the landscape, is principally due to biological control, albeit in conjunction with
manual clearing operations, fires and other natural mortalities affecting the
seeds. Over 30 years, biological control, and multiple fire cycles, have reversed
the aggressiveness of A. longifolia which, in most situations, is now generally
considered to have been rendered insignificant as an invasive species (Moran
and Hoffmann 2012) (see Fig. 19.1).
8. The small serotinous tree, Hakea sericea (Proteaceae; Silky Hakea) is still a
problematic invader in the Fynbos Biome, but 50 years ago it was far worse.
Extensive clearing and burning operations were pursued in the 1980s and the
distribution of dense infestations of hakea declined very significantly (Gordon
and Fourie 2011) from about 530,000 ha in 1979 (Fugler 1979) to less than
190,000 ha in 2001 (Grand et al. 2007). The point that may have been
overlooked is that these gains were made possible because management practices were backed up by increasing levels of biological control brought about by
two species of seed-feeding biological control agents that were first released in
South Africa in 1970. Esler et al. (2010), using models developed by Le Maitre
et al. (2008), concluded that: “In the virtual absence of mechanical control
programs after the mid-1980s, the presence of seed-attacking insects
[as biological control agents] provides an explanation for the failure of the
previously invading species [i.e. H. sericea] to expand its range”; and that “. . .
biological control was largely responsible for the failure of the species to
re-colonize cleared sites, or to spread to new areas following unplanned
wildfires.”
9. As a result of biological control, and other management efforts, that date back to
the mid-1970s, Eichhornia crassipes (Pontederiaceae; Water Hyacinth) is also
far less of a problem than it used to be (Coetzee et al. 2011a) (see Box 19.3). In
some areas and situations, biological control has completely supressed the
species (Hill and Coetzee 2017a). In other areas biological control has significantly reduced plant populations and their impact, so that alternative management methods, such as herbicide applications and manual removal, are required,
but far less frequently (Hill and Coetzee 2017b). Biological control of water
hyacinth has been hindered by eutrophication of waters that promotes plant
growth and by colder climates that slow the development of the insect biological
control agents (Hill and Olckers 2001).
10. Other previously-problematic plant species have become uncommon, restricted,
or rare under biological control and are now seldom mentioned in discussions on
invasive species. Examples include: Ageratina riparia (Asteraceae; Mistflower)
(Heystek et al. 2011), Pistia stratiotes (Araceae; Water Lettuce) (Coetzee et al.
2011a) (see Fig. 19.2), Azolla filiculoides (Azollaceae; Red Water Fern)
(McConnachie et al. 2004; Hill et al. 2008; Coetzee et al. 2011a), and
Myriophyllum aquaticum (Haloragaceae; Parrot’s Feather) (see Box 19.4)
(Cilliers 1999; Coetzee et al. 2011a). This is also the certain prognosis for
Cylindropuntia fulgida var. fulgida (Cactaceae; Chain-Fruit Cholla), and
Cylindropuntia fulgida var. mamillata (Cactaceae; Boxing-Glove Cactus),
564
M. P. Hill et al.
manual clearing operations, fires and other natural mortalities affecting the
seeds. Over 30 years, biological control, and multiple fire cycles, have reversed
the aggressiveness of A. longifolia which, in most situations, is now generally
considered to have been rendered insignificant as an invasive species (Moran
and Hoffmann 2012) (see Fig. 19.1).
8. The small serotinous tree, Hakea sericea (Proteaceae; Silky Hakea) is still a
problematic invader in the Fynbos Biome, but 50 years ago it was far worse.
Extensive clearing and burning operations were pursued in the 1980s and the
distribution of dense infestations of hakea declined very significantly (Gordon
and Fourie 2011) from about 530,000 ha in 1979 (Fugler 1979) to less than
190,000 ha in 2001 (Grand et al. 2007). The point that may have been
overlooked is that these gains were made possible because management practices were backed up by increasing levels of biological control brought about by
two species of seed-feeding biological control agents that were first released in
South Africa in 1970. Esler et al. (2010), using models developed by Le Maitre
et al. (2008), concluded that: “In the virtual absence of mechanical control
programs after the mid-1980s, the presence of seed-attacking insects
[as biological control agents] provides an explanation for the failure of the
previously invading species [i.e. H. sericea] to expand its range”; and that “. . .
biological control was largely responsible for the failure of the species to
re-colonize cleared sites, or to spread to new areas following unplanned
wildfires.”
9. As a result of biological control, and other management efforts, that date back to
the mid-1970s, Eichhornia crassipes (Pontederiaceae; Water Hyacinth) is also
far less of a problem than it used to be (Coetzee et al. 2011a) (see Box 19.3). In
some areas and situations, biological control has completely supressed the
species (Hill and Coetzee 2017a). In other areas biological control has significantly reduced plant populations and their impact, so that alternative management methods, such as herbicide applications and manual removal, are required,
but far less frequently (Hill and Coetzee 2017b). Biological control of water
hyacinth has been hindered by eutrophication of waters that promotes plant
growth and by colder climates that slow the development of the insect biological
control agents (Hill and Olckers 2001).
10. Other previously-problematic plant species have become uncommon, restricted,
or rare under biological control and are now seldom mentioned in discussions on
invasive species. Examples include: Ageratina riparia (Asteraceae; Mistflower)
(Heystek et al. 2011), Pistia stratiotes (Araceae; Water Lettuce) (Coetzee et al.
2011a) (see Fig. 19.2), Azolla filiculoides (Azollaceae; Red Water Fern)
(McConnachie et al. 2004; Hill et al. 2008; Coetzee et al. 2011a), and
Myriophyllum aquaticum (Haloragaceae; Parrot’s Feather) (see Box 19.4)
(Cilliers 1999; Coetzee et al. 2011a). This is also the certain prognosis for
Cylindropuntia fulgida var. fulgida (Cactaceae; Chain-Fruit Cholla), and
Cylindropuntia fulgida var. mamillata (Cactaceae; Boxing-Glove Cactus),
564
M. P. Hill et al.