(Source: M. Chenje)
Many alien species, including some that are invasive, have had tremendous economic value for Africa. However, overall their impact on the sustainability of the resources, upon which livelihoods and development are often based, has been adverse, undercutting opportunities, human well-being and contributing to increased human vulnerability. Invasive alien species (IAS) are a serious impediment to the sustainable use of global, regional and local biodiversity; this has implications for freshwater and marine resources, tourism, and forests and woodlands.
Invasive alien species may affect livelihood and other economic opportunities in multiple ways. In addition to their impact on the supply of environmental goods, they also affect the integrity of ecosystems, undercutting essential environmental services. The various chapters of Section 2: Environmental State-and-Trends: 20-Year Retrospective considers the environmental goods-and-services provided by the atmosphere, land resources, freshwater systems, coastal and marine environments, forests and woodlands, and biodiversity and the opportunities these resources present for development. Thus IAS, through their impact on the environment, contribute indirectly to poverty, food insecurity, ill health and poor water quality. They have multiple level and complex impacts on human wellbeing and the ability to achieve development targets, such as those set out in the the Millennium Development Goals (MDGs).
Food security, land degradation, agriculture and livestock production
Invasive alien species impact on land resources, and agriculture and livestock production systems, in multiple ways, potentially threatening food security.
Weeds may affect the productive capacity of the land and increase agricultural labour time, affecting human well-being by threatening the availability of food as well as reducing the time people have for recreation and other non-work activities, such as participation in community events. Most often the responsibility for weeding falls on women and children. In many societies women are the last to eat in times of food shortages.
Some IAS transform grasslands that support grazing. For example, Lantana camara poisons cattle and destroys understorey species. The conflict tree, which is seedy and thornless, can form dense monospecific thickets. It is difficult to eradicate once established, making extensive areas unusable and inaccessible, and threatening native plants.
(Source: Brummet 2002)
Chromolaena odorata – first introduced to Côte d’Ivoire as a biological control – also forms dense thickets. It is particularly virulent in disturbed ecosystems, and thus can be associated with agriculture, and in particular slash-and-burn activities. When it is dry it is highly combustible, promoting flash fires. In Africa, it is known to be a problem in Benin, CAR, Congo, Côte d’Ivoire, DRC, Liberia, Mauritius, Nigeria, Senegal, South Africa, Swaziland and Togo. Some tree species, such as the black wattle, also affect the viability of grass species.
Many IAS grow faster than native plants and reproduce quickly, and thus replace indigenous plants and completely alter the composition of the area they have colonized. It has been reported that agricultural and grazing land, as well as protected areas, are threatened by rapidly growing species of plants that were introduced during colonialism as garden plants and windbreaks.
Parthenium hysterophorus (congress weed) invades disturbed land, including overgrazed and recently cleared or ploughed land. Once present, it is easily spread through seed dispersal – its seeds can remain viable for up to two years and buried seeds can stay dormant for up to 20 years – and as a result of its allelopathic character. Because it is unpalatable to livestock its colonization of rangelands results in grazing shortages, placing livestock production at risk. In some countries, such as Ethiopia, where it was originally introduced through contaminated food imports, it has had devastating impacts on agriculture – earning it the local name, “no crop”. It is also a problem in subtropical areas, affecting sugar cane and banana plantations in South Africa, Mozambique, Swaziland, Zimbabwe and Madagascar.
Viruses, such as Rinderpest and Avian Influenza Virus, can become invasive, seriously placing livestock production and livelihoods at risk. The spread of Avian Influenza Virus is closely associated with the live bird, and in particular the poultry, trade. Outbreaks of Avian Influenza in Nigeria were probably the result of illegal poultry trade with China and Turkey.
Freshwater and livelihoods
(Source: UNEP 2003a, McNeely and others 2001, GISP 2004)
As discussed in Freshwater resources in Africa, these systems have a central role in local livelihoods, providing food and water, water for agriculture, tourism, recreation and hydrological power. The introduction of IAS has placed these opportunities under threat. Non-native species of fish and plants have been introduced into freshwater systems to enhance food production, control pests such as mosquitoes, and to promote water purification. However, in many cases they have had adverse effects.
Although the use of alien species in aquaculture has had many positive effects, when these species escape or become invasive they can cause significant ecosystem damage. The Mozambique tilapia is an invasive in Northern Africa and has spread worldwide through introductions for aquaculture, as well as into the coastal waters of several WIO countries. Established populations in the wild are the result of intentional releases as well as escapes from fish farms. The same is true of the Nile tilapia, from Northern Africa, which has become an IAS in Southern Africa. This species can hybridize with the Mozambique tilapia, threatening its very existence in its native habitat.
Procambarus clarkii (Louisiana crayfish) has been introduced for aquaculture as well as a biological control agent for snail hosts of bilharzia. It escaped from aquaculture sites and is responsible for the disappearance of water lilies and submerged vegetation, as well as many species of snails, in the wetlands of Eastern and Southern Africa (Howard and Matindi 2003). It threatens the existence of smaller fish, and its habit of burrowing can result in damage to dams and reservoirs. It tolerates a wide range of salinities, oxygen-poor conditions, high pollution and fluctuating water levels. These factors, along with the ability of adult crayfish to travel long distances across land, have made it an effective invasive species. In Kenya, it was introduced into Lake Naivasha where it has supported a lucrative export industry to Europe. The crayfish has become a keystone species in the lake, resulting in a cyclical boom-or-bust scenario with plants and other fish disappearing and then recovering.
Services, infrastructure and shelter
The changes IAS cause in the environment may threaten human settlements. Not only do IAS reduce the availability of environmental goods-and-services, but they may also increase the physical threats to human habitat. For example, many invasives enhance the frequency and intensity of fires. Numerous invasive grasses produce a great deal of flammable standing dead material and many resprout quickly after fires, giving them a competitive advantage over native species. This may threaten homes and other infrastructure.
Aquatic IAS may clog waterways used for transportation and enter hydropower facilities, threatening the provision of electricity.
Invasive termites damage homes and other infrastructure. Coptotermes formosanus (Formosan subterranean termite) lives in damp, woody conditions. It is native to China, but has spread around the world, including to many African countries. It not only feeds on timber in buildings but may also nest in homes and other structures. The species is very aggressive and out-competes native termite species. The Louisiana crayfish, through its burrowing habits, can cause extensive damage to dams and reservoirs.
(Source: J. C. Mohamed-Katerere)
The conversion of native biotic communities to invasive-dominated communities also has aesthetic and cultural impacts and this directly affects tourism. Some IAS directly threaten the habitat of species that are key to the tourism industry. Chromolaena odorata, for example, affects the nesting sites of crocodiles, directly placing these populations at risk. Water hyacinth, by clogging waterways, affects water-based recreational activities. Species loss also has adverse impacts on tourism. For example on the Seychelles’ Bird Island, where Anoplolepis gracilipes (crazy ant) displaced about 60,000 pairs of Sterna fuscata (sooty terns), tourism was adversely affected.
The relationship between human health and IAS is complex, with patterns of human settlement, economic activities, environmental change and disease virulence, as well as the interactions between these, being crucia. Where IAS pose human health threats it may place added strain on already fragile health systems.
International trade introduced the Asian tiger mosquito, which carries dengue fever, to the Americas and to Africa. First introduced in South Africa, it is now present in Cameroon, Madagascar, Nigeria, Réunion, the Seychelles, Kenya, Mozambique, Djibouti and Somalia. Increased exposure to vector diseases such as malaria, dengue fever, schistosomiasis (bilharzia) and trypansomiasis (African sleeping sickness) is associated with large development projects, environmental change such as forest loss, and human settlement. Forest loss, for example, has widened the transmission of some diseases previously restricted to wild animal hosts. Trade has also spread the life-threatening bacteria E. coli in meat exports.
Vibrio cholerae (Asiatic cholera) is the bacteria that causes cholera and is an IAS. Cholera is endemic or epidemic in areas with poor sanitation. Although cholera may cause mild or unapparent infections, in its extreme manifestation cholera is one of the most rapidly fatal illnesses known. It occurs in both marine and freshwater habitats including lagoons, estuaries, lakes and wetlands in association with aquatic animals. In coastal regions it may persist in shellfish and plankton. It may also be associated with algal blooms (plankton), which are influenced by the temperature of the water. Rising water temperature, through for example climate change, may increase the potential risk of this. Cholera is spread through the live food trade, and the contamination of water sources.
(Source: de Bakker 2003, IUCN 2001, IUCN/SSCI/ISSG 2004, Musil 1993, Preston 2004)
Many countries face high costs – economical, environmental and social – due to the presence of IAS. The financial and management costs associated with their eradication are astronomical and their proliferation affects the potential of countries to meet their development and environmental objectives. Resources spent on trying to control IAS could be redirected to other development initiatives, such as the implementation of the MDGs. This is an important reason to adopt approaches which control and prevent introductions.
(Source: Versfeld and others 1998)
Invasive alien species cost millions of US dollars annually in terms of lost revenue and expenditure on control measures. While the actual costs of IAS are unknown, they are believed to be substantial. The global economic costs of IAS are estimated by IUCN to be about US$400,000 million annuallyl IUCN also finds that IAS threaten the success of current and planned World Bank projects to the value of more than US$13,000 million.
Currently, Africa spends an estimated US$60 million annually on the control of IAS. The African Ministerial Conference on the Environment (AMCEN) plans to raise a further US$265 million to fund various projects related to IAS in Africa over the next three to five years.
In South Africa, alien plant species now cover more than 10.1 million hectares (ha), threatening indigenous plants. (See Figure 1). Freshwater systems and the Cape Floral Kingdom (a global centre of biodiversity) are particularly at threat and the South African government therefore established the “Working for Water” programme. This programme seeks to remove IAS infestations from water catchment, and at the same time provide poverty relief (van Wilgen and others 1998). In South Africa, in addition to altering water-flow, IAS have had other important impacts on endemic biodiversity and ecosystem services. Nitrogenfixing plants such as Acacia saligna alter the nitrogen cycle, impacting on native plants adapted to low nutrient conditions, such as, for example, many of the fynbos species. The costs associated with eradicating IAS, as shown in Box 2, have exceeded US 100 million.
In the Western Indian Ocean (WIO) countries, IAS pose a serious threat to forests and thus place biodiversity, including many endemics, at risk. Among these invasive woody species are Paraserianthes falcataria (Albizia), Adenanthera pavonia (Agati), Clidemia hirta (Creole name: Faux Watouk), Cinnamomum verum, Chrysobalanus icaco (Prune de France), Psidium cattleianum (wild guava), Syzygium jambos, Astonia macrophylla (Bois jaune) and Tabebuia pallida (Calice du pape). In addition to the costs to biodiversity, governments incur substantial financial and management costs. In the Seychelles, for example, the Ministry of Environment is involved in a programme for the eradication of IAS, including those listed below, and the replanting of indigenous species; public education is seen as an important aspect of this (Ministry of Environment Seychelles, undated):
- Clidemia hirta grows quickly, particularly in disturbed areas, and displaces native plants. It competes effectively for light and soil nutrients and is therefore a successful invasive.
- Cinnamomum verum, introduced in the 1970s for economic reasons, has spread so rapidly that today it is the most widely distributed and probably the most numerous plant in the Seychelles.
- Chrysobalanus icaco was originally introduced to prevent erosion on steep slopes. Dense thickets of this species have now become established on many steep erosion slopes. It is difficult to get rid of this species once it has become established. It also invades areas where the indigenous forest had been cleared.
- Syzygium jambos (Jambrosa) tends to replace the naturally occurring vegetation, including forests in river ravines. Jambrosa is native to Indo-Malaysia and was recorded as being well established in the Seychelles as early as the 1870s. It is still planted by many people for its edible fruits.
Across Africa, IAS in the genus Striga have a direct impact on local livelihoods, affecting more than 100 million people and as much as 40 percent of arable land in the savannahs. The cost of eradicating it is reportedly between US$7-13,000 million annually. These invasives stunt maize plant growth by attacking the roots and sucking nutrients and water and thus in addition to the direct financial costs have implications for food security.
Small island developing states
While islands may not be more susceptible to invasions by alien species than continental landmasses, they are, however, considered to be particularly vulnerable to the impacts of such invasions. On islands, IAS are now on a par with habitat loss as the lead driver of species extinctions over the last 20 years.
Important opportunities exist for effective control for terrestrial IAS. These can be effectively controlled through customs and border monitoring; these measures have greater potential for success on islands than in countries that share boundaries. In terms of responses on islands, research shows that the experience of one island country can be invaluable in managing IAS on another island even where there are major differences in climate and ecological systems. Key similarities such as the role and nature of trade may be significant. Areas where a cooperative initiative on island IAS may be especially valuable include:
- Undertaking the eradication of IAS;
- Undertaking the management of IAS where eradication is not currently feasible, to low levels that allow recovery of biodiversity values;
- Training and other capacity enhancement activities; and
- Undertaking quarantine and contingency response activities to prevent the establishment of new populations of IAS that might threaten ecosystems or species (including the control of movement in the country).
- Baillie, J.E.M., Hilton-Taylor, C. and Stuart, S.N., 2004. 2004 IUCN Red List of Threatened Species. A Global Species Assessment. IUCN – the World Conservation Union, Gland.
- Birdlife International, 2006. Illegal imports probable cause of Nigeria flu. BirdLife International.
- CBD, 2003. Pilot Assessments: the Ecological and Socio-Economic Impact of Invasive Alien Species on Island Ecosystems. UNEP/CBD/SBSTTA/9/INF/33. United Nations Environment Programme/Convention on Biological Diversity/Subsidiary Body on Scientific,Technical and Technological Advice.
- CBD, 2005. Invasive Alien Species. Convention on Biological Diversity.
- GISP, 2004. Africa Invaded: The Growing Danger of Invasive Alien Species. Global invasive Species Programme, Cape Town.
- Hall, J., 2003. Southern Africa: Alien plant species invade region. Inter Press Service, 27 June.
- Howard, G.W. and Matindi, S.W., 2003. Alien Invasive Species in Africa’s Wetlands – Some threats and Solutions. IUCN – the World Conservation Union, the Ramsar Convention on Wetlands, and the Global Invasive Species Programme. IUCN – the World Conservation Union Regional Office for Eastern Africa, Nairobi.
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- Ithula, M., 2004. Winning the war. Executives: East African Standard, May 5, 2004. Standard Newspaper Group, Nairobi.
- IUCN/SSC/ISSG, 2000. IUCN Guidelines for the Prevention of Biodiversity Loss Caused by Alien Invasive Species. IUCN – the World Conservation Union Species Survival Commission, Invasive Species Specialist Group.
- IUCN/SSC/ISSG, 2004. Global Invasive Species database. IUCN – the World Conservation Union Species Survival Commission, Invasive Species Specialist Group.
- NEPAD, 2003. Action Plan for the Environment Initiative. New Partnership for Africa’s Development, Midrand.
- UNEP, 2003a. Lake Victoria: Dynamics of an Ecosystem. United Nations Environment Programme, Nairobi.
- UNEP, 2003b. Treaty on International Trade in GMOs to become Law. Press Release, 13 June. United Nations Environment Programme, Nairobi.
- UNEP, 2004. Invasive aliens threaten biodiversity and increase vulnerability in Africa. Call to Action 1(1). United Nations Environment Programme, Nairobi.
- UNEP, 2006. Africa Environment Outlook 2.
- UNEP, 2006. Africa Environment Outlook 2, Annexes.
- van der Waal, B., 2002. Another fish on its way to extinction? Science in Africa.
- van Wilgen, B.W., Cowling, R.M. and Le Maitre, D.C., 1998. Ecosystem services, efficiency, sustainability and equity: South Africa’s Working for Water programme. Trends in Ecology and Evolution. 13, 378.
- Wittenberg, R. and Cock, M.J.W. (eds. 2001). Invasive Alien Species: A Toolkit of Best Prevention and Management Practices. CAB International, Wallington, Oxon.
This is a chapter from Africa Environment Outlook 2: Our Environment, Our Wealth (e-book).
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