The fight against invasive alien species in Eastern Africa has only just begun

Gaby Allheilig

Whether tiny fauna like the army worm, tomato leaf miner, and papaya mealybug or flora like the common water hyacinth and Lantana camara: More and more non-native species in Africa are becoming invasive. Spreading rapidly, they damage ecosystems and cause severe crop failures – as seen with the army worm and maize and sorghum crops, two Eastern African staples.

Deliberate introduction of Prosopis juliflora

Not every alien animal or plant species was introduced by accident. Prosopis juliflora, for example – a mimosa plant native to Latin America – was deliberately brought to Eastern Africa about 35 years ago and initially grown on plantations. The tree, called mathenge in Kenya, mrashia in Tanzania, and Woyane hara or Derg hara in Ethiopia, is well-adapted to arid environments. Its roots penetrate up to 50 metres into the ground, enabling it to grow in places too dry for other plants.

Exclusive focus on potential benefits

This made Prosopis juliflora seem promising for water-scarce, overgrazed areas like Baringo County in Kenya, where it was intended to help contain desertification while producing firewood, charcoal, and timber. In areas highly prone to soil erosion, like Afar Region in eastern Ethiopia, the often shrub-like tree was planted in hopes of stabilizing local soils.

Displacement of other plants

But the hoped-for benefits have long since morphed into a nightmare for the local population. The sweet fruits of Prosopis juliflora, initially considered a welcome fodder, cause tooth decay and loss in livestock if they eat the pods unprocessed.

Even more worrisome, the plant spreads rapidly in arid regions, forming nearly impenetrable thickets and displacing other vegetation – including grasses on grazing land. Not only is the fast-growing and deep-rooted newcomer hard to beat in the struggle for water, there is also evidence that suggests it chemically inhibits the growth of grasses and herbs.

Management is difficult and expensive

Prosopis juliflora trees not only produce millions of seeds that remain fertile for years, they also reproduce vegetatively. Indeed, removing the plants is not enough to prevent regrowth. The only way to kill them is to treat every single plant with selected herbicides. This is enormously time-consuming and expensive.

Conflicts among livestock farmers

“The impacts of invasive species are particularly severe in Eastern Africa,” says CDE scientist Sandra Eckert. “Many people’s livelihoods are still based on agriculture and depend heavily on grassland, arable land, natural water sources, and forests.” Loss of usable land aggravates conflicts between sedentary farmers and nomadic pastoralists.

Despite the growing problems caused by Prosopis juliflora and other invasive trees and shrubs, there has been a lack of reliable information on where, how, and to what extent they affect natural resources and people’s livelihoods. “This makes it difficult to take coordinated action,” says Eckert.

Research reveals the extent of invasion

“Woody Weeds”, a collaborative research project involving scientists from Switzerland, Ethiopia, Kenya, Tanzania, and South Africa, aims to improve this situation. In a recently published partial study, the scientists found that Prosopis juliflora has already invaded 1.17 million hectares of land in Afar Region. More recent results from a comparison with previous land uses show that about 30 per cent of this was once grassland – the basis of the traditional extensive pastoralism practised by many people in the region.

Using data to develop strategies for control and prevention

“Given the size of the area affected, it is not economically viable to exterminate Prosopis juliflora everywhere,” says Eckert, putting a damper on excessively high expectations. Instead, as the remote-sensing specialist explains, the aim is to generate robust data to support the development of effective, efficient, and realistic strategies to prevent the alien species from spreading further. The data can also help to identify and protect areas that have not yet been affected and are particularly important for farming, pastoralism, and biodiversity conservation.

New method generates new insights

To support balanced containment, the scientists have determined both the geographic extent of invaded areas and the density of Prosopis juliflora in these areas. As Eckert explains, “the negative impacts of the plant only outweigh its benefits when it grows too densely.” To make optimum use of limited financial means, it is important to determine benefits and damage locally and assess them together with local communities.

Besides involving local communities, control efforts must be adapted to each specific situation. This requires knowledge of how the plant spreads. Here, too, the scientists have gained new insights: Unlike earlier studies, their detailed mapping revealed that some of the affected areas are far away from the original plantations. “Prosopis invasion is strongest where people and animals move: in settlements and along transport routes. In addition, it affects riparian areas because watercourses transport the seeds as well,” says Eckert.

First concrete achievements

The new data are meant to lay the foundations for political authorities to promote sustainable land management practices. Collaboration with the relevant authorities is off to a good start. In Tanzania, the minister for the environment has appointed three project scientists to a task force in charge of preparing a national strategic plan for management of invasive alien species. In Kenya and Ethiopia, the relevant ministries have also responded and taken first steps. Nevertheless, the fight against invasive alien species in Eastern Africa is only just beginning.