geographic regions (Keet et al. 2017). These observations indicate that hostswitching between co-introduced rhizobia and wattles may allow those wattles not
co-introduced with their Australian bradyrhizobia to overcome the potential negative
effects associated with ecological fitting, potentially resulting in a form of invasion
meltdown (Le Roux et al. 2017; Warrington et al. 2019).
14.3.2 Pollination
Almost 90% of all flowering plant species rely to some extent on pollinators for seed
set (Ollerton et al. 2011). Pollination is, therefore, a potentially important barrier to
establishment and subsequent invasion for alien plants (Blackburn et al. 2011).
Intuitively, autonomous self-fertilisation should be less likely to limit invasiveness
as it allows plants to escape the negative consequences of small population sizes,
mate availability, and Allee effects (Baker 1955; Stebbins 1957). Nonetheless, many
invasive plant species are pollinator-dependent (e.g. van Kleunen and Johnson
2005).
It has been argued that native pollinator systems in South Africa are more
specialised than the global average (Johnson and Steiner 2003). This suggests that
introduced plants requiring specialist pollinators are less likely to receive pollinator
services, in line with the Missed Mutualisms Hypothesis (Catford et al. 2009). In
contrast, alien plants that attract a wide range of pollinators, i.e. generalists, are
expected to easily form novel interactions with pollinators in the introduced range
(the New Associations Hypothesis), which will enhance establishment and invasion
success (Baker and Stebbins 1965; Baker 1974). However, these expectations do not
always hold up. For example, Tree Tobacco, Nicotiana glauca, is pollinated by
hovering hummingbirds in its native range in the Americas (Nattero and Cocucci
2007; Ollerton et al. 2012) and its tubular flowers exclude insects and other potential
pollinators. In South Africa, N. glauca is pollinated by hovering sunbirds, which is
surprising (Fig. 14.3a). Sunbirds have a perching lifestyle and native plants provide
them with perches (Anderson et al. 2005), suggesting that a switch to a hovering
lifestyle in response to novel resources (N. glauca nectar) might be adaptive (Geerts
and Pauw 2009). It is likely that the outcome of N. glauca introductions may have
been dramatically different if native pollinators did not adopt it as a resource. For
example, in countries like Greece where bird pollinators are absent, N. glauca has
adapted increased selfing ability (shorter stigma-to-anther distances) compared to
plants in the native range (Ollerton et al. 2012).
Another alien plant in South Africa that requires specialist pollinators is the
Formosa Lily, Lilium formosanum. In its native range in Taiwan, the lily is pollinated by the Long-tongued Convolvulus Hawkmoth, Agrius convolvuli. In
South Africa, the species experiences reduced pollination in small populations, but
self-fertilization sufficiently compensates for this, alleviating any potential Allee
effect (Rodger et al. 2013). In denser populations in South Africa, L. formosanum is
readily pollinated by A. convolvuli, since this hawkmoth is native to much of the Old
14 Biotic Interactions as Mediators of Biological Invasions
405
co-introduced with their Australian bradyrhizobia to overcome the potential negative
effects associated with ecological fitting, potentially resulting in a form of invasion
meltdown (Le Roux et al. 2017; Warrington et al. 2019).
14.3.2 Pollination
Almost 90% of all flowering plant species rely to some extent on pollinators for seed
set (Ollerton et al. 2011). Pollination is, therefore, a potentially important barrier to
establishment and subsequent invasion for alien plants (Blackburn et al. 2011).
Intuitively, autonomous self-fertilisation should be less likely to limit invasiveness
as it allows plants to escape the negative consequences of small population sizes,
mate availability, and Allee effects (Baker 1955; Stebbins 1957). Nonetheless, many
invasive plant species are pollinator-dependent (e.g. van Kleunen and Johnson
2005).
It has been argued that native pollinator systems in South Africa are more
specialised than the global average (Johnson and Steiner 2003). This suggests that
introduced plants requiring specialist pollinators are less likely to receive pollinator
services, in line with the Missed Mutualisms Hypothesis (Catford et al. 2009). In
contrast, alien plants that attract a wide range of pollinators, i.e. generalists, are
expected to easily form novel interactions with pollinators in the introduced range
(the New Associations Hypothesis), which will enhance establishment and invasion
success (Baker and Stebbins 1965; Baker 1974). However, these expectations do not
always hold up. For example, Tree Tobacco, Nicotiana glauca, is pollinated by
hovering hummingbirds in its native range in the Americas (Nattero and Cocucci
2007; Ollerton et al. 2012) and its tubular flowers exclude insects and other potential
pollinators. In South Africa, N. glauca is pollinated by hovering sunbirds, which is
surprising (Fig. 14.3a). Sunbirds have a perching lifestyle and native plants provide
them with perches (Anderson et al. 2005), suggesting that a switch to a hovering
lifestyle in response to novel resources (N. glauca nectar) might be adaptive (Geerts
and Pauw 2009). It is likely that the outcome of N. glauca introductions may have
been dramatically different if native pollinators did not adopt it as a resource. For
example, in countries like Greece where bird pollinators are absent, N. glauca has
adapted increased selfing ability (shorter stigma-to-anther distances) compared to
plants in the native range (Ollerton et al. 2012).
Another alien plant in South Africa that requires specialist pollinators is the
Formosa Lily, Lilium formosanum. In its native range in Taiwan, the lily is pollinated by the Long-tongued Convolvulus Hawkmoth, Agrius convolvuli. In
South Africa, the species experiences reduced pollination in small populations, but
self-fertilization sufficiently compensates for this, alleviating any potential Allee
effect (Rodger et al. 2013). In denser populations in South Africa, L. formosanum is
readily pollinated by A. convolvuli, since this hawkmoth is native to much of the Old
14 Biotic Interactions as Mediators of Biological Invasions
405