number of quarter degree grid cells occupied per species

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4. the StatuS of aLieN SPeCieS

4.3. The extent of alien species in south africa

4.3.2. number of quarter degree grid cells occupied per species

Quarter degree grid cells (Qdgcs) provide the most convenient way to assess the extent of occurrence of alien species in south africa, as most distribution data are available at that scale. There are 1 966 Qdgcs in south africa, and each cell measures 15 minutes of latitude by 15 minutes of longitude. The cells are approximately 676 km2 in area (cells in the north of south africa are approximately 11% larger than those in the south). data on the extent are available for some, but not all, terrestrial taxa; essentially, these data only had a high degree of completeness and accuracy in the case of birds and terrestrial and freshwater plants (Table 4.2). distribution data were not available at the scale of Qdgcs for marine species.

large numbers of species have relatively restricted distributions (figure 4.3), and only in the case of plants and birds are there widespread species found in > 500 Qdgcs. at least one alien reptile (Python natalensis x molurus, a hybirid of the southern african and burmese python) and two alien terrestrial invertebrate species (Cornu aspersum, the common garden snail and Vanessa cardui, the painted lady) are relatively widespread (occurring in > 100 Qdgcs), although the data coverage is poor, so there is a low level of confidence in these estimates.

alien species in other taxa (amphibians, freshwater invertebrates and mammals) appear to be less widespread.

There are no reliable data to illustrate the distribution of freshwater fish, fungi and microbial species at this scale.

Tus of biological invasions and Their managemenT in souTh africa2017

TeRReSTRIAl AND fReSHWATeR PlANTS (n=787)

occupancy (by QDgc)

1 2 5 10 20 50 100 200 500

Species (ordered by occupancy of QDgc)

MAMMAlS (n=4)

Species (ordered by occupancy of QDgc)

occupancy (by QDgc)

5 5 10

BIRDS (n=44)

occupancy (by QDgc)

1 5 10 50 100 500 1000

Species (ordered by occupancy of QDgc)

RePTIleS (n=25)

occupancy (by QDgc)

1 2 5 10 20 50 100

Species (ordered by occupancy of QDgc)

AMPHIBIANS (n=20)

occupancy (by QDgc)

1 2 5 10 20 50 100

Species (ordered by occupancy of QDgc)

fReSHWATeR fISH (n=18)

occupancy (by QDgc)

1 2 5 10 20 50 100

Species (ordered by occupancy of QDgc)

TeRReSTRIAl INveRTeBRATeS (n=29)

occupancy (by QDgc)

1 2 5 10 20 50

Species (ordered by occupancy of QDgc)

fReSHWATeR INveRTeBRATeS (n=18)

occupancy (by QDgc)

1 2 5 10

Species (ordered by occupancy of QDgc)

Figure 4.3 The distribution in alien range sizes for alien species in South Africa. Note the range sizes are plotted on a log scale.

QDgc = quarter degree grid cell.

ChaPter 4IThe sTaTus of alien sP

Box 4.1 TEN EXAmplES Of WIdESpREAd INvASIvE AlIEN SpECIES IN SOuTh AfRICA There are 556 alien taxa listed as invasive in South Africa’s Alien and Invasive Species Regulations, and there are also many more invasive species that are not listed (Appendix 3). A relatively small subset of these species has become particularly widespread and often problematic, and hundreds of millions of rands have been spent annually on attempts to control some of them. A selection of ten of these species is presented here to illustrate the problem.

Prosopis glandulosa var. torreyana (honey mesquite):

These trees were introduced from north and central America as a source of fodder for livestock in arid areas. They have subsequently invaded extensive areas in the karoo and arid savanna, where they impact negatively on biodiversity, rangeland condition and groundwater resources (Box 4.2).

Because of their usefulness as a source of fodder, they are listed as category 3 (may be retained but not replaced) in the Northern Cape, but as category 1(b) elsewhere (must be controlled). Although there is some biological control available for this species, it is not effective. Endeavours to mechanically control the species have also not been effective, and it continues to spread.

Photograph: L. Otto. Map: L. Henderson.

Acacia mearnsii (black wattle):

These trees were introduced from Australia to provide a source of tannins from bark and for wood products.

They have extensively invaded the relatively humid parts of South Africa, notably along rivers. They have negative impacts on water resources. Because of their commercial value, they are listed as category 2 (may be retained, and traded, with a permit, but should be controlled elsewhere).

Biological control aimed at reducing seed production appears to be increasing in effectiveness, but soil-stored seed banks will probably persist for many years.

Photograph: SANBI. Map: L. Henderson

Tus of biological invasions and Their managemenT in souTh africa2017

Pinus patula, P. pinaster and P. radiata (pine trees):

These trees were introduced from Europe and North America as a source of timber. They have invaded extensively in the fynbos biome, where they impact negatively on water runoff from mountain catchments, reduce biodiversity and increase the intensity of fires. Because of their commercial value, they are listed as category 2 (may be retained, and traded, with a permit, but should be controlled elsewhere). Some progress has been made with regard to mechanical control in some areas, but at a broader scale these species continue to spread rapidly, and pose a major long-term threat to the integrity of fynbos ecosystems and the the ability of fynbos-clad catchments to deliver water runoff to dams. Biological control options have not yet been implemented for these species.

Photograph: SANBI. Map: L. Henderson

Eucalyptus camaldulensis (river red gum):

These trees were introduced from Australia to provide a variety of useful products and services. They have become highly invasive along rivers in places throughout the country, where they often dominate the riparian vegetation. Eucalypts are known to use large amounts of water, so they probably reduce water levels in rivers, as well as reduce the biodiversity of natural vegetation. The species is listed as category 1(b) (must be controlled), but as category 2 (may be retained, and traded, with a permit) in a range of habitats including plantations, windrows, bee forage areas, woodlots and in gardens, to cater for various uses. The effectiveness of attempts to control this species has not been assessed, nor are there any biological control agents available for this species.

Photograph: H. Klein. Map: L. Henderson

Chromolaena odorata (triffid weed):

This shrub originates from north, south and central America, and was probably accidentally introduced to South Africa. It has spread along much of the KwaZulu-Natal Coast and the escarpment and lowveld of the mpumalanga and limpopo provinces. It can form dense thickets and is regarded as an ecosystem transformer, almost certainly impacting negatively on range condition and biodiversity. It is placed in category 1(b) (must be controlled). There has been some success in reducing their populations in protected areas (see section 6.4.2).

Biological control options are available, but their effectiveness has also not been assessed.

Photograph: T. Schoch. Map: L. Henderson

ChaPter 4IThe sTaTus of alien sP

Parthenium hysterophorus (famine weed):

This annual herb is indigenous to tropical America, and has been present in South Africa for over 100 years. It has however only recently begun to spread rapidly, and it now occurs extensively in northern KwaZulu-Natal, Swaziland, and mpumalanga. The species causes severe allergic reactions in many people who come into contact with it, as well as in livestock and wildlife. It has the potential to substantially reduce rangeland condition. It is placed in category 1(b) (must be controlled). Serious attempts to control this species have only recently begun. Indications are that mechanical control alone will not contain this species, but biological control options are being investigated, and they hold the potential to reduce spread rates and vigour.

Photograph: SANBI. Map: L. Henderson

Lantana camara (lantana):

This shrub was originally introduced into South Africa from south and central America as a garden ornamental. It has extensively invaded the relatively humid parts of South Africa, where it can form dense thickets and transform ecosystems. The species presumably impacts negatively on biodiversity and is also poisonous. It is placed in category 1(b) (must be controlled).

much effort has been directed towards biological control of this species, where the level of control has been assessed as substantial.

Photograph: SANBI. Map: L. Henderson

Micropterus dolomieu (small-mouth bass):

This species was imported from north America to provide freshwater angling opportunities. Anglers have introduced the species to several river systems, particularly in the Western and Eastern Cape provinces. It preys on indigenous fishes and invertebrates and can change the structure of freshwater species communities. Its regulation is complex. It is placed in category 1(b) (must be controlled) in protected areas, and in category 2 or 3 in dams and rivers where it already occurs. Once established, control is not feasible except in small streams or dams where it may be possible to extirpate populations.

Photograph: R. Duane, U.S. Fish and Wildlife Service. Map: SAIAB.

Tus of biological invasions and Their managemenT in souTh africa2017

Acridotheres tristis (common mynah):

This species was brought to durban from Asia in 1888, from where it has spread to most of northeast South Africa. It favours urban environments (where populations can reach hundreds of thousands), but probably has negligible impacts on natural and rural habitats. It is listed as category 3 (does not require control, but may not be moved or traded). large-scale control would probably be impossible, but occasional removal of isolated individuals has been carried out, for example in Cape Town and the Kruger National park.

Photograph: R. Taylor. Map: ADU.

Mytilus galloprovincialis (Mediterranean mussel):

This species was accidentally introduced from Europe to South Africa’s west coast in about 1979, almost certainly by shipping. It spread rapidly to Namibia, and more slowly to the Eastern Cape.

It now dominates most of the rocky shores of the west and south coasts, where it forms dense, multi-layered beds that monopolise space on intertidal rocks. It can be beneficial as a food source to both humans and animals (for example the African oystercatcher, Haematopus ostralegus). listed as category 2 (cultivation and trade allowed with a permit). Its control would probably be impossible, given the wide range, prolific reproductive habits, and widely-dispersing larvae.

Photograph: S. Miza. Map drawn from data supplied by Dr T. Robinson.

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