Box 3.1 (continued)
accessed by the deep roots of Prosopis which sustains high-density invasions.
There are also extensive Prosopis invasions along the lower Orange River.
Besides the suite of very widespread and highly damaging invasive trees
that are currently the focus of invasive plant management in the country
(Marais et al. 2004), a large number of other tree taxa are naturalised but
have yet to invade large areas. Many of these are known to be highly invasive
in other parts of the world, including Grevillea banksii (Red Silky Oak);
Melaleuca quinquenervia (Broad-leaved Paperbark), Mimosa pigra (Giant
Sensitive Tree) and Prunus serotina (Black Cherry). Many of the taxa that
already occupy large ranges in the country also have the potential to invade
much larger areas (Rouget et al. 2004). There is thus a large invasion debt for
alien trees in South Africa and more research is needed to improve our
understanding of their invasion ecology to guide management.
3.4 Extent of Invasions
Two major assessments have been made of the spatial extent of alien plant invasions
over large parts of South Africa. Unfortunately, the two assessments used very
different methods and focused on particular taxa, types of plants, or areas. This
means that they cannot be easily compared to show changes over time (see Supplementary Appendix 3.2). Despite such challenges, the two assessments have shed
light on key aspects of plant invasions in South Africa.
Versfeld et al. (1998) reported on a rapid reconnaissance of the extent of alien
plant invasions (mainly woody plant taxa) in South Africa, undertaken mostly during
1996 and 1997 to provide information needed to support the prioritisation of control
programmes for the newly established Working for Water programme (see van
Wilgen et al. 2020b, Chap. 21). This assessment involved a combination of field
mapping (some based on historical information), desktop and workshop mapping,
and expert consultations. All the taxa known to occur in a mapping unit were listed,
most at a species level, though some at a genus level (e.g. Acacia and Eucalyptus
were recorded as wattles and eucalypts). It concluded that about 10 million ha of
South Africa (about 8% of the country) had been invaded to some degree by the
~180 species that were mapped. The Western Cape had the most extensive invasions, followed by Limpopo and Mpumalanga. KwaZulu-Natal and the Eastern
Cape were not assessed at the same level of detail as the other provinces; invasions
in these regions were considered to be close to the percentage for Mpumalanga. The
assessment showed that invasions are concentrated in the wetter regions of the
country, and that the greatest number of invasive species occured in the Western
Cape and along the eastern escarpment from KwaZulu-Natal to Limpopo.
A second national-scale assessment of the extent of alien plant invasions was the
National Invasive Alien Plant Survey. This assessment, again in support of the
Working for Water Programme, was undertaken by the Agricultural Research
78
D. M. Richardson et al.
accessed by the deep roots of Prosopis which sustains high-density invasions.
There are also extensive Prosopis invasions along the lower Orange River.
Besides the suite of very widespread and highly damaging invasive trees
that are currently the focus of invasive plant management in the country
(Marais et al. 2004), a large number of other tree taxa are naturalised but
have yet to invade large areas. Many of these are known to be highly invasive
in other parts of the world, including Grevillea banksii (Red Silky Oak);
Melaleuca quinquenervia (Broad-leaved Paperbark), Mimosa pigra (Giant
Sensitive Tree) and Prunus serotina (Black Cherry). Many of the taxa that
already occupy large ranges in the country also have the potential to invade
much larger areas (Rouget et al. 2004). There is thus a large invasion debt for
alien trees in South Africa and more research is needed to improve our
understanding of their invasion ecology to guide management.
3.4 Extent of Invasions
Two major assessments have been made of the spatial extent of alien plant invasions
over large parts of South Africa. Unfortunately, the two assessments used very
different methods and focused on particular taxa, types of plants, or areas. This
means that they cannot be easily compared to show changes over time (see Supplementary Appendix 3.2). Despite such challenges, the two assessments have shed
light on key aspects of plant invasions in South Africa.
Versfeld et al. (1998) reported on a rapid reconnaissance of the extent of alien
plant invasions (mainly woody plant taxa) in South Africa, undertaken mostly during
1996 and 1997 to provide information needed to support the prioritisation of control
programmes for the newly established Working for Water programme (see van
Wilgen et al. 2020b, Chap. 21). This assessment involved a combination of field
mapping (some based on historical information), desktop and workshop mapping,
and expert consultations. All the taxa known to occur in a mapping unit were listed,
most at a species level, though some at a genus level (e.g. Acacia and Eucalyptus
were recorded as wattles and eucalypts). It concluded that about 10 million ha of
South Africa (about 8% of the country) had been invaded to some degree by the
~180 species that were mapped. The Western Cape had the most extensive invasions, followed by Limpopo and Mpumalanga. KwaZulu-Natal and the Eastern
Cape were not assessed at the same level of detail as the other provinces; invasions
in these regions were considered to be close to the percentage for Mpumalanga. The
assessment showed that invasions are concentrated in the wetter regions of the
country, and that the greatest number of invasive species occured in the Western
Cape and along the eastern escarpment from KwaZulu-Natal to Limpopo.
A second national-scale assessment of the extent of alien plant invasions was the
National Invasive Alien Plant Survey. This assessment, again in support of the
Working for Water Programme, was undertaken by the Agricultural Research
78
D. M. Richardson et al.