Long-distance dispersal (LDD) or stratified dispersal (a combination of long- and
short-distance dispersal) is essential for species to cross environmental barriers to
new recruitment sites, and therefore for subsequent naturalisation and the development of independent outlying foci that generates invasive spread (Trakhtenbrot et al.
2005). LDD facilitates establishment far from parent plants where competition,
predation and/or fungal attack might be lower (Chimera and Drake 2010; Jordaan
et al. 2011). Birds are important vectors for LDD of plants, as they spread ingested
seeds between roosting and foraging sites (Mokotjomela et al. 2013c, 2016).
Behavioural patterns, such as local and regional migrations, may also influence the
extent of LDD (Mokotjomela et al. 2013c). In the CFR, Red-winged Starlings
(Onychognathus morio) populations consist of resident pairs and nomadic flocks;
and flocks’ movements are determined by changes in local food resources (Rowan
1955; Hockey et al. 2005). Indeed, large flocks of wintering Red-winged Starlings
shuttle between home gardens and montane environments searching for fruits, which
results in ingested seeds being dispersed over considerable distances (Mokotjomela
2012).
Generally, frugivorous birds and mammals determine the effectiveness of dispersing the seeds of alien plants, i.e. successful dispersal and germination
(Mokotjomela et al. 2016). Indeed, native frugivorous species are often responsible
for the increased invasiveness of many alien plants in South Africa (Jordaan et al.
2011; Wilson and Downs 2012; Thabethe et al. 2015; Mokotjomela et al. 2016). For
example, for the highly invasive Acacia cyclops, germination is greatly enhanced
following ingestion of its seed by two native frugivorous birds, the Knysna Turaco,
Tauraco corythaix, and the Red-winged Starling (Mokotjomela et al. 2015, 2016).
Similarly, Thabethe et al. (2015) reported enhanced seed germination for
S. mauritianum, C. camphora, P. guajava, and M. alba as a result of ingestion by
two native Tauraco species. On the other hand, highly invasive species like the
Peruvian Pepper Tree (Schinus molle) and Syringa (Melia azedarach), even though
dispersed by native frugivores in South Africa, show no germination enhancement
following passage of seeds through the gut of their novel vectors (Iponga et al. 2009;
Voigt et al. 2011). Wahlberg’s Epauletted Fruit Bats, Epomophorus wahlbergi,
consume large numbers of fruits of four invasive plant species in South Africa
(Eriobotrya japonica, M. azedarach, M. alba, and P. guajava), and with the
exception of M. azedarach, this increases seed germination rates of ingested seeds
(Jordaan et al. 2012). Two alien bird species, the Common Starling (Sturnus
vulgaris) and House Sparrow (Passer domesticus), have been recorded feeding on
the fruits of less widespread invasive species such as Pittosporum undulatum and
Myoporum tenuifolium (Mokotjomela et al. 2013b). Although the impacts of these
co-xenic associations on seed germination remain unknown, they suggest these plant
species may become widespread invaders in the future, aided by these bird dispersers. Limited evidence suggests that co-xenic associations hamper invasiveness.
We know of one example from South Africa, where invasive Rose-ringed Parakeets,
Psittacula krameri, may impede establishment of alien plants due to reduced germination of ingested seeds (Thabethe et al. 2015).
14 Biotic Interactions as Mediators of Biological Invasions
409
short-distance dispersal) is essential for species to cross environmental barriers to
new recruitment sites, and therefore for subsequent naturalisation and the development of independent outlying foci that generates invasive spread (Trakhtenbrot et al.
2005). LDD facilitates establishment far from parent plants where competition,
predation and/or fungal attack might be lower (Chimera and Drake 2010; Jordaan
et al. 2011). Birds are important vectors for LDD of plants, as they spread ingested
seeds between roosting and foraging sites (Mokotjomela et al. 2013c, 2016).
Behavioural patterns, such as local and regional migrations, may also influence the
extent of LDD (Mokotjomela et al. 2013c). In the CFR, Red-winged Starlings
(Onychognathus morio) populations consist of resident pairs and nomadic flocks;
and flocks’ movements are determined by changes in local food resources (Rowan
1955; Hockey et al. 2005). Indeed, large flocks of wintering Red-winged Starlings
shuttle between home gardens and montane environments searching for fruits, which
results in ingested seeds being dispersed over considerable distances (Mokotjomela
2012).
Generally, frugivorous birds and mammals determine the effectiveness of dispersing the seeds of alien plants, i.e. successful dispersal and germination
(Mokotjomela et al. 2016). Indeed, native frugivorous species are often responsible
for the increased invasiveness of many alien plants in South Africa (Jordaan et al.
2011; Wilson and Downs 2012; Thabethe et al. 2015; Mokotjomela et al. 2016). For
example, for the highly invasive Acacia cyclops, germination is greatly enhanced
following ingestion of its seed by two native frugivorous birds, the Knysna Turaco,
Tauraco corythaix, and the Red-winged Starling (Mokotjomela et al. 2015, 2016).
Similarly, Thabethe et al. (2015) reported enhanced seed germination for
S. mauritianum, C. camphora, P. guajava, and M. alba as a result of ingestion by
two native Tauraco species. On the other hand, highly invasive species like the
Peruvian Pepper Tree (Schinus molle) and Syringa (Melia azedarach), even though
dispersed by native frugivores in South Africa, show no germination enhancement
following passage of seeds through the gut of their novel vectors (Iponga et al. 2009;
Voigt et al. 2011). Wahlberg’s Epauletted Fruit Bats, Epomophorus wahlbergi,
consume large numbers of fruits of four invasive plant species in South Africa
(Eriobotrya japonica, M. azedarach, M. alba, and P. guajava), and with the
exception of M. azedarach, this increases seed germination rates of ingested seeds
(Jordaan et al. 2012). Two alien bird species, the Common Starling (Sturnus
vulgaris) and House Sparrow (Passer domesticus), have been recorded feeding on
the fruits of less widespread invasive species such as Pittosporum undulatum and
Myoporum tenuifolium (Mokotjomela et al. 2013b). Although the impacts of these
co-xenic associations on seed germination remain unknown, they suggest these plant
species may become widespread invaders in the future, aided by these bird dispersers. Limited evidence suggests that co-xenic associations hamper invasiveness.
We know of one example from South Africa, where invasive Rose-ringed Parakeets,
Psittacula krameri, may impede establishment of alien plants due to reduced germination of ingested seeds (Thabethe et al. 2015).
14 Biotic Interactions as Mediators of Biological Invasions
409