23 August 2010

East Africa: Invasion of the Killer Weed From South America

Nairobi — It looks nondescript. A plant with tiny white flowers. Yet the plant, parthenium, has the potential to create a pandemic that can destroy Kenya's agricultural yields by up to 40 per cent, reduce pasture by up to 90 per cent and impact negatively on human health.

"In 50 years, the whole of East Africa could be drowning in a sea of invasive species," warns Arne Witt, an invasion biologist at CABI, a not-for-profit science-based development and information organisation. The organisation helps improve people's lives by providing information and applying scientific expertise to solve problems in agriculture and the environment.

Parthenium originates from Central and South America. It is a weed but because it has an array of local insects that forage on it in its homeland, the plant is kept in check. On new continents where the climate is similar but with no insects to feed on the weed, it is having a field day edging out the local flora to the extent of becoming a threat to them.

In the past five years, this weed has exploded in Kenya, and is now common along the edges of the Nairobi National Park, Mlolongo and Athi plains. Its historical route into Africa follows that of food aid brought in by foreign agencies in war-torn and drought stricken countries. First recorded in Ethiopia in the 1980s, it is now present in Uganda, Tanzania, Swaziland, Mozambique, Zimbabwe, Somalia and South Africa.

The grain route

It could be in other African countries but because there aren't many people studying it, its full extent is not known. The weed is often associated with places where food aid is distributed. During times of drought, Kenya is one of the largest importers of food in Africa. "It's a seed contaminant," says Witt. He has been studying the invasive species since his student days at a South African university in the 1980s. South Africa has more than 300 recorded species of invasive plants.

Parthenium grows on farmed land -- including wheat and maize fields. Often, when the grain is harvested, so are the miniscule seeds of parthenium, which are then transported with the grain to other regions. Once in Africa and free of natural enemies, the plant grows unchecked, just like the infamous water hyacinth -- also from South America -- now a major problem in the region's fresh water lakes.

"A single mature plant can produce 25,000 seeds," says Witt. It explains the plant's rapid dispersal in Africa in two decades. A plant matures in a month and if the conditions are right -- enough water and sunlight -- it can survive for years. The seeds, like those of the water hyacinth, can lie dormant in the soil for years till the conditions are right for them to germinate.

"The plant is allelopathic," says Witt. "That means it releases chemicals into the soil that prevent other plants from growing there. Once it infiltrates into pastures, it can reduce the carrying capacity of the land by 90 per cent." It's a frightening figure considering that the region's economic backbone is agriculture and tourism.

If invasive species take over the savannah, they may very well spell doom for grazers like wildebeest who migrate every year from the Serengeti into the Maasai Mara and back again. The migration is described as one of the seven wonders of the modern world.

Invasive woody species also suck up thousands of gallons of water, leaving less for agriculture and in lakes and rivers. Even livestock find parthenium distasteful and won't it eat, save for when drought is at its worst. "Livestock can die if they eat large amounts of the plant. It is toxic. When nanny goats browse the flowers, which they sometimes do, their meat and milk taste disgusting," says Witt.

"In India, parthenium can reduce sorghum yields by 40 per cent. In Ethiopia, under controlled experiments, at high densities, yields showed a 97 per cent decline. Nobody knows the exact figures," says Witt. "Estimates are that between 1 and 2 million hectares of farming land is affected in Ethiopia, or approximately 1-2-per cent of the land area.

In Kenya, it is thought that a benign biotype, that was not particularly invasive, was introduced in the 1960s. But in the past five years, an invasive biotype has spread, especially along the Nairobi-Mombasa highway, Mlolongo, and around Nairobi -- the food truckers route.

Dependent on wind and water for natural dispersal parthenium spreads quickly, aided largely by a modern dispersal method -- motor vehicles. "The seeds are probably caught in the car radiators and drop off along the road. In South Africa, we are seeing parthenium infestations in rural villages and places where vehicles routinely drop off passengers and where cars are washed," says Witt.

"In Ethiopia, parthenium may be driving the Archer's lark (a rare endemic bird) into extinction because it has invaded its territory," says Witt. "In India, in some rural communities, it has driven farmers to poverty because the liquid from its noxious leaves blisters the skin, rendering one unable to work. In some cases, it is known to aggravate asthma and to induce breathing difficulties in humans.

This year in April, the Kenyan government declared parthenium a noxious weed in a gazette notice. But there has been no public awareness programme in place to ensure public awareness. The International Plant Protection Convention has a guideline on handling plants, pets and other organisms entering different countries.

Open to invasion

East African countries are signatories to the convention. But with no system in place in many countries to ward off invasive plants, the continent is open to invasion.

Plants and their products brought into Kenya must have a Phytosanitary Certificate, or a clean bill of health issued by the exporting country, which must be presented and approved by the Kenya Plant Health Inspectorate Services (KEPHIS)

"Pests are a global phenomena today," says Witt, "with new weeds coming in all the time."

"It is important to develop systems to stop the further spread of pests. Farmers and the public should be educated about invasive plants. It's cheaper to eliminate them at the beginning than when they are established."

Also important is spreading the message in schools, with the appropriate books on the trees to plant in various regions.

"It's important to have a team of people who know weeds. As soon as they find them, they should alert the authorities to eradicate them. If that fails, then it's important to try and contain the problem and stop further expansion," says Witt. An example of a plant that could have been contained at the very beginning is the water hyacinth.

Now it cannot be eradicated because it has firmly established itself in East African waters. Brought in as an ornamental plant to decorate ponds, it spread to lakes, rivers and dams. "It can take many years before an exotic plant turns problematic -- sometimes up to 30 or 40 years," says Witt. To deal with the water hyacinth today, a beetle from South America was brought in to forage on it. It feeds only on water hyacinth.

"Biocontrol then becomes a management tool," says Witt. The weed is nevertheless a pest, competing with the local plants for space, oxygen and water, besides denying fishermen their livelihood. A new weed coming from West Africa, chromolaena, is yet another invader.

It has travelled across the belt of Africa -- Democratic Republic of Congo, Uganda, and into Kenya. Initially from the Caribbean islands, the shrub with muave flowers was probably introduced as an ornamental plant. One mature plant can produce a million seeds a year.

According to researchers, the weed is found around the edges of Lake Victoria in Kenya and Tanzania. It reduces the carrying capacity of the land by 150 per cent, thus turning it into a "green desert" bare of any other species.

South Africa is notorious for the number of invasive species it has -- one reason being its colonial history from the 16th century onwards when the Dutch began to settle there, followed by the British, bringing with them many plants which have become invasive.

Since 1995, there has been a sharp increase in the number of invasive plants -- from less than 200 in the 1990s to more than 350 in 2010.

Expanding the range

In Kenya, the number of invasive species is uncertain but it's estimated that 60 introduced species are expanding their range.

With climate change, the weed problem is projected to get worse.

Many invasive species such as prosopis will benefit from increased carbon dioxide levels by growing faster and becoming more drought tolerant, while others will benefit from increased levels of disturbance -- more droughts and floods will reduce the resistance of natural communities to invasion and facilitate the spread of invasive plants.

However, unlike Kenya, South Africa has developed good systems to control invasive species and their spread, spending $60 million annually to fight the aliens.

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