Woody invasive alien species in East Africa

A village thickly overgrown with Prosopis juliflora.

Many exotic trees and shrubs have been introduced to Africa. Some of them, for example Prosopis spp. or Lantana camara, have become invasive and are threatening important ecosystem services. In South Africa alone, invasive alien plant species have been estimated to cause costs related to the loss of ecosystem services of USD 1 billion annually.

Woody invasive alien species affect ecosystems and local livelihoods

In East Africa, where peoples’ livelihoods still strongly depend on natural resources like grassland, cropland, forests, and water, the impacts of invasive species are severe. At the same time, there is a lack of reliable information on where, how, and to what extent natural resources and people are affected. Accordingly, there is also a lack of coordinated and effective sustainable management of woody invasive alien species.

The project areas: Afar (Ethiopia), Baringo (Kenya), Moshi and Amani (Tanzania). Map: Sandra Eckert; Landsat background imagery courtesy of U.S. Geological Survey

Mitigating the negative effects

The inter- and transdisciplinary research project “Woody invasive alien species in East Africa” (Woody Weeds for short) aims at mitigating the negative effects of woody invasive alien species on biodiversity, ecosystem services, and human well-being in East Africa.

To achieve this goal, the project focuses on:

  1. Understanding how different levels of invasion impact positively or negatively on natural resources and, consequently, on peoples’ livelihoods
  2. Assessing spatially and quantitatively the past, current, and potential future spread – considering different climate scenarios – of selected woody invasive alien species in three case study areas and at the country level
  3. Based on the findings, developing and testing technologies of invasive species management – prevention, early detection and rapid response, as well as biological, chemical, and physical control – jointly with local implementation groups and based on methodologies developed by the World Overview of Conservation Approaches and Technologies (WOCAT)
  4. Finding entry points into national policy processes with a view to fostering awareness and the uptake of sustainable land management practices

CDE leads the work packages that include activities 2–4 above.

First results

Spatial and quantitative assessments in Afar Region, Ethiopia, have shown the following:

  • In the 35 years since its introduction to Afar, Prosopis has invaded approximately 1.17 million hectares of land. About 30% of this was formerly grassland.
  • The most severe Prosopis invasion occurred within a strip of ten to a few hundred metres along roadsides and rivers. The banks of the Awash river, Afar’s main source of water, have been invaded to the point where local pastoralist communities and their livestock are no longer able to access the water.
  • Prosopis trees use large amounts of water. Measurements in Afar showed that one tree absorbs an average of around 6.8 litres of water daily. This adds up to 3.1 to 3.3 billion cubic metres of water annually used by Prosopis throughout the invaded areas in Afar Region.
  • Reduced access to water and the degradation of grasslands has led to conflicts among local communities.

In Baringo County, Kenya, similar assessments yielded the following findings:

  • Prosopis coverage increased from 882 hectares in 1988 to 18,792 hectares in 2016. At the same time, grasslands decreased by 6,252 hectares, cropland by 2,281 hectares, and native tree vegetation by 3,602 hectares. All three land uses are important for rural people’s livelihoods and for biodiversity.
  • Healthy and restored grasslands store significantly more soil organic carbon than land covered with Prosopis.

In addition, the project successfully identified entry points into national policy processes through which it will promote the uptake of sustainable land management practices:

  • In Kenya, the Ministry of Environment and Forestry requested project partners to identify practical solutions for Prosopis management.
  • In Tanzania, the minister for the environment appointed three project scientists into a task force in charge of preparing a national strategic plan for the management of invasive alien species.
  • In Ethiopia, the Ministry of Environment, Forestry and Climate Change is represented in the local implementation group of the Woody Weeds project.
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