how do alien species get here, and spread?

In document The sTaTus of biological invasions and Their managemenT in souTh africa 2017 (Page 192-196)

9. KeY PoLiCY-reLevaNt MeSSaGeS

9.2. how do alien species get here, and spread?

Opportunities for the introduction of high-risk alien species are increasing in line with increases in trade and travel. While effective protocols are being developed and implemented to prevent the legal introduction of high-risk alien species, there is little capacity in place to prevent unintentional and deliberate illegal introductions of high-risk alien species (see sections 3.3.1, 3.3.3, and 6.2.)

international visitors and imported goods currently enter south africa through 72 official ports of entry (harbours, airports and border posts). The volume of trade and number of people that enter through these ports is increasing; the

tHe SituAtion

The rate of introduction of new unregulated species is increasing in line with increases in trade and travel


The number currently stands at


PeR yeAR

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value of tourism, for example, has increased from around Zar130 billion in 1995 to Zar350 billion in 2017, and is predicted to increase to around Zar530 billion by 2027. greater volumes of trade and tourism can be accompanied by increasing rates of alien species introductions. The rate at which alien species are being introduced has been increasing steadily, from around 35 species per decade in the 1950s to 70 species per decade between 2000 and 2010. historically, most species have entered south africa from overseas, but the growth in trade across africa over the past decade means that an increasing number of alien species are likely to be introduced to other countries in africa and then subsequently spread from there to south africa.

intentional introductions of alien species have taken place for a range of reasons. Plants were imported for agriculture and for forestry, or as ornamentals for use in gardens and parks. animals were imported for agriculture, aquaculture or mariculture, for recreational fishing or hunting, and to supply the pet trade. some of the alien species that have become invasive were deliberately released into nature with the intention of establishing self-sustaining populations, for example trout and bass into streams and rivers. others have simply escaped cultivation or captivity, for example pine and wattle trees introduced to establish commercial forestry plantations. regulations are now in place to cover the future intentional importation of alien species, and procedures are being developed to analyse the risks posed before import permits are granted. if these procedures are adhered to, and if sufficient capacity is maintained in perpetuity, the risk posed by legal introductions will be substantially reduced.

currently, however, the dea only has a consistent presence at one of the 72 official ports of entry (occasional joint operations are carried out at other entry points, in conjunction with other departments), and the brunt of border inspections falls to the department of agriculture, forestry and fisheries (daff). The interception and prevention of import of potentially damaging invasive species might offset the cost of vigilance, and an increase in this capacity should deliver positive returns on investment.

Figure 9.1 Sniffer dogs are deployed at o.R. Tambo International Airport, where they assist in the detection of goods that are potentially illegal or harmful, including alien species.

Photograph: C. Mercado.

Tus of biological invasions and Their managemenT in souTh africa2017

Once introduced into the country, alien species can disperse rapidly along South Africa’s transport networks (see section 3.3.2)

south africa has an extensive transport network along which commodity contaminants or stowaways can be dispersed. There is also a thriving internal trade in alien species for a variety of purposes. managing the internal transport infrastructure for the purposes of preventing the dispersal of high-risk alien species is very difficult, and there has been no comprehensive analysis of the practicalities of this. This further emphasises the importance of preventing introduction in the first place.

photographer: K. faulkner.

Figure 9.2 South African roads provide many opportunities for the spread of alien species along a well-developed and heavily-used transport network.

Over 2000 alien species are present outside of captivity or cultivation in South Africa, and at least a third of these have become invasive. Many of these invasive species are now entering a phase of rapid expansion, so even if no further alien species are introduced, the problem will continue to grow due to the species already in the country (see sections 4.2 and 4.3)

most recorded invasive species are plants (574 species); other important groups include terrestrial invertebrates (107 species) and marine invertebrates (46 species). other groups (mammals, reptiles, birds, freshwater fish and amphibians) each contribute less than 20 species. for many groups of species (such as invertebrates, most marine species, and microbes) there are likely to be many alien species present that have not yet been detected and recorded. The number of invasive species is also expected to continue to increase as new species become invasive. note: for the purposes of this report, alien species are considered to be invasive if they were formally reported to have survived, reproduced, and spread unaided over considerable distances/areas, rather than as having been recorded as causing significant negative impacts.

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Number of species Terrestrial and freshwater plants Terrestrial invertebrates Marine invertebrates Microbes Birds Mammals freshwater invertebrates Reptiles freshwater fish Amphibians Marine plants0

100 200 300 400 500 600 700 800 900 1000

Non-invasive species Invasive species

Figure 9.3 Terrestrial and freshwater plants and invertebrates currently make up the bulk of known alien species in the country.

other groups make up smaller numbers. A large proportion of naturalised alien plants have gone on to become invasive.

The rate of spread of invasive species is (typically) slow as the species establishes, then rapid as it colonises new areas, slowing down as the available habitat for expansion becomes limiting. information from the southern african Plant invaders atlas (saPia) – the most reliable source of information on the distribution of invasive plants in south africa – reveals that all invasive alien plant species not subjected to biological control have increased their ranges over the past 15 years, some substantially. pompom weed (Campuloclinium macrocephalum, a herbaceous invader of grasslands) has increased in range by 670%; and famine weed (Parthenium hysterophorus, an annual invader of overgrazed rangelands and savannas) by 493%. even long established invasive tree species, that might be expected to be nearing their range limits, such as mesquite (Prosopis glandulosa) and river red gum (Eucalyptus camaldulensis), have increased in range by 180% and 61% respectively. These species have large impacts, and the impacts grow as the species spread. Thus, even if no further introductions of potentially invasive species takes place, the problems associated with invasive species will increase, a phenomenon known as

“invasion debt”.

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Area occupied

Initial slow growth

rapid expansion

Figure 9.4 The area occupied by invasive species typically initially grows at a slow rate, and then accelerates until the majority of the available habitat is occupied. The time for species to enter a phase of rapid expansion is typically in the order of several decades to centuries.

given most invasive species were introduced to South Africa in the past 200 years, the majority of these species are in or are entering the phase of rapid expansion and thus the number of species with severe impacts is set to increase.

In document The sTaTus of biological invasions and Their managemenT in souTh africa 2017 (Page 192-196)