4. Hypericum perforatum (Clusiaceae; St. John’s Wort) became increasingly
abundant and problematic in the Western Cape in the mid-1900s, but is now a
rarity. Biological control agents that had succeeded in Australia and North
America were released in South Africa in the 1960s and 1970s and stopped
the invasion of H. perforatum at an early stage (Gordon et al. 1986; Gordon and
Kluge 1991).
5. The problem of ‘short institutional memories’ is well illustrated by the example
of Sesbania punicea (Fabaceae; Red Sesbania). This plant has been conclusively
shown to have been rendered insignificant as an invader through biological
control in South Africa (Hoffmann and Moran 1998). Densities of S. punicea
declined by more than 95% as biological control took effect; woodlands of
S. punicea with closed canopies of 5 m tall trees with up to 300 mm basal stemdiameters were reduced to patches of isolated plants that seldom survived long
enough to develop beyond small bushy shrubs that are typically under 2 m tall,
with spindly stems. This situation has persisted for the last 40 years, but
S. punicea is still reported as being troublesome by landowners or landmanagers who are making their own assessments on the extent of the problem
for a suspected invasive species that they are encountering for the first time.
6. A variant on the theme discussed in (5) applies to Acacia saligna (Fabaceae;
Port Jackson Willow). Dense forests of 5–10 m tall trees with closed canopies
and barren understories once covered large areas in the Western Cape. Though
still common and plentiful in this province, long-term studies have shown that it
is a hugely-reduced problem since biological control was initiated in the late
1980s (Wood and Morris 2007) (see Box 19.3). Observations, since 1991, at
15 sites, some subjected to wild fires, have shown considerable site-specific
variations but an overall decrease of more than 60% in tree densities. Invasive
stands now consist predominantly of scattered trees and shrubs, all of which are
infected by the pathogenic biological control agent and few will live for more
than 5 years. The canopy seldom closes, and therefore other plant species can
coexist with A. saligna. Furthermore, A. saligna phyllode biomass and seed
outputs have markedly declined (Wood and Morris 2007). Given time,
populations will become even less dense and abundant and other control
measures will be needed only if landowners must clear their land for immediate
use. Yet concerns about A. saligna as a highly-problematic invasive species
persist.
7. The case study of Acacia longifolia (Fabaceae; Long-Leaved Wattle) is also
informative, although there have been no formal quantifications on the changed
status of A. longifolia invasions. Before any biological control agents were
released, A. longifolia was an abundant and widespread species in the Western
Cape. Dennill and Donnelly (1991) had shown that the biological control agents
on A. longifolia were prolific and damaging, but there was no attempt to
determine how this damage affected the population dynamics of the host
plant. Currently, A. longifolia is seldom named, or ranks low on lists of the
most-problematic invasive species (Zachariades et al. 2017). It is reasonable to
assume that this reversal, i.e. the fact that the plant has all but disappeared from
19 Biological Control Against Invasive Alien Plants
563
abundant and problematic in the Western Cape in the mid-1900s, but is now a
rarity. Biological control agents that had succeeded in Australia and North
America were released in South Africa in the 1960s and 1970s and stopped
the invasion of H. perforatum at an early stage (Gordon et al. 1986; Gordon and
Kluge 1991).
5. The problem of ‘short institutional memories’ is well illustrated by the example
of Sesbania punicea (Fabaceae; Red Sesbania). This plant has been conclusively
shown to have been rendered insignificant as an invader through biological
control in South Africa (Hoffmann and Moran 1998). Densities of S. punicea
declined by more than 95% as biological control took effect; woodlands of
S. punicea with closed canopies of 5 m tall trees with up to 300 mm basal stemdiameters were reduced to patches of isolated plants that seldom survived long
enough to develop beyond small bushy shrubs that are typically under 2 m tall,
with spindly stems. This situation has persisted for the last 40 years, but
S. punicea is still reported as being troublesome by landowners or landmanagers who are making their own assessments on the extent of the problem
for a suspected invasive species that they are encountering for the first time.
6. A variant on the theme discussed in (5) applies to Acacia saligna (Fabaceae;
Port Jackson Willow). Dense forests of 5–10 m tall trees with closed canopies
and barren understories once covered large areas in the Western Cape. Though
still common and plentiful in this province, long-term studies have shown that it
is a hugely-reduced problem since biological control was initiated in the late
1980s (Wood and Morris 2007) (see Box 19.3). Observations, since 1991, at
15 sites, some subjected to wild fires, have shown considerable site-specific
variations but an overall decrease of more than 60% in tree densities. Invasive
stands now consist predominantly of scattered trees and shrubs, all of which are
infected by the pathogenic biological control agent and few will live for more
than 5 years. The canopy seldom closes, and therefore other plant species can
coexist with A. saligna. Furthermore, A. saligna phyllode biomass and seed
outputs have markedly declined (Wood and Morris 2007). Given time,
populations will become even less dense and abundant and other control
measures will be needed only if landowners must clear their land for immediate
use. Yet concerns about A. saligna as a highly-problematic invasive species
persist.
7. The case study of Acacia longifolia (Fabaceae; Long-Leaved Wattle) is also
informative, although there have been no formal quantifications on the changed
status of A. longifolia invasions. Before any biological control agents were
released, A. longifolia was an abundant and widespread species in the Western
Cape. Dennill and Donnelly (1991) had shown that the biological control agents
on A. longifolia were prolific and damaging, but there was no attempt to
determine how this damage affected the population dynamics of the host
plant. Currently, A. longifolia is seldom named, or ranks low on lists of the
most-problematic invasive species (Zachariades et al. 2017). It is reasonable to
assume that this reversal, i.e. the fact that the plant has all but disappeared from
19 Biological Control Against Invasive Alien Plants
563