradation, and avoided ongoing control costs,
biological control was shown to be extremely economically benefitial, with estimated benefit:cost ratios ranging from 8:1 up to 3726:1 (van Wilgen and De Lange
2011).
To estimate a return on investment from mechanical and chemical alien plant
control measures, it would be necessary to know both the historic cost of control, and
the value of impacts avoided due to control. There are no reliable estimates of the
value of impacts avoided due to control. It is well known, though, that the cost per
unit area to control an invasion rises exponentially as the density of the invasion
increases (Marais et al. 2004). If invasions can be contained while they still occupy a
smaller area, at relatively low densities, returns on investment from control operations should be positive. At some point, as yet unquantified, the cost of effective
control would exceed the cost of the impact, and attempts to mechanically or
chemically control invasions at this stage would deliver negative returns on investment (Fig. 21.5). More research is needed on this aspect to gain a better
understanding.
21 The Extent and Effectiveness of Alien Plant Control Projects in South Africa
619
The use of sterile cultivars (for example in the case of Pinus and Australian Acacia
species used in commercial forestry) is sometimes proposed as a solution to the problem
of invasions that originate from commercial plantations, but there are no documented
cases of where this has been successful in South Africa. It has also been shown that large
reductions in fecundity do not necessarily adequately reduce the population growth
rates of long-lived species, which remain an invasion threat (Knight et al. 2011). In
addition, while modern technologies such as genetic modification may be used to
develop sterile varieties (Miao et al. 2012), forestry companies stand to lose environmental certification status as certifying bodies prohibit the use of genetically modified
organisms (van Wilgen and Richardson 2012). Similarly, the use of sterile cultivars of
horticultural species has been proposed as a means of reducing conflicts. There are,
however, still several open research questions as to the nature and stability of sterility
required to sufficiently reduce the risk, and whether sterility on its own would be
sufficient to prevent invasions (Richardson and Petit 2005).
21.9 Returns on Investment from Control Measures
The economic costs of plant invasions, and the economic benefits of control, have
also been the subject of a few studies in South Africa. One study (De Lange and van
Wilgen 2010) suggested that the cost of some impacts (lost water, grazing and
biodiversity) was currently about ZAR 6.5 billion per annum, but would become
much higher as invasions grow. In the case of biological control of invasive plants,
all studies have estimated very high returns on investment. By comparing the costs
of biological control research and implementation to the benefits of restored ecosystem services, or avoided ecosystem degradation, and avoided ongoing control costs,
biological control was shown to be extremely economically benefitial, with estimated benefit:cost ratios ranging from 8:1 up to 3726:1 (van Wilgen and De Lange
2011).
To estimate a return on investment from mechanical and chemical alien plant
control measures, it would be necessary to know both the historic cost of control, and
the value of impacts avoided due to control. There are no reliable estimates of the
value of impacts avoided due to control. It is well known, though, that the cost per
unit area to control an invasion rises exponentially as the density of the invasion
increases (Marais et al. 2004). If invasions can be contained while they still occupy a
smaller area, at relatively low densities, returns on investment from control operations should be positive. At some point, as yet unquantified, the cost of effective
control would exceed the cost of the impact, and attempts to mechanically or
chemically control invasions at this stage would deliver negative returns on investment (Fig. 21.5). More research is needed on this aspect to gain a better
understanding.
21 The Extent and Effectiveness of Alien Plant Control Projects in South Africa
619