There was the sound of bleating as more than 100 sheep gathered to drink from the Atbara River, a Nile tributary, herded here by nomadic shepherds.
This watering spot, shared by several tribes, is 350 kilometers east of the Sudanese capital, Khartoum, where the Blue and White Nile Rivers converge.
Framing the scene were thickly vegetated river banks. Trees and shrubs grew three meters tall, and after drinking their fill, the sheep rested, apparently contentedly in the shade.
"That tree is really terrible," remarked Abu Ali al-Amin, 30, one of the nomads, indicating a shrub whose leaf stalks extended into a sharp three-centimeter-long thorn.
"When the sheep eat the leaves their mouths become numbed. The thorns appear to be poisonous; a donkey pricked by one ended up dragging one of its legs. The milk of goats that have eaten the leaves tastes bitter and cannot be drunk fresh," Amin said.
The trees began to spread here several years ago and now in many places prevent the animals from approaching the water.
The tree's genus is Prosopis. It is more commonly known as mesquite and belongs to the Fabaceae family, which includes legumes like peas.
The commonest species in Africa grows more quickly and is more drought tolerant than those from South America, where the plant has its origins. It is often used for timber and charcoal and the pods that fall to the ground can serve as livestock feed.
Thinking it would be a good measure against desertification, bodies such as the U.N. Food and Agriculture Organization in the 1980s encouraged the full-scale planting of mesquite in countries on the Arabian Peninsula and in those bordering the Sahara Desert, including Mauritania and Niger.
However, in the 1990s, local people began to express dissatisfaction, calling the plant a major hindrance to grazing and cultivation.
With roots extending 20 to 30 meters, and able to suck up groundwater from that depth, it is said that mesquite makes it difficult for other vegetation to compete and that it causes wells to dry up. Currently, it is considered to be among the world's 100 worst invasive species, a list published by the International Union for Conservation of Nature and Natural Resources.
In 1995, the Sudanese government issued a presidential decree mandating the removal of mesquite, and the country began eradicating it.
Farther down the Atbara is New Halfa, 90,000 hectares of government-operated agricultural land irrigated by water drawn from the river. Mesquite, which was originally planted to form boundaries within the agricultural site, is now invading the cultivated plots. In 2004, a survey found that the invasive species had covered 44 percent of approximately 20,000 hectares examined. It was also said the plant was blocking irrigation canals.
Investing $25 million (1.97 billion yen) over the next two and a half years, the Sudanese state is planning to eradicate mesquite using methods including digging it up by the roots with power shovels.
"I am grateful to the state. When I tried to pull the plant out by hand the thorns cut my hand and created a terrible wound," said 53-year-old farmer Saadi Muhammad.
In 1964, when he was 5 years old, Muhammad and his parents moved to New Halfa from Wadi Halfa, a town located more than 800 kilometers away. Wadi Halfa was situated on the Nile River near the Egyptian border and was to be submerged by an artificial lake resulting from the construction of the Aswan Dam. So the Sudanese government moved the town’s inhabitants to New Halfa.
At age 11, Muhammad took over cultivation of the fields from his parents.
"First I was chased away by the dam, and then it was damage from mesquite. Now, at last, I can farm with peace of mind."
Currently, the obligation to remove mesquite lies with farmers.
"Even if I find the plant on a neighboring plot I pull it out. If not, it will eventually start growing on my land," said Muhammad. Leaving even one plant can result in a fine of 2,000 Sudanese pounds (35,000 yen, or $450), or six months imprisonment.
"This is a negative legacy of desertification countermeasures, an ironic one because the measures had good intentions," said Hiroshi Nawata, 43, an associate professor at the Research Institute for Humanity and Nature in Kyoto. "We should recognize that introducing foreign plants was a failure, and as a global society think of ways to deal with the problem."
Nawata has worked on this problem for more than 15 years.
But it is also true that mesquite—as fuel and food—can be a valuable resource in the desert.
"Because there are no other plants to rely on, some villages are dependent upon mesquite," said Mahir Salih, 55, former dean of College of Forestry and Rnage Science at the Sudan University of Science and Technology. "What is important is coexistence through skillful management, not eradication."
The Food and Agriculture Organization agrees that mesquite has some value. "It is a beneficial plant if its spread can be prevented," it has said.
Japanese researchers such as Nawata are continuing to investigate mesquite, such as its toxicity and impact on groundwater, and efficient methods for its use and extermination.
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