system as a basis, several assessments of the outcomes of biological control of
invasive plants in South Africa at the level of a plant population, have been made,
including:
1. Hoffmann (1991) who noted that of the 36 invasive plant species subjected to
biological control in South Africa at that time, 14% were under ‘complete’
control, and 22% were under ‘substantial’ control.
2. Later, Klein (2011) recorded that of 48 invasive plant species on which agents
had become established in South Africa, 21% were under ‘complete’ control and
38% under ‘substantial’ control.
3. Zachariades et al. (2017) recorded the following: “Thirteen (72%) of the 18 worst
taxa [of invasive plants in South Africa] from the work of van Wilgen et al. (2012)
have biological control agents released on them already, and seven (39%) of
these are under either complete or substantial biological control. When considering the more extensive list of species presented in van Wilgen et al. (2008), a list
of 56 most damaging species to ecosystem functioning, 19 (34%) have active
biological control and 13% are considered to be under substantial control.”
4. From the most-recent lists compiled by Zachariades (2018a), counting only those
instances in which the agents have been established for more than 10 years
(i.e. allowing enough time to draw firm conclusions about outcomes), which
coincidently also involves a total of 48 targeted, invasive plant species, 31% were
recorded as under ‘complete’ control and 40% under ‘substantial’ control.
5. Lastly, taking a different approach, Henderson and Wilson (2017) found that
there was an approximately 50% increase in the broad-scale documented range of
alien plants in South Africa between 2000 and 2016. But these authors also
concluded that “some [invasive plant] species which have been the subjects of
successful biological control programmes have shown very little expansion in
their distribution” and “in general successful biological control seems to be
associated with a reduction in the rate of spread”.
A feature of all these summaries is that they are based on the foundation of the
‘traditional’ system for describing and recording biological control achievements
against invasive alien plants. Until now this has been the only way of doing so, but
the descriptors and their definitions are vague, often subjective or ambiguous, and
convey little or no information about details or context. An elaboration of the present
system has therefore been proposed by Hoffmann et al. (2019) which is based on a
conceptual framework (see Box 19.2) that categorises the outcomes of biological
control, from barely perceptible (category CÀ), to spectacularly successful
(categorised as A+) where, over decades, the agents have had a considerable impact
on one or more of four ‘invasion parameters’ of their target hosts, namely, plant
population density, distribution (area), biomass and number of propagules (seeds),
for different regions, habitats or circumstances (see Box 19.3). Other parameters,
such as the longevity of invasive plant populations, could be accommodated if
required. This system has now been adopted in South Africa, and when implemented
it will allow for more nuanced, detailed, and accurate analyses and descriptions of
the outcomes of biological control of invasive alien plants.
19 Biological Control Against Invasive Alien Plants
559
invasive plants in South Africa at the level of a plant population, have been made,
including:
1. Hoffmann (1991) who noted that of the 36 invasive plant species subjected to
biological control in South Africa at that time, 14% were under ‘complete’
control, and 22% were under ‘substantial’ control.
2. Later, Klein (2011) recorded that of 48 invasive plant species on which agents
had become established in South Africa, 21% were under ‘complete’ control and
38% under ‘substantial’ control.
3. Zachariades et al. (2017) recorded the following: “Thirteen (72%) of the 18 worst
taxa [of invasive plants in South Africa] from the work of van Wilgen et al. (2012)
have biological control agents released on them already, and seven (39%) of
these are under either complete or substantial biological control. When considering the more extensive list of species presented in van Wilgen et al. (2008), a list
of 56 most damaging species to ecosystem functioning, 19 (34%) have active
biological control and 13% are considered to be under substantial control.”
4. From the most-recent lists compiled by Zachariades (2018a), counting only those
instances in which the agents have been established for more than 10 years
(i.e. allowing enough time to draw firm conclusions about outcomes), which
coincidently also involves a total of 48 targeted, invasive plant species, 31% were
recorded as under ‘complete’ control and 40% under ‘substantial’ control.
5. Lastly, taking a different approach, Henderson and Wilson (2017) found that
there was an approximately 50% increase in the broad-scale documented range of
alien plants in South Africa between 2000 and 2016. But these authors also
concluded that “some [invasive plant] species which have been the subjects of
successful biological control programmes have shown very little expansion in
their distribution” and “in general successful biological control seems to be
associated with a reduction in the rate of spread”.
A feature of all these summaries is that they are based on the foundation of the
‘traditional’ system for describing and recording biological control achievements
against invasive alien plants. Until now this has been the only way of doing so, but
the descriptors and their definitions are vague, often subjective or ambiguous, and
convey little or no information about details or context. An elaboration of the present
system has therefore been proposed by Hoffmann et al. (2019) which is based on a
conceptual framework (see Box 19.2) that categorises the outcomes of biological
control, from barely perceptible (category CÀ), to spectacularly successful
(categorised as A+) where, over decades, the agents have had a considerable impact
on one or more of four ‘invasion parameters’ of their target hosts, namely, plant
population density, distribution (area), biomass and number of propagules (seeds),
for different regions, habitats or circumstances (see Box 19.3). Other parameters,
such as the longevity of invasive plant populations, could be accommodated if
required. This system has now been adopted in South Africa, and when implemented
it will allow for more nuanced, detailed, and accurate analyses and descriptions of
the outcomes of biological control of invasive alien plants.
19 Biological Control Against Invasive Alien Plants
559