Invasive of the Month/Facts & Figures

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.

Another invasive alien species that may have contributed to the periodic collapse of Lake Naivasha’s aquatic plants is the coypu or nutria, Myocastor coypus. This large semi-aquatic rodent is indigenous to South America, but was introduced to East Africa, North America, Europe and Asia to be raised for its fur. Since escaping from fur farms it has established large feral populations in some areas, causing considerable damage in wetlands, rivers, dams and irrigation canals by bur rowing into banks and dykes.

Mass murder in Lake Victoria: It’s been called the biggest mass extinction of vertebrates in recorded history – at least 200 fish species erased from existence by a ravenous predator, the Nile perch

The common carp Cyprinus carpio is undoubtedly the most widespread inavasive alien fish in Africa, found in most of the continent's countries. Native to to parts of Europe and Asia, it was one of the first species to be introduced outside its natural range, and now has a global distribution. The carp was deliberately introduced for food in most cases, as it provides a cheap source of protein. In some regions of the world it was introduced for sport-fishing, but although it is considered a premier sport-fish in Europe and Asia, it is among the least favoured targets of anglers elsewhere, and is generally regarded as a pest because of the damage it causes to freshwater habitats. Furthermore, its introduction has resulted in the spread of a number of fish parasites.

Tilapia are freshwater fishes belonging to the cichlid family. The various species are indigenous to different parts of Africa and the Middle East, but a number of them have been introduced to other African areas and the rest of the world. In some instances they were introduced as sport-fish, aquarium fish, or even as biocontrol agents to control waterweed or filamentous algae, but in most cases they were intended for aquaculture. However, some species have escaped or been deliberately released from captivity, and have established invasive populations in the wild.