Africa is home to hundreds of IAS – both plant and animal – but the magnitude of the problem varies from country to country, and from ecosystem to ecosystem. In many parts, freshwater ecosystems are particularly at risk – with IAS surpassing habitat loss as the number one cause of biodiversity loss.

Invasive alien species are a problem in diverse ecosystems in Northern, Western, Central, Eastern and Southern Africa and in the Western Indian Ocean (WIO) islands: they affect both savannahs and tropical forests and they are found on land, in freshwater systems, along the coast, and in the ocean. (UNEP 2004).

Figure 1: The incidence of IAS in Africa Virtually all countries in the region are affected by IAS. In 2004, IUCN – the World Conservation Union (IUCN) identified 81 IAS in South Africa, 49 in Mauritius, 44 in Swaziland, 37 in Algeria and Madagascar, 35 in Kenya, 28 in Egypt, 26 in Ghana and Zimbabwe, and 22 in Ethiopia (IUCN/SSC/ISSG 2004). (See Figure 1). In some countries there may be under-reporting of the incidence of IAS.

Many IAS found in Africa are included on a global list of the 100 worst IAS (IUCN/SSG/ISSG 2004). These include the infamous, Eichhornia crassipes (water hyacinth) (see Box 5); economically important species including the Nile perch, Oreochromis mossambicus (Mozambique tilapia) and Acacia mearnsi (black wattle) (see Box 6); species introduced for biological control, such as Acridotheres tristis (Indian myna) (see Box 3) and Bufo marinus (cane toad); and ornamentals such as Lantana camara. There are many other IAS which present serious challenges to regional efforts to conserve the environment and to meet development objectives – the foundation of social, economic and environmental sustainability in Africa.

In some countries, IAS have become a major ecological, social and economic problem despite the existence of legal measures and substantial funding to control them. The extent of this is discussed further in this chapter in the section Challenges faced in realizing development opportunities.


Box 2: IAS, the biotic integrity of communities and the functioning of ecosystems

The Millennium Ecosystem Assessment (MA) found that trends in species introductions, as well as modelling predictions, strongly suggest that biological invasions will continue to increase in number and impact. A further concern is that multiple human impacts on biodiversity and ecosystems will decrease the natural biotic resistance to invasions and, therefore, the number of biotic communities dominated by invasive species will increase. However, research suggests that maintaining native species assemblages may diminish the ability of exotic species to become invasive.

A further important finding is that there is a positive correlation between the richness of native and exotic species across habitats – as the factors that promote the richness and coexistence of native species, such as benign climate, intermediate levels of disturbance, and habitat heterogeneity, also promote the richness and coexistence of exotic species. This suggests that hotspots for diversity are particularly at risk of invasion by introduced species, and that the loss of native species (from communities of low or high native species richness) is expected to increase.

Source: MA 2006

With increasing globalization, the threat posed by IAS is likely to increase through both intentional and accidental introductions. Human movement and the movement of goods are key drivers in the spread of IAS. With improvements in communications and infrastructure, this is likely to increase. Historically, IAS have been spread through colonization and exploration. Today, mobility through tourism, business travel and migration continues to be an important factor. Many IAS have been introduced to Africa in, for example, soil, plants, luggage, vehicles and aeroplanes (Kirby 2003). Trade – both legal and illegal – particularly in, but not limited to, plants and animals, is particularly important. Many species have been introduced through trade in manufactured goods contaminated with seeds or insects.

Trade has contributed not only to the introduction of species that colonize and fundamentally alter receiving ecosystems but that are also a factor in the growing incidence of disease. Aedes albopictus (Asian tiger mosquito), for example, is associated with the transmission of dengue fever and is believed to have been first introduced through a shipment of tyres from Japan to South Africa in 1989. By 1999 these mosquitoes were found to be present in Douala, Cameroon’s main commercial harbour (Fontennille and Toto 2001).

Invasive alien species have also been spread through the provision of humanitarian emergency food aid. For example, the weed Parthenium hysterophorus is a recent introduction to Africa through grain shipments for famine relief to Ethiopia (McNeely and others 2001). The weed was first seen in 1988 near food-aid distribution centres in Ethiopia. Buried seeds of the weed can lie dormant for as long as 20 years before germinating (GISP 2004).

Research activities and agricultural extension have also been a factor, as shown in Box 10.

Disturbed ecosystems are particularly vulnerable to invasion by alien species. In Tanzania, for example, Maesopsis eminii has become dominant in logged forests. It is also capable of regenerating in natural forests, particularly where there are large gaps caused by tree-falls (Bingelli and others 1998). In both Eastern Africa and WIO islands, the woody shrub Clidemia hirta is also increasingly common in natural forest gaps (Bingelli and others 1998). With high levels of environmental change, such as deforestation and growing extractive timber use, IAS are likely to be a growing problem. Climate change – through its impact on ecosystems – may also favour the spread of IAS. Section 2: Environmental State-and-Trends: 20-Year Retrospective considers the major environmental changes occurring in land, freshwater, coastal and marine environments, and biodiversity.

Increased trade is also associated with increased transportation. As already noted, ballast water, and its associated sediment, has been identified as an important route for the introduction of marine IAS: 14 billion tonnes of ballast water are transferred globally each year and more than 7 000 species of marine organisms may be present in ballast water at any given time (GISP 2004).

Even if these human drivers are more effectively dealt with, the problem of IAS is likely to continue as natural processes, such as cyclones and water currents, may also be a factor in their distribution. The Swaziland National History Society, for example, notes that an IAS known locally as demonia weed was blown into Swaziland by a cyclone in 1984; this has subsequently rendered large areas of formerly productive agricultural land useless (IRIN 2002).