Box 5.1 (continued)
For Common Mynas in South Africa (likely for A. t. tristis) a significant
correlation was detected between distance to Johannesburg and both dispersal
and cognitive traits (Berthouly-Salazar et al. 2012b). Furthermore, sex-biased
dispersal in Common Mynas amplifies the spatial sorting of dispersal traits in
females (stronger dispersers), specifically the wing morphology (and head
size, a qualitative proxy for brain size and thus cognitive abilities), but
weakens the pattern in males (figure below). As dispersal strategies are
typically linked to mating systems, resulting in resource defence in monogamy
where males take the lead role in acquisition and defence of resources and thus
receive considerable benefits by remaining philopatric. However, this also
makes males more susceptible to predation, and consequently favour
aggression-related traits such as morphological variation in tails for male
mynas. Sex-biased dispersal also leads to less balanced sex ratios in core
populations (e.g. sex ratio is 0.45 for birds within 250 km radius to Johannesburg versus 0.49 for birds beyond the radius). No strong spatial sorting
patterns were detected for the subspecies A. t. tristoides, with no morphological traits correlated to the distance from Durban (Berthouly-Salazar et al.
2012b). Dispersal-related traits often become homogenised once the range
expansion stops so that while the spatial sorting influences morphological
variation in expanding populations, its effect will be diluted once populations
reach their equilibria. Since the introduction to Durban pre-dates the introduction to Johannesburg by nearly 30 years (Hockey et al. 2005), the
Durban expansion has potentially filled up most suitable habitats and reached
the distributional equilibrium. In addition, distinct environmental characteristics of these two introduction points could have differentially influenced their
expansion. Johannesburg is located within the grassland biome of
South Africa, whereas Durban is located within a subtropical thicket that
extends along the east coast of the country. While the open grassland or
savanna may be more conducive to dispersal, the thicket and coastal forests
surrounding Durban but also the Drakensberg mountain ridge seems impenetrable and may have contributed to prevent high levels of dispersal from this
coastal introduction point. Factors of habitat quality could affect nondispersal-related foraging traits. Specifically, urbanisation can modify the
quality and type of food resources and therefore influence bill shape (bill
length and depth) (figure below). Primary productivity (and thus the habitat
quality and food resources) was found to significantly influence the head ratio
and bill ratio in both sexes (Berthouly-Salazar et al. 2012b).
Overall, frequent LDDs often work for invasive species that are strong
dispersers, while spatial sorting normally acts upon invasive species with poor
dispersal ability (Hui and Richardson 2017). The invasion of Common Starlings in South Africa supports the role of frequent LDDs, while the invasion of
Common Mynas the role of spatial sorting.
(continued)
5 Terrestrial Vertebrate Invasions in South Africa
133
For Common Mynas in South Africa (likely for A. t. tristis) a significant
correlation was detected between distance to Johannesburg and both dispersal
and cognitive traits (Berthouly-Salazar et al. 2012b). Furthermore, sex-biased
dispersal in Common Mynas amplifies the spatial sorting of dispersal traits in
females (stronger dispersers), specifically the wing morphology (and head
size, a qualitative proxy for brain size and thus cognitive abilities), but
weakens the pattern in males (figure below). As dispersal strategies are
typically linked to mating systems, resulting in resource defence in monogamy
where males take the lead role in acquisition and defence of resources and thus
receive considerable benefits by remaining philopatric. However, this also
makes males more susceptible to predation, and consequently favour
aggression-related traits such as morphological variation in tails for male
mynas. Sex-biased dispersal also leads to less balanced sex ratios in core
populations (e.g. sex ratio is 0.45 for birds within 250 km radius to Johannesburg versus 0.49 for birds beyond the radius). No strong spatial sorting
patterns were detected for the subspecies A. t. tristoides, with no morphological traits correlated to the distance from Durban (Berthouly-Salazar et al.
2012b). Dispersal-related traits often become homogenised once the range
expansion stops so that while the spatial sorting influences morphological
variation in expanding populations, its effect will be diluted once populations
reach their equilibria. Since the introduction to Durban pre-dates the introduction to Johannesburg by nearly 30 years (Hockey et al. 2005), the
Durban expansion has potentially filled up most suitable habitats and reached
the distributional equilibrium. In addition, distinct environmental characteristics of these two introduction points could have differentially influenced their
expansion. Johannesburg is located within the grassland biome of
South Africa, whereas Durban is located within a subtropical thicket that
extends along the east coast of the country. While the open grassland or
savanna may be more conducive to dispersal, the thicket and coastal forests
surrounding Durban but also the Drakensberg mountain ridge seems impenetrable and may have contributed to prevent high levels of dispersal from this
coastal introduction point. Factors of habitat quality could affect nondispersal-related foraging traits. Specifically, urbanisation can modify the
quality and type of food resources and therefore influence bill shape (bill
length and depth) (figure below). Primary productivity (and thus the habitat
quality and food resources) was found to significantly influence the head ratio
and bill ratio in both sexes (Berthouly-Salazar et al. 2012b).
Overall, frequent LDDs often work for invasive species that are strong
dispersers, while spatial sorting normally acts upon invasive species with poor
dispersal ability (Hui and Richardson 2017). The invasion of Common Starlings in South Africa supports the role of frequent LDDs, while the invasion of
Common Mynas the role of spatial sorting.
(continued)
5 Terrestrial Vertebrate Invasions in South Africa
133