21.7.3 Management of Prosopis Species
in the Northern Cape
Trees in the genus Prosopis (Mesquite) were introduced to provide a source of
fodder for livestock in the arid areas of South Africa, and subsequently became
invasive. Historical estimates for the rate of spread of Prosopis trees in South Africa
ranged from 3.5 to 18% per year (van den Berg 2010; Wise et al. 2012), which
implied that the invaded area could double every 5–8 years. In the Northern Cape,
the estimated total invaded area increased by almost a million hectares between 2002
and 2007, which is equivalent to 27.5% per year, and this occurred at a time during
which ZAR 390 million (2012 values) was spent on control (van Wilgen et al. 2012).
A more recent update (RT Shackleton unpubl. data) found that the public works
clearing projects had treated 203,000 ha of the area invaded by Prosopis between
2000 and 2015. Each site also received on average 2.7 follow up clearings. The cost
of these measures amounted to ZAR 1.8 billion (unadjusted for inflation) over the
same period. The project started in 1995, but cost estimates prior to the year 2000 are
not available. Between 2000 and 2016, Prosopis glandulosa (Mesquite), and
Prosopis hybrids increased their range from 40 to 112, and 390 to 481 quarterdegree grid cells, increases of 180% and 23% respectively (Henderson and Wilson
2017), suggesting that control is doing little to stop the spread of these trees.
21.7.4 Invasive Plant Control in the Kruger National Park
(Mpumalanga and Limpopo Provinces)
Van Wilgen et al. (2017) provided a recent review of alien plant control in the
Kruger National Park (KNP). There have been attempts at control in the KNP since
the mid-1950s, but in the late 1990s these attempts were broadened, and between
1997 and 2016, over ZAR 300 million was spent on invasive alien plant control.
Good progress was made with the control of several species, notably Sesbania
punicea (Red Sesbania), Opuntia stricta (Australian Pest Pear), Lantana camara
(Lantana) and two species of invasive alien aquatic plants. In all of these cases,
progress with reducing populations of the invasive species was due to biological
control. Nonetheless, much effort was also directed towards species that were
subsequently recognised as being of lower priority. For example, 38% of available
funds was spent on alien annuals between 1997 and 2016. Funds were sometimes
directed towards these annuals to meet the goals of employment creation in areas
where priority species were not present. The absence of documented assessments of
the potential impact of various species also allowed managers to base their decisions
on perceptions of the relative impact of candidate species. In addition, because
management goals were focussed on inputs (funds disbursed, employment created)
21 The Extent and Effectiveness of Alien Plant Control Projects in South Africa
615
in the Northern Cape
Trees in the genus Prosopis (Mesquite) were introduced to provide a source of
fodder for livestock in the arid areas of South Africa, and subsequently became
invasive. Historical estimates for the rate of spread of Prosopis trees in South Africa
ranged from 3.5 to 18% per year (van den Berg 2010; Wise et al. 2012), which
implied that the invaded area could double every 5–8 years. In the Northern Cape,
the estimated total invaded area increased by almost a million hectares between 2002
and 2007, which is equivalent to 27.5% per year, and this occurred at a time during
which ZAR 390 million (2012 values) was spent on control (van Wilgen et al. 2012).
A more recent update (RT Shackleton unpubl. data) found that the public works
clearing projects had treated 203,000 ha of the area invaded by Prosopis between
2000 and 2015. Each site also received on average 2.7 follow up clearings. The cost
of these measures amounted to ZAR 1.8 billion (unadjusted for inflation) over the
same period. The project started in 1995, but cost estimates prior to the year 2000 are
not available. Between 2000 and 2016, Prosopis glandulosa (Mesquite), and
Prosopis hybrids increased their range from 40 to 112, and 390 to 481 quarterdegree grid cells, increases of 180% and 23% respectively (Henderson and Wilson
2017), suggesting that control is doing little to stop the spread of these trees.
21.7.4 Invasive Plant Control in the Kruger National Park
(Mpumalanga and Limpopo Provinces)
Van Wilgen et al. (2017) provided a recent review of alien plant control in the
Kruger National Park (KNP). There have been attempts at control in the KNP since
the mid-1950s, but in the late 1990s these attempts were broadened, and between
1997 and 2016, over ZAR 300 million was spent on invasive alien plant control.
Good progress was made with the control of several species, notably Sesbania
punicea (Red Sesbania), Opuntia stricta (Australian Pest Pear), Lantana camara
(Lantana) and two species of invasive alien aquatic plants. In all of these cases,
progress with reducing populations of the invasive species was due to biological
control. Nonetheless, much effort was also directed towards species that were
subsequently recognised as being of lower priority. For example, 38% of available
funds was spent on alien annuals between 1997 and 2016. Funds were sometimes
directed towards these annuals to meet the goals of employment creation in areas
where priority species were not present. The absence of documented assessments of
the potential impact of various species also allowed managers to base their decisions
on perceptions of the relative impact of candidate species. In addition, because
management goals were focussed on inputs (funds disbursed, employment created)
21 The Extent and Effectiveness of Alien Plant Control Projects in South Africa
615