“Invasive alien species are emerging as one of the major threats to sustainable development, on a par with global warming and the destruction of life-support systems. These aliens come in the form of plants, animals and microbes that have been introduced into an area from other parts of the world, and have been able to displace indigenous species.” ~ Preston and Williams, Working for Water Programme/South Africa 2003
(Source: CBD 1992, Shrine and others 2000, ESA 1998)
Alien – that is non-native – species have been introduced both accidentally and intentionally. Intentional introductions are, and have been, motivated by economic, environmental and social considerations. In the forest sector, for example, Pinus, Eucalyptus and Acacia species are important sources of pulp, timber and fuelwood, yet at the same time they have placed tremendous strain on water resources. In Southern and Eastern Africa, these species are the backbone of plantation forestry, bringing in valuable foreign currency, yet at the same time decimating land and water resources. In South Africa, for example, they consume 7 percent of available water. Many introductions, however, are unintentionally coming into countries with other goods and, in the case of marine invasive alien species (IAS), in the ballast water of ships. Although only a small percentage of these alien species will become invasive, when they do their impacts are immense, insidious and usually irreversible, and they may be as damaging to native species and ecosystems on a global scale as the loss and degradation of habitats. In Africa some important ecosystems are under threat, consequently undermining development and livelihood opportunities, increasing human vulnerability and threatening human well-being. Thus, IAS have a direct bearing on Africa’s ability to meet the Millennium Development Goals (MDGs) and their targets. (These goals and targets, and progress towards meeting them, are set out in Annex 1).
Although not all alien species will become invasive or threaten the environment, this is an area in which a clear policy approach is necessary because of its potentially wide-ranging impacts when they do become invasive, and because of the difficulties, including financial costs, in reversing its impacts. One experience which illustrates this dilemma is the introduction of Lates niloticus (Nile perch) into Eastern Africa. The Nile perch has had immense economic value in the countries where it has been introduced, but it has also wreaked havoc in the ecosystems, resulting in the loss of endemic species and altered ecosystems with knock-on effects for livelihoods. Alien species, such as Bufo marinus (cane toad) and Chromolaena odorata (bitter bush) have been used for biological control and as an ornamental, and then subsequently become invasive.
Increased mobility and human interaction have been key drivers in the spread of IAS. On the one hand, increasing global connectedness – through trade, travel and tourism – has enriched the lives of people all over the world, through increased opportunities for sharing information and knowledge as well as improved access to a range of biodiversity. Increased access to biodiversity has created new opportunities for forestry, agriculture, aquaculture, horticulture, and biodiversity-based industries including the pharmaceutical sector. However, this increasing interaction has had its costs too:
“It has broken down the natural barriers of oceans, mountains, rivers and deserts which for millennia provided the isolation essential for unique species and ecosystems to evolve. In just a few hundred years these barriers have been rendered ineffective by major global forces that combined to help alien species travel vast distances to new habitats and become alien invasive species” (IUCN/SSG/ISSG 2000).
The challenge facing Africa is how to respond – to known IAS and to new introductions of alien species that could potentially become invasive. First, Africa needs to develop systems for evaluating the risks and benefits associated with alien species, and for deciding when to use them and when to prevent their introduction or eradicate them. This entails considering the economic, development, environment and human well-being costs-and-benefits, and recognizing the close relationship between these sectors. Second, Africa faces the challenges of how to translate its policy objectives into effective management practice. When species are identified as a threat, appropriate responses may include establishing systems for their eradication, as well as for controlling and monitoring their introduction. When alien species are used, developing early warning and assessment systems regarding their behaviour as well as effective response systems is essential.
(Source: data from IUCN/SSC/ISSG 2004)
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.
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. (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. 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 Invasive alien species and development challenges in Africa.
(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. 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.
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. 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.
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. In both Eastern Africa and WIO islands, the woody shrub Clidemia hirta is also increasingly common in natural forest gaps. 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. Environmental State-and-Trends in Africa: 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.
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.
Impacts on biodiversity and ecosystems
(Source: Birdlife 2006, GISP 2004, Howard 2003, IUCN/SSC/ISSG 2004, UNEP 2004)
Although IAS come from diverse taxonomic groups they share some similar impacts. Tree species such as the black wattle from Australia, Prosopis spp. (mesquite tree) from Mexico, and Leucaena leucocephala (the conflict tree) behave in a similar way to invasive alien fish species, such as Cyprinus carpio (the common carp), Micropterus salmoides (American black bass), Oreochromis nilotica (Nile tilapia) and Mozambique tilapia, and out-compete native species and convert receiving ecosystems.
(Source: Birdlife international 2004, GISP 2004)
Invasive alien species may threaten native species as direct predators or competitors, as vectors of disease, or by modifying the habitat or altering native species dynamics. The threat posed to biodiversity by IAS is considered second only to that of habitat loss. On small islands, it is now comparable with habitat loss as the lead cause of biodiversity loss.
Invasive species may out-compete native species, repressing or excluding them and, therefore, fundamentally change the ecosystem. They may indirectly transform the structure and species composition of the ecosystem by changing the way in which nutrients are cycled through the ecosystem. Entire ecosystems may be placed at risk through knock-on effects. Given the critical role biodiversity places in the maintenance of essential ecosystem functions, IAS may cause changes in environmental services, such as flood control and water supply, water assimilation, nutrient recycling, conservation and regeneration of soils. Biodiversity in Africa discusses the complex relationship between biodiversity and the maintenance of essential ecosystem functions. Invasives may also affect native species by introducing pathogens or parasites that cause disease or kill native species.
Among other things, both old and newly established IAS contribute to land degradation through soil erosion and the drawing down of water resources, reducing resources available to people and indigenous plants. Others produce leaf litter which poisons the soil, suppressing the growth of other plants, and in particular that of the understorey. They may alter the environment in directions that are more favourable for them but less favourable to native species. This could include altering geomorphic processes (soil erosion rates, for instance, or sediment accretion), biogeochemical cycling, hydrological cycles, or fire or light regimes. For example, invading trees in the fynbos of the Cape Floral Kingdom reduce stream-flow from mountain catchment areas and change the overall hydrological regime of the entire area, which in turn prevents the germination and growth of native species.
Wattle trees and mesquite can sink their roots deeper into the soil than indigenous trees, sucking out massive volumes of water and out-competing indigenous plants for nourishment. In some environments, invasive trees, like the black wattle, increase rainfall interception and transpiration, which causes a decrease in stream-flow. The leaves and branches of the black wattle are believed to have allelopathic properties – that is the chemical inhibition of growth and seed germination of other plants. Highly combustible, fire-tolerant alien plants may also alter the fire regime, and combined with competition for light, nutrients, water and space, this is believed to be an important factor in extinctions.
Marine IAS are a growing problem in Africa’s coastal waters, estuaries and lagoons. Many of these introductions are related to sea vessels and aquaculture. Hypnea musciformis (hypnea) is red algae, originally from Trieste in Italy, and is now distributed throughout the world. It occurs in coastland, estuaries and marine habitats where it attaches to coral, stones or shells on sheltered tropical reef flats. Its success is related to its rapid growth rate, ability to epiphytize other algae and easy fragmentation. In Africa, it is present in the coastal waters of Morocco, Namibia, Angola, Congo, Gabon, São Tomé, Cameroon, Nigeria, Togo, Ghana, Côte d’Ivoire, Liberia, Sierra Leone, Guinea-Bissau, Gambia, north Senegal, the Cape Verde Islands, Mauritania, Ethiopia, Egypt (Red Sea), Djibouti, Kenya, Tanzania, Mozambique, South Africa, Madagascar, the Seychelles, Mauritius and Réunion. Invasion pathways include aquaculture and dispersal by boats and other vessels.
(Source: Seitre/Still Pictures)
The management and control of IAS present some important challenges for decision-makers. Globally, preventing their introduction is seen as the cornerstone of effective measures for dealing with IAS. This approach is believed to be the most cost-effective and environmentally-sound approach as once an invasive species becomes established, eradication may be impossible and ecological damage irreversible. This obligation to control IAS needs to be balanced against international trade obligations as well as social and economic concerns. Developing systems for making sound choices must be a priority for African governments.
Although the New Partnership for Africa’s Development Environmental Action Plan (NEPAD-EAP) identifies IAS as an important programme area, it is not clear how it will be addressed. The development of programmes and strategies will need to be based on a comprehensive analysis of IAS, and their associated costs-and-benefits. The need for strategic research to support this cannot be overemphasized. Research may include compiling a complete inventory of all alien species, including noninvasive ones, determining the impacts to date on ecosystems, and assessing the financial resources needed as against the cost of inaction. This will require new levels of investment in research. Partnerships and collaboration are essential for effective research. For example, the private sector could play a role in supporting research and development. Regional cooperation may help lower research costs.
The NEPAD-EAP will need to be complemented by national and sub-regional interventions. National strategies would need to identify the goals and objectives of an alien species plan. Such strategies will need to draw on existing knowledge and establishment management approaches. The shift that has taken place from species preservation to ecosystem integrity may form the basis of these responses. Legal and institutional frameworks at national, subregional and regional levels will need to be refined to establish complementarity between different sectors. While the region has taken significant steps to address the problem by adopting the African Convention on the Conservation of Nature and Natural Resources (ACCNNR), its successful implementation will be long-term since out of the 33 countries that have signed it, only four had ratified it by March 2006.
Legislation will need to create effective frameworks that are consistent with international obligations. These could benefit from the use of established legal approaches and principles, such as precaution, cost-recovery measures, rights of public participation, and rights of access to information. Other important management and decision-making tools that could be incorporated in national, sub-regional and regional frameworks include risk analysis and assessment systems, environmental impact assessment and cost-benefit analysis.
Partnerships, with a cross-section of actors at multiple scales, are an important aspect of developing appropriate responses. The inclusion of different stakeholders, from communities, NGOs, research organizations, the private sector and government, is important for developing appropriate policy as well as identifying effective interventions. Information, and its communication, is critical to bringing diverse sectors on board as effective partners. Environmental education initiatives should also highlight the problems of IAS and how they influence environmental change, which among other impacts, exacerbate human vulnerability. The spread of IAS is directly linked to trade and human mobility. Reconciling these, and developing mechanisms that deal effectively with this challenge, is undoubtedly a priority area for policy and response.
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- UNEP, 2006. Africa Environment Outlook 2, Annexes.
This is a chapter from Africa Environment Outlook 2: Our Environment, Our Wealth (e-book).
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