Findings on destructive invasive trees released in book

A new book on the ecology and management of one of the most destructive invasive weeds in Africa, is now available online.

The book, titled The Ecology and Management of Invasive Prosopis Trees in Eastern Africa, is the outcome of a large, decade-long collaborative research project, involving six universities and eight institutes from Ethiopia, Kenya, Tanzania, South Africa, Switzerland, and the United States of America (USA).

Between the 1970’s and the 1990’s, several Prosopis species, originally from the Americas, 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 species, 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 community members from Baringo County in Kenya brought a toothless goat to a Nairobi court as evidence of one of the many impacts of Prosopis pods on their livestock.

Professor Brian van Wilgen, one of the editors of the book and emeritus professor in invasion biology at Stellenbosch University, says to date this tree has invaded close to ten million hectares (roughly the size of the Northwest Province) of land in eastern 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,” he says.

The negative effects of this “conflict” species are so encompassing that people are forced 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 exacerbating the effects of a changing climate.

A significant finding is that the invasion in eastern Africa is still at a very early stage. If left uncontrolled, however, 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 USD 375 million per 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.

Professor Van Wilgen says the book fills a major gap in the field of invasion biology, as African-based research is virtually non-existent 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, but that only 3,4 % of these 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 warns.

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

Professor Philip Hulme, distinguished professor of plant biosecurity in the Department of Pest-Management and Conservation at Lincoln University, New Zealand, writes in the Foreword that the book presents “an outstanding overview of the problems arising from misguided policies that favoured short-term results over longer-term environmental and social consequences.

“This robust evidence base that quantifies the negative impacts sends a clear warning to those individuals, institutions and organizations that continue to consider Prosopis species as valuable multipurpose trees. The prolific growth rate and high dispersal efficiency of Prosopis trees mean that, despite the best intentions of those seeking to use it as a multipurpose tree, the ultimate outcome has been widespread invasion and a subsequent worsening of the environment as well as the quality of human life.”