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 are a serious impediment to the sustainable use of global, regional and local biodiversity (CBD 2005); this has implications for freshwater and marine resources, tourism, and forest 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 (UNEP 2004, NEPAD 2003). They have multiple level and complex impacts on human well-being and the ability to achieve development targets, such as those set out in the MDGs.


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 (IUCN/SSC/ISSG 2004). 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. 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 (IUCN/SSC/ISSG 2004). 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 (Hall 2003).

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 (GISP 2004). 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” (GISP 2004). 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 (BirdLife International 2006).