13.4 Climate
South Africa has hot summers, which, combined with aridity, produces significant
water stress. Moreover, rainfall seasonality changes dramatically across the country
(Fig. 13.1) influencing runoff and evapotranspiration (Schulze and Maharaj 2007b).
The majority of the country receives summer rains from the Intertropical Convergence Zone to the east. However, along the west and southwest coast winter rainfall
comes from westerly winds over the cold Benguela current (Chase and Meadows
2007; van Wilgen et al. 2020a, Chap. 1). Between these areas, rainfall is intermittent
throughout the year. This variation in rainfall seasonality creates distinct phenology
among native flora and fauna, and often demarcates the distribution of species
(Colville et al. 2014), or maps onto genetic disjunctions within species (Tolley
et al. 2014). But such limitations can be alleviated by human-made irrigation
schemes or dams [for a discussion of interbasin water transfer (IWT) schemes in
South Africa see Box 12.2 in Faulkner et al. (2020), Chap. 12; and Muller (1999)].
Anthropogenic changes in seasonal water availability has resulted in the range
expansion of a variety of native species (Okes et al. 2008), and facilitated invasions
(Davies et al. 2013; De Villiers et al. 2016; Measey et al. 2020, Chap. 5; Moodley
et al. 2014).
The incidence of frost also varies significantly across the country. The low-lying
coastal areas tend to be frost-free, but the high-elevation central plateau experiences
frost in most years. While frost incidence has severely limited the presence of native
trees on the Highveld, many alien trees are frost-hardy and able to invade treeless
ecosystems. Moreover, frost damage (“frost heave”) in otherwise dense and impenetrable grass swards in the Cathedral Peak area of the Drakensberg has been
implicated in creating opportunities for the establishment of Patula Pine (Pinus
patula) seedlings (Richardson and Bond 1991).
Insect establishment and spread is similarly known to be affected by abiotic
conditions. For example, Zimmermann and Moran (1991) argued that rain, hail,
and heavy wind (together with predation of eggs by native ants) provided a substantial barrier to the establishment of the biological control agent Cactus Moth
(Cactoblastis cactorum); and Singh and Olckers (2017) argued that temperature and
humidity limited the spread of the biological control agent Anthonomus santacruzi
that was introduced to control Bugweed (Solanum mauritianum).
13.4.1 Species Distribution Models
The marked gradients in temperature, mean annual precipitation, rainfall seasonality,
and frost (Fig. 13.1) mean that South Africa is arguably well suited to using species
distribution models to predict invasions (Rouget et al. 2004). Species distribution
models, which attempt to quantify the environmental niche suitable for a species,
have been used to predict the potential distribution of several major plant invaders
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J. R. Wilson et al.
South Africa has hot summers, which, combined with aridity, produces significant
water stress. Moreover, rainfall seasonality changes dramatically across the country
(Fig. 13.1) influencing runoff and evapotranspiration (Schulze and Maharaj 2007b).
The majority of the country receives summer rains from the Intertropical Convergence Zone to the east. However, along the west and southwest coast winter rainfall
comes from westerly winds over the cold Benguela current (Chase and Meadows
2007; van Wilgen et al. 2020a, Chap. 1). Between these areas, rainfall is intermittent
throughout the year. This variation in rainfall seasonality creates distinct phenology
among native flora and fauna, and often demarcates the distribution of species
(Colville et al. 2014), or maps onto genetic disjunctions within species (Tolley
et al. 2014). But such limitations can be alleviated by human-made irrigation
schemes or dams [for a discussion of interbasin water transfer (IWT) schemes in
South Africa see Box 12.2 in Faulkner et al. (2020), Chap. 12; and Muller (1999)].
Anthropogenic changes in seasonal water availability has resulted in the range
expansion of a variety of native species (Okes et al. 2008), and facilitated invasions
(Davies et al. 2013; De Villiers et al. 2016; Measey et al. 2020, Chap. 5; Moodley
et al. 2014).
The incidence of frost also varies significantly across the country. The low-lying
coastal areas tend to be frost-free, but the high-elevation central plateau experiences
frost in most years. While frost incidence has severely limited the presence of native
trees on the Highveld, many alien trees are frost-hardy and able to invade treeless
ecosystems. Moreover, frost damage (“frost heave”) in otherwise dense and impenetrable grass swards in the Cathedral Peak area of the Drakensberg has been
implicated in creating opportunities for the establishment of Patula Pine (Pinus
patula) seedlings (Richardson and Bond 1991).
Insect establishment and spread is similarly known to be affected by abiotic
conditions. For example, Zimmermann and Moran (1991) argued that rain, hail,
and heavy wind (together with predation of eggs by native ants) provided a substantial barrier to the establishment of the biological control agent Cactus Moth
(Cactoblastis cactorum); and Singh and Olckers (2017) argued that temperature and
humidity limited the spread of the biological control agent Anthonomus santacruzi
that was introduced to control Bugweed (Solanum mauritianum).
13.4.1 Species Distribution Models
The marked gradients in temperature, mean annual precipitation, rainfall seasonality,
and frost (Fig. 13.1) mean that South Africa is arguably well suited to using species
distribution models to predict invasions (Rouget et al. 2004). Species distribution
models, which attempt to quantify the environmental niche suitable for a species,
have been used to predict the potential distribution of several major plant invaders
364
J. R. Wilson et al.