turn leading to multi-trophic cascades through their knock-on effects. Indeed, since
the early 1970s, nutrient-loving tussock grasslands of Cook’s Tussock-grass, Poa
cookii, also showed a rapid decline, and habitats where tussock grassland previously
occurred could no longer support many animal colonies (Smith 1976; Smith and
Mucina 2006). The influence of domestic cats on seabirds, and thus soil nutrient
inputs, was likely the reason for the shrinkage of tussock grasslands during this
period. A successful cat eradication program launched on Marion Island in 1974 led
to the complete eradication of cats by 1991 (Bester et al. 2000; Greve et al. 2020,
Chap. 8). Since then, seabird populations have recovered, and tussock grasslands
seem to follow suite (Cooper et al. 1995). On the other hand, mice have not yet been
eradicated on Marion Island (see Greve et al. 2020, Chap. 8). Mice harvest up to
100% of the seed crop produced by some plant species (Chown and Smith 1993) and
can cause severe structural damage to keystone species such as the cushion plant,
Azorella selago (Phiri et al. 2009). They have also begun to prey on seabird chicks
(Fig. 14.3e; Jones and Ryan 2010) and consume large numbers of native insects,
including important keystone species like the flightless moth, Pringleophaga
marioni (Chown and Smith 1993). The knock-on effects of these disruptions to
multi-tropic interactions remain unknown.
14.5 Synthesis and Food for Thought
The South African situation provides unique circumstances to understand the role of
various ecological and evolutionary hypotheses related to biotic interactions in
facilitating or impeding the spread of non-native species. The country’s exceptional
biodiversity and environmental heterogeneity is reflected by an equally diverse and
impressive array of invasive organisms from all over the planet. This provides
unique opportunities to understand how different interaction types (e.g. familiar
vs. novel associations) and their evolutionary context (i.e. eco-evolutionary experience) shape the outcomes of biological invasions. We found ample examples of
studies that addressed specific ecological interactions of invasive species in
South Africa. Despite very few of these explicitly testing any of the main hypotheses
in invasion ecology that invoke biotic interactions, we found indirect evidence
supporting some of these hypotheses.
Disruption of key mutualistic requirements, such as pollination and
mycorrhization, are expected to impede invasion success. Surprisingly, in
South Africa, there is no indication that any plant invasion ever failed due to a
lack of pollinators, and at best, we speculate that plant invasions might only be
slowed down due to pollination limitation. This observation might reflect a research
bias towards species that have already become widespread, implicating that barriers
to reproduction have already been crossed. Future research should therefore focus on
comparative analyses of the pollination requirements and limitations of non-invasive
and invasive congeners that share similar introduction histories in South Africa.
Available data for insect invasions in South Africa not only support the notion that
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J. J. Le Roux et al.
the early 1970s, nutrient-loving tussock grasslands of Cook’s Tussock-grass, Poa
cookii, also showed a rapid decline, and habitats where tussock grassland previously
occurred could no longer support many animal colonies (Smith 1976; Smith and
Mucina 2006). The influence of domestic cats on seabirds, and thus soil nutrient
inputs, was likely the reason for the shrinkage of tussock grasslands during this
period. A successful cat eradication program launched on Marion Island in 1974 led
to the complete eradication of cats by 1991 (Bester et al. 2000; Greve et al. 2020,
Chap. 8). Since then, seabird populations have recovered, and tussock grasslands
seem to follow suite (Cooper et al. 1995). On the other hand, mice have not yet been
eradicated on Marion Island (see Greve et al. 2020, Chap. 8). Mice harvest up to
100% of the seed crop produced by some plant species (Chown and Smith 1993) and
can cause severe structural damage to keystone species such as the cushion plant,
Azorella selago (Phiri et al. 2009). They have also begun to prey on seabird chicks
(Fig. 14.3e; Jones and Ryan 2010) and consume large numbers of native insects,
including important keystone species like the flightless moth, Pringleophaga
marioni (Chown and Smith 1993). The knock-on effects of these disruptions to
multi-tropic interactions remain unknown.
14.5 Synthesis and Food for Thought
The South African situation provides unique circumstances to understand the role of
various ecological and evolutionary hypotheses related to biotic interactions in
facilitating or impeding the spread of non-native species. The country’s exceptional
biodiversity and environmental heterogeneity is reflected by an equally diverse and
impressive array of invasive organisms from all over the planet. This provides
unique opportunities to understand how different interaction types (e.g. familiar
vs. novel associations) and their evolutionary context (i.e. eco-evolutionary experience) shape the outcomes of biological invasions. We found ample examples of
studies that addressed specific ecological interactions of invasive species in
South Africa. Despite very few of these explicitly testing any of the main hypotheses
in invasion ecology that invoke biotic interactions, we found indirect evidence
supporting some of these hypotheses.
Disruption of key mutualistic requirements, such as pollination and
mycorrhization, are expected to impede invasion success. Surprisingly, in
South Africa, there is no indication that any plant invasion ever failed due to a
lack of pollinators, and at best, we speculate that plant invasions might only be
slowed down due to pollination limitation. This observation might reflect a research
bias towards species that have already become widespread, implicating that barriers
to reproduction have already been crossed. Future research should therefore focus on
comparative analyses of the pollination requirements and limitations of non-invasive
and invasive congeners that share similar introduction histories in South Africa.
Available data for insect invasions in South Africa not only support the notion that
414
J. J. Le Roux et al.