Study calls for rapid action to stop spread of woody weeds in Eastern Africa

Bern, 13 February 2020

Invasive alien species are threatening large tracts of rangeland and causing biodiversity loss in Eastern Africa. An international team of scientists, including of CDE, are now warning that woody weeds like Prosopis juliflora risk invading vast new areas. They recommend a coordinated management plan to curb their spread.

Lead authors of a new study in Ecosphere, Sandra Eckert, Centre for Development and Environment, University of Bern, and CABI’s Urs Schaffner, say that unless steps are taken to control Prosopis juliflora and Lantana camara it could spread to unreached areas of Kenya, Ethiopia and Tanzania.

P. juliflora and L. camara are both considered to be amongst the world’s 100 worst invasive species with the former being introduced in the 1970s and 1980s as part of dryland reforestation programmes and the latter in the 19th century as an ornamental plant.

However, the scientists, including those from the University of Nairobi, Sokoine University of Agriculture in Morogoro, and CDE’s partner institution, the Water and Land Resource Centre in Addis Ababa, suggest that these noxious weeds – exacerbated by climate change – will seriously impact upon grazing land and drive out native flora and fauna if left unmanaged.

“Lowland regions are seriously threatened”

Sandra Eckert said, “The semi-arid to arid regions of Ethiopia and Kenya, essentially all areas below about 1600m, are either currently or at risk of being invaded by P. julifora. It has also started to invade the north of Tanzania and could even affect neighbouring regions such as Sudan and South Sudan.”

A previous study co-authored by Sandra Eckert and Urs Schaffner showed that P. juliflora can spread very rapidly; in the Afar region in Ethiopia, this weed has invaded 1.2 million ha of rangeland, shrubland and crop fields within 35 years.

Sandra Eckert added, “Our data clearly show that the whole Afar and large parts of Somali region are suitable for invasion by P. juliflora, so these lowland regions, once known for their grasslands and pastoralist communities, are seriously threatened.

“In respect of L. camara, the Ethiopian and Kenyan highlands as well as northern and central Tanzania have the highest risk of invasion. Our model suggests that Zambia could also be a target in the future.”

The researchers believe that a “spatially explicit management strategy that ensures concerted communication and management across national and subnational borders” is needed to nip the woody weeds scourge in the bud.

They have been investigating the spread of invasive woody species in Eastern Africa since 2015, in a project funded by the Swiss Programme for Research on Global Issues for Development (r4d).

“Early detection and rapid response will be key”

Urs Schaffner said, “Our results reveal that 'early detection and rapid response', i.e. the identification of the first trees in new areas and a rapid removal of these trees, will be key for slowing down the fast spread of P. juliflora. Surveillance of areas still free of P. juliflora is warranted not only within the boundaries of the current climate, but also in areas which will become suitable in the context of climate change. In the case of P. juliflora, the planting of saplings and dispersal of seeds should be prohibited in areas bordering the current range of suitable habitats – for example south-western Tanzania and northern Zambia.”

“In respect of L. camara our findings indicate that this species has already colonized a significant part of its ecological niche in Eastern Africa and that prevention measures may be less warranted than with P. julifora. Rather, management should be targeted at preventing L. camara from building up high local densities, as currently observed in areas boardering Lake Victoria. We recommend that management is in line with more detailed maps of the invasive species’ current distribution – ideally with information on local abundance or cover.”

Go to the study


Please note that the photo is exclusively for use in reporting on the publication of “Niche change analysis as a tool to inform management of two invasive species in Eastern Africa”, and the photographer must be credited.

Prosopis invasion along an irrigation channel of the Awash river in Ethiopia. The dark clumps are native tree species, the rest are invasive alien species. © Hanspeter Liniger, CDE