World, including South Africa (Rodger et al. 2010). This example illustrates how the
wide native range distributions of pollinators may facilitate reproductive success of
an alien species. Similarly, invasive Peanut-butter Cassia, Senna didymobotrya in
South Africa, a shrub from tropical Africa that relies on buzz pollinators (where
pollinators must buzz at a specific frequency for pollen release and cross-pollination,
Dulberger 1981; van Kleunen and Johnson 2005), is pollinated in South Africa by
the carpenter bee (Xylocopa flavorufa). As the bee’s native range includes both
South Africa (where the plant is alien) and tropical Africa (where the plant is native),
this is neither a novel association nor a co-introduction. In contrast, other specialised
species such as the Moth Catcher (Araujia sericifera), which, as its common name
suggests, is moth pollinated, is largely visited by native honeybees, Apis mellifera, in
South Africa. South African honeybees have learnt to access the nectar of the large
moth catcher flowers (Coombs and Peter 2010). Despite the expectation that highly
specialised mutualistic interactions may hamper establishment success (i.e. Missed
Mutualisms Hypothesis, Catford et al. 2009), this example supports the emerging
view that specialised pollination requirements are not necessarily a barrier to plant
invasiveness (Richardson et al. 2000a). The examples discussed above show that
reproductive barriers can be overcome when the same pollinators, or functionally
similar pollinators are present, or if local pollinators can adapt to new resources
provided by invasive populations. On the other hand, alien plants with generalist
pollination requirements are expected to find pollinators more easily than their
specialist counterparts, whether in urban (Geerts et al. 2017) or natural environments
(Gibson et al. 2011). Generalist alien plants are assured of pollination when native
generalist pollinators are abundant. Honeybees in South Africa are important pollinators for many alien plants. Examples of invasive alien plant genera with generalist
flowers that are pollinated by honeybees in South Africa include Acacia, Banksia,
Hakea and Pueraria (Gibson et al. 2011, 2013; Moodley et al. 2016; Geerts et al.
2016).
Although generalist pollination systems promote invasiveness, Baker (1955)
postulated that selfing enhances the chances of establishment success of introduced
species as it assures reproduction when mates and/or pollinators are limited. Globally, it appears that selfing rates are higher in invasive plants than for native plants
(Richardson et al. 2000a; Burns et al. 2011). Support for this pattern in South African
comes from a study of 17 invasive woody species which showed that all were either
self-compatible or apomictic (reproducing asexually, without fertilisation)
(Rambuda and Johnson 2004). Similarly, Moodley et al. (2016) found that, although
pollinators increased seed set in four out of the five invasive Australian Banksia
species they studied, all species were capable of autonomous selfing. Interestingly,
in the Willow-leafed Hakea (Hakea salicifolia) naturalised populations received
almost four times the number of pollinator visits compared to populations that had
not naturalised (Moodley et al. 2016). This should not prevent invasion, since
H. salicifolia produces fruits via selfing in the absence of pollinators, but such spatial
variation in reproduction may explain some of the variation in the extent and rate of
naturalisation (Moodley et al. 2016). Geerts et al. (2016) found that invasive Kudzu
Vine, Pueraria montana (native to Asia), produces seed autonomously in
406
J. J. Le Roux et al.
wide native range distributions of pollinators may facilitate reproductive success of
an alien species. Similarly, invasive Peanut-butter Cassia, Senna didymobotrya in
South Africa, a shrub from tropical Africa that relies on buzz pollinators (where
pollinators must buzz at a specific frequency for pollen release and cross-pollination,
Dulberger 1981; van Kleunen and Johnson 2005), is pollinated in South Africa by
the carpenter bee (Xylocopa flavorufa). As the bee’s native range includes both
South Africa (where the plant is alien) and tropical Africa (where the plant is native),
this is neither a novel association nor a co-introduction. In contrast, other specialised
species such as the Moth Catcher (Araujia sericifera), which, as its common name
suggests, is moth pollinated, is largely visited by native honeybees, Apis mellifera, in
South Africa. South African honeybees have learnt to access the nectar of the large
moth catcher flowers (Coombs and Peter 2010). Despite the expectation that highly
specialised mutualistic interactions may hamper establishment success (i.e. Missed
Mutualisms Hypothesis, Catford et al. 2009), this example supports the emerging
view that specialised pollination requirements are not necessarily a barrier to plant
invasiveness (Richardson et al. 2000a). The examples discussed above show that
reproductive barriers can be overcome when the same pollinators, or functionally
similar pollinators are present, or if local pollinators can adapt to new resources
provided by invasive populations. On the other hand, alien plants with generalist
pollination requirements are expected to find pollinators more easily than their
specialist counterparts, whether in urban (Geerts et al. 2017) or natural environments
(Gibson et al. 2011). Generalist alien plants are assured of pollination when native
generalist pollinators are abundant. Honeybees in South Africa are important pollinators for many alien plants. Examples of invasive alien plant genera with generalist
flowers that are pollinated by honeybees in South Africa include Acacia, Banksia,
Hakea and Pueraria (Gibson et al. 2011, 2013; Moodley et al. 2016; Geerts et al.
2016).
Although generalist pollination systems promote invasiveness, Baker (1955)
postulated that selfing enhances the chances of establishment success of introduced
species as it assures reproduction when mates and/or pollinators are limited. Globally, it appears that selfing rates are higher in invasive plants than for native plants
(Richardson et al. 2000a; Burns et al. 2011). Support for this pattern in South African
comes from a study of 17 invasive woody species which showed that all were either
self-compatible or apomictic (reproducing asexually, without fertilisation)
(Rambuda and Johnson 2004). Similarly, Moodley et al. (2016) found that, although
pollinators increased seed set in four out of the five invasive Australian Banksia
species they studied, all species were capable of autonomous selfing. Interestingly,
in the Willow-leafed Hakea (Hakea salicifolia) naturalised populations received
almost four times the number of pollinator visits compared to populations that had
not naturalised (Moodley et al. 2016). This should not prevent invasion, since
H. salicifolia produces fruits via selfing in the absence of pollinators, but such spatial
variation in reproduction may explain some of the variation in the extent and rate of
naturalisation (Moodley et al. 2016). Geerts et al. (2016) found that invasive Kudzu
Vine, Pueraria montana (native to Asia), produces seed autonomously in
406
J. J. Le Roux et al.