Invasive Species Threaten Africa's Wetlands

NAIROBI, Kenya, February 5, 2003 (ENS) - Invasive species are devastating Africa's wetlandss, crowding out native species and costing billions of dollars in environmental and economic damage, warns a new report from international conservation groups. The groups have released a booklet describing the seven worst offenders, hoping to draw attention to the problem and promote ways of controlling - and perhaps profiting from - the invaders.

The beautiful, large purple and violet flowers of the South American water hyacinth make it a very popular ornamental plant, but it is also considered one of the worst aquatic weeds in the world. The invasive water hyacinth has inflicted enormous environmental and economic damage on Lake Victoria, among many other places in Africa and around the world.

Water hyacinth is one of the seven worst alien wetland invaders featured in the booklet "Invasive Alien Species in Africa's Wetlands," released today in Nairobi by IUCN - The World Conservation Union, together with the Ramsar Convention on Wetlands of International Importance and the Global Invasive Species Programme (GISP).

hyacinth invasion

South American water hyacinth on Lake Victoria in Africa. This South American native is one of the worst aquatic weeds in the world, growing so rapidly that it can block waterways and crowd out other plants and wildlife. (Photo by Geoffrey Howard, courtesy IUCN)
"The damage caused by alien invasive species to African wetlands runs into the billions of dollars annually, while the impacts of these species are only just being realized," said Geoffrey Howard, co-author of the publication and regional program coordinator for IUCN in eastern Africa.

Biological invasion is considered one of the greatest threats to biodiversity, second only to habitat loss and degradation. Invasive species can have devastating impacts on natural habitats and on the economy.

The economic damage inflicted worldwide by invasive alien species has been estimated at US$400 billion per year.

Invasive alien species are those that occur outside their natural range and threaten the existence of native plants and animals. These aliens can be plants, animals or microorganisms that are introduced intentionally for economic or agricultural purposes, or accidentally, through tourism, travel or trade, or when domestic animals become feral. Whatever the case, invasive alien species proliferate very quickly and, without controls such as their native competitors, predators, parasites and pathogens, can cause major problems for their new ecosystems.

"Invasive Alien Species in Africa's Wetlands" introduces some existing alien invaders that affect aquatic ecosystems in Africa, and describes others that feared to be on their way.

water hyacinth

Water hyacinths are also a problem in the United States, where they are damaging many of Florida's waterways. (Photo courtesy Florida Department of Environmental Protection)
The water hyacinth has inflicted immense damage on Africa's wetlands, lakes, rivers, hydro-schemes, irrigation and water supply systems, fisheries, human welfare and biodiversity. Now found in 50 countries on five continents, water hyacinth is a rapidly growing plant, with populations able to double in just 12 days.

Infestations of this weed literally block waterways, interfering with their use and preventing sunlight and oxygen from penetrating the water column and reaching submerged plants, thus reducing biological diversity.


The Louisiana crawfish was brought to Africa for aquaculture, but has escaped to make its home in many African wetlands. (Photo courtesy The Aquaculture Network Information)
The Louisiana crayfish, which owes its name to its origin in the U.S., is responsible for the disappearance of water lilies and submerged vegetation, as well as many species of snails, in the wetlands of eastern and southern Africa that it has invaded. The crayfish also threatens the existence of smaller fish, and its habit of burrowing can result in damage to dams and reservoirs.

The Louisiana crayfish escaped from aquaculture sites in Africa where it is bred to provide a specialty food. Adult crayfish can travel long distances across land, particularly in damp grass, and so spread from one wetland to another.


The Azolla or red water fern can completely carpet the surface of a waterway. (Photo credit unknown)
The Azolla, or red water fern, can hitchhike on people's shoes and vehicles; be carried by wind and water currents; or be spread by birds and mammals. Once it has reached a sheltered aquatic area, it forms mats on the water surface that block irrigation channels and waterways, increase water loss and interfere with boating and fishing.

The mats also provide havens for mosquito larvae and bilharzias carrying snails, while preventing light from reaching submerged plants and animals.


While the common or mirror carp is popular with anglers and fish farmers, it can be a real problem when it escapes into wild rivers and lakes. (Photo courtesy Travel Fish Africa)
Originally from Asia and eastern Europe, the common or mirror carp is prized by anglers as well as those seeking fish for food, and may have been introduced to Africa more than two hundred years ago. This voracious fish can grow to as long as 120 centimeters (47 inches) and weigh as much as 35 kilograms (77 pounds), preying on local fish and invertebrates.

Large carp females are reported to lay as many as a million eggs, which makes them popular for fish farms, but also allows the escaped fish to rapidly populate a lake or stream. While the carp represents a valuable source of food for African communities, the booklet stresses that the negative environmental impacts of its introduction need to be addressed.

pickerel weed

Pickerel weed is among the invasive plant species expected to soon become a problem for Africa's wetlands. (Photo courtesy South Florida Water Management District)
Other invasives described in the booklet are the Kariba weed and Nile cabbage. Water milfoil, also known as parrot's feather, and pickerel weed are cited as among Africa's potential invaders.

The publication also outlines some solutions to existing biological invasions and provides tips on traditional and new uses of these species.

"We hope that the booklet will help Africa in its efforts to counter the global threat posed by alien invasive species and will give this issue a more prominent place on the agenda of conservationists, economists and planners, and in the thinking of millions of people across the continent and worldwide," said Howard. "It is also a first step in the Ramsar process of recognising invasive species as extremely important in wetlands and needing attention in every Ramsar site."