For successful establishment and invasion, alien species must compete with
native species for available resources. Following LDD, germination and establishment of alien seeds depends on an array of factors, ranging from availability of
suitable conditions (such as those created by biophysical disturbance, e.g. anthropic
habitats), to inter-specific competition. Few studies have reported on the complete
seed dispersal cycle of both native and alien plant species in the same environment
(Wang and Smith 2002). Nonetheless, the increasing number of invasive fleshyfruited plants in South Africa indicates that their seeds are effectively dispersed, and
establishment success is high (Mokotjomela et al. 2015). Because of the commonly
smaller seed size of invasive alien fruits (Gosper and Vivian-Smith 2010;
Mokotjomela et al. 2013a), more seeds can be dispersed by vertebrates than those
of native species, implying that each dispersal event will likely carry more seeds of
alien than native species. High propagule pressure has been reported to drive rates of
recruitment of many bird-dispersed invasive species such as Schinus molle (Iponga
et al. 2009). Similarly, the rapid spread of invasive Prickly Pear in South Africa’s
Kruger National Park was mainly driven through seed dispersal by elephants and
baboons (Lotter et al. 1999; Foxcroft and Rejmánek 2007). Sixty percent of
Prickly Pear seeds sampled from baboon faeces led to successful seed germination
and seedling establishment (Lotter et al. 1999).
Patterns of seed dispersal of alien plants also influence the impacts that the alien
species may have in invaded ecosystems. For example, dispersal of alien Schinus
molle seeds by native birds in semi-arid savannas in South Africa has resulted in
recruitment of this species mainly beneath native acacias (Vachellia tortilis), the
dominant tree in this vegetation type. Initially V. tortilis may act as nurse plants for
S. molle seedlings (Iponga et al. 2008). Subsequently, growth of S. molle and its
superiority in competition for light over V. tortilis trees results in the gradual
replacement of V. tortilis by S. molle, leading to a change in woodland structure
and altered ecosystem processes (Iponga et al. 2008). Seed dispersal dynamics are
altered when alien plants replace native plants in South African ecosystems. An
example of this is where invasive alien Prosopis trees replace native Vachellia
species in arid savanna. Differences in branch height and angle between Prosopis
and Vachellia alter the habitat for birds, resulting in the loss of suitable perch sites for
key frugivorous birds (Dean et al. 2002).
14.4 Selected Examples of Impacts on Native Species Biotic
Interactions and Ecological Networks
Irrespective of the pathways and dynamics underlying interaction reassembly of
alien species, it will certainly have consequences for native taxa. That is, native
species may experience altered biotic interactions as invaders increase in abundance
and range, which may include losses and gains of old and new associations,
respectively. Establishing interactions is one thing, but their effectiveness is equally
410
J. J. Le Roux et al.
native species for available resources. Following LDD, germination and establishment of alien seeds depends on an array of factors, ranging from availability of
suitable conditions (such as those created by biophysical disturbance, e.g. anthropic
habitats), to inter-specific competition. Few studies have reported on the complete
seed dispersal cycle of both native and alien plant species in the same environment
(Wang and Smith 2002). Nonetheless, the increasing number of invasive fleshyfruited plants in South Africa indicates that their seeds are effectively dispersed, and
establishment success is high (Mokotjomela et al. 2015). Because of the commonly
smaller seed size of invasive alien fruits (Gosper and Vivian-Smith 2010;
Mokotjomela et al. 2013a), more seeds can be dispersed by vertebrates than those
of native species, implying that each dispersal event will likely carry more seeds of
alien than native species. High propagule pressure has been reported to drive rates of
recruitment of many bird-dispersed invasive species such as Schinus molle (Iponga
et al. 2009). Similarly, the rapid spread of invasive Prickly Pear in South Africa’s
Kruger National Park was mainly driven through seed dispersal by elephants and
baboons (Lotter et al. 1999; Foxcroft and Rejmánek 2007). Sixty percent of
Prickly Pear seeds sampled from baboon faeces led to successful seed germination
and seedling establishment (Lotter et al. 1999).
Patterns of seed dispersal of alien plants also influence the impacts that the alien
species may have in invaded ecosystems. For example, dispersal of alien Schinus
molle seeds by native birds in semi-arid savannas in South Africa has resulted in
recruitment of this species mainly beneath native acacias (Vachellia tortilis), the
dominant tree in this vegetation type. Initially V. tortilis may act as nurse plants for
S. molle seedlings (Iponga et al. 2008). Subsequently, growth of S. molle and its
superiority in competition for light over V. tortilis trees results in the gradual
replacement of V. tortilis by S. molle, leading to a change in woodland structure
and altered ecosystem processes (Iponga et al. 2008). Seed dispersal dynamics are
altered when alien plants replace native plants in South African ecosystems. An
example of this is where invasive alien Prosopis trees replace native Vachellia
species in arid savanna. Differences in branch height and angle between Prosopis
and Vachellia alter the habitat for birds, resulting in the loss of suitable perch sites for
key frugivorous birds (Dean et al. 2002).
14.4 Selected Examples of Impacts on Native Species Biotic
Interactions and Ecological Networks
Irrespective of the pathways and dynamics underlying interaction reassembly of
alien species, it will certainly have consequences for native taxa. That is, native
species may experience altered biotic interactions as invaders increase in abundance
and range, which may include losses and gains of old and new associations,
respectively. Establishing interactions is one thing, but their effectiveness is equally
410
J. J. Le Roux et al.