Alien plants, fish and shellfish introduced into Africa are spreading unchecked and exterminating local species in lakes, rivers and wetlands, a report published yesterday said.
”The damage caused by alien invasive species to African wetlands runs into the billions of pounds annually, while the impacts of these species are only just being realised,” said Geoffrey Howard, co-author of the report.
The World Conservation Union and the Ramsar Convention on Wetlands of International Importance and the Global Invasive Species Programme (GISP) published the report in an attempt to control the menace.
It says that after the loss of forests and other natural vegetation, biological invasion is the greatest threat posed to biodiversity.
The invasion has devastating consequences for the global economy, with the worldwide damage inflicted estimated at £250-billion ($410-billion) a year.
Invasive alien species are defined as those that occur outside their natural range and threaten the existence of native plants and animals.
These can be introduced intentionally for economic or agricultural purposes, or accidentally through tourism, travel, or trade. Without the usual competitors – predators, parasites and pathogens — they find in their native habitats, they can quickly get out of control.
The report lists the species causing most damage. Top comes the water hyacinth ( Eichhornia crassipes ) which is found in 50 African countries. Populations of the weed, which chokes waterways, are known to double in only 12 days. Controlling the hyacinth is difficult.
Collecting the plant does not work because it grows faster than it can be gathered but a combination of two beetles which eat it, fungi and herbicides can keep it under control.
Next is the Louisiana crayfish ( Procambarus clarkii ), which owes its name to its origin and 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. It also threatens smaller fish, and its habit of burrowing can result in damage to dams and reservoirs.
The crayfish is bred to provide a speciality food but breathes air and can walk distances to find a new home. Only the drainage of water courses and capture have been effective in controlling it.
An ornamental plant that has become a dangerous weed is the the Azolla, or Red water fern ( Azolla filiculoides ). It can hitchhike on people’s shoes and vehicles, be carried by wind and water currents or spread by animals.
It forms mats on the water surface that block irrigation channels, increase water loss and interfere with boating and fishing. The mats also provide havens for mosquito larvae. – Guardian Unlimited Â