(Hill et al. 2020b, Chap. 19, Sect. 19.3). Between 2000 and 2015, H. sericea
increased its occurrence in quarter degree grid cells from 77 to 85, an increase of
10% (Henderson and Wilson 2017). During the same period, the ecologically similar
pine trees [Pinus pinaster (Cluster Pine) and P. radiata (Monterey Pine), for which
no biological control is available] increased from 85 to 108, and from 70 to
95 QDGCs, or 27% and 21% respectively.
Marais et al. (2004) reported that good progress had been made with clearing
certain species (at a cost of ~ZAR 2.3 billion between 1996 and 2004, costs unadjusted
for inflation), but also that at current estimated rates of clearing, many of the targeted
species would not be brought under control within the next half century. They stressed
that their estimates were preliminary, given the incomplete data on the project
management system. In 2012, van Wilgen et al. (2012) reported that control operations
were in many cases only applied to a relatively small portion of the estimated invaded
area (2–5% depending on the species), despite substantial spending (ZAR 3.2 billion
in 2012 values).
21.7 The Effectiveness of Management in Selected Areas
21.7.1 Monitoring of Control Effectiveness
The effectiveness of control measures in a particular area (for example a protected
area, a catchment area, a farm, or a stretch of river) needs to be assessed against the
goal of the management, with such assessments based on regular monitoring of
outcomes. However, while almost all alien plant control projects in South Africa
have an implicit goal of reaching a “maintenance level”, this goal is seldom stated
explicitly in terms of the desired final extent or density of invasion (van Wilgen
et al. 2016; Fill et al. 2017). The concept of a maintenance level recognises that,
for most invasions, eradication is infeasible, but that invasions can be reduced to a
level where the negative impacts are negligible and control costs are relatively low
in perpetuity. In the vast majority of South Africa’s government-funded alien plant
control projects, the indicators used to monitor progress and set targets include the
amounts of money to be spent, the number of people to be employed, and the areas
to be treated. These are input or output indicators, rather than outcomes in terms of
changes in the levels of plant invasions (Wilson et al. 2018). In the absence of a
monitoring programme that is focussed on outcomes, it is difficult to assess
effectiveness objectively. However, several studies have been conducted, particularly over the past decade, in which the effectiveness of management has been
assessed, and these are summarised here. These studies provide a limited basis
from which to derive broad conclusions about the effectiveness of control
measures.
612
B. W. van Wilgen et al.
increased its occurrence in quarter degree grid cells from 77 to 85, an increase of
10% (Henderson and Wilson 2017). During the same period, the ecologically similar
pine trees [Pinus pinaster (Cluster Pine) and P. radiata (Monterey Pine), for which
no biological control is available] increased from 85 to 108, and from 70 to
95 QDGCs, or 27% and 21% respectively.
Marais et al. (2004) reported that good progress had been made with clearing
certain species (at a cost of ~ZAR 2.3 billion between 1996 and 2004, costs unadjusted
for inflation), but also that at current estimated rates of clearing, many of the targeted
species would not be brought under control within the next half century. They stressed
that their estimates were preliminary, given the incomplete data on the project
management system. In 2012, van Wilgen et al. (2012) reported that control operations
were in many cases only applied to a relatively small portion of the estimated invaded
area (2–5% depending on the species), despite substantial spending (ZAR 3.2 billion
in 2012 values).
21.7 The Effectiveness of Management in Selected Areas
21.7.1 Monitoring of Control Effectiveness
The effectiveness of control measures in a particular area (for example a protected
area, a catchment area, a farm, or a stretch of river) needs to be assessed against the
goal of the management, with such assessments based on regular monitoring of
outcomes. However, while almost all alien plant control projects in South Africa
have an implicit goal of reaching a “maintenance level”, this goal is seldom stated
explicitly in terms of the desired final extent or density of invasion (van Wilgen
et al. 2016; Fill et al. 2017). The concept of a maintenance level recognises that,
for most invasions, eradication is infeasible, but that invasions can be reduced to a
level where the negative impacts are negligible and control costs are relatively low
in perpetuity. In the vast majority of South Africa’s government-funded alien plant
control projects, the indicators used to monitor progress and set targets include the
amounts of money to be spent, the number of people to be employed, and the areas
to be treated. These are input or output indicators, rather than outcomes in terms of
changes in the levels of plant invasions (Wilson et al. 2018). In the absence of a
monitoring programme that is focussed on outcomes, it is difficult to assess
effectiveness objectively. However, several studies have been conducted, particularly over the past decade, in which the effectiveness of management has been
assessed, and these are summarised here. These studies provide a limited basis
from which to derive broad conclusions about the effectiveness of control
measures.
612
B. W. van Wilgen et al.