Home Africa News Prosopis invasions: Battle against Africa’s worst weed

Prosopis invasions: Battle against Africa’s worst weed

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When it comes to tackling one of Africa’s most destructive invasive weeds — the Prosopis species — there are no clear-cut answers.

The weeds were introduced to South Africa in the late 1800s and widely distributed and planted up to the 1960s for shade and fodder during a time of severe drought, according to a 2015 study. Invasive Prosopis stands, comprising several species and their hybrids, now cover very large areas of arid and semi-arid parts of the country, with extensive invasions in the Northern Cape and Western Cape. 

“First of all you must stop them from spreading to new areas, so you get them when there’s still only a few of them, and you try to eliminate them,” said Brian van Wilgen, an emeritus professor in invasion in biology at Stellenbosch University

“The problem is that people don’t realise they’re a problem until it is too late … and then it becomes too expensive. We’re trying to raise some awareness here about what you’re letting yourselves in for if you let them get out of control.”

Van Wilgen is one of the editors of a new book on the ecology and management of one of the most damaging invasive weeds on the continent. It is the outcome of a decade-long collaborative research project, involving six universities and eight institutes from Ethiopia, Kenya, Tanzania, South Africa, Switzerland and the United States.

The invasion is mainly in the Northern Cape, Van Wilgen said. “It’s the biggest province but it’s pretty dry, so they’re invading there and in parts of the Free State, the Western Cape and bits of the North West.”

Prosopis is estimated to have invaded 1.8 million hectares (1.5%) of South Africa and to have spread by 3.5% to 8% a year, according to the study.

“This implies that invaded areas can double every five to eight years. In the Northern Cape, Prosopis invasions increased by almost one million ha between 2002 and 2007, which is equivalent to 27.5% per year,” it said.

In terms of land area invaded, Prosopis is ranked as the second-worst invasive alien plant taxon in South Africa after Australian Acacia species. “The genus also ranks highly for its negative impacts on biodiversity and ecosystem services.”

Prosopis is also invasive in Namibia and Botswana, Van Wilgen said. “These trees come from Central America; they’re pretty hardy. They can grow in pretty dry places and they do quite well. They would provide firewood, the seedpods are eaten by cattle and are cheap and so the government in the 1950s promoted them. 

“They said: ‘Here farmers, have some weeds. You can plant them on your farm’, and they supplied seeds and everything and, of course, they got out of control. They didn’t predict it at the time. Now they form these dense impenetrable thickets; they shade out all of the grass so there’s nothing for the cattle to eat.”

With their very deep tap roots, the weeds also consume a lot of water that can cause farmers’ wells to dry up.

Fig6.2 S
Prosopis juliflora tree invasions along a river in the Bogoria District, Kenya. Photo: Urs Schaffner

Eastern Africa

From the 1970s to the 1990s, several Prosopis species were introduced on a large scale to countries in Eastern Africa by international aid agencies to provide benefits such as timber, fodder and shade, and to combat desertification.

One of those, Prosopis juliflora, subsequently became invasive and is now regarded as a classic example of a well-intentioned action gone wrong. 

In 2006, the detrimental effects of Prosopis invasions made international headlines when people in Baringo County in Kenya brought a toothless goat to a Nairobi court as evidence of one of the many harmful effects of Prosopis pods on their livestock.

According to Van Wilgen, this tree has invaded close to 10 million ha of land in East Africa. “Although these trees were initially introduced for the benefits that they could provide, these benefits have subsequently been cancelled out and are now considerably exceeded by negative impacts.” 

The negative effects of this “conflict” species are so encompassing that people have to leave their land to try and make a living elsewhere. The trees fundamentally change the structure and functioning of communal rangelands, croplands, wetlands and protected areas. Open savannas become dense, impenetrable stands of trees with little or no grass below them. 

In densely invaded areas, Prosopis trees can consume about 50% of the annual rainfall of the region, thus severely depleting groundwater resources and worsening the effects of a changing climate.

A significant finding is that the invasion in East Africa is still at a very early stage. If left uncontrolled, it is likely to expand to cover three quarters of Kenya and almost half of Ethiopia and Tanzania, respectively. The economic costs of such a scenario could grow to $375 million a year. 

Kenya stands out as having the largest proportion of suitable areas for future invasions: almost all its arid and semi-arid lands are at risk of being invaded if spread is not contained.

Efforts to control Prosopis include cutting down trees, poisoning stumps and using biological control methods such as introducing host-specific insects that only target the invasive trees. But managing these invasions is costly and difficult, particularly for farmers who lack the resources to carry out large-scale control efforts.

“That’s a huge job [cutting trees], especially when you think of the vast area that’s now been invaded. It’s beyond affordability for most of those farmers; they’re living on the edge anyway. They’re not rich people and to pay and then to keep them under control is quite challenging,” Van Wilgen said.

Fig7.3undernourishedlivestock2(1) S
Undernourished cattle in a rangeland invaded by Prosopis juliflora in Kenya. Note the total absence of grasses and other herbaceous plants. Photo: Urs Schaffner

Filling a major gap

He said the book fills a major gap in the field of invasion biology, as African-based research is limited in the peer-reviewed literature.

A review of the field of invasion biology in 2008 established that 2 670 research papers had been published in the peer-reviewed literature since 1980. Only 3.4 % were from Africa, with three-quarters of those being from a single country: South Africa.

“This dearth of relevant understanding has dire consequences for the management of a significant environmental problem on the African continent,” he said.

The authors hope the book will fill this gap and become the “go-to” source for policy- and decision-makers in Africa.

Many of the findings and insights reported in this book arise from the research under the Woody Weeds project that ran from 2015 to 2024. It was funded by the Swiss National Science Foundation and the Swiss Agency for Development and Cooperation under the Swiss Programme for Research on Global Issues for Development, as well as the Swiss Agency for Development and Cooperation.

Van Wilgen said: “The project that we worked on was funded by the Swiss government, so Swiss aid money and it because up until fairly recently, a lot of research and effort was going into South Africa and very very little in Eastern Africa. It was an attempt to try and help those countries a bit.

About 10 doctoral students and about 10 master’s students were involved in the project. “They’ve all written their work up and they’ve published in various journals all over the place. All of this information is scattered; nobody had put all of this information in one place and that’s what we tried to do.”

In densely invaded areas, the trees can consume about 50% of the annual rainfall of the region, severely depleting groundwater