intense land use (e.g. overgrazing) leads to reduced frugivore abundance, which, in
turn, means that fewer fruit of the invasive Bugweed are eaten and dispersed by
animals and, by inference, reduced rates of spread. However, increased land use
intensity can also lead to an increase in the abundance of particular alien species such
as Australian Pest Pear (Opuntia stricta var. stricta) (Strum et al. 2015).
In protected areas the number of settlements and cultivated fields as well as the
intensity of grazing are generally far lower. Opportunities for long-distance dispersal
via road corridors are also significantly curtailed since the road network is less
extensive and used less. The boundaries of protected areas can, therefore, provide
an effective filter to the spread of invasive species which generally decline in
abundance inside the reserve. In their study of the Kruger National Park, for
example, Foxcroft et al. (2011) concluded that the park boundary provided an
effective barrier to invasions since the records of invasive plant species declined
rapidly beyond 1.5 km inside the park.
Proximity to highly-disturbed environments has an important influence on the
cover and richness of alien plants. In general, urban areas act both as points of
introduction and as bridge-heads for alien species (Gaertner et al. 2017; Potgieter
et al. 2020, Chap. 11). This is evident both at regional scales and at landscape scales
(Donaldson et al. 2014; Milton et al. 2007). Small towns in particular contain a high
diversity of alien plants and opportunities for spread into neighbouring natural
ecosystems (McLean et al. 2017, 2018). Roads and railways can also both facilitate
the spread of alien plants in South Africa and, by providing disturbance, provide
sites for establishment [cf. Faulkner et al. (2020, Chap. 12); though see also Kalwij
et al. (2008)]. For example, the distribution of the invasive Fountain Grass
(Pennisetum setaceum) closely tracks the road-network and associated disturbances
(Rahlao et al. 2010), while Schinus molle was often planted as a road-side shade tree,
creating foci for invasions (Richardson et al. 2010). Other human modifications of
the environment, e.g. fencing and the construction of telegraph poles, will presumably have had a similar range of impacts on invasions and their spread. The
development of highly disturbed agricultural fields poses a particular problem for
the spread of invasive aliens (van Rensburg et al. 2018), which is perhaps greatest for
riparian zones. Meek et al. (2010) surveyed the vegetation of a river corridor passing
through different types of land use in the fynbos. They showed that alien plants were
significantly more abundant at sites adjacent to agricultural fields and urban areas as
compared with natural areas or grazing lands.
Perhaps the best example of how land use disturbance facilitates the spread of
invasive species is the role that plantation forestry has played in the expansion of
alien conifers in South Africa. Van Wilgen and Richardson (2012) estimated that the
extent of invasive conifer stands, mostly pines, is more than four times greater than
the extent of formal forestry plantations (0.66 vs. 2.9 million ha), but that formal
plantations continue to provide propagules for invasive conifers to expand their
range into natural environments across South Africa.
The type and intensity of land use and how it has been practised over time are,
therefore, useful predictors of the distribution of alien species. For management
purposes, the influence of particular land use practices on the abundance and rate of
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J. R. Wilson et al.
turn, means that fewer fruit of the invasive Bugweed are eaten and dispersed by
animals and, by inference, reduced rates of spread. However, increased land use
intensity can also lead to an increase in the abundance of particular alien species such
as Australian Pest Pear (Opuntia stricta var. stricta) (Strum et al. 2015).
In protected areas the number of settlements and cultivated fields as well as the
intensity of grazing are generally far lower. Opportunities for long-distance dispersal
via road corridors are also significantly curtailed since the road network is less
extensive and used less. The boundaries of protected areas can, therefore, provide
an effective filter to the spread of invasive species which generally decline in
abundance inside the reserve. In their study of the Kruger National Park, for
example, Foxcroft et al. (2011) concluded that the park boundary provided an
effective barrier to invasions since the records of invasive plant species declined
rapidly beyond 1.5 km inside the park.
Proximity to highly-disturbed environments has an important influence on the
cover and richness of alien plants. In general, urban areas act both as points of
introduction and as bridge-heads for alien species (Gaertner et al. 2017; Potgieter
et al. 2020, Chap. 11). This is evident both at regional scales and at landscape scales
(Donaldson et al. 2014; Milton et al. 2007). Small towns in particular contain a high
diversity of alien plants and opportunities for spread into neighbouring natural
ecosystems (McLean et al. 2017, 2018). Roads and railways can also both facilitate
the spread of alien plants in South Africa and, by providing disturbance, provide
sites for establishment [cf. Faulkner et al. (2020, Chap. 12); though see also Kalwij
et al. (2008)]. For example, the distribution of the invasive Fountain Grass
(Pennisetum setaceum) closely tracks the road-network and associated disturbances
(Rahlao et al. 2010), while Schinus molle was often planted as a road-side shade tree,
creating foci for invasions (Richardson et al. 2010). Other human modifications of
the environment, e.g. fencing and the construction of telegraph poles, will presumably have had a similar range of impacts on invasions and their spread. The
development of highly disturbed agricultural fields poses a particular problem for
the spread of invasive aliens (van Rensburg et al. 2018), which is perhaps greatest for
riparian zones. Meek et al. (2010) surveyed the vegetation of a river corridor passing
through different types of land use in the fynbos. They showed that alien plants were
significantly more abundant at sites adjacent to agricultural fields and urban areas as
compared with natural areas or grazing lands.
Perhaps the best example of how land use disturbance facilitates the spread of
invasive species is the role that plantation forestry has played in the expansion of
alien conifers in South Africa. Van Wilgen and Richardson (2012) estimated that the
extent of invasive conifer stands, mostly pines, is more than four times greater than
the extent of formal forestry plantations (0.66 vs. 2.9 million ha), but that formal
plantations continue to provide propagules for invasive conifers to expand their
range into natural environments across South Africa.
The type and intensity of land use and how it has been practised over time are,
therefore, useful predictors of the distribution of alien species. For management
purposes, the influence of particular land use practices on the abundance and rate of
376
J. R. Wilson et al.