spread of invasive species provides practical insights. These include insights into
how alien species are introduced [e.g. as ornamentals introduced in small numbers to
multiple foci (towns) or as forestry species introduced in large numbers to a few
locations (Donaldson et al. 2014)]; and insights into how they are spread around [e.g.
through road maintenance equipment (Geerts et al. 2016; Kaplan et al. 2014)].
Understanding such mechanisms can be used to prioritise surveillance and control
measures (Wilson et al. 2017). However, despite there being widespread recognition
that the human footprint is a key determinant of the success of alien species (Thuiller
et al. 2006), very few details of how land use affects alien species distributions in
South Africa are known.
13.8 Conclusion
Various hypotheses in invasion science are related primarily to environmental
factors—habitat filtering; environmental heterogeneity; increased resource acquisition; disturbance; dynamic equilibrium model; opportunity window; and resourceenemy release (Catford et al. 2009; Jeschke and Heger 2018). Many of these
hypotheses are underpinned by the notion that abiotic factors can both promote
and limit invasions, and that there is a “sweet-spot”. For example, Buckley et al.
(2007) argued that at an intermediate level of disturbance there is a “weed-shaped
hole” when there is sufficient disruption of native communities to create opportunities for alien plant species without conditions being so adverse as to prevent
establishment at all.
In light of this, it is not surprising that the probability of invasions in South Africa
is profoundly influenced by environmental factors in often complex ways. In
general, climate is most influential at a broad-scale; microsite conditions at a local
scale; and the influence of humans operates across scales by determining where alien
species are introduced and where they can establish and spread. Nonetheless, most
invasions are context-specific. In consequence: (1) rules of thumb often do not have
the discriminatory power needed to reliably inform management and policy; and so
(2) there is still much to be gained from autecological studies [e.g. see Richardson
et al. (2000) for a discussion of the interacting factors that determine the distribution
of Prosopis spp. invasions in South Africa]. Will it inevitably always be this way?
Possibly, but it certainly seems that the interaction between invasions and environmental factors is likely to become more complicated as it plays out in the context of
other global change drivers.
Acknowledgements We acknowledge support from the DSI-NRF Centre of Excellence for
Invasion Biology. DMR acknowledges support from the National Research Foundation (grant
85417). SG acknowledges support from the CPUT University Research Fund. The South African
Department of Environment, Forestry, and Fisheries (DEFF) are thanked for funding the South
African National Biodiversity Institute noting that this publication does not necessarily represent the
views or opinions of DEFF or its employees.
13 The Role of Environmental Factors in Promoting and Limiting Invasions
377
how alien species are introduced [e.g. as ornamentals introduced in small numbers to
multiple foci (towns) or as forestry species introduced in large numbers to a few
locations (Donaldson et al. 2014)]; and insights into how they are spread around [e.g.
through road maintenance equipment (Geerts et al. 2016; Kaplan et al. 2014)].
Understanding such mechanisms can be used to prioritise surveillance and control
measures (Wilson et al. 2017). However, despite there being widespread recognition
that the human footprint is a key determinant of the success of alien species (Thuiller
et al. 2006), very few details of how land use affects alien species distributions in
South Africa are known.
13.8 Conclusion
Various hypotheses in invasion science are related primarily to environmental
factors—habitat filtering; environmental heterogeneity; increased resource acquisition; disturbance; dynamic equilibrium model; opportunity window; and resourceenemy release (Catford et al. 2009; Jeschke and Heger 2018). Many of these
hypotheses are underpinned by the notion that abiotic factors can both promote
and limit invasions, and that there is a “sweet-spot”. For example, Buckley et al.
(2007) argued that at an intermediate level of disturbance there is a “weed-shaped
hole” when there is sufficient disruption of native communities to create opportunities for alien plant species without conditions being so adverse as to prevent
establishment at all.
In light of this, it is not surprising that the probability of invasions in South Africa
is profoundly influenced by environmental factors in often complex ways. In
general, climate is most influential at a broad-scale; microsite conditions at a local
scale; and the influence of humans operates across scales by determining where alien
species are introduced and where they can establish and spread. Nonetheless, most
invasions are context-specific. In consequence: (1) rules of thumb often do not have
the discriminatory power needed to reliably inform management and policy; and so
(2) there is still much to be gained from autecological studies [e.g. see Richardson
et al. (2000) for a discussion of the interacting factors that determine the distribution
of Prosopis spp. invasions in South Africa]. Will it inevitably always be this way?
Possibly, but it certainly seems that the interaction between invasions and environmental factors is likely to become more complicated as it plays out in the context of
other global change drivers.
Acknowledgements We acknowledge support from the DSI-NRF Centre of Excellence for
Invasion Biology. DMR acknowledges support from the National Research Foundation (grant
85417). SG acknowledges support from the CPUT University Research Fund. The South African
Department of Environment, Forestry, and Fisheries (DEFF) are thanked for funding the South
African National Biodiversity Institute noting that this publication does not necessarily represent the
views or opinions of DEFF or its employees.
13 The Role of Environmental Factors in Promoting and Limiting Invasions
377