sites, but do not establish or persist, as they require enhanced nutrients and moisture,
as well as long fire-free intervals (Manders and Richardson 1992).
5. The Influence of Fire Regimes
The effect of fires on invasions also depends on fire frequency. Invasive alien trees
and shrubs, such as Pinus and Hakea species, invade the treeless fynbos because
they are pre-adapted to fire-prone ecosystems, and can establish and reach reproductive maturity between fires. The frequency and intensity of fires in Africa has also
been postulated as the main reason why African grasses are widespread invaders
elsewhere in the world, but alien grasses are relatively unsuccessful in Africa (Visser
et al. 2016). Higher-rainfall Grassland and Savanna Biomes can burn every second
year, killing most serotinous species before they reach reproductive age, thus
preventing invasions [Table 13.2, though cf. species such as Pompom Weed
(Campuloclinium macrocephalum) and American Bramble (Rubus cuneifolius)
that are tolerant of frequent fires and thus able to invade]. Fynbos, on the other
hand, usually burns at intervals of 10 years or more (Kraaij and van Wilgen 2014)
which allows serotinous plants to mature and invade. The short fire cycles explain
why Pinus patula is not as aggressively invasive in the Grassland Biome (despite
widespread plantations) as other Pinus species in the Fynbos. Importantly, however,
invasive species can also alter fire frequencies to their advantage, e.g. frequent fires
can prevent resprouting or reseeding species such as Australian wattles (Acacia
species), but once such invasive species become dominant, they shade out grasses,
remove the primary fuel for fires, and alter fire frequency in a way that gives them a
competitive advantage (Gaertner et al. 2014).
Of course, fire does not act alone in promoting invasions, and there are strong
interactions between herbivory, fire, and invasion. Over-grazing removes fuel before
the vegetation can burn, leading to the removal of fire from the dynamics of the
ecosystem and allowing some invasive plants to colonise areas disturbed by overgrazing (O’Connor and van Wilgen 2020, Chap. 13). In the Fynbos, the interaction
between fire and wind has been crucial in shaping the invasion window for alien
trees and shrubs such as hakeas and pines (Richardson and Brown 1986).
13.7 Interactions Between Land Use and Other Drivers
Land use reflects the socioeconomic function of land (Martinez and Mollicone 2012)
and refers to the multitude of ways in which people utilise, manipulate, manage, or
unintentionally modify the environment, usually to obtain a product that can be
consumed, traded or sold. Abiotic factors, such as climate, geomorphology and soils,
play a key role in determining the nature and intensity of land use as well as the
influence that land use has on invasives. In South Africa, most of the environment
(~70%) is used either for livestock production or wildlife ranching (Meissner et al.
2013) although crop cultivation (dryland and irrigated) also occurs over a significant
part (~11–13%) of the country (Schoeman et al. 2013). The remaining area is used
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as well as long fire-free intervals (Manders and Richardson 1992).
5. The Influence of Fire Regimes
The effect of fires on invasions also depends on fire frequency. Invasive alien trees
and shrubs, such as Pinus and Hakea species, invade the treeless fynbos because
they are pre-adapted to fire-prone ecosystems, and can establish and reach reproductive maturity between fires. The frequency and intensity of fires in Africa has also
been postulated as the main reason why African grasses are widespread invaders
elsewhere in the world, but alien grasses are relatively unsuccessful in Africa (Visser
et al. 2016). Higher-rainfall Grassland and Savanna Biomes can burn every second
year, killing most serotinous species before they reach reproductive age, thus
preventing invasions [Table 13.2, though cf. species such as Pompom Weed
(Campuloclinium macrocephalum) and American Bramble (Rubus cuneifolius)
that are tolerant of frequent fires and thus able to invade]. Fynbos, on the other
hand, usually burns at intervals of 10 years or more (Kraaij and van Wilgen 2014)
which allows serotinous plants to mature and invade. The short fire cycles explain
why Pinus patula is not as aggressively invasive in the Grassland Biome (despite
widespread plantations) as other Pinus species in the Fynbos. Importantly, however,
invasive species can also alter fire frequencies to their advantage, e.g. frequent fires
can prevent resprouting or reseeding species such as Australian wattles (Acacia
species), but once such invasive species become dominant, they shade out grasses,
remove the primary fuel for fires, and alter fire frequency in a way that gives them a
competitive advantage (Gaertner et al. 2014).
Of course, fire does not act alone in promoting invasions, and there are strong
interactions between herbivory, fire, and invasion. Over-grazing removes fuel before
the vegetation can burn, leading to the removal of fire from the dynamics of the
ecosystem and allowing some invasive plants to colonise areas disturbed by overgrazing (O’Connor and van Wilgen 2020, Chap. 13). In the Fynbos, the interaction
between fire and wind has been crucial in shaping the invasion window for alien
trees and shrubs such as hakeas and pines (Richardson and Brown 1986).
13.7 Interactions Between Land Use and Other Drivers
Land use reflects the socioeconomic function of land (Martinez and Mollicone 2012)
and refers to the multitude of ways in which people utilise, manipulate, manage, or
unintentionally modify the environment, usually to obtain a product that can be
consumed, traded or sold. Abiotic factors, such as climate, geomorphology and soils,
play a key role in determining the nature and intensity of land use as well as the
influence that land use has on invasives. In South Africa, most of the environment
(~70%) is used either for livestock production or wildlife ranching (Meissner et al.
2013) although crop cultivation (dryland and irrigated) also occurs over a significant
part (~11–13%) of the country (Schoeman et al. 2013). The remaining area is used
374
J. R. Wilson et al.