Scientists tackle one of Africa’s most destructive invasive trees

An important new book on the ecology and management of one of Africa's most destructive invasive plants is now available online.The book, titled 'The Ecology and Management of Invasive Prosopis Trees in Eastern Africa', is the culmination of a decade-long research project involving six universities and eight institutions from Ethiopia, Kenya, Tanzania, South Africa, Switzerland and the USA.Between the 1970s and 1990s, international aid organisations introduced various Prosopis species, originally from the Americas, on a large scale to countries in Africa, including Namibia, as a countermeasure against desertification, highlighting their benefits such as wood, livestock fodder and shade.One of the species introduced to eastern Africa, Prosopis juliflora, subsequently became an invasive problem and is now considered a classic example of a well-intentioned plan gone wrong.According to Atlas Namibia, there are three species of prosopis in Namibia: Prosopis glandulosa is most common. Prosopis trees were introduced to Namibia from the arid Americas for shade and fodder. Although drought-tolerant and nutritious, they use a lot of groundwater and have invaded pans and rivers such as the Fish, Nossob and Orange. They outcompete indigenous riverine trees by depriving them of groundwater, and in places, they have completely choked the riverbed. Livestock eat the pods, which helps spread the seeds.“In Namibia, the Prosopis species is a highly invasive plant and has been labelled as a great concern in this country,” Stephanie De Lange wrote in the article 'The Prosopis – a formidable opponent'.Harmful impactIn 2006, the harmful impact of Prosopis invasions gained international attention when community members from Baringo County in Kenya brought a toothless goat to a Nairobi court as evidence of one of the many negative effects of Prosopis pods on their livestock.Prof Brian van Wilgen, one of the editors of the book and an emeritus professor in invasion biology at Stellenbosch University's Department of Botany and Zoology, says this tree has, to date, taken over approximately ten million hectares of land in eastern Africa – roughly the size of South Africa's North West Province.“Although these trees were initially planted for the benefits they offered at the time, those benefits have since been negated and are now significantly outweighed by the negative impacts,” he explains.The detrimental effects of these “conflict” species are so extensive that people are being forced to abandon their land and attempt to rebuild their lives elsewhere.The trees have fundamentally altered the structure and functioning of communal grazing areas, farmlands, wetlands and protected areas. Open savanna landscapes have become densely vegetated with impenetrable trees, leaving little to no grass beneath them. In areas of dense invasion, Prosopis trees can use approximately 50% of the annual regional rainfall, completely depleting groundwater resources and exacerbating the impacts of climate change.A significant finding is that the invasion in eastern Africa is still at a very early stage. However, if left uncontrolled, it is likely to expand and eventually cover three-quarters of Kenya’s surface area and nearly half of Ethiopia’s and Tanzania’s territories.The estimated economic cost of such a scenario could reach up to US$375 million per year.Kenya is the country most at risk in terms of potential future invasion, with nearly all its arid and semi-arid lands vulnerable to being overtaken if the spread is not controlled.Managing the troublesome weedVan Wilgen says the book addresses a significant gap in the field of invasion biology, as Africa-based research is almost non-existent in peer-reviewed literature.“A 2008 review of invasion biology as a discipline found that 2 670 research articles had been published in peer-reviewed journals since 1980, but only 3.4% of these came from Africa – and three-quarters of those from a single country, South Africa. This lack of relevant understanding has severe implications for managing a major environmental problem on the African continent,” he warns.The authors hope the book will fill this knowledge gap and become a go-to resource for Africa’s policymakers and decision-makers.Prof. Philip Hulme, distinguished professor of plant biosecurity in the department of pest management and conservation at Lincoln University in New Zealand, writes in the foreword that the book “provides an excellent overview of the problems arising from misguided policies that prioritised short-term outcomes over longer-term environmental and social consequences.“This robust evidence base, which quantifies the negative impacts, serves as a clear warning to those individuals, institutions, and organisations that continue to regard Prosopis species as valuable multipurpose trees. Due to their rapid growth rate and high dispersal efficiency, widespread invasion by Prosopis trees, along with resulting environmental degradation and a decline in overall human quality of life, has been the outcome—despite the best intentions.”The 272-page book follows 10 years of extensive research, and many of the findings and insights in this book are the result of research conducted during the ten-year-long Woody Weeds project between 2015 and 2024.This project was funded by the Swiss National Science Foundation and the Swiss Agency for Development and Cooperation (as part of the Swiss Programme for Research on Global Development Issues, as well as the Swiss Agency for Development and Cooperation).The book is available online at The Ecology and Management of Invasive Prosopis Trees in Eastern Africa | CABI Invasives Series.- Stellenbosch University

Scientists tackle one of Africa’s most destructive invasive trees

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Namibian Sun 2025-04-03

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Scientists tackle one of Africa’s most destructive invasive trees

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Namibian Sun 2025-04-03

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