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Chromolaena odorata – commonly called chromolaena, triffid weed or Siam weed – is one of the worst invasive plant species in the humid tropics and sub-tropics of the world. Its native range extends from Florida in the United States to northern Argentina, but it has invaded south-east Asia, parts of Oceania, and West, Central and southern Africa, where it is a major threat to biodiversity, agriculture and human welfare.
The genus Acacia comprises some 1500 species, and close to 1000 of these are indigenous to Australia, where they are commonly known as wattles. Many other Acacia species naturally occur in Africa, spreading to other parts of the continent where they are considered serious invaders. Because of the vast number of acacia species, only a few are highlighted below, with a focus on some of the more serious Australian invasive species introduced to South Africa, highlighting a variety of negative consequences deriving from these introductions.
Parthenium hysterophorus, commonly called parthenium or congress weed, is an aggressive invader that is native to Mexico. The weed was first seen growing in Ethiopia in 1988 near food-aid distribution centres, so it is presumed that imported wheat grain was contaminated with its seeds. Once introduced, the weed was able to spread rapidly, as the seeds are readily dispersed in mud adhering to vehicles, machinery and animals, as well as by water and wind. The seeds can remain viable on the soil surface for up to two years, while buried seeds can stay dormant for as long as 20 years before germinating.
The genus Prosopis, commonly known as mesquite, includes more than 40 species, most of which are indigenous to an area ranging from Argentina to the southern United States. Several species have become invasive in Africa and other parts of the world, particularly the sub-tropical Prosopis glandulosa and P. velutina and the tropical P. juliflora and P. pallida. These species have been widely introduced as a source of fuelwood, fodder and shade, and are also used for sand stabilisation, soil improvement, or for hedges to contain livestock.
Leucaena leucocephala is another tree species that has been promoted by international agroforestry organisations as a fodder and firewood resource, but is widely reported as an invasive weed.
The water hyacinth Eichhornia crassipes is considered the world’s worst invasive aquatic weed. Indigenous to the Amazon Basin of South America, it was introduced to many parts of the world as an ornamental plant, and today occurs in more than 50 countries on five continents.In Africa, it was first recorded in the 1890s from the River Nile in Egypt, but has since become widespread throughout the continent. The plant thrives in still and slow-moving water-bodies that have become nutrient-enriched through eutrophication, and dense mats of water hyacinth now blanket many of Africa’s dams, lakes, rivers and canals.
Caulerpa taxifolia is a green seaweed that is widely distributed in the world’s tropical seas. In Africa it occurs naturally in the Gulf of Guinea in the west, and Tanzania, Kenya, Somalia and the Red Sea in the east, as well as Madagascar, the Maldives and Seychelles. However, a robust, cold-tolerant strain has become an aggressive invader outside the natural range of the species. It has spread throughout the northern Mediterranean, where it has had a devastating impact on local biodiversity. It has also been found in the coastal waters of Tunisia in North Africa, as well as the United States and Australia. Now there are concerns that it might be introduced to other parts of Africa, and threaten marine communities there too
Algal blooms are a natural phenomenon in the Benguela region off the coasts of Namibia and South Africa, where wind-induced upwelling results in nutrient enrichment of coastal waters. However, some algal blooms have harmful effects such as shellfish poisoning and marine mortalities, which can adversely affect coastal tourism, mariculture operations and fisheries. There has also been speculation that recent fish kills off the Kenyan coast were caused by a harmful algal bloom. It is quite possible that some of the species responsible for harmful algal blooms were introduced to African waters in the ballast of visiting ships, and there are fears that new and more problematic species might arrive in the future.
The European green crab Carcinus maenas, also called the shore crab, is a voracious predator of the marine environment. Indigenous to the Atlantic coast of Europe and North Africa, it has invaded numerous coastal communities outside its native range, including South Africa, Australia, and both coasts of North America. It was discovered in Cape Town harbour, South Africa, in 1983, and has since invaded the coastal waters of the surrounding Cape Peninsula.
The Louisiana crayfish Procambarus clarkii, also known as the red swamp crayfish, supports a lucrative aquaculture industry in its native range, and is a popular component of the region’s Cajun cuisine. Over the last 50 years it has been introduced to Africa, Europe and Asia, in most cases with negative consequences.