Native South African ants also play an important role in the invasion of alien
plants adapted for myrmecochory. For example, they are responsible for shortdistance dispersal and seed burial of the Port Jackson Willow, Acacia saligna
(Holmes 1990). While other agents are more important for long-distance dispersal
in this species, burial protects seeds from predation and fire (Richardson et al.
2000a). Introduced livestock are key agents for the dispersal of many widespread
invasive plant species, especially in rangelands, notably species of the genus
Prosopis in South Africa (Richardson et al. 2000a). Dispersal mutualisms recorded
in South Africa include several novel interactions involving native bird functional
groups not recorded to disperse the plant species elsewhere, e.g. Barn Swallows
(Hirundo rustica; Underhill and Hofmeyr 2007) and Black Korhaans (Eupodotis
afra; Knight and Macdonald 1991) dispersing Acacia cyclops seeds, and Pied Crows
(Corvus albus) dispersing Opuntia seeds (Dean and Milton 2000). The presence of
wide-ranging native mammals such as African Elephants (Loxodonta africana) in
some of South Africa’s protected areas has resulted in unique patterns of invasion.
For example, long-distance dispersed seeds of Prickly Pear, Opuntia stricta, by
elephants and Chacma Baboons (Papio ursinus) from a few initial foci in the Kruger
National Park, facilitated the rapid spread of the species; a very different invasion
scenario compared to that in other parts of the invasive range of this cactus (Foxcroft
et al. 2004; Foxcroft and Rejmánek 2007).
Several factors that influence competition for dispersal agents have been identified in South Africa. For example, Knight (1986) reported that bird-dispersed alien
fleshy-fruited plants in the CFR have fruit displays that are more conspicuous and
more attractive to native birds than those of co-occurring native plants. Another
factor promoting the preference of fruits of invasive species over those produced by
native species by birds is that some invasive species offer higher nutritional rewards
(e.g. Cinnamomum camphora, Lantana camara, Morus alba, Psidium guajava,
Solanum mauritianum; Jordaan et al. 2011; Mokotjomela et al. 2013a; Thabethe
et al. 2015). The reproductive phenologies of some invasive plant species also ensure
that their fruits or seeds are available for longer periods compared to many native
species (Knight 1988; Mokotjomela 2012). For example, the invasion of Sand
Blackberry (Rubus cuneifolius) in South Africa depends on dispersal by frugivorous
birds and mammals (Denny and Goodall 1991), which exploit its prolific fruit crop
throughout the year (van Kleunen and Johnson 2007). Similarly, invasion of
Bugweed (S. mauritianum) in South Africa is driven by the abundance of fruit,
small seediness, and high sugar content of its berries, making the species’ fruit a
more attractive resource than that provided by co-occurring natives (Mokotjomela
et al. 2013a).
The importance of habitat quality in the assembly of mutualisms has been well
documented (Muller-Landau and Hardesty 2005). In South Africa, Schor et al.
(2015) found that preferential foraging on berries of the invasive S. mauritianum
by native birds declined with increasing presence of native resources (fruits) in
farmlands in KwaZulu-Natal. Rejmánek (1996) argued that such contextdependency of novel resource utilisation may explain why tropical forest habitats
suffer less from plant invasions than other vegetation types.
408
J. J. Le Roux et al.
plants adapted for myrmecochory. For example, they are responsible for shortdistance dispersal and seed burial of the Port Jackson Willow, Acacia saligna
(Holmes 1990). While other agents are more important for long-distance dispersal
in this species, burial protects seeds from predation and fire (Richardson et al.
2000a). Introduced livestock are key agents for the dispersal of many widespread
invasive plant species, especially in rangelands, notably species of the genus
Prosopis in South Africa (Richardson et al. 2000a). Dispersal mutualisms recorded
in South Africa include several novel interactions involving native bird functional
groups not recorded to disperse the plant species elsewhere, e.g. Barn Swallows
(Hirundo rustica; Underhill and Hofmeyr 2007) and Black Korhaans (Eupodotis
afra; Knight and Macdonald 1991) dispersing Acacia cyclops seeds, and Pied Crows
(Corvus albus) dispersing Opuntia seeds (Dean and Milton 2000). The presence of
wide-ranging native mammals such as African Elephants (Loxodonta africana) in
some of South Africa’s protected areas has resulted in unique patterns of invasion.
For example, long-distance dispersed seeds of Prickly Pear, Opuntia stricta, by
elephants and Chacma Baboons (Papio ursinus) from a few initial foci in the Kruger
National Park, facilitated the rapid spread of the species; a very different invasion
scenario compared to that in other parts of the invasive range of this cactus (Foxcroft
et al. 2004; Foxcroft and Rejmánek 2007).
Several factors that influence competition for dispersal agents have been identified in South Africa. For example, Knight (1986) reported that bird-dispersed alien
fleshy-fruited plants in the CFR have fruit displays that are more conspicuous and
more attractive to native birds than those of co-occurring native plants. Another
factor promoting the preference of fruits of invasive species over those produced by
native species by birds is that some invasive species offer higher nutritional rewards
(e.g. Cinnamomum camphora, Lantana camara, Morus alba, Psidium guajava,
Solanum mauritianum; Jordaan et al. 2011; Mokotjomela et al. 2013a; Thabethe
et al. 2015). The reproductive phenologies of some invasive plant species also ensure
that their fruits or seeds are available for longer periods compared to many native
species (Knight 1988; Mokotjomela 2012). For example, the invasion of Sand
Blackberry (Rubus cuneifolius) in South Africa depends on dispersal by frugivorous
birds and mammals (Denny and Goodall 1991), which exploit its prolific fruit crop
throughout the year (van Kleunen and Johnson 2007). Similarly, invasion of
Bugweed (S. mauritianum) in South Africa is driven by the abundance of fruit,
small seediness, and high sugar content of its berries, making the species’ fruit a
more attractive resource than that provided by co-occurring natives (Mokotjomela
et al. 2013a).
The importance of habitat quality in the assembly of mutualisms has been well
documented (Muller-Landau and Hardesty 2005). In South Africa, Schor et al.
(2015) found that preferential foraging on berries of the invasive S. mauritianum
by native birds declined with increasing presence of native resources (fruits) in
farmlands in KwaZulu-Natal. Rejmánek (1996) argued that such contextdependency of novel resource utilisation may explain why tropical forest habitats
suffer less from plant invasions than other vegetation types.
408
J. J. Le Roux et al.