for a wide range of other purposes such as nature conservation, forest plantations,
mines, roads, and urban settlements (Fairbanks et al. 2000).
The impact of land use on invasions is context-dependent and changes across
spatial and temporal scales depending on regional climatic, habitat, and local
disturbance factors (Cabra-Rivas et al. 2016; Gonzalez-Moreno et al. 2014; Walker
et al. 2017). At large spatial scales environmental factors, especially climate, seem
most important (Terzano et al. 2018), while at landscape or habitat scales, local landuse practices also influence the establishment and spread of alien species (Thuiller
et al. 2006). Examining plant invasions in South Africa, Rouget and Richardson
(2003) found that there is a stronger response to environmental factors at large
spatial scales. Exploring this in more detail for invasive tree species that are also
commercially important crops in South Africa, Rouget et al. (2002) found that the
distribution of invasive stands was largely explained by climatic factors, even when
key factors that are known to drive invasions at the landscape scale, such as
propagule pressure from plantations and landuse, were included in models. In a
study at the landscape scale, Rouget et al. (2001) found soil pH to be the most
important variable for explaining invasive pine distribution in a highly fragmented
semi-arid shrubland. Similarly, Goodall et al. (2011) showed that the presence of the
herbaceous Pompom Weed in the grasslands of Gauteng Province is affected by
environmental factors such as rainfall, topography, and soil texture at large spatial
scales. However, at a more local level, historical contingencies, and specific land use
practices, were more important in determining where plants are found. In their study,
degraded rangelands, fallow fields, and drained wetlands exhibited a greater dominance of Pompom Weed than did rangelands that were covered by a healthy grass
sward. Well-managed rangelands in relatively good condition, therefore, were better
able to resist invasion by Pompom Weed than overgrazed, frequently-burned
rangelands in poor condition.
In rangelands, herbivory on both native and alien species affects the abundance
and rate of spread of invasive species. Steinschen et al. (1996) found that heavy
grazing in Namaqualand’s rangelands promoted the spread of annual alien grasses
such as Japanese Brome (Bromus pectinatus). Continuous, heavy grazing removes
the competitive dominance of perennial shrubs, which, in turn, promotes the spread
of annual grasses, with concomitant negative impacts for sheep production in the
affected region. However, local environmental conditions also influence specific
outcomes and biotic interactions are mediated in complex ways by abiotic factors
such as climate and soil. For example, a field experiment in the arid savanna of the
Northern Cape involving the manipulation of seedlings of the naturalised Peruvian
Peppertree (Schinus molle) showed that browsing reduced the establishment, growth
and survival of seedlings (Iponga et al. 2009). The precise outcome was strongly
influenced by soil type (greater success in fertile versus low nutrient status soils) and
microsite (greater survival under large native tree canopies than in the open).
The land use type (e.g. livestock production, arable lands) and the intensity of
land use do not always affect the establishment and rate of spread of invasive species
in intuitive ways. For example, Schor et al. (2015) showed how disturbance influences the spread of the invasive Bugweed in KwaZulu-Natal. They suggest that
13 The Role of Environmental Factors in Promoting and Limiting Invasions
375
mines, roads, and urban settlements (Fairbanks et al. 2000).
The impact of land use on invasions is context-dependent and changes across
spatial and temporal scales depending on regional climatic, habitat, and local
disturbance factors (Cabra-Rivas et al. 2016; Gonzalez-Moreno et al. 2014; Walker
et al. 2017). At large spatial scales environmental factors, especially climate, seem
most important (Terzano et al. 2018), while at landscape or habitat scales, local landuse practices also influence the establishment and spread of alien species (Thuiller
et al. 2006). Examining plant invasions in South Africa, Rouget and Richardson
(2003) found that there is a stronger response to environmental factors at large
spatial scales. Exploring this in more detail for invasive tree species that are also
commercially important crops in South Africa, Rouget et al. (2002) found that the
distribution of invasive stands was largely explained by climatic factors, even when
key factors that are known to drive invasions at the landscape scale, such as
propagule pressure from plantations and landuse, were included in models. In a
study at the landscape scale, Rouget et al. (2001) found soil pH to be the most
important variable for explaining invasive pine distribution in a highly fragmented
semi-arid shrubland. Similarly, Goodall et al. (2011) showed that the presence of the
herbaceous Pompom Weed in the grasslands of Gauteng Province is affected by
environmental factors such as rainfall, topography, and soil texture at large spatial
scales. However, at a more local level, historical contingencies, and specific land use
practices, were more important in determining where plants are found. In their study,
degraded rangelands, fallow fields, and drained wetlands exhibited a greater dominance of Pompom Weed than did rangelands that were covered by a healthy grass
sward. Well-managed rangelands in relatively good condition, therefore, were better
able to resist invasion by Pompom Weed than overgrazed, frequently-burned
rangelands in poor condition.
In rangelands, herbivory on both native and alien species affects the abundance
and rate of spread of invasive species. Steinschen et al. (1996) found that heavy
grazing in Namaqualand’s rangelands promoted the spread of annual alien grasses
such as Japanese Brome (Bromus pectinatus). Continuous, heavy grazing removes
the competitive dominance of perennial shrubs, which, in turn, promotes the spread
of annual grasses, with concomitant negative impacts for sheep production in the
affected region. However, local environmental conditions also influence specific
outcomes and biotic interactions are mediated in complex ways by abiotic factors
such as climate and soil. For example, a field experiment in the arid savanna of the
Northern Cape involving the manipulation of seedlings of the naturalised Peruvian
Peppertree (Schinus molle) showed that browsing reduced the establishment, growth
and survival of seedlings (Iponga et al. 2009). The precise outcome was strongly
influenced by soil type (greater success in fertile versus low nutrient status soils) and
microsite (greater survival under large native tree canopies than in the open).
The land use type (e.g. livestock production, arable lands) and the intensity of
land use do not always affect the establishment and rate of spread of invasive species
in intuitive ways. For example, Schor et al. (2015) showed how disturbance influences the spread of the invasive Bugweed in KwaZulu-Natal. They suggest that
13 The Role of Environmental Factors in Promoting and Limiting Invasions
375