It was introduced as a force for good. But the mesquite tree has taken to Ethiopia's soil with such relentless enthusiasm that it threatens the livelihood of thousands.

Caroline Irby reports
27 August 2004

In Ethiopia they call it the Devil Tree. It is both an addiction and an affliction. In this largely pastoral country there are many people who can't do without it: it provides shelter, building materials, fencing for livestock, firewood, charcoal and shade from the fierce African sun.

But the Devil Tree drives a Mephistophelian bargain. In the 30-odd years since it was introduced from Mexico, it has started to take over the rural landscape. Now it has eaten up as much as a quarter of all arable grazing land in some areas, leaching the earth of the nutrients that once nurtured the grass that pastoralists rely on to graze their cattle, their livelihood.

We know it as mesquite, or - to give it its real name - Prosopis juliflora. It was imported in the late 1970s and early 1980s to rural areas where the tree trunks were used to shore up irrigation dykes and as firewood.

But over time the abundant access to water and - as with so many other introduced species - a lack of natural competition has allowed the Devil Tree to thrive. It had a reputation as a tree which could grow anywhere - that was part of its appeal as a fuel wood in the first place. But Ethiopians report that it will grow up through the floors of their huts. Its canopy has taken over ground cover, making it hard for other plants to compete.

But it is the thorns, which grow along tendrils to three or four inches in length, which make life nearly impossible for anyone who has to live in an infested area. It's not just herdsmen's feet that are affected - farmers have taken to driving on solid tyres, according to a report from Oxfam, which also reports that injuries from the thorns are far worse than the injuries that a normal thorn would cause because of their size and the wound's tendency to become infected more easily.

As you'd expect, people are looking for someone to blame for the introduction of Devil Tree to Ethiopia and there are different stories as to who brought the tree to the country. Some say it was the UN's Food and Agriculture Organisation. Others blame state forestry development agencies. But whoever is at fault, it is generally agreed that the intentions were good. No one could have foreseen the plant's incredible invasive power.

The tree reproduced slowly during its initial 20 years in Ethiopia. Farmers' first complaints about it came in the mid-1980s and had nothing to do with the tree's propagation: its thorns were stabbing their feet. Only after two gradual decades of incubation did it begin to reproduce rampantly. But by then it had begun to threaten the very livelihoods of the people it was meant to help.

Ahemedu Koka used to depend on his 30 cattle to support himself, his wife and eight children. For centuries, his ancestors have lived as herders, wringing a living out of the barren region of Afar, eastern Ethiopia, by walking hundreds of miles each week in search of grassland to graze their livestock. Last year, driven from his traditional living by the unavailability of grazing land, Mr Koka, 38, opted for a different way of life: he now earns a living making and selling charcoal from the Devil Tree; his two remaining cattle stand forlornly tethered outside his hut.

For Mr Koka, life in the charcoal trade is now easier than it was roving the arid semi-desert of Afar with camels, goats and cattle, looking for pasture in the mesquite-blighted landscape. He earns around Birr 500 (£35) each month from his charcoal sales: not enough to send his children to school but sufficient to feed them. Yet Mr Koka is a reluctant businessman: "I prefer to keep livestock," he says, "but the environment forces us to change."

There are 44 species of prosopis of which one, Prosopis juliflora, was introduced to Ethiopia. Prosopis juliflora has shown itself to be an aggressive pioneer when transplanted from its home environment. It can withstand high temperatures, shortage of water and saline soils; it is also a monoculture: it roots out moisture, grows quickly to 6m tall and reaches out its branches to join neighbouring Prosopis trees, forming a canopy that denies endemic species beneath the water and light necessary for their survival.

The invasiveness of the species has been aggravated in Afar by extensive grazing. Prosopis is spread by livestock who eat its pods and distribute them undigested in their faeces as they roam. Consistent with its selfish character, prosopis does not share its nutrients with the animals that eat its pods, nor are its leaves edible. But the plant profits from the softening process that occurs in the animals' gut for its own eventual germination. Prosopis answers to none of herders' traditional needs: "If an animal has ever eaten its leaves, only Allah has seen; if there's a use for this tree, only Allah knows it," says Ibrahim Hamadou, who walks on average 25 miles each day in search of land free of the Devil Tree on which to graze his cattle.

The threat that prosopis represents for herders falls against a backdrop of encroachments on their traditional grazing area. The flooding of the Awash river, the introduction of agriculture to the region, the sectioning-off of land for national parks and the advancing of Issa herders from the Somali region of Ethiopia into Afar grazing territory are also putting pressure on the pastoral land available to Afar people.

It wasn't meant to be this way: natural forests of prosopis are treasured in its native South America. On the basis of its environment-enriching properties, prosopis has been introduced to arid climates from South Africa and Australia, to India and the US. Its 10ft roots suck minerals from the earth and fertilise the soil as the inedible leaves decay on the surface. The deep roots also reduce salinity in the soil by decreasing the ground-water table level and the tree serves both as a shelter for humans and animals.

But as in Africa, the Australians have begun to find the Devil Tree an unwelcome guest that is almost impossible to get rid of. It has colonised more than 800,000 hectares of arable land in northern Australia and the authorities are racking their brains to try to keep it under control. Various chemical solutions have been tried without success, and at present it is only the use of controlled burning that is inhibiting its spread.

It is the same story in the US, where infestations in New Mexico have reduced the carrying capacity of arable land by up to 75 per cent.

A recent study concluded that prosopis infestations directly cost the US agricultural sector about $300m (£170m), with consequent losses to economic activity amounting to three times that amount.

The Afar regional government in Ethiopia and the non-governmental organisation Farm Africa are working to turn the invasive plant into profit for the Afar people. One solution with commercial potential is to market the wood internationally as high-quality flooring. One company, backed by the Ethiopian government, is negotiating to build log-processing plants in the Afar region twinned with a final product-processing plant in Addis Ababa, Ethiopia's capital. The end product would then be shipped from Djibouti to the US, Europe and the Middle East.

But the commercialisation of prosopis opens up a conflict of interest between settlers and pastoralists: between those who want to use prosopis and those whose very lives are seriously affected. "Pastoralists want to see the plant eliminated. They don't want to hear about any negotiations with prosopis," says Dr Taffese Mesfin, pastoral programme adviser for Farm Africa.

Ahemedu Koka's returns from selling charcoal illustrate one initiative that appears to be benefiting pastoralists - if modestly. Charcoal production is currently illegal in most of Ethiopia, due to the virtual wiping out of the country's neem and acacia forests, but the government has allowed one charcoal association to open in the Middle Awash Valley, the area where prosopis has propagated most fervently, and where Mr Koka has settled.

Another enterprise which is designed to help pastoralists to profit from their enemy will open nearby later this year. The regional government has bought four machines to crush prosopis pods, which will both reduce the number of seeds germinating and create protein-rich feed for cattle, to be sold by the pastoralists. Communities have begun to collect pods and are being trained to operate the machinery. It won't make a big impact on the spread of the tree: even where no livestock is present to disseminate the many pods, warthogs run wild sowing the seed.

But Dr Taffese is determined to work with the invader. "It's no good condemning it," he states. "If we don't do anything about prosopis, we can't talk about development. We have to understand its better uses."

The pastoralist way of life is moulded on the ecosystem in which they live. Prosopis is now part of the ecosystem; it is an alien species for which pastoralists have no traditional use, but the plant has potential. Though each of the countries where it has been introduced is struggling to control infestations, the marketability of prosopis is also exploited: prosopis is used to make timber and charcoal in the US, animal fodder in Brazil, gum in India and honey in Mexico, where restrictions are now in place on the plant for fear of its overuse.

Dubale Admassu, an animal scientist working on the control of prosopis in Afar, says: "We'll take the Afars to Mexico and the Mexicans to Afar: then we'll learn what to do with this plant!" Pastoralism is all about adapting to change, and faced with the near impossibility of eliminating the tree from their land, pastoralists could begin to consider the thorny invader as a blessing very well disguised.

PLANTS THAT OUTGREW THEIR WELCOME

BANMARA PLANTS

Invaded central and eastern Nepal from India. Named the "forest killer" by locals, these plants are one of the biggest threats to biodiversity in Nepal and cause economic damage. As cattle do not eat them, the farmers are forced to travel farther in search of new grazing areas.

KILLER ALGAE

Originally a tropical alga that needs water temperatures above 20C, it jumped from the aquariums of the Ocean Museum in Nice into the Mediterranean. Here it mutated to cope with the cooler water temperatures and now covers more than 6,000 hectares of seabed.

GIANT HOGWEED

Originated in the Caucasus and was introduced as a garden plant to England in the 19th century. From here it spread along riverbeds from Scandinavia to France and Switzerland. The shrub produces a toxin which is potentially dangerous to humans and animals.

FIREWEED AND FOUNTAIN GRASS

A major threat to the habitat in Hawaii. Fireweed is toxic to animals and causes severe liver damage in cattle. Fountain grass overgrows local plants and poses a major fire risk.

HIMALAYAN BALSAM

Made its way from India into English gardens in the 19th century. The plant spreads quickly along riverbed. The plant juice is toxic to humans.

Stephan Schepers