Alien species 'cost Africa billions'

Plants and animals introduced from other continents are placing a huge burden on Africa, conservationists say.

They put the cost of the damage caused by alien species in African wetlands at billions of dollars every year.

" They get here in soil, plants, luggage, vehicles, even aircraft "
Dr Geoffrey Howard, IUCN

While there are ways of controlling the invaders, complete eradication is seldom possible.

And even the exploitation of useful species will not itself be enough to control them.

The gravity of the threat is described in a report by IUCN, the World Conservation Union, entitled Alien Invasive Species in Africa's Wetlands.

It was published with the Ramsar Wetlands Convention and the Global Invasive Species Programme, and launched here during the annual meeting of the governing council of the United Nations Environment Programme (Unep).

Water menace

The report says the continent's wetlands are increasingly recognised as important for both humans and wildlife.

The alien species, which also include micro-organisms, have been introduced intentionally or by chance.

They arrive without their native controls, like predators, parasites and competitors.

One of the best-known is the water hyacinth, a native of the Amazon basin brought to Africa as an ornamental plant.

It has now spread to most of the continent's lakes and rivers, and can form huge mats of floating vegetation covering thousands of hectares.

These deprive life beneath the surface of light and oxygen, and reduce the variety of fish species.

Disease spread

The hyacinth can make fishing impossible and seriously affect water supplies, shipping and power generation.

Able to double its mass in 12 days, it grows faster than mechanical cutters can clear it.

Herbicides do work, but they endanger other wildlife. The report says the best option is biological control using two beetle species, a moth, a mite and pathogenic fungi.

Another problem is azolla, the red water fern. Its floating mats are a haven for mosquito larvae and for snails carrying bilharzia, which infects about 300 million people.

A flea beetle and a weevil, both leaf-eating insects, have worked well in South Africa to clear azolla.

Unwelcome fish

Other water plants listed in the report are the water lettuce or Nile cabbage, and the water fern, also known as Kariba weed.

An alien that grows on land is the giant sensitive plant, a prickly shrub named for the way its leaves fold inwards when the plant is touched.

It grows close to water, developing into thickets which smother other plants and prevent animals from reaching the water's edge.

One highly mobile alien is the Louisiana crayfish, which destroys native African plants, snails and crustaceans. It can travel long distances over land, and the burrows it digs damage dams and reservoirs.

Even the common carp is unwelcome, the report says. Although it is a valuable food source, it causes many problems because it eats local fish, invertebrates and vegetation to extinction.

Native loss

Dr Geoffrey Howard of IUCN told BBC News Online: "We have hundreds of invasive species in Africa. Often we don't recognise them because we've grown up with them.

"They get here in soil, plants, luggage, vehicles, even aircraft."

IUCN estimates the cost of the worldwide damage from invasive species at $400bn a year.

Dr Klaus Toepfer, executive director of Unep, said they were one of the greatest threats to Africa's wildlife.

A study for Unep says the Nile perch, an alien introduced to Lake Victoria in East Africa, constituted 1% of the annual catch soon after its introduction in the 1960s.

It is now 80% of the catch, and is thought to have helped to drive more than 200 native fish species to extinction.

Images by Geoffrey Howard/IUCN and Luc de Vos