Hybridisation also complicates the compilation of an alien flora for South Africa.
A notable example is the genus Prosopis. Published records detail the introduction
of at least seven Prosopis species (P. cineraria, P. glandulosa, P. juliflora,
P. laevigata, P. pubescens, P. tamarugo and P. velutina) (Poynton 1990). However,
preliminary molecular studies, together with variation in seed morphology, suggest
that most populations in South Africa are hybrids, and that at least one previously
unrecorded species, P. hassleri, is present (Mazibuko 2012). The presence of
P. chilensis, P. glandulosa, and P. laevigata was confirmed, but neither
P. juliflora nor P. velutina, were identified using the selected molecular markers.
While the taxonomy of the genus remains problematic, there is no doubt that
multiple species were introduced into South Africa (Poynton 1990). Moreover,
Mazibuko’s (2012) results, suggest that most Prosopis taxa hybridise freely in
South Africa and that invasive populations represent a hybrid swarm.
The challenges associated with producing an accurate and definitive alien flora
reviewed above point to two main conclusions: (1) lists of alien species for
South Africa (like elsewhere) have substantial errors, although the actual error
rates are unknown. While some listed species are likely not present, lists generally
substantially underestimate the number of alien species that have been introduced;
and (2) lists need to be regularly updated based on agreed definitions, current
nomenclature, and evidence that species are still present. The production of a register
of alien species is a requirement of the national regulations, and the goal is for this to
form part of the triennial reports on the status of biological invasions led by the
South African National Biodiversity Institute (Wilson et al. 2017a; van Wilgen and
Wilson 2018; Wilson et al. 2018).
3.3.2 A Preliminary Enumeration of South Africa’s
Naturalised Flora
We used the list of naturalised plant taxa for South Africa produced for the first
national status report on biological invasions for the purposes of this chapter
(Appendix 3 in van Wilgen and Wilson 2018). We made a few minor modifications
based on our knowledge of the introduction status of many taxa (i.e. their position on
the introduction-naturalisation-invasion continuum; Richardson and Pyšek 2012),
using published and unpublished information, and correspondence with colleagues.
We also made some changes to accommodate recent taxonomic treatments. The list
in Supplementary Appendix 3.1 includes 759 taxa, including all 327 plant taxa listed
in the national legislation. Even though many taxa have only naturalised in the last
few decades, the number of taxa listed here is well below the “at least 1000 candidate
species” considered by Wells et al. (1986). This is due to our strict requirement for
inclusion as naturalised, namely that there had to be evidence for populations that
were self-sustaining for at least 10 years (Pyšek et al. 2004).
3 The Biogeography of South African Terrestrial Plant Invasions
75
A notable example is the genus Prosopis. Published records detail the introduction
of at least seven Prosopis species (P. cineraria, P. glandulosa, P. juliflora,
P. laevigata, P. pubescens, P. tamarugo and P. velutina) (Poynton 1990). However,
preliminary molecular studies, together with variation in seed morphology, suggest
that most populations in South Africa are hybrids, and that at least one previously
unrecorded species, P. hassleri, is present (Mazibuko 2012). The presence of
P. chilensis, P. glandulosa, and P. laevigata was confirmed, but neither
P. juliflora nor P. velutina, were identified using the selected molecular markers.
While the taxonomy of the genus remains problematic, there is no doubt that
multiple species were introduced into South Africa (Poynton 1990). Moreover,
Mazibuko’s (2012) results, suggest that most Prosopis taxa hybridise freely in
South Africa and that invasive populations represent a hybrid swarm.
The challenges associated with producing an accurate and definitive alien flora
reviewed above point to two main conclusions: (1) lists of alien species for
South Africa (like elsewhere) have substantial errors, although the actual error
rates are unknown. While some listed species are likely not present, lists generally
substantially underestimate the number of alien species that have been introduced;
and (2) lists need to be regularly updated based on agreed definitions, current
nomenclature, and evidence that species are still present. The production of a register
of alien species is a requirement of the national regulations, and the goal is for this to
form part of the triennial reports on the status of biological invasions led by the
South African National Biodiversity Institute (Wilson et al. 2017a; van Wilgen and
Wilson 2018; Wilson et al. 2018).
3.3.2 A Preliminary Enumeration of South Africa’s
Naturalised Flora
We used the list of naturalised plant taxa for South Africa produced for the first
national status report on biological invasions for the purposes of this chapter
(Appendix 3 in van Wilgen and Wilson 2018). We made a few minor modifications
based on our knowledge of the introduction status of many taxa (i.e. their position on
the introduction-naturalisation-invasion continuum; Richardson and Pyšek 2012),
using published and unpublished information, and correspondence with colleagues.
We also made some changes to accommodate recent taxonomic treatments. The list
in Supplementary Appendix 3.1 includes 759 taxa, including all 327 plant taxa listed
in the national legislation. Even though many taxa have only naturalised in the last
few decades, the number of taxa listed here is well below the “at least 1000 candidate
species” considered by Wells et al. (1986). This is due to our strict requirement for
inclusion as naturalised, namely that there had to be evidence for populations that
were self-sustaining for at least 10 years (Pyšek et al. 2004).
3 The Biogeography of South African Terrestrial Plant Invasions
75