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Preface Annex 1
IMPACTS ON BIODIVERSITY AND ECOSYSTEMS
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.
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).
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).