But Crawley (1989), among others, has emphasised that measures of agent numbers
and the levels of damage that they inflict, while useful determinants of progress,
are not relevant to the population dynamics of the target plants themselves, i.e. to
the outcomes of biological control. This plant-population perspective has been
recognised by practitioners and has helped to change the emphasis in assessing the
accomplishments of biological control of invasive alien plants.
The outcomes of biological control of invasive alien plants can be measured in
many ways, including, for example, reductions in density of the invasive species, the
extent of their distribution, changes in biomass, their longevity, and the numbers of
seeds, or other propagules, produced and accumulated. Furthermore, as Hoffmann
et al. (2019) have noted: “Each of these measures must be considered in a context of
scale (e.g. in a defined locality such as a reserve or forest, in a functional habitat
such as a river catchment or mountain range, or at a landscape level that might
include a biome, region or country).” In the latest world catalogue of biological
control of “weeds” (Winston et al. 2014), the “scale of impact” of biological control
is categorised by subjective estimates as: “localized, regional, widespread throughout [the] range [of the target plant], and unknown”, and descriptive notes are
provided to support these opinions for each of 1555 intentional releases of “weed”
biological control agents, worldwide. Thus, the impacts of biological control are
invariably complex and often site-specific. For example, on Chromolaena odorata
(Asteraceae; Triffid Weed), in South Africa, agent-impacts are sporadic and highly
localised (Zachariades et al. 2011), and the situation is dynamic and not yet understood (Zachariades et al. 2016). Similarly, on the various taxa in the L. camara
hybrid complex, agents undergo ‘boom and bust’ cycles, dependent on varietal types
of the plants, and on altitudinal and other effects (Urban et al. 2011).
For all of these reasons, accurate quantitative measures of plant- and seedpopulation dynamics, and accompanying experimental evaluations of outcomes in
biological control, are expensive and difficult, and require years or decades of study.
The key here, with more recent South African programmes since the 1970s, has been
sustained funding for biological control of invasive plants and the career-longcontinuity of many of the research- and support-personnel involved. This has
encouraged long-term research programmes (albeit never enough) and a disproportionately high number of carefully-quantified and unambiguous evaluations. For
example: the 25-year evaluation of biological control of Opuntia stricta (Cactaceae;
Australian Pest Pear; see Box 19.1) (Hoffmann et al. 1998; Paterson et al. 2011);
16 years of assessment on the biological control of Sesbania punicea (Fabaceae; Red
Sesbania) at 22 sites (Hoffmann and Moran 1998); ongoing observations over
28 years on a fungal-pathogen biological control agent used against Acacia saligna
(Fabaceae; Port Jackson Willow) (Wood and Morris 2007); continuing country-wide
surveys over the past decade on several species of invasive aquatic plants (Coetzee
et al. 2011a); and finally, the assessments of the degree of biological control against
L. camara, ongoing since the 1970s (Cilliers 1987; Cilliers and Neser 1991; Urban
et al. 2011). In all of these cases, and for the other cases cited throughout the text,
conclusions about the status of the individual biological control projects, which have
19 Biological Control Against Invasive Alien Plants
557
and the levels of damage that they inflict, while useful determinants of progress,
are not relevant to the population dynamics of the target plants themselves, i.e. to
the outcomes of biological control. This plant-population perspective has been
recognised by practitioners and has helped to change the emphasis in assessing the
accomplishments of biological control of invasive alien plants.
The outcomes of biological control of invasive alien plants can be measured in
many ways, including, for example, reductions in density of the invasive species, the
extent of their distribution, changes in biomass, their longevity, and the numbers of
seeds, or other propagules, produced and accumulated. Furthermore, as Hoffmann
et al. (2019) have noted: “Each of these measures must be considered in a context of
scale (e.g. in a defined locality such as a reserve or forest, in a functional habitat
such as a river catchment or mountain range, or at a landscape level that might
include a biome, region or country).” In the latest world catalogue of biological
control of “weeds” (Winston et al. 2014), the “scale of impact” of biological control
is categorised by subjective estimates as: “localized, regional, widespread throughout [the] range [of the target plant], and unknown”, and descriptive notes are
provided to support these opinions for each of 1555 intentional releases of “weed”
biological control agents, worldwide. Thus, the impacts of biological control are
invariably complex and often site-specific. For example, on Chromolaena odorata
(Asteraceae; Triffid Weed), in South Africa, agent-impacts are sporadic and highly
localised (Zachariades et al. 2011), and the situation is dynamic and not yet understood (Zachariades et al. 2016). Similarly, on the various taxa in the L. camara
hybrid complex, agents undergo ‘boom and bust’ cycles, dependent on varietal types
of the plants, and on altitudinal and other effects (Urban et al. 2011).
For all of these reasons, accurate quantitative measures of plant- and seedpopulation dynamics, and accompanying experimental evaluations of outcomes in
biological control, are expensive and difficult, and require years or decades of study.
The key here, with more recent South African programmes since the 1970s, has been
sustained funding for biological control of invasive plants and the career-longcontinuity of many of the research- and support-personnel involved. This has
encouraged long-term research programmes (albeit never enough) and a disproportionately high number of carefully-quantified and unambiguous evaluations. For
example: the 25-year evaluation of biological control of Opuntia stricta (Cactaceae;
Australian Pest Pear; see Box 19.1) (Hoffmann et al. 1998; Paterson et al. 2011);
16 years of assessment on the biological control of Sesbania punicea (Fabaceae; Red
Sesbania) at 22 sites (Hoffmann and Moran 1998); ongoing observations over
28 years on a fungal-pathogen biological control agent used against Acacia saligna
(Fabaceae; Port Jackson Willow) (Wood and Morris 2007); continuing country-wide
surveys over the past decade on several species of invasive aquatic plants (Coetzee
et al. 2011a); and finally, the assessments of the degree of biological control against
L. camara, ongoing since the 1970s (Cilliers 1987; Cilliers and Neser 1991; Urban
et al. 2011). In all of these cases, and for the other cases cited throughout the text,
conclusions about the status of the individual biological control projects, which have
19 Biological Control Against Invasive Alien Plants
557