Research leads to national strategy on invasive plants

Interview: Gaby Allheilig

Prosopis juliflora was introduced to East Africa during and after the colonial era – as animal feed and a source of wood, as well as for soil stabilization and reforestation. While well-intentioned, this experiment went badly wrong: The alien tree species has spread invasively, jeopardizing entire ecosystems. How bad is the situation? 

When I was first asked to work on the “Woody Weeds” research project in 2015, I was sceptical – I thought there were more pressing challenges in East Africa than neophytes. As a geographer, I didn’t fully grasp the scale of the problem. But reality quickly proved me wrong. In the Ethiopian Afar region alone, Prosopis juliflora, the species of the Prosopis genus that’s invasive in East Africa, has spread across an area about a quarter of the size of Switzerland. Each plant consumes an average of seven litres of water per day. Extrapolated to this region, that’s three billion cubic metres per year. 

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“Models show that the situation could drastically worsen” 

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So it’s mainly about water consumption? 

Not just. Prosopis trees grow roots up to 30 metres long. Their enormous water consumption lowers the groundwater level, displacing other plants and destroying biodiversity: When Prosopis juliflora cover reaches 60 per cent, there’s almost no plant diversity left. In Marigat sub-county in Kenya, for example, 40 per cent of the loss of native Vachellia trees between 1988 and 2016 was caused by Prosopis.

And the situation could drastically worsen: Models show that the Prosopis invasion in East Africa is at an early stage. Around half of Tanzania is suitable for the growth of Prosopis, and in Kenya it’s even three quarters of the entire country area, around 450,000 out of 580,000 square kilometres. 

The research project also analysed the socio-economic effects. What were your main findings? 

We found that the Prosopis invasion didn’t affect all members of a community in the same way. Prosopis is what we call a “conflict plant” – some groups, such as charcoal producers, benefit from it and therefore have little interest in fighting it. Others, mainly pastoralists, suffer greatly from its negative effects. Prosopis threatens their livelihoods, especially when it attacks the dry season pastures they rely on. But farmers are also affected. In Kenya’s Baringo and Tana River counties, Prosopis has spread across important irrigated cropland, where farmers are now forced to spend a lot of time and energy trying to keep the invasive plant in check. This reduces productivity.

You mentioned the economic consequences… 

Yes, because we’re talking about a considerable economic burden for the affected rural households and communities. In Ethiopia, the cost of the Prosopis invasion at the national level is currently estimated at USD 6 million per year – a figure that is forecast to rise to USD 75 million in 30 years if no measures are taken to stop the spread. In Tanzania, costs are expected to reach USD 100 million over the same period, while in Kenya, they are projected to reach up to USD 175 million per year. 

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“Sometimes international organizations still recommend reforestation with Prosopis today – because of the climate” 

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Why wasn’t anything done sooner? 

When Prosopis is first introduced in a given place, the local population appreciates the tree’s benefits. It’s only once the negative consequences become apparent that awareness of the problem begins to rise – but by then it’s often too late to eradicate the plant. In addition, as I mentioned, opinions differ depending on the group as to how urgent and necessary it is to manage Prosopis. This makes it difficult to take effective action. 

And unfortunately, this confusion is sometimes reinforced by international organizations that recommend Prosopis for carbon sequestration. These recommendations overlook the fact that healthy grasslands and savannahs store almost as much carbon as forests – and are also highly biodiverse ecosystems that provide important services. 

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“Eradicating Prosopis is almost impossible” 

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Are there even any ways of getting rid of Prosopis in East Africa? 

Eradication is almost impossible in areas that are heavily infested. So, the question is how to manage Prosopis. Our research indicates that we should pursue three main objectives. Firstly, prevention. That is, preventing the species from taking hold in new areas. Secondly, systematic monitoring of areas where Prosopis occurs only sporadically, and immediate removal of saplings. And thirdly, control. This means stopping the spread at the edges of heavily infested regions and protecting important resources such as water sources or water holes, as well as infrastructure. 

That sounds simple enough... 

In reality, the management of Prosopis is highly complex. One preventive measure that would actually be quite easy to implement is to quarantine cattle on their migration routes, so that they excrete all Prosopis seeds before moving on to an area that’s not yet infested. The difficulty with this is that people don’t realize the importance of preventive measures in areas where Prosopis isn’t yet causing a problem.

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“Experts believe that further spread can be stopped” 

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Yes, there are various techniques to achieve this. On small areas, Prosopis can be removed mechanically. This is very labour-intensive, as the rootstock has to be removed to a depth of 50 centimetres.

On larger areas and under certain conditions, bulldozers can be used. However, this only works if you immediately plant crops or sow grasses on the cleared area – and systematically remove new shoots. Treatment with chemicals is also possible. We describe these techniques in a small handbook. But for very large areas, experts believe that only biological control offers an effective solution. 

Which means? 

Releasing natural controllers of Prosopis. Trials in South Africa and Australia have demonstrated that although complete eradication may not be feasible, this approach can effectively halt the plant’s spread and, importantly, do so at a very low cost. 

At the risk of replacing one evil with another, and creating a new problem? 

These days, testing is extremely rigorous and must show that the organisms used – usually insects – are highly specialized and will die rather than attack another plant species. 

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“A study on the water consumption of Prosopis was a bombshell” 

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How did the countries concerned react to the research results? 

Findings by Hailu Shiferaw, an Ethiopian doctoral student, on the enormous water consumption of Prosopis came as a bombshell to the affected areas in Kenya. Local authorities then alerted the national government.

At the same time, Urs Schaffner, project coordinator of CABI Switzerland, continuously fed the results back to the national level, raising awareness and encouraging action. Finally, the Kenyan cabinet adopted a national Prosopis strategy, which had previously been developed with project partners. Tanzania’s national invasive species strategy was also significantly influenced by our project and co-designed by our project partners. 

And how is the strategy now being implemented on the ground?

In Kenya, we investigated ways of implementing it in three districts that together cover an area larger than Switzerland. We worked with implementation groups comprising representatives of various local stakeholder groups to jointly develop spatially explicit Prosopis management plans. The authorities of the three districts have integrated these plans into their official planning and are now eligible to apply for national-level budgets to combat Prosopis. 

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“Pilot initiatives achieved significant success” 

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At the same time, pilot initiatives have achieved significant success. For example, it was possible to remove all Prosopis trees in the Lake Bogoria National Reserve, where they were a deadly trap for flamingos due to their long thorns. 

The results have previously been published in scientific journals. You’ve now released a book. Why? 

There is indeed a wealth of research literature on Prosopis. Eight doctoral theses were written in the “Woody Weeds” project alone. But this knowledge is scattered across various individual articles in different journals. This makes it difficult to access for decision-makers and people in the field. In addition, the book integrates the experiences and findings from seven years of research, and demonstrates the link between research and implementation in a comprehensible way.