21.6 National-Scale or Species-Specific Assessments
of Management Effectiveness
The most comprehensive national-scale assessment of management efficacy at a
species level to date (Henderson and Wilson 2017) was based on the Southern
African Plant Invaders Atlas (SAPIA). The 773 alien plant taxa recorded in
SAPIA was an increase of 172 taxa over the last assessment in 2006 (Henderson
and Wilson 2017). Between 2000 and 2016 there was also an approximately 50%
increase in the broad-scale documented range of alien plants in SAPIA. Several
species (Campuloclinium macrocephalum, Parthenium hysterophorus, Opuntia
engelmannii, Cryptostegia grandiflora, Pennisetum setaceum, Tecoma stans,
Sagittaria platyphylla, Gleditsia triacanthos, and Trichocereus spachianus) were
considered to be of particular concern as they had increased substantially in distribution over the past decade. Henderson and Wilson (2017) reported further that
approximately 126 taxa were targeted for clearing by the Department of Environmental Affairs’ Natural Resource Management (NRM) programmes (formerly
“Working for Water”) between 2000 and 2012, although most effort was focussed
on a relatively small number of widespread taxa (Table 21.1). Examination of the
data suggested that whether a species was targeted for control or not made little
difference, as both targeted and neglected species continued to spread at comparable
rates. Henderson and Wilson (2017) concluded that this outcome was perhaps not
surprising, given the lack of evidence of a general strategic approach to NRM’s
activities, and the absence of dedicated strategic efforts to contain specific invasive
plants, or to reduce the rate at which they invade particular areas. By contrast, they
found a clear signal that biological control had reduced rates of spread of several
important invasive alien plant species. Notably, however, SAPIA was not designed
as a tool to monitor management effectiveness, but rather as a means of collating
information on alien plant distributions and how that distribution has changed over
time. For reliable assessments of management efficacy over time, SAPIA would
need to be augmented by monitoring specifically designed for this purpose. There
have been few examples of such monitoring to date.
A species-specific study on the integrated control of Hakea sericea (Sweet
Hakea) was conducted in the Western Cape by Esler et al. (2010). The control
included a combination of felling and burning, augmented by biological control (van
Wilgen et al. 1992). Data from two surveys, 22 years apart, suggested that the
distribution of the species was reduced by 64%, from ~530,000 to ~190,000 ha
between 1979 and 2001. The species either decreased in density, or was eliminated
from 492,113 ha, while it increased in density, or colonised 107,192 ha. It was
concluded that the initial mechanical clearing, integrated with the judicious use of
prescribed burning, in the 1970s and 1980s by the then Department of Forestry (van
Wilgen et al. 1992) was responsible for reducing the density and extent of infestations, and that biological control was largely responsible for the failure of the species
to re-colonise cleared sites, or to spread to new areas following unplanned wildfires
21 The Extent and Effectiveness of Alien Plant Control Projects in South Africa
611
of Management Effectiveness
The most comprehensive national-scale assessment of management efficacy at a
species level to date (Henderson and Wilson 2017) was based on the Southern
African Plant Invaders Atlas (SAPIA). The 773 alien plant taxa recorded in
SAPIA was an increase of 172 taxa over the last assessment in 2006 (Henderson
and Wilson 2017). Between 2000 and 2016 there was also an approximately 50%
increase in the broad-scale documented range of alien plants in SAPIA. Several
species (Campuloclinium macrocephalum, Parthenium hysterophorus, Opuntia
engelmannii, Cryptostegia grandiflora, Pennisetum setaceum, Tecoma stans,
Sagittaria platyphylla, Gleditsia triacanthos, and Trichocereus spachianus) were
considered to be of particular concern as they had increased substantially in distribution over the past decade. Henderson and Wilson (2017) reported further that
approximately 126 taxa were targeted for clearing by the Department of Environmental Affairs’ Natural Resource Management (NRM) programmes (formerly
“Working for Water”) between 2000 and 2012, although most effort was focussed
on a relatively small number of widespread taxa (Table 21.1). Examination of the
data suggested that whether a species was targeted for control or not made little
difference, as both targeted and neglected species continued to spread at comparable
rates. Henderson and Wilson (2017) concluded that this outcome was perhaps not
surprising, given the lack of evidence of a general strategic approach to NRM’s
activities, and the absence of dedicated strategic efforts to contain specific invasive
plants, or to reduce the rate at which they invade particular areas. By contrast, they
found a clear signal that biological control had reduced rates of spread of several
important invasive alien plant species. Notably, however, SAPIA was not designed
as a tool to monitor management effectiveness, but rather as a means of collating
information on alien plant distributions and how that distribution has changed over
time. For reliable assessments of management efficacy over time, SAPIA would
need to be augmented by monitoring specifically designed for this purpose. There
have been few examples of such monitoring to date.
A species-specific study on the integrated control of Hakea sericea (Sweet
Hakea) was conducted in the Western Cape by Esler et al. (2010). The control
included a combination of felling and burning, augmented by biological control (van
Wilgen et al. 1992). Data from two surveys, 22 years apart, suggested that the
distribution of the species was reduced by 64%, from ~530,000 to ~190,000 ha
between 1979 and 2001. The species either decreased in density, or was eliminated
from 492,113 ha, while it increased in density, or colonised 107,192 ha. It was
concluded that the initial mechanical clearing, integrated with the judicious use of
prescribed burning, in the 1970s and 1980s by the then Department of Forestry (van
Wilgen et al. 1992) was responsible for reducing the density and extent of infestations, and that biological control was largely responsible for the failure of the species
to re-colonise cleared sites, or to spread to new areas following unplanned wildfires
21 The Extent and Effectiveness of Alien Plant Control Projects in South Africa
611