Box 4.2 (continued)
quarantine in South Africa by the Centre for Biological Control at Rhodes
University. Permission for the fly’s release was granted in June 2018, following the results of no-choice and paired choice tests which indicated that the
physiological host range of the fly is limited to species within the
Hydrocharitaceae, with a significantly higher preference and performance on
its host plant. Additionally, continuation tests showed that none of the
non-target species was able to sustain H. egeriae populations for more than
three generations (Smith et al. 2019).
Mass rearing of the fly commenced at the Waainek Mass Rearing Facility at
Rhodes University, shortly after permission for its release was granted. The
Nahoon River in East London was chosen as the first release site for the fly
largely due to the size of invasive populations of E. densa, and because it was
the first population identified in South Africa during annual countrywide
surveys, in 2008. It is also a site that has undergone a regime shift driven by
biological control, from a floating plant dominated state of Water Hyacinth to a
submerged stable state of E. densa. The fly was released on 12 October 2018,
and the first post-release survey a month later confirmed its establishment in
the system (RS, pers. obs.). Further releases will be made at invaded sites
around the country.
4.7 Regime Shifts and Alternate Stable States
The integrated control programme against invasive macrophytes in South Africa has
been highly successful, as measured by an increase in the number of sites under
biological control, coupled with a significant reduction in the cover of these invasive
plants and a degree of recovery of ecosystem services (Hill and Coetzee 2017;
Zachariades et al. 2017). However, unless the primary driver of invasions (i.e.,
eutrophication by nitrates and phosphates) in aquatic ecosystems is addressed, we
anticipate a succession of invasions by a new suite of emergent and submerged
invasive aquatic plant species (Coetzee et al. 2011a, b).
Ecosystems that are successfully colonised by non-native species often remain in
long-term stable degraded states (Scheffer et al. 2003). However, there is evidence
that the successful control of floating invasive plants can facilitate the proliferation
of a new suite of invaders, inducing a secondary degraded stable state (Strange et al.
2018). As a result of successful biological control and the subsequent decomposition
of floating plant biomass, there is an increase in available nutrients, light and space
within the water column. Invasive submerged plants can successfully capitalise on
this new abundance of resources and proliferate (Chimney and Pietro 2006; James
et al. 2006; Longhi et al. 2008). This is confounded by high levels of external
nutrients that facilitate plant growth and help to sustain a new stable regime of
submerged invasive plant dominance (Duarte 1995). The systems thus have two
4 Invasive Alien Aquatic Plants in South African Freshwater Ecosystems
109
quarantine in South Africa by the Centre for Biological Control at Rhodes
University. Permission for the fly’s release was granted in June 2018, following the results of no-choice and paired choice tests which indicated that the
physiological host range of the fly is limited to species within the
Hydrocharitaceae, with a significantly higher preference and performance on
its host plant. Additionally, continuation tests showed that none of the
non-target species was able to sustain H. egeriae populations for more than
three generations (Smith et al. 2019).
Mass rearing of the fly commenced at the Waainek Mass Rearing Facility at
Rhodes University, shortly after permission for its release was granted. The
Nahoon River in East London was chosen as the first release site for the fly
largely due to the size of invasive populations of E. densa, and because it was
the first population identified in South Africa during annual countrywide
surveys, in 2008. It is also a site that has undergone a regime shift driven by
biological control, from a floating plant dominated state of Water Hyacinth to a
submerged stable state of E. densa. The fly was released on 12 October 2018,
and the first post-release survey a month later confirmed its establishment in
the system (RS, pers. obs.). Further releases will be made at invaded sites
around the country.
4.7 Regime Shifts and Alternate Stable States
The integrated control programme against invasive macrophytes in South Africa has
been highly successful, as measured by an increase in the number of sites under
biological control, coupled with a significant reduction in the cover of these invasive
plants and a degree of recovery of ecosystem services (Hill and Coetzee 2017;
Zachariades et al. 2017). However, unless the primary driver of invasions (i.e.,
eutrophication by nitrates and phosphates) in aquatic ecosystems is addressed, we
anticipate a succession of invasions by a new suite of emergent and submerged
invasive aquatic plant species (Coetzee et al. 2011a, b).
Ecosystems that are successfully colonised by non-native species often remain in
long-term stable degraded states (Scheffer et al. 2003). However, there is evidence
that the successful control of floating invasive plants can facilitate the proliferation
of a new suite of invaders, inducing a secondary degraded stable state (Strange et al.
2018). As a result of successful biological control and the subsequent decomposition
of floating plant biomass, there is an increase in available nutrients, light and space
within the water column. Invasive submerged plants can successfully capitalise on
this new abundance of resources and proliferate (Chimney and Pietro 2006; James
et al. 2006; Longhi et al. 2008). This is confounded by high levels of external
nutrients that facilitate plant growth and help to sustain a new stable regime of
submerged invasive plant dominance (Duarte 1995). The systems thus have two
4 Invasive Alien Aquatic Plants in South African Freshwater Ecosystems
109