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.

Box 3: Invasive bird species

Over the last 500 years, IAS have been partly or wholly responsible for the extinction of at least 65 bird species, making this the most common contributory factor in recent losses to the world’s avifauna (BirdLife International 2006).

Invasive bird species include: Passer domesticus, (house sparrow) which out-competes many small, native African birds, Corvus splendens, (Indian house crow) which has spread from the Tanzanian coast inland over the last 20 years, and Acridotheres tristis (common or Indian myna).

The Indian house crow destroys the habitat of many other birds and as a result in the Tanzanian capital, Dar es Salaam, there are now only a few other common bird species (Howard 2003). The house crow came with ships from India – probably as early as the late 1800s. It now extends to Cape Town and has been recorded in Port Sudan and even in Cairo. This bird kills other species, destroys nests, and steals eggs and chicks of the domestic chicken. It also spreads disease and is generally a serious pest in towns along the coast of Eastern Africa (Howard 2003).

Indian mynas were introduced to reduce the insect population in agricultural areas. Mynas inflict damage to grape and other fruit crops like apricots, apples, pears, strawberries and gooseberries (IUCN/SSC/ISSG 2004). They also reduce biodiversity by competing for nesting hollows, destroying chicks and eggs and evicting small mammals.

Source: Birdlife 2006, GISP 2004, Howard 2003, IUCN/SSC/ISSG 2004, UNEP 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 (MA 2006). The threat posed to biodiversity by IAS is considered second only to that of habitat loss (CBD 2005). On small islands, it is now comparable with habitat loss as the lead cause of biodiversity loss (Baillie and others 2004).

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 (McNeely and others 2001). 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 (GISP 2004, Levine and others 2003). Chapter 7: Biodiversity 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 (UNEP 2004). 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 (MA 2006; Levine and others 2003). 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 (MA 2006).

Box 4: Aliens from Planet Earth
  • IAS, especially predators, directly threaten more than 300 bird species.
  • One of the most notorious plant IAS in Africa is the water hyacinth. It costs some countries tens of millions of dollars annually to control. The water hyacinth grows quickly and harms wetland ecosystems by blocking sunlight and oxygen, altering water-flows and increasing evapotranspiration.
  • The water fern enhances the breeding of mosquitoes and snails that carry bilharzia, which infects about 300 million people annually in the tropics.

Source: BirdLife International 2004, GISP 2004

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 (Preston and Williams 2003). In some environments, invasive trees, like the black wattle, increase rainfall interception and transpiration, which causes a decrease in stream-flow (IUCN/SSC/ISSG 2004).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 (Richardson and van Wilgen 2004).

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